Are involuntary autobiographical memory and déjà vu natural products of memory retrieval?

Abstract

Involuntary autobiographical memories (IAMs) and déjà vu are phenomena that occur spontaneously in daily life. IAMs are recollections of the personal past, whereas déjà vu is defined as an experience in which the person feels familiarity at the same time as knowing that the familiarity is false. We present and discuss the idea that both IAMs and déjà vu can be explained as natural phenomena resulting from memory processing and, importantly, are both based on the same memory retrieval processes. Briefly, we hypothesise that both can be described as “involuntary” or spontaneous cognitions, where IAMs deliver content and déjà vu delivers only the feeling of retrieval. We map out the similarities and differences between the two, making a theoretical and neuroscientific account for their integration into models of memory retrieval and how the autobiographical memory literature can explain these quirks of daily life and unusual but meaningful phenomena. We explain the emergence of the déjà vu phenomenon by relating it to well-known mechanisms of autobiographical memory retrieval, concluding that IAMs and déjà vu lie on a continuum.

1. Introduction

It's exactly the same as the last time, it's all come back to me … it's behind, it's always thoughts from childhood, it's always visual, it's a place behind the house, the field where my father put his car, near a lake … It's not always the same countryside; I've forgotten the story of this countryside … Yes, it's pleasant because we were going to get the car from behind the house, it's a happy memory, it's never unpleasant.

Vignal, Maillard, McGonigal, and Chauvel (2007, p. 92)

I have the impression that everything around me has been here before, that it has already happened, I feel as if I am going backwards in time.

Vignal et al. (2007, p. 92)

I could be carrying out an everyday activity – e.g., driving/attending lecture – and visual images appear in my head – possibly of people I know/places I've been (with no relevance to what's going on in reality). I could be doing the housework and certain melodies I hadn't heard for a while pop into my head.

Kvavilashvili and Mandler (2004, p. 89)

It feels like you're living an experience that you've already lived through or perhaps dreamed. As each small detail of the experience unfolds it seems as though you knew it was going to happen--as if you could have described the scene before it even existed. Each time I feel a strong conviction that I've seen all of it happen before.

Jersakova, O'Connor, and Moulin (2015, data available online)

These quotes capture two different spontaneous cognitions, in two different populations. The first two refer to reports of spontaneous thoughts cued by electrical stimulation of the brain (for a review, see Curot et al., 2017). The second two are subjective reports of the same phenomena in healthy people. One phenomenon concerns the reproduction of a fully formed scene from the personal past, an involuntary autobiographical memory (IAM; Berntsen, 1996, 2010), the other concerns a vaguer feeling described as déjà vu (Moulin, 2018). Here, we outline how these phenomena are related, with the view that theories of memory function should take them into account.

The starting point for our synthesis comes from Penfield's (e.g., Penfield & Perot, 1963) intracortical stimulation studies, whereby current applied to the temporal cortex produced “interpretative illusions.” In their review of 520 stimulations of patients' temporal lobes, Penfield and Perot (1963) reported that 7% of stimulations produced “responses” devoid of content, and 10% of stimulations produced “hallucinations,” which included sensory content, and which were mostly autobiographical experiences. In contemporary work, Vignal et al. (2007) showed that stimulation of the same electrical contact in the temporal lobe either produced déjà vu (with lower voltages and more restricted discharges) or a more fully formed specific memory. It seems that at least some forms of retrieval from the personal past and déjà vu share underlying neurological and/or cognitive processes, and the difference between them may be expressed on a continuum. Where sufficient activation exists from a cue in the environment, there is the experience of effortless retrieval, whereas where retrieval fails it can prompt unusual and seemingly out-of-context feelings. These experiences are not restricted to electrical stimulation; involuntary memories and déjà vu occur in healthy participants, and can be provoked in the laboratory.

We map out the similarities and differences between these phenomena (see Table 1), and provide an account for their integration into theories of memory retrieval. Both can be described as “involuntary” or “spontaneous cognitions,” one delivering content, and the other delivering only a feeling. The development of a theory which integrates these spontaneous cognitions into a memory retrieval framework is thus of value, especially since a novel development in the field is that such phenomena are underpinned by the default mode network (DMN; e.g., Andrews-Hanna, Reidler, Huang, & Buckner, 2010; Buckner, Andrews-Hanna, & Schacter, 2008; Raichle, 2015; Stawarczyk, Majerus, Maquet, & D'Argembeau, 2011; Van Calster, D'Argembeau, Salmon, Peters, & Majerus, 2017), and changes in spontaneous cognition might be one of the earliest signs of temporal-lobe pathology (Kvavilashvili, Niedźwieńska, Gilbert, & Markostamou, 2020). Any account of memory retrieval ought to consider these two phenomena.

Table 1. The summary of similarities and differences between déjà vu and involuntary autobiographical memories (partially inspired by Kvavilashvili, 2015, Figure 1).

There has also been a shift of attention in cognitive psychology from studying goal-oriented, voluntary, and effortful forms of human cognition to studying phenomena that come to mind spontaneously and/or without any preceding intention to think about them. This has resulted in the emergence of several independent research fields on (1) mind wandering and daydreaming (e.g., Christoff, Irving, Fox, Spreng, & Andrews-Hanna, 2016; Smallwood & Schooler, 2015), (2) IAM (e.g., Berntsen, 2009; Mace, 2007) and automatically retrieved voluntary memories (Barzykowski & Staugaard, 2016; Uzer, Lee, & Brown, 2012), (3) spontaneous future thinking (Cole & Kvavilashvili, 2019, 2021), (4) spontaneous retrieval of prospective memory tasks (Gilbert, Hadjipavlou, & Raoelison, 2013; Kvavilashvili & Rummel, 2020; Niedźwieńska & Barzykowski, 2012; Niedźwieńska, Rendell, Barzykowski, & Leszczyńska, 2014), (5) involuntary semantic memories or mind-pops (Kvavilashvili & Mandler, 2004), (6) intrusive memories (Marks, Franklin, & Zoellner, 2018), (7) involuntary musical imagery (earworms; Hyman et al., 2013), (8) déjà vu and associated spontaneous metacognitions (Moulin, 2018), and (9) game transfer phenomena (Ortiz De Gortari & Griffiths, 2015). As yet, these spontaneous phenomena from daily life are studied separately (but see Barzykowski, Hajdas, Radel, Niedźwieńska, & Kvavilashvili, 2021a, 2022; Barzykowski, Radel, Niedźwieńska, & Kvavilashvili, 2019b; Plimpton, Patel, & Kvavilashvili, 2015; Vannucci, Pelagatti, Hanczakowski, & Chiorri, 2019), significantly impeding our understanding of their nature and shared underlying mechanisms. To move towards an integration of these phenomena, we start with IAMs and déjà vu; specific forms of spontaneous cognition for the personal past, where there have been significant developments recently in the literature.

1.1. Involuntary autobiographical memories (IAMs)

Autobiographical memory is the ability to remember events from our personal past (Conway & Pleydell-Pearce, 2000). Theories distinguish between involuntary and voluntary retrieval in autobiographical memory as a consequence of conscious intention (i.e., wanting to remember). IAMs are the reminiscence of personal events that come to mind without any conscious attempt at retrieval (Berntsen, 1996, 2010). They are contrasted with voluntary autobiographical memories (e.g., Barzykowski & Staugaard, 2016, 2018; Schlagman & Kvavilashvili, 2008). Voluntary memories typically involve an effortful and time-consuming strategic search (Botzung, Denkova, Ciuciu, Scheiber, & Manning, 2008; Burgess & Shallice, 1996; Conway, 1990, 2005; Conway & Loveday, 2010; Conway & Pleydell-Pearce, 2000), but they may also be directly recalled (i.e., automatically and effortlessly) in a similar, yet still somewhat different, way as involuntary memories (Uzer et al., 2012; see also Barzykowski & Staugaard, 2016; Harris, O'Connor, & Sutton, 2015; Uzer & Brown, 2017). When we use our memory intentionally, for example, when trying to recall whether we have already paid a conference fee, we retrieve voluntary autobiographical memories. However, memories may sometimes pop into our mind without any retrieval attempt, for example, when driving, a memory of meeting our partner for the first time may enter into consciousness without being sought-for.

Involuntary memories are now treated as a common example of remembering (e.g., Berntsen, 2010, 2015; Brewin, Gregory, Lipton, & Burgess, 2010; Clark, Mackay, & Holmes, 2013; Moulds & Krans, 2015) and as a phenomenon worthy of investigating in and outside the laboratory (e.g., Berntsen, 1996; Roberts, McGinnis, & Bladt, 1994; Schlagman & Kvavilashvili, 2008). IAMs were overlooked in cognitive psychology for several decades (e.g., Miller, 1962/1974), but starting from the nineties (e.g., Berntsen, 1996; Roberts et al., 1994), they have been investigated using three main methods (see Berntsen, 2009, for a more detailed review): Survey methods (e.g., Berntsen, Rubin, & Salgado, 2015), structured diaries (e.g., Johannessen & Berntsen, 2010; Mace, 2005), and experimental methods (e.g., Ball, 2007; Barzykowski & Niedźwieńska, 2016, 2018a; Mace, 2006; Schlagman & Kvavilashvili, 2008; Vannucci, Batool, Pelagatti, & Mazzoni, 2014).

The laboratory method developed by Schlagman and Kvavilashvili (2008; with further modifications, e.g., Barzykowski, Niedźwieńska, & Mazzoni, 2019a; Plimpton et al., 2015; Vannucci et al., 2019) allows the examination of the retrieval phase of this spontaneous phenomenon under controlled conditions (e.g., Barzykowski, 2014; Barzykowski & Niedźwieńska, 2012). Participants are engaged in a monotonous vigilance task requiring little attention (such as detecting 15 infrequent target vertical lines in a stream of 785 slides with horizontal lines) while exposed to irrelevant word phrases (e.g., buying a baguette, a wonderful smile, an unpleasant conversation), some of which may incidentally trigger involuntary memories (e.g., I remember buying fresh and still warm bread from the bakery's back window when getting back from a crazy all night party; this was something I very much needed when going home at 4am). Participants are instructed to write down any spontaneously occurring thought and/or memories during the vigilance task. Schlagman and Kvavilashvili's (2008) participants reported on average approximately six involuntary memories (ranging from 1 to 24) during laboratory sessions.

While IAMs were treated at first as a relatively rare phenomenon (e.g., Davachi & Dobbins, 2008), they are now considered common in daily life (Berntsen, 2010, 2015; Brewin et al., 2010; Clark et al., 2013; Moulds & Krans, 2015). Involuntary remembering may be even more frequently experienced in daily life than voluntary and effortful autobiographical retrieval (e.g., Rasmussen & Berntsen, 2011; Uzer et al., 2012). The high prevalence of IAMs is also demonstrated by people's tendency to classify automatically (i.e., effortlessly) retrieved voluntary memories as involuntary memories even though these memories were retrieved in response to a direct instruction to do so (e.g., Barzykowski, Staugaard, & Mazzoni, 2021b; Sanson, Cardwell, Rasmussen, & Garry, 2020). IAMs may be considered as a universal well-known and frequently experienced phenomenon that commonly occurs in the general population and which occurs in healthy autobiographical memory.¹ While typical frequency estimates vary depending on the method used (e.g., Laughland & Kvavilashvili, 2018), the average frequency of IAMs is in the order of a dozen per day (e.g., roughly 20 memories per day; Rasmussen & Berntsen, 2011).

The results of naturalistic diary studies, where participants are instructed to record and describe IAMs and their accompanying context immediately as they occur in everyday life (e.g., Berntsen, 1996; Laughland & Kvavilashvili, 2018), have shown that IAMs are more likely to be reported when attention is diffuse (Berntsen, 1996; 2009), and when the individual is engaged in an automatic activity with low attention and low cognitive demands (e.g., washing-up, walking, ironing). They also arise in response to incidental external and/or internal cues that usually overlap with key features of the memory content (e.g., seeing a given mug on a desk may trigger a certain past episode of receiving it as a gift; Berntsen, 1998). This is supported by data from laboratory settings: The content of IAMs directly corresponds to experimentally provided cues (e.g., Barzykowski & Niedźwieńska, 2018b; Plimpton et al., 2015; Vannucci, Pelagatti, Hanczakowski, Mazzoni, & Paccani, 2015, 2019). This idea, that IAMs are in fact a response to cues in the environment even though they are experienced as involuntary, is a critical aspect of our argument and we explain this focussing on the notion of cognitive control.

It could be argued that the key difference between involuntary and voluntary memories is the presence or not of cognitive control. When trying to deliberately retrieve a given memory (e.g., eating Ramen with close friends two weeks ago) in response to a certain cue (e.g., eating Ramen), one has to actively and strategically search for a representation that specifically relates to that cue. Thus, there is a cascade of cognitive processes leading to successful memory retrieval: (1) Actively elaborating on the cue, (2) monitoring the stream of consciousness looking for a thought corresponding to a cue, (3) deciding whether the thought relates to the searched episode from the personal past and (4) deciding to terminate search or continue the search until a better match is found (see also Mace, Clevinger, Delaney, Mendez, & Simpson, 2017, for mental strategies used in voluntary recall).

Involuntary retrieval circumvents such search processes as there are no pre-set requirements to be met and the involvement of cognitive control in the memory retrieval is minimised. The presence of cognitive control is also reflected by longer retrieval latencies for voluntary than involuntary memories (e.g., Barzykowski et al., 2021b; Schlagman & Kvavilashvili, 2008). In addition, Barzykowski et al. (2021b) recently also reported data suggesting that voluntary retrieval entails not only increased cognitive control but also more awareness during memory retrieval, which allows participants to retrospectively reflect upon and evaluate their retrieval. Involuntary memories thus come to mind rapidly, and thus somewhat devoid of phenomenology, since there is no feeling of effort.

Whilst there are only a few neuroscientific studies which help elucidate the nature of the difference between voluntary and involuntary memories, there is a consensus view on the neuroscience of cognitive control more generally. Traditionally, the frontal lobes are seen as a critical structure in a network of regions (essentially a frontal–cingulate–parietal network) responsible for cognitive control (e.g., Cai et al., 2016). This network is involved in bringing to attention task-relevant information, distributing attentional resources and suppressing inappropriate behavioural responses. In a review, Menon and D'Esposito (2022) outline six different large-scale functional networks involved in cognitive control, proposing that cognitive control is implemented by dynamic interactions among prefrontal cortex (PFC) networks embedded in a “global brain architecture.” In memory retrieval more specifically, Eichenbaum (2017) proposes that during recall, contextual cues are processed by the ventral hippocampus, which sends this information to the prefrontal cortex, which in turn “biases” the retrieval of context-appropriate memories from the dorsal hippocampus.

These neuroscientific descriptions of the complex, dynamic interactions between the prefrontal cortex and the temporal lobe are described without reference to comparison between voluntary and involuntary retrieval, and so it is difficult to know the implication of the prefrontal cortex in involuntary retrieval, despite causal mechanisms being identified in the literature (i.e., prefrontal activation occurring before, or leading to, cognitive control and memory retrieval [Cai et al., 2016; Eichenbaum, 2017]). In their Trace Transformation Theory, Sekeres, Winocur, and Moscovitch (2018) propose a similar interaction between the same structures, but also specify difference between the anterior and posterior hippocampus in humans, from more coarse representations in the anterior hippocampus and more fine-scaled representations in the posterior hippocampus (e.g., Brunec et al., 2018).

The neuroscientific literature describes a network with a to-and-fro between retrieval and control mechanisms driven by cues in the environment, which inspires our view here. But note that most neuroscientific accounts of this process do not differentiate between voluntary and involuntary retrieval (perhaps because these are more subtle differences in the non-human brain and are difficult to operationalise experimentally). However, cognitive control is a key component of neuroscientific accounts of retrieval and our focus here is what occurs cognitively when memories are involuntarily retrieved: Does this imply less control, and possibly less of a causal activation derived from the prefrontal cortex?

To examine this, Hall et al. (2014) compared voluntary and involuntary retrieval, finding that voluntary retrieval elicited greater activity in dorsal frontal regions while keeping other sub-components of the retrieval network (e.g., medial temporal, ventral occipitotemporal, and ventral parietal regions) similarly engaged. Previous studies (e.g., Hall, Gjedde, & Kupers, 2008; Hall et al., 2014; Kompus, Eichele, Hugdahl, & Nyberg, 2011; for review, see Kvavilashvili et al., 2020) suggest important roles for the posterior cingulate cortex (involved during both involuntary and voluntary retrieval), dorsolateral prefrontal cortex (inactivated during involuntary but activated during voluntary retrieval), posterior parietal regions (e.g., inferior parietal lobule),² and medial temporal-lobe areas in the involuntary retrieval processes. Therefore, involuntary retrieval compared to voluntary retrieval may not need additional engagement of the prefrontal cortex.

The phenomenology of IAMs is of them being spontaneous and this is corroborated by the neuroimaging findings that converge on a lack of cognitive control in involuntary retrieval. In this way, they might be thought of as part of a family of “stimulus unrelated thoughts” including mind wandering, which is described as being thought decoupled from current perceptual inputs (e.g., Mills, Zamani, White, & Christoff, 2021; Schooler et al., 2011). However, as stated above, IAMs are in fact often related to easily identifiable cues in the environment (e.g., Plimpton et al., 2015; Schlagman & Kvavilashvili, 2008). Mace (2004) showed that in everyday life, IAMs were most likely to be triggered by abstract cues (e.g., thoughts, language-based referents to the original past event) rather than sensory/perceptual cues (e.g., raw sensory experiences such as taste or smell). This finding suggests that although sensory and perceptual experiences may sporadically trigger an involuntary memory, more often than not they are evoked by objects, scenes, themes, abstract linguistic-based cues, and activation of autobiographical memory schemas. Several authors stress the relationship between mind wandering and involuntary memories (see Cole & Kvavilashvili, 2019; Plimpton et al., 2015), stating that retrieving an IAM is not related to the ongoing task, but it is related to information in the environment. To summarise, we propose that retrieval in involuntary memory is relatively rapid and although cued by the environment, arrives without an awareness of memory search, and is as such a relatively pure, content-rich type of retrieval, as indicated in Figure 1.

Figure 1. Flow chart representing the memory phenomena (autobiographical memory, déjà vu, tip of the tongue, feeling of knowing), as a result of (1) retrieval intentionality (involuntary vs. voluntary), (2) memory content accessibility (accessible/inaccessible), and (3) feeling of familiarity (present/absent). Outcomes of the retrieval process are: No memory (whereby nothing is retrieved that is experienced as a memory), déjà vu, involuntary memory, voluntary memory, and the feeling of knowing/tip-of-the-tongue, this latter, which is evaluated as plausible despite the lack of retrieved content is experienced as a frustrating sense of familiarity for a currently unretrieved representation. It is a sensation which provokes a search in memory, hence its link to voluntary retrieval process. Access to content: Access to content implies complete recollective retrieval of the personal past including the phenomenology of remembering and a sense of successful retrieval. Feeling of familiarity: Feeling of familiarity implies a subjective experience of fluency devoid of any content. Voluntary and involuntary retrieval: These labels refer to generic memory processes which are either will fully engaged (e.g., memory search, cue elaboration, generation of associations) or which are provoked by cues in the environment.

1.2. Déjà vu

Déjà vu is a brief experience in which the person experiences false familiarity at the same time as knowing that the familiarity is false. A reasonable estimate is that about 80–90% of the population experience it at least once, and no more than 10 times a year (see Brown, 2003). It is therefore experienced far less frequently than IAMs. Questionnaire studies have examined the possible triggers of déjà vu, and although it appears to be more likely for scenes, conversations, and for familiar places and people, no real patterns emerge (see Moulin, 2018, for a review). It is not the case that déjà vu has a uniquely visual trigger: Even people who are blind report experiencing déjà vu³ (O'Connor & Moulin, 2006). The current conceptualisation is that déjà vu arises from an inappropriate activation of familiarity: It feels as if a memory is being accessed, but no such representation from the personal past is present (Brown, 2004). This idea that déjà vu might result when the relevant memory cannot be retrieved has inspired the paradigms developed for studying déjà vu.

Definitions converge on the idea of a clash in mental evaluations and a conflict in appraisals (e.g., Urquhart, Sivakumaran, Macfarlane, & O'Connor, 2021). For instance, one may experience déjà vu when entering a flat, having a strong feeling of having been in this flat before, although at the same time knowing that this is actually the first time you have visited your friend's place. The déjà vu experience is thus also metacognitive: It arises out of a higher order interpretation of retrieval processes; it is not possible to have a déjà vu experience and not be aware of it. This critical feature distinguishes it from a false memory.

Whilst some authors have suggested the conflict in déjà vu arises from false familiarity which is top–down, or neurological in origin (the decoupled familiarity hypothesis; Illman, Butler, Souchay, & Moulin, 2012). This sees déjà vu as essentially an infrequent or random experience caused by a neurophysiological event, not unlike the electrical stimulations which start this article. Others, however, have suggested it is cued by stimuli which present some undetected conceptual or perceptual overlap with stored representations (e.g., the Gestalt Similarity account; Cleary et al., 2012). Déjà vu is a spontaneous mental event, in that it is not directly related to the current goals of conscious processing, and it is not sought for. However, as with IAMs, it can be triggered by cues in the environment.

The idea is that, for example, when entering the friend's flat for the first time, the configuration of the window, fireplace, and sofa match a stored representation, and give a certain configural fluency (e.g., Cleary et al., 2012; Cleary & Claxton, 2018), such that a feeling of familiarity for the environment is generated which relates strictly to one's personal past, even though it is known that this is the first visit to the apartment. The room feels familiar because it reminds us of something we have already encountered in our past, but we are not able to retrieve the source of the familiarity: We are unaware of the resemblance. Similar theories have been proposed since the nineteenth century (e.g., Knight, 1895), and may involve anodyne processes such as reading a description of a place and then visiting the same place or somewhere similar (e.g., Hawthorne, 1863).

Such experiences can be provoked in the laboratory. In the first of a series of influential experiments, Brown and Marsh (2008, see also 2009, 2010) used unattended processing to change beliefs about memory and prior occurrence, using a superficially presented stimuli to produce a déjà vu-like experience. This would be comparable with having a quick glance at a scene, and then subsequently processing the information more fully – what has been described as a cognitive “double take.” Critically, the same information is actually processed twice, which is presumably not the case for our example in our friend's flat. Cleary and colleagues (e.g., Cleary, Ryals, & Nomi, 2009) sought to explain déjà vu, not in terms of an exactly matching unattended processing of a stimulus, but as a perceptual overlap, basing their empirical work on the recognition without identification paradigm (see Cleary, 2008). Their hypothesis was that déjà vu was provoked by configural overlap in perception.

In their first experiment, they presented line drawings of scenes, such as an airstrip with a plane coming into land. This image is matched with second “test” image which shares the same general layout and elements, a picture of a pond. Instead of having the aeroplane above the horizon in the centre, there is a dragonfly, and the converging lines of the airstrip tapering into the distance are replaced by a similar pattern on the surface of the water. This gives two unique scenes which share a large perceptual similarity. Participants study the scenes with labels (e.g., airstrip) and at test they see the novel configurally similar scenes, which they cannot name (e.g., pond), since they were never studied. The configural similarity generates a feeling of familiarity, but when combined with the failure to produce a name (i.e., recognition without identification), it generates déjà vu. Subsequently, using virtual reality, Cleary et al. (2012) used conceptually the same method but used rooms and scenes (exactly like the flat example above), again producing déjà vu in their participants.

Drawing on this virtual reality approach, and using navigational paths through a virtual environment, Cleary and colleagues have mapped out the range of attributions and subjective experiences that accompany déjà vu (e.g., Cleary & Claxton, 2018; Cleary, Huebert, McNeely-White, & Spahr, 2019; Cleary, McNeely-White, Huebert, & Claxton, 2021a). In these experiments, Cleary and colleagues sought to explain the feeling of “prescience” in déjà vu – the ability to be able to predict the future. In the first work in this series, Cleary and Claxton showed that a feeling of familiarity with a turn in a path generated feelings akin to déjà vu, replicating their earlier work, but this feeling did not lead to above-chance ability to predict the next turn. Critically, however, déjà vu was accompanied by increased feelings of knowing the direction of the next turn, leading Cleary and colleagues to describe déjà vu as an illusion of prediction. In a follow up, Cleary, McNeely-White, Huebert and Claxton showed that prescience was not a ubiquitous experience in déjà vu, but that it was related to the level of familiarity felt; the more intense the phenomenology of familiarity, the more the participant felt like they could predict what was coming next. Cleary, Huebert, McNeely-White and Spahr went one stage further, asking participants to report on how things unfolded after the feeling of déjà vu, showing a “postdictive bias” as well as the illusion of prediction. Participants were more likely to say that the novel route unfolded in a way that they remember from the déjà vu: Something which, as in the earlier task, was not borne out in their ability to identify the right path.

Most recently, Huebert, McNeely-White, and Cleary (2022) tested the idea that feelings of familiarity in cued recall might provoke illusory feelings of recollection – being able to recall contextual specifics from a past event, in line with the observation of recollective confabulation patients, who have been described as having permanent déjà vu (Moulin, 2013). Again using the recognition without recall paradigm, Huebert et al. manipulated the amount of cue-target feature overlap. Increasing familiarity led to increased confidence in knowing a contextual detail of some other information that was presented at study. In sum, using various iterations of the recognition without identification paradigm, Cleary and colleagues have repeatedly provoked feelings of familiarity in the laboratory which are described as déjà vu by participants. Crucially for our argument below, this feeling of familiarity has implications for illusions of prediction and recollection.

In a different experimental framework, Urquhart and colleagues (Urquhart & O'Connor, 2014; Urquhart et al., 2021) generated a feeling of familiarity in word learning tasks using conceptually related stimuli. They use Deese–Roediger–McDermott (DRM)-like stimuli (Roediger & McDermott, 1995), in which one critical non-presented item (e.g., SLEEP) is related to a series of other targets (e.g., BLANKET, REST, DOZE, DREAM, BED). At test, the non-presented item will feel familiar to the participant, and in an oft-replicated finding, a large proportion of people report a false memory for this item. Urquhart and O'Connor (2014) produced an analogue of déjà vu by adding a task at study whereby participants also had to report and note the number of words beginning with a certain letter, in our example, “S.” Because SLEEP was not presented, and no other word was presented that began with an “S,” it created a situation where the participants had knowledge about the study phase: “no words began with an ‘S’.” Thus, when presented the item SLEEP in the test phase, participants found the item very familiar but were also confronted with the information that they had not studied any words beginning with an “S,” and participants were likely to report having an experience of déjà vu, hence O'Connor and colleagues' emphasis on the role of conflict in the déjà vu experience.

As a final note, déjà vu is a subjective experience where the key characteristic is its phenomenology and as such it is not only difficult to verify people's experience, it is also possible that experimental inductions exaggerate the rate of déjà vu generation, or that participants confuse different types of phenomenological familiarity (O'Connor & Moulin, 2010). One concern is that participants report having had déjà vu even in a control condition. Central to this problem is the notion of demand characteristics, a response bias generated by social desirability (Orne, 1962), sometimes referred to as reactivity. The fact that participants are asked whether they have experienced déjà vu repeatedly adds to this possibility (e.g., Winkler, Kanouse, & Ware, 1982).

Jersakova et al. (2015) examined the possibility that reports of déjà vu were influenced by demand characteristics. They reasoned that participants could interpret being asked about whether they experience déjà vu as indicative that they should be experiencing it. This could arise with a participant who is unsure of what exactly she is experiencing or how to describe it. Also, it could be that the communication between the experimenter and participants about the exact nature of the studied experience is not clear. Cleary et al. (2009) report a cross-experiment comparison of déjà vu rates for otherwise identical experiments when they do and do not provide a definition of déjà vu. They observed changes in how the participants responded to their induction of familiarity, noting that “… participants’ pre-experimental notions about déjà vu may make them more inclined to equate déjà vu with standard feelings of familiarity” (p. 1087).

Jersakova et al. carried out online experiments with an ongoing continuous recognition task, reporting whether a given item was old or new. Interspersed with this task were questions about other experiences (notably déjà vu and the tip of the tongue [TOT]), which were manipulated between subject, and there was also a control condition with no questions asked during the recognition phase. All participants had a common final post-experimental questionnaire that asked about the incidence of subjective experiences, the hypothesis being that asking frequently about déjà vu would lead to an elevated level of déjà vu reporting. Across experiments, between 33 and 58% of participants reported experiencing déjà vu or TOT – even if they had not been in a condition where they had been asked repeatedly about those conditions. Changing the definition of déjà vu or asking participants to bring to mind a real-life instance of déjà vu or TOT before completing the task had no impact on reporting rates in the post-experimental questionnaire. Jersakova et al. also compared their “laboratory” déjà vu and the real-world experience finding that participants rated salience, intensity, and emotionality higher for real-life déjà vu and TOT experiences as compared to their experimentally generated analogues.

Our aim here is not to question the outcome of experiments examining déjà vu formation, but rather explore how demand characteristics, and “pre-experimental notions” might influence responding of such a subjective and difficult to describe state. It should be noted that déjà vu research is not alone in possibly being influenced by demand characteristics – as we have noted here, for example, the expectancy of generating an involuntary memory also influences the rate of this experience too. However, all experimental approaches to déjà vu formation involve asking the participant directly if they have had déjà vu, and often on a trial-by-trial basis. This “provocation” of déjà vu is therefore somewhat at odds with our characterisation of déjà vu as spontaneous. Our point here is about the phenomenological spontaneity and unpredictability of the experience, which lends itself to the design used to provoke involuntary memories. Whilst we characterise déjà vu in the real world as being an unexpected, spontaneous experience, we do not yet know the extent to which it is experienced as surprising or spontaneous in experimental settings. It seems that two lines of experimentation might address this issue. The first would be to adapt the existing successful paradigms by Cleary and colleagues reviewed above such that questions are asked about the spontaneity and “automatic” nature of the experience. Second, we propose below an approach whereby a mundane ongoing task leaves space to experience spontaneous phenomena such as involuntary memories to complement the existing work.

2. Retrieval processes in involuntary autobiographical memory and déjà vu

Whereas IAMs are mostly discussed within the context of autobiographical memory (where recall tasks are predominant), déjà vu has been mostly discussed in the context of familiarity processes in recognition memory decision making. To reconcile these literatures, our hypothesis is that, given that both déjà vu and IAMs relate to our personal past, they reveal something about human retrieval processes, in general, and autobiographical memory retrieval, in particular. A complete account of retrieval should incorporate both these phenomena and demonstrate how they relate to each other. Importantly, while the idea that IAMs and déjà vu may be similar phenomena has been suggested in the literature (e.g., Bradley, Moulin, & Kvavilashvili, 2013; Conway & Loveday, 2015; Illman et al., 2012; Mazzoni & Hanczakowski, 2011), to our best knowledge, the present paper is the first that provides an account for the possible underlying mechanisms of both. Of note, in the only empirical work on the relationship, Moulin et al. (2014) find that retrospective questionnaire reports of the frequency of déjà vu and IAMs in daily life are related, at least in older adults (mean age 71; r[44] = 0.723).⁴

2.1 Familiarity and feelings of fluency

The experimental induction of déjà vu converges on one theoretical entity: Familiarity. The implication is that retrieval is incomplete: Participants find a stimulus familiar but at the same time, are unable to retrieve its source (the Brown, Marsh, and Cleary experiments), or know that the familiarity is false (the Urquhart and O'Connor experiments). In terms of memory retrieval, as pointed out by Renoult, Irish, Moscovitch, and Rugg (2019), familiarity is often defined by default – it is what occurs in the absence of the rich evocative mental time travel which is experienced in episodic remembering (Renoult et al., 2019). For a definition of familiarity, we turn to the recognition memory literature, where it is described as a recognition memory decision-making process based on the evaluation of trace strength, a judgement of prior occurrence based simply on a feeling, rather than the retrieval of information (e.g., Yonelinas, 2002). A robust literature examining this trace strength account using signal detection models and receiver operating curve analysis exists (for a review, see Yonelinas & Parks, 2007; see also Delay & Wixted, 2021, for an example application to recognition memory making the case for a continuous process).

The focus here however is on phenomenology rather than recognition memory performance, and perhaps the idea of familiarity is most intuitive when we talk about first-person experiences, such as “the butcher on the bus”:

Consider seeing a man on a bus whom you are sure that you have seen before; you “know” him in that sense. Such a recognition is usually followed by a search process asking, in effect, Where could I know him from? Who is he? The search process generates likely contexts … Eventually the search may end with the insight, That's the butcher from the supermarket!

Mandler (1980, pp. 252–253)

In Figure 1, we propose that such phenomenological familiarity occurs when there is (temporarily) no content retrieved. It is felt in both involuntary and voluntary retrieval, but as we will explain below, it serves to guide and motivate retrieval processes: A sense of familiarity arises without any intention, but the subsequent search for corresponding information can be intentional. Before examining this metacognitive account in more detail, we quickly review the notion of familiarity in recognition memory.

Recognition of a cue in the environment arguably relies on two retrieval modes or processes, familiarity and recollection (e.g., Yonelinas, 2002). The neural and cognitive organisation of these entities is often debated (for an overview of this debate, see Moulin, Souchay, & Morris, 2013, and for a proposal for the integration of these ideas, see Bastin et al., 2019). Simons, Ritchey, and Fernyhough (2022) propose that early characterisations (such as Yonelinas's [2002] account of a hippocampal mechanism and a simple threshold that gives rise to recollection) are helpful for explaining binary (laboratory-based) memory decision making. However, with more complex representations of the personal past, they propose that additional to this hippocampal reinstatement, cortical regions such as the angular gyrus are recruited to generate a precise representation of the episode (e.g., Richter, Cooper, Bays, & Simons, 2016). As such, they describe remembering as drawing upon two different dimensions: Retrieval success and the precision of the details retrieved.

We emphasise separable phenomenological experiences of familiarity and recollection, and we are agnostic as to whether these map onto one or two underlying processes, although as will be seen, our ideas converge on the idea of a trace-strength account based on the continuity between familiarity and recollection. A nuanced view would be that complex interactions exist between networks traditionally thought responsible for familiarity (extra-hippocampal regions) and recollection (the hippocampus) such as proposed by Quamme, Yonelinas, and Norman (2007), with also the recruitment of an area around the left angular gyrus which yields the multisensorial information experienced in remembering as outlined by Simons et al. (2022). As proposed by Montaldi and Mayes (2010), we suggest that recollection and familiarity are related: “…each is a complex function, likely to depend on several different processes that are probably mediated by different structures that are functionally connected in a system” (p. 1294). Whereas the consideration of recollection and familiarity as processes favours an interaction between shared neural and cognitive processes in a continuum, for the experient, recollection and familiarity are separable signals which differ in their phenomenology and content.

The critical issue, as identified by Bastin et al. (2019), is that these signals are interpreted differently, and have consequences for retrieval. They propose an integrative model with a representational hierarchy and an attributional system. The first is a “core” system that stores specific representations based on interactions between a large network of computational operations from the whole brain, split into an entity representation system (guided by feelings of fluency – see below) and a relational representation system (responsible for binding and pattern completion) centring on the hippocampus. A second system acts on the current context and “translates content reactivation into a subjective experience” (p. 5) and includes input from networks in the parietal and prefrontal cortexes. Critically, their model rests on differences in the outputs and representations of their core systems, and an interaction between processes and computations, resonating with the Montaldi and Mayes “kinds” of memory proposal and our view about separable phenomenology. In autobiographical memory, the retrieval of specifics from the personal past is accompanied by a feeling of mental time travel or autonoesis (see Conway, 2009), directly comparable with the concept of recollection: A detailed representation of a moment in the personal past. The idea of the “self in past” (Conway, 2009; Klein, 2013; Marshall, Halligan, & Wade, 1995; Moulin & Souchay, 2014) is that it enables distinguishing between imagined, false, and actual events and gives episodic memory its particular character (Perrin & Rousset, 2014). Recollection gives the feeling of completeness, and the resolution of the feeling of familiarity. In the butcher on the bus, if we are able to identify the person we have seen, we resolve the feeling of familiarity and the feeling of recollection confirms successful retrieval. The phenomenology of remembering is indeed associated with high degrees of confidence (Dunn, 2004).

Recognition decisions made based on familiarity are proposed to be faster and more automatic than those based on recollection (e.g., Besson, Ceccaldi, Didic, & Barbeau, 2012; Hintzman, Caulton, & Levitin, 1998; Hintzman & Curran, 1994; Mandler, 2008; see Dewhurst, Holmes, Brandt, & Dean, 2006, for a discussion of this issue, and Berry, Shanks, Speekenbrink, & Henson, 2012, for a single-process explanation of this pattern of results). In tasks where recollection and familiarity are compared by asking participants to make decisions based on the information they are able to retrieve, Yonelinas and Jacoby (1994) found that recollection decisions peaked at 800–1100 ms, whereas familiarity decisions peaked earlier at between 600 and 800 ms. In EEG paradigms (e.g., Duzel, Yonelinas, Mangun, Heinze, & Tulving, 1997; Rugg & Curran, 2007), two separate neural signatures are found for old/new decisions in episodic memory. The FN400 effect (between 300 and 500 ms, e.g., Curran, 1999; a negative potential in the frontal areas) is proposed to reflect familiarity processing, whereas the parietal old/new effect (sometimes called the late parietal effect) is a positive potential in the parietal area arising slightly later, after about 500 ms (e.g., Tsivilis et al., 2015).

Familiarity is often described as a low-level process working on perceptual inputs⁵ (Bastin et al., 2019). This part of the memory system tries to match, as quickly and effortlessly as possible, the contents of mental representations stored in memory with the current contents of perception; consistent with the classic neuroanatomical view of the temporal-lobe memory system as being the last point in the ventral visual stream (e.g., Suzuki & Amaral, 1994). Viewed like this, familiarity decision making is the last stage in perception – once we have composed and identified a scene or environment, we can “read off” whether we have encountered it before. More recent neuroanatomical accounts of memory retrieval propose a similar organisation. Moscovitch (2008) describes two phases in episodic retrieval. A first stage involving the hippocampus is non-conscious, based on a rapid interaction between a retrieval cue and a stored representation, whereas a second stage, featuring a more complex interaction between the hippocampus, prefrontal, and parietal cortex, is required for the retrieved information to be re-experienced. These accounts are inspired by the notion of an organisational hierarchy of connectivity (e.g., Margulies et al., 2016). Irish and Vatansever (2020, p. 47) propose a “…concrete to abstract representation spectrum along modality-specific and default mode brain networks which collectively support rich and detailed memories,” such that rather than specifying different stores or types of information as in the standard episodic/semantic distinction (see also Renoult et al., 2019), the memory system emerges as part of a larger scale network with retrieval of specific events from the personal past as being a type of interrogation of the stored representations based on a common network, rather than something restricted to a specific zone, region or process in the brain. As such, memories might be thought of as reflecting a dynamic pattern of local to regional connectivities, varying along the continuum from abstract to specific and more or less involving the sensorimotor regions (Irish & Vatansever, 2020). For an up-to-date review of the neural processes in the subjective experience of remembering taking on board both the large-scale networks and traditional “regions” involved in memory, see Simons et al. (2022).

The existing cognitive frameworks resonate with the neuroscientific view, but discuss phenomenology in a way which is somewhat lacking in neuroscientific accounts (see Renoult et al., 2019). In Whittlesea's SCAPE (Selective Construction and Preservation of Experience) model (Whittlesea & Williams, 2001), the memory system is continuously trying to make sense of its inputs, so that it can interpret any signals arising from low-level processing of the environment. Familiarity is thus a subjective feeling arising from the fluent processing of a stimulus. This theory draws upon various illusions of familiarity: Where the fluency of processing leads to the (erroneous) decision that we have seen something before, or to “know” an answer (e.g., Goldinger & Hansen, 2005; Newman, Garry, Bernstein, Kantner, & Lindsay, 2012; Whittlesea & Williams, 1998). Phonological and structural regularity can be used to induce feelings of fluency which are misinterpreted as meaning we have seen the information before. For example, in the hension effect, people are proposed to be more likely to make false positives for pseudowords such as hension (which is regular and resembles a real word) than stofwus (for a critique, see Cleary, Morris, & Langley, 2007). The idea is that the ease at which they read the word is misinterpreted as it being an item from a study list. Golding and Hansen push the idea one step further: Participants respond to meaningless subliminal vibration cues, interpreting them as a signal they have seen the word before. There is some debate about whether the critical element is “surprise” in false memories, and it seems unlikely that Whittlesea's view could explain all kinds of false memory. For instance, in the DRM paradigm, the critical lures for which participants have false memories are expected as targets, and so the false memory cannot be described as surprising (Karpicke, McCabe, & Roediger, 2008). Moreover, Cleary et al. (2007) point out that structural regularity usually leads to benefits in recognition memory, not an increase in false positives, suggesting that empirical support for Whittlesea's ideas on false memories is weak. Cleary et al. (2007) found that the hension effect disappeared completely once controlling for inter-item similarity in the recognition test materials, and so interestingly, this effect which formed the basis of the original theorising receives little empirical support. The role of phenomenological fluency in recognition memory is something which will need continued evaluation, even though the basic idea, that the ease of processing leads to an attribution of prior exposure, is well established (e.g., Jacoby & Dallas, 1981; but see also Berry et al., 2012).

Whilst there is not unequivocal empirical support from recognition memory studies, Whittlesea describes an interaction of attributions of memory and processing fluency, something that is critical for thinking about how cognitive “feelings” guide retrieval processing (Bastin et al., 2019; Moulin & Souchay, 2014). The idea that we adopt here is that the current context generates a top–down expectation of processing fluency. When an expectation of fluency is violated, it triggers a search for as to why that has arisen. This is the search triggered in the case of the butcher on the bus; the person feels intensely familiar precisely because the source of the familiarity is unknown. In this way, we align our reasoning with Whittlesea's observation that strangers feel familiar, but our friends do not (Whittlesea & Williams, 1998): The phenomenology of familiarity is produced by a mismatch between processing fluency and the expectations about fluency. The spontaneity and phenomenology of the experience are one and the same: It feels familiar because it was unexpected.

Fluency is also a variable of interest in IAM studies. Sanson et al. (2020; see also Barzykowski et al., 2021b) investigated the influence of fluency on classification of intention in a paired-associates design. Participants studied a series of phrases, half of which were accompanied by a phrase-related photo. During retrieval, participants were presented with the phrase only and then instructed to retrieve additional information about it. Crucially (Experiment 3; Sanson et al., 2020, p. 12), participants reported if they experienced retrieval as spontaneous (“the information came to mind without intent”) or deliberate (“the information was brought to mind intentionally”). Although they were explicitly instructed to intentionally retrieve memories, fluency overruled intentionality when classifying their memories. When participants intentionally, yet fluently, brought a memory to mind, they were prone to judge the retrieval as spontaneous. Similar results were obtained in study by Barzykowski et al. (2021b) who investigated the effect of retrieval effort while classifying memories. They created experimental conditions that maximise the probability of one type of retrieval (e.g., involuntary or voluntary) while minimising the probability of the other. Participants recalled autobiographical memories in each condition, retrieval time was measured, and they classified their memories as either voluntary or involuntary and rated them on retrieval effort. There were four categories of memories: Experimentally defined voluntary and involuntary memories with an objective measure of effort (retrieval time) and subjectively classified involuntary and voluntary memories with a subjective measure of effort. Similar to Sanson et al., the majority of memories were classified as involuntary, whether they were retrieved in the experimentally defined voluntary or involuntary condition. Interestingly, the relationship between the rating of subjective fluency and objective retrieval time was only significant in the voluntary conditions, indicating that people are more aware of subjective effort when deliberately trying to retrieve a memory than for an involuntary memory. In contrast with existing theories, these results indicate that the subjective feeling of fluency is more important than intention for the involuntary–voluntary distinction. Such experiments suggest that low-level feelings are used when making explicit decisions about memory.

These findings question the interpretability of existing studies on involuntary memories in which fluency has not been controlled for. Sanson et al. (2020, p. 2) argued “[…] subjects themselves classify their memories as voluntary or involuntary, and so the validity of what scientists know about involuntary memories rests upon the validity of this task. How well do subjects perform this task, then? Here, we show that, at least under certain circumstances, the answer is not very well.” Our view is more moderate: Classifications are internally consistent in terms of retrieval latency and retrieval effort, especially during voluntary retrieval. At the same time, it is much less of an issue during involuntary retrieval, since the vast majority of participants' classifications align with the classification set by the experimental condition. As such, these findings do not undermine previous results on involuntary and voluntary memories but rather extend our understanding of retrieval. Accordingly, the involuntary–voluntary distinction is not as clear-cut as the conceptual foundation of intentional versus non-intentional retrieval (or indeed retrieval with or without cognitive control) suggests. As such, there are no involuntary/voluntary memories but a memory representation that is constituted by certain factors operating during retrieval, an attributional process. We see retrieval as a dynamic process consisting of interactions between different dimensions (e.g., intention, retrieval effort, memory accessibility, and monitoring processes) that all together make up the particular “flavour” of a retrieved memory.

We see familiarity as metacognitive; a feeling that is generated from the fluent processing of a stimulus.⁶ It is a signal which guides memory processing – orientating us to information in the environment which we have already encountered. As such, it can be described as an epistemic feeling (Moulin & Souchay, 2014). Arango-Muñoz (2011) defines epistemic feelings as quick-acting intuitive processes, based on how things feel, and déjà vu is often used by philosophers as an example of an epistemic feeling (Arango-Muñoz, 2014; Bortolotti, 2010; de Sousa, 2009; Gerrans, 2014). Epistemic feelings neatly map onto a set of metacognitive phenomena and paradigms that are better researched and understood than déjà vu, such as the TOT state (Greely, 2021) and the feeling of knowing (FOK; Souchay, Moulin, Clarys, Taconnat, & Isingrini, 2007). These are instances of metacognitive awareness at the moment of retrieval failure (Fig. 1). When familiarity is high and retrieval is incomplete, this will lead to such feelings, even for the personal past. Critically, in the TOT (or the FOK) for the personal past, the experient must evaluate retrieval as plausible, since otherwise the experience would be one of conflict, possibly resulting in déjà vu.

Souchay and Moulin (2009) synthesised the existing proposals into a schematic representation about the relationship between familiarity and recollection (Fig. 2). Various similar proposals exist in the literature (see Hintzman & Curran, 1994; Koriat & Levy-Sadot, 2001; Reder, 1987; Reder & Ritter, 1992).⁷ It explains the search for information given a cue in the environment, and pertains most clearly to the FOK. In the FOK, a rapid initial feeling of familiarity with a cue triggers the attempted retrieval of episodic detail about the cue, and any associated information. In the absence of any contextual information, people can make accurate predictions of future recognition based on the strength of familiarity for the cue, and what “partial information” about the cue or searched-for-target comes to mind. Such research shows that even when information is not currently retrievable, outputs from the recognition memory system (in particular fluency) can be acted on metacognitively in order to accurately predict the likelihood of subsequent retrieval, suggesting the existence of a sequential arrangement as shown in Figure 2, which is why it is helpful to think of familiarity and recollection as lying along a continuum.

Figure 2. Representation of the metacognitive evaluation of familiarity and recollection in recognition memory decision making.

In the context of IAMs, it is possible to retrieve information directly without feeling anything until the moment there is a fully formed mental representation. We would argue that this is due to there being very little delay between the first familiarity stage and the complete retrieval. Memory retrieval in this case feels spontaneous because the subject had both the feeling of familiarity and recollected specifics at approximately the same time. Familiarity is most strongly felt when we are not able to retrieve contextual specifics. In the butcher on the bus, familiarity is high but the recollected specifics are (momentarily) absent, whereas in déjà vu, the familiarity is similarly high, but a metacognitive evaluation favours the interpretation that the familiarity is false or impossible. Here we suggest for the first time that this is based on access to information about the personal past.⁸

2.2 Autobiographical retrieval mechanisms

Here, we examine in more detail autobiographical memory retrieval processes with a focus on metacognition, and propose a threshold mechanism. According to Conway and Pleydell-Pearce (2000; for later modifications, see also Conway, 2008, 2009; Conway & Jobson, 2012), autobiographical memory consists of a hierarchical network of interconnected nodes that differ in terms of their level of specificity. At the bottom are fragments of events with specific sensory details (e.g., details experienced when riding a bike for the first time). Such vivid information is further connected to super-ordinate levels of general events (e.g., riding a bike to work), common themes (riding, commuting to work), and, at the highest level, important periods in one's life (e.g., when I moved to France). The activation of autobiographical information spreads across the network resulting in the construction of a particular memory. Such activation may be elicited by different types of cues, internal or external. While voluntary autobiographical memories are the result of a top–down search process that eventually arrives at an episode, directly retrieved (i.e., involuntary) memories are thought to circumvent the search process and enter consciousness very quickly. Conway and Pleydell-Pearce argued that fragments of memory representations are constantly activated at the bottom level of the hierarchy, but the vast majority of such memories never reach consciousness due to being suppressed by constantly operating executive control system. Moreover, some low level and/or mundane information will be completely in line with expectancies (cf. Whittlesea's SCAPE) and will not be raised to awareness. Only some of these activated memories, especially those that are consistent with current self-goals, may occasionally get through this executive control mechanism and reach awareness.

This implies that IAMs occur as a result of two concurrent processes, namely spreading activation and inhibition that works against such activation (see Ball & Hennessey, 2009). The retrieval of IAMs depends on absent or inefficient inhibitory control. For instance, when entering a hotel room for the very first time (i.e., we have never been to the city, in general, nor to this hotel chain), the environment is being continuously processed and automatically matched with any corresponding representation in memory. Aside of deliberative searches in memory cued by conscious thoughts (e.g., being asked by the receptionist if we have ever stayed at the hotel before), we propose that there are several possible outcomes, which lie on a continuum: (a) An involuntary memory may be triggered, (b) one may get only a general feeling of familiarity, or (c) the room does not trigger any retrieval process. The key premises delivered from this model are that (1) autobiographical memory information is constantly and automatically activated by cues and that (2) only some of them may enter awareness.

Given that IAMs occur automatically in response to incidental external and internal cues, one may ask why are we not constantly flooded by memories? According to the principle of cue overload, it is most likely that a cue will match several past events. Berntsen (2009) proposed a mechanism of cue-item discriminability, defined as “how easily a given cue isolates an item” (Rubin, 1995, p. 151 as cited in Berntsen, 2009, p. 107; see also Norman & Bobrow, 1979). That is, the more events that are associated with a particular cue, the less efficient this cue will be in triggering any one of them. Berntsen, Staugaard, and Sørensen (2013) examined the relationship between cue and memory and were able to predict the occurrence of an involuntary episodic memory based on a manipulation at encoding. Across four experiments, they found that involuntary episodic memories were retrieved more often in response to unique compared to repeated cues, in keeping with the principle of cue-overload.⁹

The déjà vu field has yet to manipulate such variables, although a neuroscientific account relating déjà vu to novelty mechanisms has been advanced by O'Connor, Lever, and Moulin (2010). At least in clinical samples, delusional beliefs of prior occurrence are more likely to appear in novel locations and events (i.e., environments with unique cues) than those that are familiar. Our prediction is that déjà vu would arise in situations where the cues are relatively unique, since in situations where there is cue overload, the attribution will be that any sense of familiarity is normal given the current situation.

It can be argued that cues that are unique and distinctive are infrequent in daily life and most cues map onto many different memories and events. Given how frequent IAMs are, it must be the case that other processes are at play. In response, it has been suggested that priming processes (i.e., increasing the activation of a memory information by prior encounter with the contents of memory representation) may not only increase cue-item discriminability allowing for efficient activation of a particular involuntary memory (e.g., Barzykowski & Niedźwieńska, 2018b; Mace, 2005) but also increase the familiarity of a given cue.¹⁰ It is thus possible that on some occasions, a new event/setting may consist of cues that map onto many different past memories, or even that there is a relevant configural, contextual similarity between the current situation and some past events (exactly as proposed in the Gestalt Similarity hypothesis; Cleary et al., 2009). While these cues/contexts are not efficient in triggering any given memory, it is nonetheless possible that some of them may induce the feeling of familiarity, leading to the experience of déjà vu. This would be the case if the memory is vague, or non-specific: Associated with many previous events/information in the AM organisational hierarchy and unable to enter awareness. The activation is not strong enough to trigger a given memory, but it is strong enough to trigger a feeling of familiarity. This suggests that while on some occasions the memory content (e.g., being in a similar hotel room in Rome a few years ago) does not enter consciousness, leaving us without a straightforward reference to our past, the feeling of familiarity still appears. Conway and Pleydell-Pearce (2000) assumed an inhibitory control mechanism suppresses irrelevant memory information, preventing it from reaching consciousness. Findings from several recent studies (e.g., Barzykowski et al., 2019b, 2021a, 2022) do not provide full support for the existence of such a direct control mechanism,¹¹ and favour an account based on a retrieval threshold (see Barzykowski & Staugaard, 2016, 2018; Barzykowski et al., 2019a).

In recognition memory decision making, activation/threshold models have long been the norm, stemming from signal detection theory (e.g., Banks, 1970; Wixted, 2007). In autobiographical memory, it has recently been proposed that in order to reach consciousness, a memory must pass an awareness threshold that determines the minimum amount of activation for awareness. Entering awareness can be achieved either when a memory reaches levels of activation that is greater than a given threshold, or when the threshold is lowered by some additional factors. For activation, for instance, it may be easier for a phenomenologically pronounced and intense memories to pass the threshold because they may be especially good at drawing one's attention (see also Barzykowski & Staugaard, 2016, 2018; Barzykowski et al., 2019a). For the threshold level, it is argued that it may be momentarily modified by different factors (e.g., expecting memory to happen, placing the focus of attention on one's stream of thoughts), which would consequently modify the experience of otherwise less accessible memories. The threshold hypothesis states that, while both highly and poorly accessible memories can be retrieved either voluntarily or involuntarily, the processes operating during memory retrieval can influence the frequency of each type of retrieval by modifying the awareness threshold. Barzykowski and Staugaard (2018; also, Barzykowski et al., 2019a) proposed these processes to be intention (i.e., wanting to retrieve a memory) and selective monitoring (i.e., expecting a memory to appear). This idea has been verified in several empirical studies (e.g., Barzykowski & Mazzoni, 2022; Barzykowski, Skopicz-Radkiewicz, Kabut, Staugaard, & Mazzoni, 2023; Barzykowski & Staugaard, 2016, 2018; Barzykowski et al., 2019a, 2021b). For instance, involuntary memories compared to voluntary memories were rated by participants as more phenomenologically accessible; namely, more clear, unusual, personally relevant, important, recent, and more frequently rehearsed in the past (e.g., Barzykowski & Staugaard, 2016; Barzykowski et al., 2019a). These findings corroborate the idea that when not being actively engaged in memory retrieval, a memory has to be phenomenologically sound to pass the threshold. Also, Barzykowski and Niedźwieńska (2016) demonstrated that involuntary memories retrieved in a selective monitoring condition (i.e., when one is expecting a memory to appear) were less clear, less detailed, less vivid, and elicited less intense physical reactions compared to memories retrieved without selective monitoring. In another study, selective monitoring decreased the personal importance and emotional intensity of memories (Barzykowski & Staugaard, 2018). Finally, these results pertain not only to the phenomenological characteristics of memories but also to their objectively observable descriptions (Barzykowski et al., 2023). When looking at the descriptions of memories, on average involuntary memories were indeed more accessible (i.e., rated by independent judges as having more indicators of memory accessibility such as being scored higher on the emotional impact scale, etc.) than voluntary memories and intentionality lowered the awareness threshold and increased the number of low-accessibility memories.

Note that these threshold processes are metacognitive, about expectations and intentions. Our proposal is that in complete retrieval, there is little need to reflect too much on the process of retrieval: Information comes to mind, and is recollectively experienced, whether or not it was sought for. Naturally, the recall of contextual specifics may even justify to the experient why the information has come to mind, for example, “it was in this post office where I got my foreign currency last year.” However, in cases where retrieval is incomplete or low-level signals violate expectations (i.e., finding something familiar in a novel location), then intense feelings are generated.

Autobiographical retrieval is therefore determined by a complex interplay between factors. Intention and monitoring can be conceived as processes that enable access to otherwise less accessible memories. The most important assumption of the threshold hypothesis is that because of certain phenomenological properties (e.g., emotional intensity, clarity, vividness, personal relevance, unusualness), some mental contents may be especially good at drawing one's memory-related attention and, thus, they may pass the awareness threshold more easily and be more likely to be reported. However, on occasions where we may not be aware of a given memory content, but where activation is sufficient, we can still feel familiarity. If the familiarity is logically impossible and unexpected, it will trigger a search in memory to explain the feeling. Thus, both déjà vu and IAMs start from the same point in shared retrieval processes.

We turn our attention to how these processes can be split down into their component parts. Autobiographical memory retrieval has the following four stages (e.g., Barzykowski & Mazzoni, 2022; Wilckens, Erickson,& Wheeler, 2012): Pre-retrieval, retrieval, post-retrieval, and retrieval outcome report stages.¹² The pre-retrieval stage is associated with any cognitive processes that either facilitate or impair retrieval. For example, during this stage, an individual may be in “retrieval mode” in which “the cognitive system is prepared for or expects memory construction and recollection” (Conway, 2001, p. 1379). The effect of priming would occur here, enhancing the likelihood that the memory will be triggered and/or will enter a person's awareness. The retrieval stage relates to the forming and developing of an autobiographical memory, but without explicit self-reflection; namely, a given memory might have been formed but one may not be explicitly aware of it yet. During this stage, a memory is triggered by and/or accessed via a given cue. Once the memory is formed, during the post-retrieval phase, people may become aware of having the memory in mind. Thus, this stage relates to the ability to, for example, extract autobiographical content from the stream of consciousness to explicitly become aware of having a memory that is autobiographical. One is fully aware that an autobiographical memory was actually retrieved, and in voluntary retrieval, if the memory content meets the given criteria, then the search may be terminated. In the last stage, the retrieved memory may be shared with others and reported by giving a verbal account of the content.

This four-stage account highlights two main factors, the first being intentionality during the pre-retrieval phase, the second being a metacognitive process (see Koriat & Goldsmith, 1996; Mazzoni & Kirsch, 2002). Koriat and Goldsmith, inspired by signal detection models, describe a situation whereby the subject can decide whether to report retrieved information (p. 181): “people monitor the subjective likelihood that an item of information that comes to mind is correct and then apply a control threshold to the monitoring output in order to decide whether to volunteer that item or not.” Their emphasis, necessitated by the use of a forced/free report recall procedure (see Goldsmith & Koriat, 1999), is on the participant evaluating the quality of information before choosing to output it, whereas our proposal is that internally such a process also exists to motivate and regulate the search for information for the subject.

When looking at these retrieval stages, we argue that both déjà vu and IAMs are natural forms of involuntary and spontaneous processing of cues that can be found in one's surroundings, and that memory information when retrieved is not only accompanied by the exact memory content but also the feeling of familiarity that, importantly, may arise and/or be experienced separately from each other. In particular, déjà vu can be thought of as arising at the retrieval stage, and before the person is aware of having any content in mind.

A critical issue is to understand why in some cases, retrieval is incomplete and déjà vu arises rather than the retrieval of an IAM. Here, we advance two hypotheses which are not mutually exclusive and may map onto the top–down/bottom–up descriptions of déjà vu in the literature. These are proposals based on cue familiarity/trace strength and the metacognitive evaluation of cues in the environment. It is beyond the scope of this paper to delve too deeply into retrieval failure per se on a mechanistic level, but it is important to note that the dynamic patterns of inhibition and activation seen in episodic memory (e.g., Anderson, Bjork, & Bjork, 1994; for a review, see Kuhl & Wagner, 2009) are likely to contribute to the failed retrieval referred to here. For instance, it is known that competition between similar cues leads to an inhibition process whereby the retrieval of one memory actively inhibits the retrieval of another. It is not impossible that information is not simply forgotten or lost according to this process, but that it gives rise to phenomenological states such as referred to here, something that would warrant further investigation.

Our first hypothesis is that there is only weak activation of the associated cues and memories from the environment, and thus there is not sufficient activation for any content to enter into consciousness. This is not unlike the experience of information in the TOT (developed in detail below), where knowledge or lexical information is partially activated and frustratingly will not come to mind, even if we are aware of its existence. In this weak version of déjà vu, a parallel is seen in autobiographical memory. A cue reminds us of something stored in autobiographical memory, but the cue is undetected and does not yield sufficient activation, giving a feeling of familiarity without a known source. Accordingly, it would be expected that déjà vu will most likely occur in a context that is rather familiar/known, in contradiction to what we proposed above when discussing cue discrimination. Whilst Brown and Marsh (2008) compared mundane and distinctive scenes in their experimental analogue of déjà vu, finding that mundane scenes were more likely to generate feelings of having visited a place before, this rather common-sense interpretation of déjà vu in mundane, familiar locations has yet to be tested (although questionnaire studies suggest that in healthy groups, déjà vu happens more in familiar than novel contexts; see Moulin, 2018). One possibility to test this idea in experimental analogues of déjà vu is to further probe the déja vu once experienced. For instance, one may amplify the weak activation of the associated cues and memories by lowering the awareness threshold and/or drawing the attention to the possible sources of déjà vu. This way, the feeling of familiarity caused by weak activation of memory content could be overcome leading eventually to memory content retrieval; that is, reinstating cues experimentally may shift the experience from déjà vu to a memory of the past. This weak activation account also suggests that the déjà experience may be more likely to arise when one is engaged in attention demanding activities where ongoing task performance elevates the awareness threshold making weakly activated memories difficult to enter consciousness.

Our second hypothesis centres on metacognitive evaluations and posits that déjà vu is felt precisely because it is unexpected, aligning with the top–down view. In certain contexts, the expectation of familiarity should be low. In such cases, any involuntary process of retrieval, especially when it does not result in any retrieved information will be unexpected and will lead to a metacognitive evaluation which will be raised to consciousness. Until this feeling of familiarity is resolved or explained, it will grow and the experiment may well search for a reason “why” they have this feeling. Experimentally, it would be relatively simple to manipulate expectations about the likelihood of finding information familiar. In Cleary-type recognition without identification experiments, the idea that expectations about familiarity lead to the feeling of conflict should mean that déjà vu is more intense and more frequent when the expectation is lower. This could either be manipulated by giving explicit instructions to this effect to participants or manipulating the amount of similarity between the initial scene and its analogue in the recognition phase. It would be intense when the context is surprising, such as in the unrelated experiments conceit (e.g., Lakin & Chartrand, 2003): A first study phase is conducted in the context of one task and then the test phase is presented as a separate experiment. This design could assess the role of the experimental context (either presented as the same experiment or a separate experiment) on the report of déjà vu.

There is little existing research to discriminate between our two hypotheses, but Urquhart and colleagues successfully manipulated both the novelty and the activation (which they described as “familiarity”) of the critical non-presented stimulus. For novelty, they manipulated how specific the letter string was in referring to the critical lure (e.g., monitor for words that begin “SLE” in the list, vs. monitor words that begin with “B”). In the test phase, the string SLE was highly novel because it did not correspond to any items presented in the study phase, whereas the less specific “B” string applied to two words in the studied list, blanket and bed. For activation of the cue, they used results from previous versions of the DRM task: Some lists of associates are more likely to activate the critical lure than others: Thus they had more strongly and more weakly activated critical lures. In their experiment, the recognition memory results replicated the expected patterns. However, for the rates of experiencing déjà vu, there were no effects or interactions involving activation: The strength activation of the critical lure in the presented lists had no bearing on the generation of déjà vu. Thus, in this case, a weakly activated associate was not more likely to generate déjà vu than a more strongly activated one. The effect of novelty was however significant: Participants were more likely to experience déjà vu when the critical lure was unambiguously impossible to have been viewed in the study phase due to its unique string.

Our explanation of déjà vu as being “unexpected” would involve processes downstream, corresponding with the post-retrieval stage. During this stage, one becomes aware of having either an involuntary memory in mind or having a feeling of familiarity. More precisely, during both the post-retrieval and final stage, the unexpected feeling of familiarity without having any particular memory in mind may start metacognitive processing (e.g., thinking of plausibility of the event, etc.) leading people to explicitly experience déjà vu, especially in cases when the current situation is judged as novel and, crucially, “encountering it in the past is in fact highly implausible” (Mazzoni & Hanczakowski, 2011, p. 104). This would be the conflict described by Urquhart and O'Connor (2014): Familiarity is generated bottom–up from the environment, but higher-level autobiographical belief or knowledge opposes the feeling. This should be a rare occurrence, since as Conway (2005) proposes, autobiographical remembering is typically coherent with current goals and processing, but this would be a situation where the experience is incoherent. An interesting way to test this idea would be to re-evaluate reported déjà vu experiences in questionnaires according to self-reports of implausibility and novelty, on one hand, and also their stability across time. More precisely, although a given situation may provoke déjà vu, it may do so because in fact, there is not sufficient access to information relating directly to a past situation. However, such access may be restored later, and this additional information may change the evaluation of what was initially experienced as déjà vu, raising the question of the stability of déjà vu over time. In the only work related to this idea, Milton, Butler, and Zeman (2011) report a case of epileptic amnesia where the spontaneous retrieval of previously inaccessible autobiographical memories was “heralded” by a period of déjà vu. Finally, we may also hypothesise that if déjà vu experiences arise in response to the unexpected feeling of familiarity without having any particular memory in mind, then increasing the expectancy of a feeling of familiarity in response to the current situation should lower the frequency of reporting déjà vu experiences.

In our proposal, memory retrieval is not an all-or-nothing process, and numerous forms of partial or incomplete memory retrieval exist.¹³ Notably, memory content and a feeling of familiarity are separable (e.g., Ryals & Cleary, 2012). Studies on the TOT phenomenon (e.g., Brown, 1991; Heine, Ober, & Shenaut, 1999; Schwartz, 2001, 2002, 2011) demonstrate that on some occasions while we cannot access memory content (e.g., trying to recall the last time we were in cinema), we have a strong feeling of familiarity, with the attribution that this memory exists somewhere. Whilst this phenomenon is usually discussed in terms of linguistic or semantic processes, our proposal is that it occurs for the personal past too. Thus, it may occur for a given cue (e.g., a colourful beach ball): There is a strong feeling of that there is something in our past relating strongly to this cue or that this cue reminds us of something that cannot be yet recalled.

In Figure 1, we propose a flow chart summarising the possible memory phenomena on three dimensions: (1) Retrieval intentionality (involuntary vs. voluntary), (2) memory content accessibility (the presence vs. absence of memory content), and (3) phenomenological familiarity (feeling familiar or not). It shows that TOTs/FOKs are the result of unrecalled content despite having strong feelings of familiarity. They can arise both when retrieval is intentional and unintentional, and in this way, they act as a bridge between more controlled and less-controlled processes. That is, an involuntary retrieval may provoke a sense of familiarity, that once detected will lead to an intentional retrieval or at least a wilful memory search. In this way, metacognition plays a functional role in recruiting intentional retrieval processes to something for which retrieval was cued in an involuntary manner.

Figure 1 makes the case that familiarity is something that is experienced (or not) on the way to retrieving content, and that there is something of a sequential relationship between feelings of familiarity and the retrieval of content. We think that this is necessarily the case in failed retrieval, where there is a phenomenological response despite there being a lack of content, but this is not necessarily the case in fluent, successful retrieval where the presentation of content to consciousness is so rapid as to feel direct. The organisation may be sequential, but it is also recursive: Feelings of familiarity and content retrieval may jump back-and-forth in a manner inspired by the large-scale synchronisations of activation in the DMN; more precisely, this phenomenology-content loop is inspired by the interactions between retrieval and control networks stated in various models of memory retrieval described above (e.g., Eichenbaum, 2017; Irish & Vatansever, 2020; Sekeres et al., 2018).

Our view is that one underlying retrieval process yields two different experiences: Phenomenological familiarity is experienced when retrieval is incomplete, but it is not experienced when retrieval is fluent and routine. That is why our friends do not feel familiar, since we are able to rapidly retrieve specific information about them. That is, high trace strength memories can be accessed directly without giving rise to phenomenological fluency. In Figure 1, voluntary memories can be retrieved with familiarity or more directly, but involuntary memories never give rise to familiarity during retrieval. The study of déjà vu and IAMs helps us arrive at this conclusion since we need to understand why there is sometimes unbidden retrieval of the personal past without giving rise to any particular phenomenology, and on the other hand, in déjà vu, there is the opposite: An intense feeling of familiarity with the conclusion that the familiarity is erroneous, and devoid of content.

In sum, both déjà vu and IAMs are products of involuntary retrieval but, as presented in Figure 1, while IAMs are accompanied by memory content, déjà vu lacks the access to memory content and the feeling of familiarity is judged as implausible (if judged plausible, then one would most likely experience FOK/TOT).¹⁴ An interesting consequence emerging from Figure 1 is the possibility of differentiating involuntary and voluntary instances of FOK/TOT, which for now in our characterisation are the same experience whether or not they are encountered in intentional or unintentional retrieval. Although they have not been explicitly contrasted with each other in previous studies, such a distinction seems to us to be possible. Finally, all these phenomena (involuntary/voluntary memories, déjà vu, FOK/TOT) can be described along the same retrieval dimensions and so the integration of déjà vu into autobiographical retrieval has opened up new way of thinking about and classifying the range of possible retrieval experiences. We discuss this now, pointing out where research is needed.

3. What is already known and what still needs to be known

Whereas déjà vu research has been aligned with familiarity processing, IAMs have been studied from the viewpoint of autobiographical recall. It seems to us that to reconcile many naturalistic phenomena, a more circumspect view is needed. For instance, in déjà vu, the role of conflict favours the view that a feeling of retrieval is at odds with what is known or recalled about the current situation. Very little in cognitive science speaks to our rapid ability to reject things as unknown or unexperienced (but see Kolers and Palef [1976], “knowing not”). Déjà vu cannot simply be only about familiarity but also about the failure of the familiarity to reproduce any stored representation which can explain it: Recall (or at least attempted recall) is involved in déjà vu. Likewise, a stereotypical view of involuntary memory is that information arrives in consciousness unbidden, and it is as such uniquely a recall phenomenon. However, as we have seen, it is likely that cue activation and information in the environment drives this recall, however effortlessly, and the view we present here is that there should be a continuum between finding something familiar in the environment and spontaneously recalling information related to it. Along this continuum are varying degrees of our three dimensions: Intentionality, accessibility of content, and the feeling of familiarity. The classification by the experient of a retrieval as being déjà vu or an IAM is a metacognitive evaluation applied at a later stage to more-or-less usual memory processes, which like low-level familiarity mechanisms, we consider to be somewhat permanently active, scanning the environment for novelty and familiarity (in line with Bastin et al.'s [2019] proposition). This, of course, implies a modulation between responding to the external environment and generating internal thoughts; a fundamental property of human cognition. In cognitive psychology, this is captured in the difference between internal and external attention (Chun, Golomb, & Turk-Browne, 2011), and in cognitive neuroscience, this issue is represented in the discussion of the role of the DMN in “perceptually decoupled thought” (Smallwood et al., 2013; see also Kvavilashvili et al., 2020).

One possibility is that purely spontaneous retrieval without feelings of familiarity and metacognitive evaluations of intentionality should be related to greater activity in the DMN, such as medial pre-frontal cortex and the posterior cingulate, whereas stimulus-driven retrieval accompanied by familiarity, even when the retrieval of content is low, should be indexed by higher parahippocampal/medial temporal-lobe activation. Of note in Urquhart et al.'s (2021) fMRI investigation of conflict-based déjà vu, undetected false memory decisions (false positives to unstudied critical lures) reveal activations which are indistinguishable from genuine memory decisions (see also McDermott, Gilmore, Nelson, Watson, & Ojemann, 2017). It remains to be seen whether the prefrontal activations seen for decisions where conflict is detected are due to modulations of internal/external attention or align more broadly with metacognitive processes which too are proposed to lead to activations in prefrontal cortex (e.g., Morales, Lau, & Fleming, 2018).

Secondly, although both IAMs and déjà vu experiences have been studied for several years, one basic but yet central aspect of these two spontaneous phenomena was surprisingly overlooked; namely, their phenomenology. Therefore, there is still a need for experimentally oriented systematic studies to further examine exactly what it is like to have IAMs and déjà vu. Are they experienced similarly to each other and only the presence or lack of the content makes them distinct or, alternatively, are there more differences in the way they are phenomenologically experienced? It would be also interesting to examine how these two experiences are described by people. This leads to an intriguing question of the role of metacognitive beliefs in experiencing these spontaneous instances (for similar studies on mind wandering, see Zedelius, Protzko, & Schooler, 2021).

In addition, there is a need to examine the role of conflict in déjà vu. This “fact checking” control process is the thing that is hardest to reconcile with existing involuntary memory theory. We are obliged to propose that a later, metacognitive stage is able to reject cues in the environment as being false. In our schematic (Fig. 1), chief among the higher-order evaluations in memory retrieval is plausibility. In implausible situations, we are able to quickly reject the current situation as being novel or unique, when this happens quickly and decisively, we imagine that it is what generates déjà vu, as opposed to a search in memory for a prior event. Given the notion of the continuity between imagination and memory retrieval, in counterfactual memory (e.g., De Brigard, Spreng, Mitchell, & Schacter, 2015) or in representations of the future (e.g., Schacter et al., 2012), it seems important to identify the control mechanisms that allow us to experience memories as veridical or as having occurred in the past. Are these the same control mechanisms implied in the conflict generated in déjà vu? It has been proposed that déjà vu acts to correct faulty or overactive feelings of familiarity, and it follows that if one is less likely to experience déjà vu, then one should be more likely to falsely recognise situations and events.

Our proposal rests on the ability to judge the plausibility of retrieved material and even the plausibility (or likelihood) that the present situation should generate memories or feelings of familiarity. In comparison to investigations of familiarity or recollection, this is an underdeveloped field, and in general, there is a lack of studies evaluating metacognition in autobiographical memory (although plausibility evaluations have gained some interest, for example, Scoboria, Mazzoni, Kirsch, & Relyea, 2004). One way of rapidly judging plausibility online during retrieval would be by conceptual stores of “personal semantics” (e.g., Renoult, Davidson, Palombo, Moscovitch, & Levine, 2012). In any one situation (or “event”), we activate schemas which can help us rapidly retrieve appropriate information as a scaffold (Irish & Piguet, 2013) but which also would create a metacognitive model of expectancies about fluency and the likelihood of retrieval, thus contributing to our evaluation of plausibility and a metacognitive representation of the situation or event.

The notion of a continuum should lead to some empirical tests of this idea, and the two domains may learn from each other. Most notably, the IAM literature has converged on a laboratory task which reliably yields IAMs. An obvious implication of our ideas is that in these mundane and repetitive tasks, participants should be experiencing metacognitive experiences and unresolved feelings of familiarity as well as more well-developed IAMs. Thus, when studying involuntary memories, one could ask for instances of déjà vu and familiarity to test some of the ideas suggested in the present paper. Also, questionnaire and diary studies should investigate the incidence of the two phenomena to examine whether those people who experience more IAMs also experience déjà vu more often.

Testing people with temporal-lobe pathology on tasks which generate IAMs would also be of interest (for a first case study for déjà vu, see Cleary et al., 2021b). Martin et al. (2012) have identified familiarity and recollection tasks which distinguish between people with temporal-lobe epilepsy who do and do not experience déjà vu, and according to our continuum hypothesis, people who experience more déjà vu in temporal-lobe epilepsy should also be more likely to experience involuntary memories. Such populations would help converge on the relationship between bottom–up and top–down processes in retrieval. Patients with selective parahippocampal impairments which lead to feeling less familiarity should, by our view, experience more spontaneous retrieval, and be less able to metacognitively act on cues in the environment. Such information is as yet unknown, primarily because, as with the existing IAM tasks in healthy populations, most memory measures concentrate on objective indices of declarative memory, and very little on subjective report or incomplete retrieval. Because we argue that familiarity operates to tell us that something in the environment “reminds” us of something, it should be possible to experimentally induce these feelings through priming and fluency manipulations, and measure the resultant effects on phenomenology and subjective experience.

To our knowledge, there are as yet no similar studies investigating involuntary memory retrieval in the context of brain damage, but there are studies demonstrating autobiographical memory retrieval deficits relating to clinical states (e.g., Múnera et al., 2014; Philippi, Tranel, Duff, & Rudrauf, 2013). We propose that it is a matter of time until case studies with people lacking involuntary retrieval will be reported. From the opposite end, there is already one case study by Parker, Cahill, and McGaugh (2006) describing AJ's memory as “nonstop, uncontrollable, and automatic” which is an example of extreme domination of involuntary retrieval, described as most likely resulting from a variant of a neurodevelopmental frontostriatal disorder.

It would also be fruitful to study both the mental content and feeling of familiarity as two dimensions that may influence memory retrieval independently from each other. For example, one may try to manipulate familiarity and memory accessibility during involuntary retrieval by the use of different types of antecedental/previously prepared cue-sets such as, for example: (1) Cues that are highly efficient in quickly triggering an autobiographical memory and are also rated as highly familiar/typical, (2) cues that are less efficient in triggering any concrete memory while being rated as familiar (i.e., that slowly but eventually leads to a memory retrieval), (3) cues that are less efficient and weakly familiar. As a result, one may expect that most IAMs will be experienced with the first type of cue, while déjà vu would be experienced with the second type.

Next, if we agree on the spontaneous nature of IAMs and déjà vu, then one may ask about the relationship between mind wandering and IAMs and déjà vu. As, in general, mind wandering relates to thoughts not related to the ongoing task per se, then one may expect to observe instances of IAMs and déjà vu during typical mind-wandering episodes. For instance, as Kvavilashvili et al. (2020; also Cole & Kvavilashvili, 2019; Plimpton et al., 2015) directly treat IAMs as an instance of spontaneous mind wandering, it may be argued that at least some examples of déjà vu may constitute the content of mind-wandering instances. Importantly, we propose that déjà vu and IAMs may both have multiple causes. For instance, it is still possible that on some occasions, déjà vu experiences are due to false recognition (as suggested by Mazzoni & Hanczakowski, 2011) or decoupled familiarity (Illman et al., 2012) on a neural level. It may also be true, especially when metacognitive abilities are impaired, that déjà vu is a result of the lack of access to already activated autobiographical memories that are below the awareness threshold (e.g., Conway & Loveday, 2015; Moulin et al., 2014).

Integrating déjà vu into autobiographical memory has opened interesting avenues for future research. For instance, recent advances and newer theoretical developments such as those proposed by Rubin and Umanath (2015; also Rubin, 2022, p. 10) propose a common theoretical ground and “home for many homeless categories of memory” including déjà vu. As they aim to improving and integrating theories arising from laboratory work, they create a conceptual “corner” space for such integration. Specifically, Rubin and Umanath (2015) proposed the theory of event memory to classify and understand “a mental construction of a scene recalled as a single occurrence” (p. 1).¹⁵ In these terms, déjà vu may be understood as an example of self-reference (i.e., about past events involving the person recalling them) implicit scene memory that “includes recognizing people and objects as a function of their context, traversing familiar routes without explicit memory” (Rubin, 2022, p. 7). Importantly, implicit scene memories are accompanied by feelings of familiarity and reliving without an access to memory of past experience. Although it is unknown whether déjà vu is associated with scene processing the same way as involuntary memories are, given the fact that scenes are known to be a common trigger of déjà vu (Brown, 2004; Cleary & Brown, 2022), it is highly possible that scenes, in general, may be a special form of triggering cues of both IAMs and déjà vu. This shows another possible way in which these two phenomena may have something in common and future studies should investigate this possibility.

Finally, we mentioned briefly several other memory-related phenomena (e.g., FOK, TOT, see Fig. 1) that strongly depend on the feeling of familiarity which were not discussed in the same detail as déjà vu and IAMs. Future research might want to consider how these primarily metacognitive experiences fit into a framework of thoughts and feelings associated with spontaneous retrieval. A key issue is their spontaneity – it would be normal that feelings of imminent retrieval and retrieval failure as well as feelings of familiarity guide retrieval processes, and our hypothesis here is that whilst we can ask participants to consciously and wilfully report their likelihood of retrieval in an experimental task (e.g., in the FOK task), the TOT, the butcher on the bus, and FOK all also exist as naturally occurring spontaneous phenomena in daily life, and are far more frequently experienced than déjà vu.

4. Conclusions

In the present paper, we discussed the idea that IAMs and déjà vu can be described and explained as natural forms of memory processing and that both are based on the same retrieval processes. According to our continuum hypothesis, déjà vu is, at least on some occasions, an underdeveloped form and special case of involuntary memory; it draws upon the personal past and shares some crucial properties with IAMs, and surprisingly these two instances have not yet been comprehensively considered alongside each other in cognitive psychological discussions of human memory, in general, and autobiographical memory, in particular.

While there are a few possible origins of déjà vu and IAMs, not least in intercortical stimulation studies, we argue that one may be routine: A product of the ongoing automatic processing of cues in the environment which may present some conceptual or perceptual overlap with stored memory representations. As our cognitive system tries to match, as quickly and effortlessly as possible, the contents of mental representations stored in memory with the current contents of perception, one would expect déjà vu and involuntary memories to arise. This way, on some occasions, such an overlap may successfully trigger an involuntary memory of a given past personal event. This should be indeed expected from time to time, especially given the fact of “the vastness and richness of the autobiographical memory base” (Mace & Atkinson, 2009, p. 202). On some other occasions though, the overlap may be too weak and/or diffused to successfully trigger a given personal memory leaving one with only a feeling of familiarity, which we conceive as something like the TOT but for the personal past. Importantly, this should be also reasonably expected to happen given the fact that a great number of cues in the environment may coincide with a great number of memories stored in mind (i.e., the cue-overload principle) not being able to successfully match and trigger one given memory.

In terms of the function of these phenomena, we propose that they are the result of a continuously active memory system that automatically and rapidly scans the environment for matching representations. The purpose and evolutionary advantage of such a system is clear, quickly raising pertinent information to consciousness without effort. Yet, our characterisation is this system is of it being a constant iteration driven by feelings of familiarity and successful retrieval. Déjà vu arises when the familiarity arises in an inappropriate context, and its function is to signal that there is unwarranted activation of familiarity in the absence of retrieved contextual specifics which could link the present moment to the personal past.

Interestingly, there are neuropsychological patients which support this view. When familiarity is pathologically overactive, it can yield feelings of recollective confabulation, described by carers and medics as like permanent déjà vu (see Moulin, 2013, for a series of cases). The account of such patients is that rather than experiencing a conflict in mental evaluations as proposed here, they confabulate contextual specifics to justify the feeling of confabulation (Turner, Shores, Breen, & Coltheart, 2017). Neuroanatomically, in the most detailed analysis of the brain of such cases, Craik et al. (2014) describe extensive grey matter atrophy in frontal and medial temporal areas as shown by MRI, again converging on the notion that inappropriate familiarity was not “discounted or edited out” (p. 367).

Here, we have rarely discussed conceptual knowledge in memory retrieval, focusing on the personal past. Déjà vu is studied exclusively from an episodic memory viewpoint, whereas involuntary memories are most studied in the context of a richer, more complex construct: Autobiographical memory encompasses both conceptual and event-specific information. As such, for example, whereas we might discuss the integration of episodic and semantic memory in autobiographical retrieval (e.g., Mace & Unlu, 2020), and can even propose purely semantic involuntary cognitions (e.g., Kvavilashvili & Mandler, 2004), there is not, to the best of our knowledge, a similar parallel in déjà vu: It seems to uniquely include an interpretation of a prior experience rather than information or knowledge. Although familiarity is an epistemic feeling which can be experienced for both semantic and episodic materials (see Kempnich, Urquhart, O'Connor, and Moulin [2017] for evidence of dual process retrieval processes in episodic word lists and general knowledge; or Bowles and Köhler [2014] for studies of familiarity in semantic memory), it is unclear what a “semantic” déjà vu would consist of. This issue still warrants attention in future work, although it would need to be discussed in the context of evidence shifting away from the existence of truly separable semantic and episodic systems (e.g., Renoult et al., 2019).

By relating to well-known and already elaborated mechanisms of autobiographical memory retrieval, we were able to explain the generation of déjà vu. This theoretical and conceptual integration will facilitate future studies on every-day spontaneous phenomena, and in doing so underlines the importance of three important dimensions on which memories are experienced: Intentionality, plausibility, and phenomenological experiences during retrieval, notably fluency.