We extend the discussion in the target article about distinctions between extrinsic coding (external references to known things, as required by information theory) and the alternative we and the target article both favor, intrinsic coding (internal relationships within sensory and motor signals). 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.

Although the E-Z Reader model accounts well for eye-tracking data, it will be judged by new predictions and consistency with evidence from brain imaging methodologies. The stage architecture proposed for lexical access seems somewhat arbitrary and calculated timings are conservatively slow. There are certain effects in the literature that seem incompatible with the model.