FLASH VISUAL EVOKED POTENTIALS IN THE MONKEY

Abstract

To delineate the spatiotemporal pattern of intracortical generators of the flash visual evoked potential (VEP), recordings of the VEP and concurrent multiple unit activity (MUA) were made from closely spaced sites through the cortex. The depth profiles of the VEP were subjected to current source density (CSD) analysis to localize the transmembrane current flows contributing to the generation of the filed potentials.;The cortically generated binocular flash VEP consists of four consistently recordable components, all of which are arise predominantly within cortical lamina IV. The earliest component is a surface negativity peaking at 24 msec (N24) generated within sublamina IVA. The next two components are also surface negative, and are generated within lamina IVCb. The first, N40, inverts across lamina IVCb, while N55 remains negative in polarity in all cortical layers. The subsequent waveform feature is a positivity, P65, which also inverts across and is generated within lamina IVCb, and is associated with a coincident decrease in MUA to below the prestimulus baseline. Thus, this component is hypothesized to represent an intracortically generated inhibitory process. Later components are more variable and are usually generated within lamina III, and less frequently in lamina V.;Monocular stimulation of the eye providing the dominant input to a cortical dominance domain results in a laminar VEP pattern virtually identical to that seen with binocular stimulation. Stimulation of the non-dominant eye yields a simpler pattern, consisting only of N40 and P65, neither of which clearly invert across sublamina IVCb as they do with binocular and dominant eye stimulation. In addition, N40 has a shorter onset latency with stimulation of the contralateral eye compared with ipsilateral eye stimulation.;Intracortical injection of the GABA antagonist bicuculline resulted in activation of the previously inactive lamina IVCa, implicating inactivation of an intracortically generated inhibitory process. P65 virtually disappears with bicuculline and no longer inverts, suggesting that this component represents a GABAergically mediated process. Finally, late activity within laminae III and V increases sharply with bicuculline, implicating GABAergic modulation of the response properties of the neurons in these layers.