Visual evoked potentials to a faint light: signal propagation analyzed with peak latency and topographic mapping.

Accurate localization for the process of recognition of a light stimulus is yet to be determined. We studied 19-channel VEP from nine healthy volunteers, evoked by physiologically faint light less than 200 Cd/m2, using a light emitting diode (5 mm phi, 0.3 degrees, energized for 5 msec). VEP was bandpass filtered from 0.16 to 120 Hz, and analyzed from 20 msec pre-stimulus to 184.2 msec post-stimulus. The grand average VEP suggested an initial positive peak at 115.8 msec at the frontal poles and at 136 msec over the occipital areas. An initial negative peak was noted at 156 msec at the frontal poles and at 179.2 msec over the occipital areas. This might suggest that the potentials evoked had propagated from the frontal poles to the occipital areas, taking about 20 msec. However, the topographic maps contained little evidence for such a propagation, but rather indicated the waxing and waning of positive or negative extremes.