Trichromacy may result from an adaptation to the regularities in terrestrial illumination. However, we suggest that a complete characterization of the challenges faced by colour perception must include changes in surface surround and illuminant changes due to inter-reflections between surfaces in cluttered scenes. Furthermore, our trichromatic system may have evolved to allow the detection of brownish-reddish edibles against greenish backgrounds. [Shepard]

This commentary addresses an additional feature of biorobotic modeling: usefulness in prosthetic design. By implementing structurally and behaviorally accurate models as prosthetics, biologically accurate restoration can be approximated. This has potential to reestablish important peripheral elements of the sensorimotor control system, including limb biomechanics, proprioception, and vision. Examples given include musculoskeletal prosthetics and retinal implants.

The dynamic systems approach simulates a wide range of effects and generates novel predictions, but it fails to explain age-related behavioral changes in psychological terms. We argue that the roles of conscious control and explicit knowledge must be addressed in any model of A-not-B performance, and a fortiori, in any model of goal-directed action.

The arguments of Stoffregen & Bardy for studying perception basedon the global array are intriguing. This theory can be examined innonhuman species using nonverbal tasks. We examine how monkeysmaster a skill that incorporates a two-dimensional/three-dimensionalinterface. We feel this provides excellent support for Stoffregen& Bardy's theory.

A mature science strives to provide causal explanations of observed phenomena rather than focusing on taxonomic descriptions of data. A field theory model is a step towards providing a truly scientific account of development. However, the model is under-constrained in that it ignores the boundary conditions defined by the physical constraints imposed by the infant's developing brain and body.

We argue that confusability between items should be distinguished from generalization between items. Shepard's data concern confusability, but the theories proposed by Shepard and by Tenenbaum & Griffiths concern generalization, indicating a gap between theory and data. We consider the empirical and theoretical work involved in bridging this gap. [Shepard; Tenenbaum & Griffiths]

Stoffregen & Bardy reject the likelihood that infants aresensitive to the global array, implying that intersensoryintegration is not possible in early development. We argue thatinfants are sensitive to unimodal arrays and are able to integratethem through the active participation of their nervous system andthat the observed developmental changes are due to experience andbrain development.

As modelling becomes a popular approach in the study of biological systems it is necessary to clarify its concepts and dimensions. This helps to characterize and to distinguish models but cannot establish their quality. The virtue of a model depends on the insight gained in respect to a specific scientific question, and it is hard to measure this with a theory.

The dynamical view of the brain that Tsuda presents is thoroughly substantiated by theory and computer simulations, but strong experimental evidence for chaotic brain processes is still lacking. New methods are called for. It is also important to make a distinction between the generating mechanisms and the observed behavior, which is complicated by a mixing of stochastic and deterministic processes.

This commentary suggests: (1) experimentalists must expand their subject pools beyond university students; (2) the pollution created by deception would not be a problem if experimentalists fully used non-student subjects; (3) one-shot games remain important and repeated games should not ignore social learning; (4) economists need to take better control of context; and (5) using computers in experiments creates potential problems.

Aristotle's four causes frame Webb's question. Comprehension requires specification of trigger, function, mechanism, and representation. Robots are real models of function. Physical, biological, and epigenetic constraints delimit the hypothesis space for candidate mechanisms. Robots constitute a simplified system more susceptible to formal representation than the target system. They thus constitute an important tool in a constructivist development of scientific knowledge.

I suggest that contemporary economics shares many of the characteristics of methodological behaviourism in psychology, with its emphasis on the influence of motivation, learning, and situational incentives on behaviour, and minimal interest in the details of the cognitive processes that transform input (information) into output (behaviour). The emphasis on these characteristics has the same strengths and weaknesses in economics as in behaviourist psychology.

Shepard's exponential law provides a functional explanation of generalization. The account complements the more common mechanistic models. The elegant and powerful analyses answer one of Tinbergen's (1963) four whys of behavior: a benefit conferred on the animal by generalizing in this way. A complete account might address evolutionary and developmental questions in addition to mechanistic and functional ones. [Shepard]

This commentary discusses a strategy by which investigators canexamine whether observers perceive properties of the global arrayindependently of properties in individual sensory arrays. Researchshowing that perception of complex relationships appears to beindependent of the perception of individual components isconsidered. Ashby and Townsend's (1986) methods for identifyingperceptual independence are important tools for studying the globalarray.

Studies have failed to yield definitive evidence for the existence and/or role of well-defined chaotic attractors in real brain systems. Tsuda's transients stabilized on unstable manifolds of unstable fixed points using mechanisms similar to Ott's algorithmic “control of chaos” are demonstrable. Grebogi's order in preserving “strange nonchaotic” attractor with fractal dimension but Lyapounov is suggested for neural network tasks dependent on sequence.

The proposed model is put forward as a template for the dynamical systems approach to embodied cognition. In order to extend this view to cognitive processing in general, however, two limitations must be overcome. First, it must be demonstrated that sensorimotor coordination of the type evident in the A-not-B error is typical of other aspects of cognition. Second, the explanatory utility of dynamical systems models must be clarified.

We discuss whether low-dimensional chaos and even nonlinear processes can be traced in the electrical activity of the brain. Experimental data show that the dimensional complexity of the EEG decreases during event-related potentials associated with cognitive effort. This probably represents increased nonlinear cooperation between different neural systems during sensory information processing.

James Gibson's theory of information, as specific to (but notnecessarily “like”) its sources, is especially indebted to the studyof occlusion for its core examples. In occlusion, one is “amodally”aware of hidden surfaces. Is this still too related to one modalityto count as a good case for Stoffregen & Bardy?

Biorobots may model the causation of relatively simple behaviors, but many animal behaviorists are concerned with complex cognitive traits and their evolution. Biorobotics seems limited in its ability to model cognition and to provide evolutionary explanations. Also, if robots could model complex traits, such as theory of mind, underdetermination could be problematic. Underdetermination is also a challenge for comparative psychologists.

Contra Shepard we argue, first, that his presentation of a three-dimensional representational (psychological or phenomenal) colour space is at odds with many results in colour science, and, second, that there is insufficient evidence for Shepard's stronger claim that the three-dimensionality of colour perception has resulted from natural selection, moulded by the particulars of the solar spectrum and its variations. [Shepard]