As illustrated by research Koehler himself cites (Dawes et al. 1993), the purpose of experiments is to choose between contrasting explanations of past observations – rather than to seek statistical generalizations about the prevalence of effects. True external validity results not from sampling various problems that are representative of “real world” decision making, but from reproducing an effect in the laboratory with minimal contamination (including from real world factors).

Heyman's target article makes a strong case for a behavioral approach to addiction, yet some important assumptions require justification, and promising behavioral alternatives to the author's melioration approach should be considered. In particular, the behavioral economic approach to addiction appears well developed and comprehensive. How does the melioration approach complement or improve on a behavioral economic account?

Koriat & Goldsmith raise important questions about memory, but there is need for caution: first, if we define accuracy by output measures, there is a danger that a perfectly accurate memory can be nearly useless. Second, when we focus on correspondence, there is a danger that syntactic correspondence will be mistaken for historical truth.

This article reviews models of the cerebellum and motor learning, from the landmark papers by Marr and Albus through those of the present time. The unique architecture of the cerebellar cortex is ideally suited for pattern recognition, but how is pattern recognition incorporated into motor control and learning systems? The present analysis begins with a discussion of exactly what the cerebellar cortex needs to regulate through its anatomically defined projections to premotor networks. Next, we examine various models showing how the microcircuitry in the cerebellar cortex could be used to achieve its regulatory functions. Having thus defined what it is that Purkinje cells in the cerebellar cortex must learn, we then evaluate theories of motor learning. We examine current models of synaptic plasticity, credit assignment, and the generation of training information, indicating how they could function cooperatively to guide the processes of motor learning.

Heyman's two crucial theses are that people over-value immediate rewards, and that addictive substances “subvert the value of competing commodities.” These perennial ideas were discussed by Plato. Whereas Heyman provides scientific clarification and support for the first, the second remains problematic. I outline how this deficiency might be remedied via evolutionary considerations.

The persistence of many contrasting notions of climbing fiber function after years of investigation testifies that the issue of climbing fiber contributions to cerebellar transactions is still unresolved. The proposed capabilities of the climbing fibers cover an impressive spectrum. For many researchers, the climbing fibers signal errors in motor performance, either in the conventional manner of frequency modulation or as a single announcement of an “unexpected event”. More controversial is the effect of these signals on the simple spike modulation of Purkinje cells. In some hands, they lead to a long-term depression of the strength of parallel fiber synapses, while, in other hands, they lead to a short-lasting enhancement of the responsiveness of Purkinje cells to mossy fiber inputs or contribute to the often-seen reciprocal relation between complex and simple spike modulation. For still other investigators, the climbing fibers serve internal timing functions through their capacity for synchronous and rhythmic firing. The above viewpoints are presented in the spirit of trying to reach some consensus about climbing fiber function. Each point of view is introduced by summarizing first the key observations made by the respective proponents; then the issues of short-lasting enhancement, reciprocity between complex and simple spikes, and synchrony and rhythmicity are addressed in the context of the visual climbing fiber system of the vestibulocerebellum.

The important question is how people process probabilistic information, not whether they process it in accordance with a normative model that we never should have expected them to be capable of following. Experience is not the cure, as widely thought, to problems with utilizing base rate information.

Koriat and Goldsmith clearly show the need for an alternative to the storehouse metaphor; however, the alternative metaphor they choose – the correspondence metaphor – is problematic. A more suitable one is the capacity metaphor.

I support the major theme of Heyman's target article that behavioral choice theory can enhance our understanding of drug dependence, but I raise concerns about the critique of the operant model of drug dependence, the underscoring of melioration to the exclusion of other theories of choice, and assertions about the unique effects of drug reinforcement.

(1) There is enough contradictory evidence regarding the role of base rates in category learning to confirm the nonexistence of biases in such learning. (2) It is not always possible to activate statistical reasoning through frequentist representation. (3) It is necessary to use the concept of systematic processing in reconsidering the published work on biases.

Although there is increasing agreement that the cerebellum plays an important role in motor learning, the basic substance of what constitutes motor learning has been difficult to define. Unless motor learning is somehow radically different from other forms of learning, it must involve relatively simple stimulus-stimulus and stimulus-response associations. All forms of learning, including purely sensory associations and cognitive learning as well as motor learning, effect changes in behavior. However, a singular characteristic of motor learning is that it adjusts joint and limb mechanics by altering the neural input to muscles through practice and mental rehearsal. The hypothesis proposed here is that the cerebellum plays an important role in motor learning by forming and storing associated muscle activation patterns for the time-varying control of limb mechanics. By modulating the cocontraction of agonist-antagonist muscles through adjustments in the timing and amplitude of muscle activity, the viscoelastic properties of joints can be appropriately regulated throughout Movement and adapted for transitions between postures and movements. Optimal control of joint viscoelastic properties cannot be achieved by online corrections initiated by reflex feedback because of the delays and consequent instabilities incurred. Instead, strategies for optimizing muscle activation patterns or synergies must be learned from the temporal association of proprioceptive stimuli signaling muscle lengths and forces and the rates of changes in these parameters, with reinforcement occurring when the movement achieves its objective. Such strategies would involve varying degrees of cocontraction or reciprocal inhibition of agonist-antagonist muscles that ultimately contribute to joint and limb stiffness. Evidence from neural recordings and clinical and experimental lesion studies are presented, suggesting that the cerebellum uses teleceptive and proprioceptive feedback as feedforward conditioned stimuli for specific muscle activation patterns contributing to joint stiffness (i.e., agonist-antagonist muscle synergies) for particular tasks and postures. A wide variety of observations are thought to be consistent with such a role for the cerebellum, but ultimately additional experiments could confirm or disconfirm this hypothesis.

Koriat & Goldsmith's distinction between encoding processes and metamnemonic decision processes is theoretically and practically important, as is their methodology for separating the two. However, their accuracy measure is a conditional statistic, subject to the unfathomable selection effects that have hindered analogous measures in the past. We also find their arguments concerning basic and applied research mostly beside the point.

Heyman's analysis of addiction suggests that drug taking is irrational. The irrationality of drug taking, however, may depend on the acceptance of mainstream society's view of what is valuable. Consideration of the addict's viewpoint and cultural aspects of drug taking may be useful in trying to fathom the “rationality” of drug taking.

Brindley proposed that we initially generate movements “consciously”, under higher cerebral control. As the movement is practiced, the cerebellum learns to link within itself the context in which the movement is made to the lower level movement generators. Marr and Albus proposed that the linkage is established by a special input from the inferior olive, which plays upon an input-output element within the cerebellum during the period of the learning. When the linkage is complete, the occurrence of the context (represented by a certain input to the cerebellum) will trigger (through the cerebellum) the appropriate motor response. The “learned” movement is distinguished from the “unlearned” conscious movement by its now being automatic, rapid, and stereotyped. The idea is still controversial, but has been supported by a variety of animal studies and, as reviewed here, is consistent with the results of a number of human PET and ablation studies. I have added to the idea of context-response linkage what I think is another important variable: novel combinations of downstream elements. With regard to the motor system and the muscles, this could explain how varied combinations of muscles may become active in precise time-amplitude specifications so as to produce coordinated movements appropriate to specific contexts. In this target article, I have further extended this idea to the premotor parts of the brain and their role in cognition. These areas receive influences from the cerebellum; they are active both in planning movements that are to be executed and in thinking about movements that are not to be executed. From recent evidence, the cerebellar output extends even to what has been characterized as the ultimate frontal planning area, the “prefrontal” cortex, area 46. The cerebellum thus may be involved in context-response linkage, and response combination even at these higher levels. The implication would be that, through practice, an experiential context would automatically evoke a certain mental action plan. The plan would be in the realm of thought, and could – but need not – lead to execution. The specific cerebellar contribution would be one of the context linkage and the shaping of the response, through trial and error learning. The prefrontal and premotor areas could still plan without the help of the cerebellum, but not so automatically, rapidly, stereotypically, so precisely linked to context, or so free of error. Nor would their activities improve optimally with mental practice.

The base rate literature has an opposite twin in the social psychological literature on stereotypes, which concludes that people use their preexisting beliefs about probabilistic category attributes too much, rather than not enough. This ironic discrepancy arises because beliefs about category attributes enhance accuracy when the beliefs are accurate and diminish accuracy when they are not. To determine the accuracy of base rate/stereotype beliefs requires research that addresses specific content.

Koriat & Goldsmith's abstract correspondence metaphor is unlikely to prove useful to memory science. It aims to motivate and inform the investigation of everyday memory, but that movement has prospered without it. The irrelevance of its competitor – the more concrete storehouse metaphor – as a guiding force in memory research presages a similar fate for the correspondence perspective.

In contrast to traditional research on base-rate neglect, an ecologically-oriented research program would analyze the correspondence between cognitive algorithms and the nature of information in the environment. Bayesian computations turn out to be simpler when information is represented in frequency formats as opposed to the probability formats used in previous research. Frequency formats often enable even uninstructed subjects to perform Bayesian reasoning.

We ask what cerebellum and basal ganglia “do,” arguing that cerebellum tunes motor schemas and their coordination. We argue for a synthesis of models addressing the real-time role and error signaling roles of climbing fibers. “Synthetic PET” bridges between regional and neuro-physiological studies, while “synaptie eligibility” relates the neurochemis-try of learning to neural and behavioral levels. [CRÉPEL et al.; HOUK et al.; KANO; LINDEN; SIMPSON et al.; SMITH; THACH; VINCENT]

Neither the storehouse nor the correspondence metaphor is an appropriate conceptual framework for memory research. Instead a meaning-based account of human memory is required. The correspondence metaphor is an advance over previous suggestions but entails an oversimple view of “accuracy.” Freud's account of memory may provide a more fruitful approach to memory and meaning.

I discuss various theoretical issues concerning maximizing, matching, and melioration. The model of addiction based on melioration has the key feature that a reduction in drag use increases both the value of drag-taking and the value of conventional activities. I found Heyman's target article stimulating. I don't feel competent to comment on drags but I do have some thoughts, both general and specific, on the theoretical issues.