Effects of wavelength on the timing and laminar distribution of illuminance-evoked activity in macaque V1.

Responses to full-field colored flashes (red, blue, and green) were compared with those to illuminance-matched white flashes in area V1, optic radiations, and the lateral geniculate nucleus of two alert macaques. Laminar profiles of visual evoked potentials (VEPs), current source density, and multiunit activity were obtained using multicontact electrodes capable of sampling from all layers of cortex or lateral geniculate nucleus, simultaneously. In striate cortex, stimulation with colored flash enhanced transmembrane current flow dramatically in both layer 4c and the supragranular laminae. Stimulation with red evoked the largest enhancement in every electrode penetration. The mean peak amplitudes of current sinks evoked by red were 203% and 537% of those evoked by white light in layer 4c and the supragranular laminae, respectively. Color effects in V1 were preceded by an initial epoch of wavelength-insensitive activity. In layer 4c, the red effect reached significance, on average, at 47 ms, or approximately 24 ms after the onset of transmembrane current flow. In the supragranular layers, the red effect reached significance, on average, at 55 ms, or approximately 14 ms after the onset of current flow. Recordings from optic radiations in the white matter below V1 and from lateral geniculate nucleus showed no significant difference in the responses to color and illuminance-matched white light. Enhancement of supragranular current flow with color stimulation increased the contribution of these laminae to the generation of the surface VEP. Comparison of the surface VEP wave forms evoked by white and color stimuli may, therefore, help to differentiate the responses of the granular and supragranular laminae.