Flash-evoked activity in the macaque form and color pathway

Abstract

Goals. These experiments investigated neuroelectric activity within the primate visual pathways. One goal was to examine the degree of which the temporal activation sequences across visual regions adheres to a hierarchical scheme. We compared onset latencies of neural ensembles using techniques sensitive to both postsynaptic potentials and action potentials. The second goal was to investigate the timing and laminar distribution of activity in Area V4 and determine the contribution of this area to the surface-recorded visual evoked potential (VEP). The third goal of these experiments was to examine the effects of wavelength manipulation in the timing and laminar distribution of activity within and across Areas V1, V4 and inferotemporal cortex (IT).;Methods. Three complementary measures of neuroelectric activity were obtained from awake monkeys by simultaneous sampling at incremental cortical depths with a multicontact electrode. Laminar profiles of visual evoked potentials (VEP) and concomitant multiunit activity (MUA) were recorded. Current source density (CSD) analyses of the VEP profiles were implemented to determine the timing, direction and density of transmembrane current flow.;Results. Small onset latency differences found between some visual areas require involving parallel processing mechanisms for visual system organization. The color studies suggested that differences in the timing of activity evoked by different stimulus features, such as color and luminance, represent an additional dimension of visual system organization.;The study of V4 revealed that this higher-order visual area receives a direct thalamic projection, the effect of which is inhibitory. Later excitatory activity in V4 is consistent with input from lower cortical stages. The major contribution of V4 to the surface-recorded VEP is to the longer latency components, as would be expected for an upstream visual region.;The major findings of the color studies in V1, V4 and IT are that: (1) color stimuli can increase transmembrane current flow and multiunit activity; this enhancement is always largest for red stimuli and (2) the onset of color related activity occurs after an epoch of luminance-sensitive activity in all stages of the form and color pathway tested.