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 co mpared with those to illuminance-matched white flashes in area V1, opt ic radiations, and the lateral geniculate nucleus of two alert macaque s. Laminar profiles of visual evoked potentials (VEPs), current source density, and multiunit activity were obtained using multicontact elec trodes capable of sampling from all layers of cortex or lateral genicu late nucleus, simultaneously. In striate cortex, stimulation with colo red flash enhanced transmembrane current flow dramatically in both lay er 4c and the supragranular laminae. Stimulation with red evoked the l argest enhancement in every electrode penetration. The mean peak ampli tudes of current sinks evoked by red were 203% and 537% of those evoke d by white light in layer 4c and the supragranular laminae, respective ly. Color effects in V1 were preceded by an initial epoch of wavelengt h-insensitive activity. In layer 4c, the red effect reached significan ce, on average, at 47 ms, or approximate to 24 ms after the onset of t ransmembrane current flow. In the supragranular layers, the red effect reached significance, on average, at 55 ms, or approximate to 14 ms a fter the onset of current flow. Recordings from optic radiations in th e white matter below V1 and from lateral geniculate nucleus showed no significant difference in the responses to color and illuminance-match ed white light. Enhancement of supragranular current flow with color s timulation increased the contribution of these laminae to the generati on 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.