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Programs of the brain.
J. Z. Young 1978
http://www.amazon.co.uk/exec/obidos/ASIN/0198575459/

The primary visual cortex is in the occipital region at the back of the head (Fig. 12.5). Here the pattern of the retina is enormously enlarged, there are 5000 cortical cells for each cell of the thalamus, which sends on the signals from the optic nerve. The cortex is, as it were, a greatly expanded version of the retina, the fovea being especially extensively represented. Each degree of retinal field at the fovea is represented by 6 mm of this primary visual cortex (Daniel and Whitteridge 1961). But the visual cortex is much more extensive even than this. Beyond the primary visual area is a whole series of'secondary' areas, each carrying a further full topographical map or representation of the retina and having cells with distinctive functions. Zeki (1974) recognizes at least second, third, fourth and fifth visual areas, probably there are more. The later members of the series receive connections from the earlier ones.

All the areas of the system are made up of columns of nerve cells, each sensitive to some particular visual feature, say a line placed at 30 to the horizontal (p. 50). In some areas there are cells censitive only to one colour. So we can say that the signals sent from the retina by red light are 'decoded' by the fact that they activate certain particular 'red' cells in the cortex. Like all codes this one is pre-established, being laid down initially by heredity and improved by use. A kitten or monkey is born with cells in the brain sensitive to contours set at various angles, but any that are not used will disappear (Chapter 3).

As signals from the retina pass through the various visual areas they are recombined in different ways. This is the process by which the words of the neural language are joined in 'grammatical' ways to give meanings. No doubt there is a hereditary element in the development of the grammar and it is also greatly influenced by experience. As yet all we know about the processes involved is that they somehow depend upon topological relations and that they are disturbed by local injuries to the cortex.

Programs of the brain.
J. Z. Young 1978
http://www.amazon.co.uk/exec/obidos/ASIN/0198575459/