Kubovy and Epstein distinguish between systems that follow rules, and those that merely instantiate them. They regard compliance with the principles of kinematic geometry in apparent motion as a case of instantiation. There is, however, some reason to believe that the human visual system internalizes the principles of kinematic geometry, even if it does not explicitly represent them. We offer functional resemblance as a criterion for internal representation. [Kubovy & Epstein]

We make three points. First, even if Finlay et al.'s proposed developmental mechanisms hold, there remains great scope for selection on specific brain structures. Second, the positive covariance among the size of brain structures provides far less support for the proposed developmental mechanisms than Finlay et al. acknowledge. Third, even if the proposed mechanisms are the primary size determinants for most brain structures, these structures should not be considered “spandrels.”

Humphreys and Forde fail to account for the ontology of the “features” that they claim are constitutive of concepts. This failure is common to decompositional theories of conceptual representation. Category-specific deficits can be better explained by a theory that takes inferential relations among atomic concepts to be the key characteristic of conceptual representation and processing.

The Theory of Event Coding (TEC) presented in Hommel et al.'s target article provides a useful heuristic framework for stimulating research. Although the authors present TEC as providing a more integrated view of perception and action than classical information processing, TEC is restricted to the stage often called response selection and shares many features with existing theories.

Shepard's article presents an impressive application of the mathematics of symmetry to the understanding of motion. However, there are basic psychological phenomena that the model does not handle well. These include the importance of the orientations of rotational motions to salient reference systems for the understanding of the motions. An alternative model of the understanding of rotations is sketched. [Shepard]

I argue that, while Finlay et al, are correct to suggest that there are developmental regularities (or constraints) acting on brain component evolution, they are incorrect to infer from this that a developmental explanation necessarily implies that structural changes preceded functional use. Developmental and functional (adaptationist) explanations are complementary, not alternatives.

A temporal account of the mental capacities for processing information may not be relevant in a context where the goal is to search for storage capacity expressed in chunks. However, if mental capacity and information processing is the question, the time issue can be rehabilitated. A very different temporal viewpoint on capacity limit is proposed in this commentary.

Humphreys and Forde conceptualize object representations as structural descriptions, without discussing the implications of structural description models. We argue that structural description models entail two major assumptions – a part-structure assumption and an invariance assumption. The invariance assumption is highly problematic because it contradicts a large body of findings which indicate that recognition performance depends on orientation and size. We will delineate relevant findings and outline an alternative conception.

This discussion argues that for many word meanings, the child has to assemble a new category, using relatively slow information-sifting processes. This does not cause high semantic errors, because children probably hold off using a word until much such sifting has occurred, rather than producing the new word as soon as they have any information on it.

We argue that the strengths of the Theory of Event Coding (TEC) can usefully be applied to a wider scope of cognitive tasks, and tested by more diverse methodologies. When allied with a theory of conceptual representation such as Barsalou's (1999a) perceptual symbol systems, and extended to data from eye-movement studies, the TEC has the potential to address the larger goals of an embodied view of cognition.

Cowan's review shows that a short-term memory limit of four items is consistent with a wide range of phenomena in the field. However, he does not explain that limit, whereas an existing theory does offer an explanation for capacity limitations. Furthermore, processing capacity limits cannot be reduced to storage limits as Cowan claims.

It is suggested that a connection between neurogenesis and brain part size is unsurprising. It is argued that neurogenesis cannot, however, be the only factor contributing to brain size. Highly individual post-natal experience radically shapes individual brains, leading to dramatic increases in brain size. The role of comparatively coarse statistical techniques in addressing these subtle biological issues is questioned.

Shepard proposes that the human mind relies on screw displacement because of its adaptive simplicity and uniqueness. I discuss this hypothesis by assessing screw displacement with respect to (1) other plausible spatial transformations, (2) a variety of criteria for adaptive efficiency and utility, and (3) a variety of psychological conditions in which observed responses discriminate amongst alternative spatial procedures. [Shepard]

Although Humphreys & Forde's HIT provides a comprehensive account of category-specific deficits, standard models of categorization and identification may also be able to explain many aspects of such deficits. The assumptions of an exemplar-based account of category- specific deficits are presented, and it is argued that exemplar models may be able to explain key findings on impaired object identification and categorization.

We commend the argument that perception and action are tightly coupled. We claim that the argument is not new, that uniting stimulus and response codes is not a problem for a cognitive system, only for psychologists who assume them, and that the Theory of Event Coding (TEC)'s event-codes are arbitrary and ungrounded. Affordances and information offer the common basis for perception-action (and even for event-codes).

Research on children's and adults' concepts embodies very different assumptions of how concepts are structured, as reflected in their experimental designs. Developmental studies seem to assume that categories contain highly similar objects that can all be identified from one or two examples. If concepts are more like those tested in adult experiments, research on word learning may be misleading.

Future work is needed to establish that pure short-term memory is a coherent individual difference attribute that is separable from traditional compound short-term memory measures. Psychometric support for latent pure short-term memory capacity will provide an important starting point for future fine-grained analyses of the intrinsic factors that influence individual differences in math skills.

The article of Finlay et al. is an excellent example of identifying constraints in the development of the brain, and their implications on brain architecture in evolution. Here we further illustrate the importance of constraints by presenting a few examples of how a small number of biophysical mechanisms or even a single life history parameter can have an enormous impact on brain evolution.

Bloom's domain general theory remains strictly cognitive and individualistic. By ignoring the contribution of social interaction and collective construction of concepts, he fails to solve the word learning problem.

Recent investigations and theorising about category-specific deficits have begun to focus upon patients with progressive brain disease such as semantic dementia and Alzheimer's disease. In this commentary we briefly review what insights have been gained from studying patients of this type. We concentrate on four specific issues: the sensory/functional distinction, correlation between features, neuroanatomical considerations, and confounding factors.