Evoked potential contrast sensitivity in the parafovea: spatial organization.

Visual evoked potential contrast sensitivity functions (VEP/CSFs) were determined for counterphase flickered sine-wave gratings in circular fields up to 8 degrees in diameter centered on the fovea. VEP sources responding to 16 c/deg gratings appeared to be concentrated in the central 2 degrees of the visual field while sources responding to lower spatial frequencies appeared to be distributed over progressively wider areas of the visual field as spatial frequency decreased. It was also found that independently determined VEP/CSFs for non-overlapping annular regions of the visual field centered on the fovea summed to equal the VEP/CSF obtained when both regions were stimulated simultaneously.