ABSTRACT
The organization of associational cortex If we compare the cerebral hemispheres of a series of animals from different evolutionary stages, what is striking when we get to Man is not just the expansion in the absolute mass of neural tissue, but the dramatic changes in the relative proportions of the cortex devoted to different functions. Very little of a rat’s cortex is not
either primary motor or a projection area for one of the senses; in Man, by contrast, most areas of the cortex neither respond in an obvious way to simple sensory stimulation, nor produce movements when electrically activated: they are what have sometimes been called silent areas. :
Now these are precisely the properties we would expect from neural levels in the middle of the model of the brain shown above, that was first presented in Chapter 1. Because a neuron in any level is activated only by a particular pattern of activity in the preceding layer, as we penetrate deeper into the sensory side we find that individual neurons become fussier and fussier about what they respond to, and eventually the chance of our finding out, in an experiment of finite duration, what they do actually do becomes vanishingly small. Stimulation is equally frustrating: unless we happen to stimulate them in a pattern that makes some kind of neural sense, corresponding to what is needed to activate the next layer along, nothing will happen at all. Both these problems are accentuated in cortical areas, where there is an immense degree of convergence and divergence from neurons in one column to those in others.
