THE TANGENTIAL ORGANIZATION OF THE VISUAL CORTEX.

The internal organization of the cerebral cortex has been extensively studied by numerous investigators (see Sholl, 1956). All existing detailed morphological descriptions of cortical neurons, however, draw their information only from sections cut at right angles to the pial surface. Each one of such sections shows only one dimension of the spread of fibres tangential to the pial surface. Although theoretically the total tangential organization could be reconstructed from this material, this organization has never been adequately described and indeed it can only be directly visualized and ultimately confirmed on sections cut tangential to the pial surface. Study of such sections is essential for an adequate three dimensional description of cortical neurons and their branches. A better understanding of the tangential organization is particulary important in such situations as the primary visual cortex where the topographical projection of the retina upon the cortex is such that a map of the 'cortical retina' is displayed tangential to the pial surface. The relationships between the various points of the map can only be adequately seen in sections cutting successively through each 'horizontal' cortical layer in that plane. A study of the dendritic fields of the neurons of the optic lobe of the octopus has shown that they are elongated in shape in the tangential plane with orientations mainly in two directions at right angles to each other, with few obliques (Young, 1960). The author has related this morphological organization to the mechanism of pattern discrimination suggested by the behavioural evidence in this animal which has difficulty in discriminating between two obliquely orientated rectangles but can easily distinguish between a horizontal and a vertical rectangle (Sutherland, 1957). A similar study is rendered particularly interesting in the cat in view of the physiological properties of individual cortical cells recently described by Hubel & Wiesel (1959, 1962). The authors have shown that, though the visual fields of individual cells of the retina and lateral geniculate body are circular in shape, the centre and periphery having different excitatory and inhibitory polarities (Hubel, 1960), those of cortical cells are elongated in shape. They have suggested that the mechanism responsible for this transformation lies in the convergence of lateral geniculate cell axons, having receptive fields arranged along a straight line on the retina, upon a single cortical cell ('simple fields'). This convergence could be achieved in either of two ways. Either: (1) the axons from a specific set of geniculate neurons (say a row) make their own way to each