Foveal striate cortex of behaving monkey: single-neuron responses to square-wave gratings during fixation of gaze.

1. Singleand multiunit activity was recorded from fovea1 striate neurons in six hemispheres of three rhesus monkeys trained to maintain fixation of a small luminous spot superimposed on patterned backgrounds. 2. Ongoing multiunit activity was monitored throughout all penetrat;bns, and a region of sustained high-frequency discharge was consistently encountered. Histological reconstructions revealed that this region was coextensive with layer IVc. 3. In a sample of 369 neurons, 283 were activated by square-wave gratings. Of those activated, 85% were sensitive to the orientation of gratings, and of those sensitive to orientation, 25% were sensitive to direction of movement. 4. The responses to moving gratings of 177 neurons were analyzed from interspike intervals collected by computer during the presentation of gratings of different spatial frequencies. From these responses, most cells could be identified as “modulated” or “fusional .” The discharge patterns of modulated cells reflected the temporal frequency of luminance change at all effective spatial frequencies. Fusional cell responses, in contrast, showed little or no temporal modulation at any but the lowest spatial frequencies tested. All neurons showed some degree of spatial-frequency selectivity to moving gratings. 5. Nearly all fusional cells and a majority of modulated cells responded to stationary as well as moving gratings. Responses to stationary gratings were strong and regular for fusional cells, relatively weak for many modulated cells. 6. Spatial-frequency sensitivity curves were