Spectral and orientation specificity of single cells in foveal striate cortex of the vervet monkey, Cercopithecus aethiops.

1. The spectral sensitivity, orientation specificity and inhibitory surround of seventy-three cells were studied in the vervet monkey. The eye was in the dark or illuminated with steady white or spectral light. The cells were in the striate cortex corresponding to the foveal representation. Nearly all the cells gave on- or on/off-responses.2. More than a third of the cells (41%) responded over a narrow spectral band, in the blue, green, yellow or red section. Three quarters of them were orientation specific with flanking inhibitory surround and half of these were of the ;stopped-end' variety as well. The effect of the wave-length of the background indicated that only a half were activated by such excitatory-inhibitory colour pairs as have been described in the geniculate nucleus.3. A third of the cells (36%) responded to most colours but with the greatest response to green, yellow or red. Less than half were orientation specific. Unlike the narrowband cells, the response decreased with the intensity of the light. Two thirds were activated by the excitatory-inhibitory colour pairs that have been described in the geniculate nucleus.4. In both groups of cells the wave-length of the spectral peak could be different when the stimuli were weak compared with when they were strong.5. One quarter of the cells (23%) gave on/off-responses to all spectral flashes; half were not orientation specific. The difference in orientation specificity between narrow- and broadband cells is significant (P < 0.01).