The projection from the lateral geniculate nucleus onto the visual cortex in the cat. A quantitative study with horseradish‐peroxidase

Horseradish-peroxidase (HRP) was injected (9-18 microng in 0.03-0.06 micronl) into cortical areas 17, 18 or 19 of 11 adult cats. After survival times of 17 hours to 7 days, the thalamus was examined for retrogradely HRP labelled nerve cells in serial transverse sections. From these sections, the percentage of labelled cells occurring in each subdivision of the dorsal lateral geniculate nucleus (LGNd) was calculated for each animal. One case each for injections in areas 17, 18, and 19 was then chosen for nerve cell size measurements in each LGNd subdivision. The perikaryal area of each labelled cell (N=689), and of representative samples of unlabelled cells (N=1137), was measured by planimetry. Size distribution histograms, mean values, standard deviation, and statistical significance levels were obtained by computer. It was found that area 17 receives a projection almost exclusively from laminae A and Al, and that the projecting cells belong to all cell size classes. Area 18 receives a projection mainly from laminae C and Al, and from the medial interlaminar nucleus (MIN). The projecting cells belong mainly to the large cell size classes. Area 19 receives a projection largely from MIN, and also from the C-laminae and extrageniculate cell groups. The projecting cells belong to all cell size classes, with some emphasis on the large cells of lamina C. A significant projection was found to exist from the parvocellular laminae of LGNd onto area 19 and, to a lesser degree, area 18. In conclusion, as one goes from area 17 and 18 and to 19 the projection source shifts from the A-lamine through the C-laminae on to MIN and extrageniculate cell groups. The cells which project to area 18 are on the whole larger than those which project to areas 17 and 19. A significant proportion of the contralateral visual input to area 18 is relayed via lamina C. These results provide a quantitative confirmation and extension of previous anatomical findings, and are in close relationship with physiological results regarding parallel channel processing in the visual system.