Van Gelder's hard line against representations is not supported or supportable, and his soft line in favor of dynamical systems thinking as a supplement to representational models of cognition is good advice, but not revolutionary, as he seems to think.

Apart from the computationally appealing properties of representation by similarities, it is possible to extend this form of representation when needed to include object parts as well as the correspondence between subobject parts.

Chow's book should be read only by those who already have a firm enough grasp of the logic of significance testing to separate the few valid, insightful points from the many incorrect statements and misrepresentations.

Developmental counts of axons in the splenium of the rat corpus callosum are compatible with the hypothesis that estrogen may be acting late in development to sculpt the female nervous system.

The learnability of features and their dependence on task and context do not rule out the possibility that primitives used for constructing new features are as small as pixels, nor that they are as large as object parts, or even entire objects. In fact, the simplest approach to feature acquisition may be to treat objects not as if they are composed of unknown primitives according to unknown rules, but rather as if they are what they seem: patterns of atomic features, standing in various similarity relationships to other objects, which serve as holistic features.

Although Pessoa et al. imply that many aspects of the filling-in debate may be displaced by a regard for active vision, they remain loyal to naive neural reductionist explanations of certain pieces of psychophysical evidence. Alternative interpretations are provided for two specific examples and a new category of filling-in (of visual detail) is proposed.

It is argued that there is a role for the representational conception of vision, and that this is compatible with the task-level account advocated by Pessoa et al. However, the role of representations must be understood independently of our conscious experience of vision.

Research in attitudes and social cognition exemplifies van Gelder's distinction between the computational and dynamical approaches. The former emphasizes linear measurement and rational decision-making. The latter considers processes of associative memory and self-organization in attitude formation and social influence. The study of dynamical processes in social cognition has been facilitated by connectionist approaches to computation.

The fixed-feature viewpoint Schyns et al. are opposing is not a widely held theoretical position but rather a working assumption of cognitive psychologists – and thus a straw man. We accept their demonstration of new-feature acquisition, but question its ubiquity in category learning. We suggest that new-feature learning (at least in adults) is rarer and more difficult than the authors suggest.

We argue that the notion of distal similarity on which Edelman's reconstruction of the process of perception and the nature of representation rests is ill defined. As a consequence, the mapping between world and description that is supposedly at stake is, in fact, a mapping between two different descriptions or “representations.”

There is some very limited evidence for a role of estrogens in human psychosexual masculinization; its interpretation is uncertain. Fitch & Denenberg's demonstration of a role for estrogens in the behavioral feminization of nonhuman mammals implicitly suggests an answer to a riddle posed by the syndrome of congenital adrenal hyperplasia in women.

Most psychometricians believe that the significance test is counterproductive. I have read Chow's book to see whether it addresses or rebuts any of the key facts brought out by the psychometricians. The book is empty on this score; it is entirely irrelevant to the current debate. It presents nothing new and is riddled with errors.

We argue that the dynamical and computational hypotheses are compatible and in fact need each other: they are about different aspects of cognition. However, only computationalism is about the information-processing aspect. We then argue that any form of information processing relying on matching and comparing, as cognition does, must use discrete representations and computations defined over them.

Chow illustrates the important role played by significance testing in the evaluation of research findings. Statistics and the goals of research should be treated as both interrelated and separate parts of the research evaluation process – a message that will benefit all who read Chow's book. The arguments are especially pertinent to the debate over the relative merits of confidence intervals and significance tests.

Feature-based theories of concept formation face two dilemmas. First, for many natural concepts, it is hard to see how the concepts of the features could be developmentally more basic. Second, concept formation must be guided by “abstraction heuristics,” but these can be neither universal principles of rational thought nor natural conventions.

This informative and conceptually stimulating target article is very useful. I merely query whether the term “illusory contours” is appropriate for gap filling; “illusory surfaces” seems better – and “fictional surfaces” better still. These seem to be rule based rather than knowledge based, suggesting indeed the importance of distinguishing rules (analogous to syntax in language) from knowledge (equivalent to semantics) for classifying perceptual phenomena.

Indirect routes by which gonadal hormones influence sexual differentiation are considered. In rats, differentiation may depend on the way in which the mother responds to the hormonal condition of her pups, and this has implications for the interpretation of the data for humans. Interaction between gonadal hormones and light experience in chicks is compared with the mammalian systems covered in Fitch & Denenberg's review.

The representation of similarities is a viable concept for a cognitive extension of visual psychophysics to the recognition of shapes, bringing issues such as similarity and categorization back into that field. However, as a framework it appears too general to place constraints on a particular process model for categorization. In particular, a preference for Chorus-like schemes with respect to structure-oriented approaches is unwarranted.

Van Gelder has presented a position that he ties closely to a broad class of models known as dynamical models. One can support many of his broader claims about the importance of this class (as has been argued by connectionists for quite some time), but there are a number of unique characteristics of his brand of dynamicism. These characteristics engender difficulties for his view.

The fact that the female mammalian brain remains responsive to estrogens throughout life may open the way for other instigators of neuronal plasticity, making the female brain different from that of the male in its response to the actions of a number of hormones, to injury and to aging.