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In contrast to those of the cortex, the layers of the retina are far from constant in thickness. In both monkey and human the ganglion-cell layer near the fovea (bottom layer, top photograph) is many cell bodies thick, perhaps eight or ten, whereas far in the periphery, say 70 to 80 degrees out, (bottom photograph) there are too few ganglion cells to make one layer. This should be no surprise since foveal ganglion-cell field centers are tiny; they are larger in the periphery (just as in the cortex). Thus in the fovea, compared with the periphery, it takes more cells to look after a unit area of retina.
How does this affect the overall shape of the striate cortex? Although I have repeatedly called the cortex a plate, I have not necessarily meant to imply that it is a plane. If there were no distortion at all in shape, the striate cortex would be a sphere, just as the eyeball is and just as any map of the earth, if undistorted, must be. (Strictly, of course, the striate cortex on one side maps about half of the back halves of the two eyes, or about a quarter-sphere.) In stretching, so as to keep thickness constant and yet manage many more messages from the crowded layers of ganglion cells at the fovea, the cortex becomes distorted relative to the spherical surface that it otherwise would approximate.
If we unfold and smooth out the creases in the cortex, we discover that it is indeed neither spherical nor flat; it has the shape of a very distorted quartersphere, rather like a pear or an egg. This result was predicted in 1962 by Daniel and Whitteridge, who determined experimentally the magnification in area 17 as a function of distance from the to veal representation, as mentioned on page31, and used the result to calculate the three-dimensional shape. They then made a rubber model of the cortex from serial histological sections and literally unfolded it, thus verifying the pear shape they had predicted. We can see the shape in the illustration on this page. Till then no one had reasoned out the question so as to predict that the cortex would unfold into any reasonable shape, nor, to my knowledge, had anyone realized that for any area of cortex, some shape or other must exist whose configuration should follow logically from its function. Presumably the folds, which must be smoothed out (without stretching or tearing) to get at the essential shape, exist because this large, distorted quarter-sphere must be crumpled to fit the compact box of the skull.
The foldings may not be entirely arbitrary: some of the details are probably determined so as to minimize the lengths of cortico-cortical connections.
In the somatosensory cortex the problems of topography can become extreme to the point of absurdity. The cortex corresponding to the skin covering the hand, for example, should have basically a glove shape, with distortions over and above that to allow for the much greater sensory capacities of the finger tips, as compared with the palm or back of the hand. Such a distortion is analogous to the distortion of the foveal projections relative to the periphery, to allow for its greater acuity. Would the hand area of the cortex—if we modeled it in rubber and then stood inside and blew gently to get rid of the artificial creases—really resemble a glove? Probably not. Determining the map of the somatosensory cortex has turned out to be a daunting task. The results so far suggest that the predicted shape is just too bizarre; instead, the surface is cut up into manageable pieces as if with scissors, and pasted back together like a quilt so as to approximate a flat surface.

   
 


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In the somatosensory cortex the problems of topography can become extreme to the point of absurdity. The cortex corresponding to the skin covering the hand, for example, should have basically a glove shape, with distortions over and above that to allow for the much greater sensory capacities of the finger tips, as compared with the palm or back of the hand. Such a distortion is analogous to the distortion of the foveal projections relative to the periphery, to allow for its greater acuity. Would the hand area of the cortex—if we modeled it in rubber and then stood inside and blew gently to get rid of the artificial creases—really resemble a glove? Probably not. Determining the map of the somatosensory cortex has turned out to be a daunting task. The results so far suggest that the predicted shape is just too bizarre; instead, the surface is cut up into manageable pieces as if with scissors, and pasted back together like a quilt so as to approximate a flat surface.

 
 
 
 
 





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