Happily, by the end of this chapter, we will see the choir coalesce into sections, each of which sings the complete song. Each section is surrounded by neighbors, sections that are similarly diverse. In cortex , it looks as if one string-quartet section occupies a space that is hexagonal in shape and about 0.5 mm across. It could constitute the most elementary version of Hebb's cell-assembly , one that could represent a word, a face, or a pronunciation. To get there, however, we first need to consider a few more aspects of the geometry and its relevant neurophysiology.
Antagonistic surrounds serve to concentrate things.
An example of this second type of physiological focusing is the projection from a thalamic neuron, one specializing in just one finger tip, to the hand map in cerebral cortex. Its axon terminal branches seem to span much of the hand's map, but, when you look at the cortical neurons they're feeding, you find that they typically have small receptive fields, little larger than those of thalamic neurons. There should be a marked stability of the triangular arrays formed by the hot spots, even under various perturbations. Let us suppose that a triangular array is firing in a repeated cycle. And that one point in the midst of a triangular array tries to fire out of sync, later than its neighbors.