We extend the discussion in the target article about distinctions between extrinsic coding and the alternative we and the target article both favor, intrinsic coding. Central to our thinking about intrinsic coding is population coding and the concept of high-dimensional neural response spaces.

The target article does not consider neural data on primate spatial representations, which we suggest provide grounds for believing that navigational space may be three-dimensional rather than quasi–two-dimensional. Furthermore, we question the authors' interpretation of rat neurophysiological data as indicating that the vertical dimension may be encoded in a neural structure separate from the two horizontal dimensions.

The relative contributions of biological and cultural factors in determining category characteristics almost certainly vary for different categories, so that the results of these simulations on color categories don't necessarily generalize. It is suggested here that categories that pick out structure in the environment of strong behavioral significance to individual agents will be predominantly biologically determined and will converge without interagent communication, whereas those categories that serve primarily to coordinate behavior in a population will require communication to converge.