Receptive-field properties of neurons in the macaque inferior pulvinar

The responses to visual stimuli of neurons in the inferior pulvinar were studied in immobilized monkeys anesthetized with nitrous oxide. Most cells had well-defined receptive fields and responded strongly to stationary or moving spots and slits of light. Receptive fields ranged, on the average, from 1 to 5° in diameter. Although nonvisual stimuli could alter the pattern of spontaneous activity, they did not evoke repeatable, time-locked responses comparable to those evoked by visual stimuli. About a third of the cells were insensitive to stimulus orientation. These cells responded to spots or slits moved through the receptive field in any direction and to stationary slits flashed at any orientation. Two-thirds of the cells were sensitive to the orientation of a moving stimulus. About half of these responded equally well to the two directions of movement of a properly oriented slit; most were also sensitive to the orientation of a stationary flashed slit. The remaining cells were sensitive to the direction of movement: they responded best to stimulus movement in one direction and were inhibited or failed to respond for movement in the opposite direction. A third of these directional cells were also sensitive to the orientation of stationary slits. Most cells showed a pronounced moment-to-moment fluctuation in their visual responsiveness. Responsiveness was often correlated with the level of arousal as reflected in the electroencephalogram (EEG). A number of orientation-sensitive cells exhibited a pronounced afterdischarge, some continuing to discharge for up to 10 s after stimulus offset. The results support the notion that the inferior pulvinar receives visual information from striate cortex. It is suggested that the response of pulvinar cells to this input may be modulated both by the general level of arousal and by visuooculomotor information derived from the superior colliculus.