Do the relative mapping densities of the magno- and parvocellular systems vary with eccentricity?

Abstract

Two recent papers on the macaque visual system have concluded that in the lateral geniculate body the ratio of the number of cells in the magnocellular system to the number in the parvocellular system representing the same area of visual field increases by a factor of 20 between the fovea and the far periphery. In the primary visual cortex the relative cell densities of the 2 systems change little with eccentricity. These calculations therefore predict a 20-fold change in the relative densities of the inputs to the visual cortex from the 2 subdivisions of the lateral geniculate body. To test this prediction, we asked if the following vary with eccentricity: (1) the ratio of the number of magnocellular to parvocellular neurons innervating a given area of striate cortex and (2) the relative density, in the magno- and parvo-recipient sublaminae of layer 4C, of radioactivity transported from the eye to the cortex. Neither of these ratios showed any significant variation with eccentricity. These results seem to throw doubt on the contention that the ratio between the magnocellular and parvocellular layers of the number of cells per degree2 of visual field varies significantly with eccentricity.