Functionally distinct groups of X‐cells in the lateral geniculate nucleus of the cat

The latencies and visual response properties of 202 X‐cells in the A‐laminae of the cat dorsal lateral geniculate nucleus (LGN) were examined to investigate the recent claim (Mastronarde, '85,'87a) that functionally different groups of X‐cells reside there. Two groups of X‐cells were found, which differed in their extracellularly recorded responses to spots of light flashed within their receptive fields. One group, constituting one‐third of the sample, responded to spot onset with a profound and often long‐lasting dip in discharge rate, such that cell discharge usually did not reach half maximum until ≥ 100 msec after spot onset. About 70% of these cells also displayed a transient discharge at spot offset. These cells correspond to Mastronarde's lagged X‐cells, and we similarly refer to them as XL‐cells. The second group, constituting the remainder of the X‐cell population, generally responded to spot onset with a short latency (≤ 60 msec) brisk discharge, no detectable XL‐type dip, and a rapid reduction in firing at spot offset. We refer to these neurons as nonlagged (XN) X‐cells; this group probably encompasses all of Mastronarde's non‐XL‐cells.