Stimulus contaminants in visual electrophysiology

Evoked cortical potentials produced by flash pattern stimulation contain contrast- and luminance-dependent components. In order to elicit contrast-specific responses, reversing checkerboards used to elicit steady-state visually evoked responses (VER's) are designed to have luminance changes in adjacent checks that are equal and opposite so that the net change in the overall checkerboard luminance is zero. Data are presented which demonstrate that commercially available, electronically generated reversing checkerboards on television monitors may contain luminance artifacts sufficiently large to elicit flicker VER's closely resembling genuine contrast VER's. These hidden luminance contaminants may result in erroneous conclusions when assessing visual mechanisms or function.