Timing differences between the light responses of X cells recorded simultaneously in cat lateral geniculate nucleus

Using two microelectrodes, recordings were made from pairs of like-signed cells at different depths in single layers (A or A1) of the cat's lateral geniculate nucleus (LGN). The cells were chosen to have near or overlapping receptive fields so that they could be stimulated simultaneously with a single spot or bar of light. Under these controlled conditions, paired X cells (n = 32 pairs) showed differences in latency from less than 10 to about 80 ms, but no latency differences were observed between paired Y cells (n = 3 pairs). Within XY pairs (n = 11) the Y cells responded faster than the X cells. A separate analysis of previously reported, singly recorded X cells (n = 131) also showed wide differences in X cell latencies. The results confirm that temporal differences in signalling occur within the X pathway in the cat LGN (Mastronarde, 1981, 1987a; Humphrey & Weller, 1988a) and they show explicitly that the differences occur simultaneously, in single layers and at single retinotopic loci. No consistent relationship was found between timing and depth, but the possibility of weak, depth-dependent trends is considered in the Discussion.