Striate cortical contribution to the surface-recorded pattern-reversal VEP in the alert monkey

The striate cortical contribution to the surface pattern-reversal visual evoked potential (VEP) was investigated in awake monkeys during performance of a visual fixation task, by examining laminar profiles of VEP, current source density (CSD) and concomitant multiunit activity (MUA) in Area 17, recorded simultaneously at incremental depths using multicontact electrodes. Stimuli were black/white bar gratings centered on the fixation point. The typical surface pattern-reversal VEP over striate cortex consists of a prominent positivity peaking at 50-70 msec (P60), followed by a large negativity peaking at approx. 80 msec (N80), and then by a late broad positivity, peaking between 120 and 150 msec (P125). P60 is often preceded by a small negativity peaking at 45-55 msec (N50), and on rare occasions a small positivity (P40) is also observed. N50 is generated primarily by current sinks in Lamina 4C. P60 arises from large current sources in the supragranular laminae. N80 and P125 appear to be composite waveforms reflecting complex contributions from local activity and from activity occurring outside of the foveal/immediate parafoveal representation in Area 17. The basic physiologic sequence elicited by patterned stimulation is similar to that elicited by diffuse luminance or by electrical stimulation, but is characterized by more prominent supra- and infragranular activation. It is consistent with the cellular and synaptic anatomy of Area 17: initial activation of the thalamorecipient subdivisions of Lamina 4C, followed by activation of mid/upper Lamina 4 and of supra- and infragranular laminae. Our results suggest the possibility of differentiating synaptic stages and cellular processes reflected in the human VEP, based on homologies with simian VEP components.