We distinguish the question whether only human minds are equipped with a language of thought (LoT) from the question whether human minds employ a single uniquely human learning mechanism. Thus separated, our answer to both questions is negative. Even very simple minds employ a LoT. And the comparative data reviewed by Penn et al. actually suggest that there are many distinctively human learning mechanisms.

We argue that formal analogical reasoning is not a uniquely human trait but is found in chimpanzees, if not in monkeys. We also contest the claim that the relational matching-to-sample task is not exemplary of analogical behavior, and we provide evidence that symbolic-like treatment of relational information can be found in nonhuman species, a point in contention with the relational reinterpretation hypothesis.

Penn et al. argue that the complexity of relational learning is beyond animals. We discuss a model that demonstrates relational learning need not involve complex processes. Novel stimuli are compared to previous experiences stored in memory. As learning shifts attention from featural to relational cues, the comparison process becomes more analogical in nature, successfully accounting for performance across species and development.

The history of comparative psychology is replete with proclamations of human uniqueness. Locke and Morgan denied animals relational thought; Darwin opened the door to that possibility. Penn et al. may be too quick to dismiss the cognitive competences of animals. The developmental precursors to relational thought in humans are not yet known; providing animals those prerequisite experiences may promote more advanced relational thought.

The prevailing view that there is significant cognitive continuity between humans and other animals is a result of misinterpretations of the role of evolution, combined with anthropomorphism. This combination has often resulted in an over-interpretation of data from animal experiments. Comparative psychology should do what the name indicates: study the cognitive capacities of different species empirically, without naive evolutionary presuppositions.

In our target article, we argued that there is a profound functional discontinuity between the cognitive abilities of modern humans and those of all other extant species. Unsurprisingly, our hypothesis elicited a wide range of responses from commentators. After responding to the commentaries, we conclude that our hypothesis lies closer to Darwin's views on the matter than to those of many of our contemporaries.

A necessary consequence of the nature of neural transmission systems is that as change in the physical state of a time-varying event takes place, delays produce error between the instantaneous registered state and the external state. Another source of delay is the transmission of internal motor commands to muscles and the inertia of the musculoskeletal system. How does the central nervous system compensate for these pervasive delays? Although it has been argued that delay compensation occurs late in the motor planning stages, even the earliest visual processes, such as phototransduction, contribute significantly to delays. I argue that compensation is not an exclusive property of the motor system, but rather, is a pervasive feature of the central nervous system (CNS) organization. Although the motor planning system may contain a highly flexible compensation mechanism, accounting not just for delays but also variability in delays (e.g., those resulting from variations in luminance contrast, internal body temperature, muscle fatigue, etc.), visual mechanisms also contribute to compensation. Previous suggestions of this notion of “visual prediction” led to a lively debate producing re-examination of previous arguments, new analyses, and review of the experiments presented here. Understanding visual prediction will inform our theories of sensory processes and visual perception, and will impact our notion of visual awareness.

An attention shift from a stationary to a changing object has to occur in feature – rather than physical – space, in order to bind these stimuli into a unitary percept. This time-consuming shift leads to the perception of a changing stimulus further ahead along its trajectory. This attentional framework is able to accommodate the flash-lag effect in its multiple empirical manifestations.

To successfully interact with a dynamic world, our actions must be guided by a continuously changing anticipated future. Such anticipations must be tuned to the processing delays in the nervous system as well as to the slowness of the body, something that requires constant adaptation of the predictive mechanisms, which in turn require that sensory information be processed at different time-scales.

Neural delays, which are generally defined as visuomotor delays in interceptive actions, must be compensated to enable accurate timing in movement. Visuomotor delays can depend on the kind of task, the use of information, and the skill of the performer. The compensation for such delays does not necessarily require prediction or representation but can be made by an attunement of some parameters in what is called a law of control.

3D FORMOTION, a unified cortical model of motion integration and segmentation, explains how brain mechanisms of form and motion processing interact to generate coherent percepts of object motion from spatially distributed and ambiguous visual information. The same cortical circuits reproduce motion-induced distortion of position maps, including both flash-lag and flash-drag effects.

The hypothesis that there has been selection pressure for mechanisms which enable us to perceive the present tends to be conflated with the hypothesis that there has been selection pressure for mechanisms that compensate for inevitable neural delay. The relationship between the two is more subtle, because increases in neural delay can be advantageous for building more useful perceptions.

Analogous to prism adaptation, sensorimotor compensation for existing neural delays has been clearly demonstrated. This system can also adapt to new delays, both internal and external. This seems to occur at least partially in the sensor systems, and works for discrete, stationary events. This provides additional evidence for visual prediction, but not in a manner that is consistent with spatial extrapolation.

Nijhawan argues that neural compensation is necessary to account for couplings of perception and action. Although perhaps true in some cases, computational tolerance for asynchronously arriving continuous information is of more importance. Moreover, some of the everyday venues Nijhawan uses to argue for the relevance of prediction and compensation can be better ascribed to skill.

Although prediction is one of the key tasks of intelligent brains, it often proves impossible in an unpredictably changing world. Hence, brains often decide what happened retrospectively. This framework of postdiction, the opposite of prediction, stands as an alternative or complimentary framework to prediction. I further show how motor-sensory recalibration demonstrates delay compensation at the perceptual level.

Nijhawan's theory rests on the assumption that transient signals compete with predictive signals to generate the visual percept. We describe experiments that show that this assumption is incorrect. Our results are consistent with an alternative theory that proposes that vision is instead postdictive, in that the perception of an event is influenced by occurrences after the event.

Evidence for motion extrapolation at motion offset is scarce. In contrast, there is abundant evidence that subjects mentally extrapolate the future trajectory of weak motion signals at motion offset. Further, pointing movements overshoot at motion offset. We believe that mental and sensorimotor extrapolation is sufficient to solve the problem of perceptual latencies. Both present the advantage of being much more flexible than motion extrapolation.

Nijhawan redraws our attention to the problem of accurately perceiving an ever-changing visual world via a sensory system that has finite and significant communication times. The quandary is compelling and stark, but the suggestion that the visual system can compensate for these transmission delays by extrapolating the present is not so unequivocal. However, in this current airing of contradictory issues, accounts, and findings, Nijhawan trades spatial extrapolation – a rather specific concept introduced earlier (in Nijhawan 1994) for visual prediction – a far more expansive notion that forces the issue of both the perceived reality and functional significance of compensation.

Neural activity in the middle temporal area (MT) is strongly correlated with motion perception. I analyzed the temporal relationship between the representation of direction in MT and the actual direction of a stimulus that continuously changed direction. The representation in MT lagged the stimulus by 45 msec. Hence, as far as the perception of direction is concerned, the hypothesis of lag compensation can be rejected.

The extent to which visual processing can proceed in the visual hierarchy without awareness determines the magnitude of perceptual delay. Increasing data demonstrate that primary visual cortex (V1) is involved in consciousness, constraining the magnitude of visual delay. This makes it possible that visual delay is actually within the optimal lengths to allow sufficient computation; thus it might be unnecessary to compensate for visual delay.

The time delay problem – that perception lives slightly in the past as a result of neural conduction – has recently attracted a considerate amount of attention in the context of the flash-lag effect. The effect refers to a visual illusion wherein a brief flash of light and a continuously moving object that physically align in space and time are perceived to be displaced from one another – the flashed stimulus appears to lag behind the moving object (Krekelberg & Lappe 2001). In the target article, Nijhawan compellingly argues that delay compensation could be undertaken by a predictive process in the feedforward pathways in the vision system. Before jumping into the quest for the mechanism of delay compensation, however, I would like to argue that the magnitude of delay has been overestimated, and that it might even be unnecessary to compensate for such a delay.