The distribution of on- and off-centre X- and Y-like cells in the A layers of the cat's lateral geniculate nucleus.

Responses to light were recorded extracellularly from single cells in the A laminae of the dorsal lateral geniculate nucleus (d.l.g.n.) in the cat. Micro-electrodes were passed through the nucleus parallel to the laminar borders allowing the depths of the cells to be measured accurately and the depth distributions of the different cell types to be determined. 1200 cells were recorded in thirty cats along ninety-nine tracks through layers A and A1. The cells were identified physiologically as on- or off-centre cells (hereafter called on or off cells) and most (n = 960) were classified as X- or Y-like cells (hereafter called X or Y cells). Both on and off cells were found throughout layers A and A1 but their proportions changed in a graded fashion with depth. On cells were the predominant type at the tops of the layers, the on and off cells were in balance in the centres, and off cells were the predominant type at the bottoms. The steepest gradient and the maximum differences in proportions occurred in layer A while in A1 the same pattern appeared to be present but was weak and not statistically significant in the present sample. X and Y cells were also distributed differently through the depths of the layers but in patterns that differed from those of the on and off cells. The Y cells were concentrated at the borders and there was a complementary increase of X cells toward the centres of the layers. Again, the pattern was strong in A but weak and not statistically significant in A1. The distribution of X-on cells peaked near the top of layer A while the distribution of X-off cells peaked near the bottom. This was not so in A1 where the distributions of X-on and X-off cells were nearly uniform and similar to one another except for a slight predominance (56%) of X-on over X-off cells that was not related to depth. The Y-on and Y-off cells showed similar patterns of distribution in both layers A and A1 with Y-on cells predominant at the tops of the layers and Y-off cells predominant at the bottoms. The differences in the distributions of the four cell types (X-on, X-off, Y-on, Y-off) through the depths of the layers result in unique combinations of cells at different depths. It is suggested in the Discussion that the changing balance between the cells may have functional consequences.