Discharges of 315 relay cells of the lateral geniculate nucleus (LGN) during spontaneous eye movements were studied in alert cats. 2. When tested in a stationary patterned field, 114 cells showed sustained discharges related to the direction of gaze (S cells) and to local differences in luminance; 109 cells showed transient response to quick shifts of retinal image during saccades (T cells); ninety-two cells showed mixed responses (M cells), i.e. transient responses to rapid shifts of retinal image and sustained firing related to local differences in luminance. 3. Following saccades occurring in the light, T and M cells showed a burst discharge, while spontaneous discharges of S cells were completely suppressed for 150-200 msec. 4. When tested in total darkness, modifications in activity which were apparent in light disappeared completely. This was true for all 315 relay cells. 5. T cells responded to optic chiasm stimulation at shorter latencies (X = 1.15 msec) than S cells (X = 1.77 msec). M cells showed a latency distribution in between those for S and T cells with a mean latency 1.40 msec. 6. When tested with moving grating stimulation, S cells responded in only one manner; with discharges to each stripe of the grating (primary response), while T and M cells showed two different responses: a primary response to a slower motion and a non-specific burst in response to a faster motion. The burst did not reflect the stimulus pattern (secondary response). 7. When tested with diffuse light switched on and off over the tangent screen, S cells showed a sustained response either to light or darkness, whereas T and M cells responded transiently either to the onset or offset of the light, or to both. M cells occasionally showed a mixture of transient and sustained responses either to light or darkness. 8. In over-all response properties, most S cells correspond to X (sustained) cells and most T cells to Y (transient) cells previously known from acute experiments. M cells had intermediate response properties between X and Y cells. 9. Functional roles of these classes of cells in relation to previously proposed functions are discussed.
