Spatial summation and conduction latency classification of cells of the lateral geniculate nucleus of macaques

Cells in the lateral geniculate nucleus (LGN) of the macaque monkey were investigated with microelectrodes in an attempt to develop an overall classification scheme. We classified cells in the parvocellular (P) and magnocellular (M) layers according to (non)linearity of spatial summation, shock latency, and chromatic organization of center and surround. We also measured the spatial and temporal tuning to counterphasing and drifting sine wave gratings and tested for periphery effects. Our results showed that no strict laminar segregation existed for any cell property studied. Our results can be summarized as follows: 1. Most P layer cells showed a linear summation (98%) and color-opponent responses (80%), while other cells showed a nonlinear summation (Y-cells, 2%) and broad band responses (28%). In contrast, 37% of the M layer cells were linear summators and the remainder were nonlinear. Therefore, there are overlapping distributions of X- and Y-cells in P and M layers but not a strict segregation. 2. P layer cells had longer shock latencies than M layer cells. X-cells conducted more slowly (2.4 +/- 0.7 msec) than Y-cells (1.6 +/- 0.8 msec), but there were overlapping distributions. Latency shortened gradually, rather than abruptly, with increasing depth. 3. The first harmonic of X- and Y-cell responses was maximally sensitive to spatial frequencies of about 2 cycles/deg. Each type of cell modulated about a mean rate to a drifting grating, although Y-cells had higher distortion than X-cells. Response amplitudes to drifting gratings were higher for MX- and MY- than for PX-cells. No DC elevation to high spatial frequencies was seen. Spatial bandwidths averaged 2 to 5 octaves. X-cells were maximally tuned to temporal frequencies around 11 Hz, and Y-cells were tuned to about 19 Hz;. temporal bandwidths for both averaged 2.8 octaves. 4. Periphery effects were detected in 4% of the X-cells and 25% of the Y-cells. 5. These data indicate that gradual changes occur between dorsal and ventral layers: summation changes from linear to nonlinear; conduction latencies shorten; peak temporal tuning increases; response amplitudes increase; the periphery effect becomes more prevalent. Spatial tuning does not change. No strict laminar segregation or specificity exists for any of the properties that we studied.