Contrast adaptation in human retina and cortex.

PURPOSE Although cortical contrast adaptation has been extensively studied with both psychophysical and electrophysiological techniques, little is known about retinal contrast adaptation in humans. METHODS Retinal and cortical long-term contrast adaptation was assessed with simultaneous measurement of pattern electroretinogram (PERG) and cortical visual evoked potentials (VEPs). This study involved three approaches: sampling of the contrast transfer function from 2.7% to 98% with adaptation to high (98%) and low (7.3%) contrasts, linearity of adaptation effects, and transfer of contrast adaptation between parallel and orthogonal grating orientations. RESULTS Contrast adaptation affected retinal and cortical recordings quite differently. The VEP showed a sigmoid contrast transfer function, which was shifted toward higher contrasts (by a factor of 1.9), whereas amplitudes at higher test contrasts were enhanced to 127%. The PERG decreased in amplitude to approximately 90%, and the latency was significantly reduced by 4 to 6 msec (P < 0.05). All measured effects were linear with adaptation contrast. Orientation played no role in the PERG results, whereas the VEP was enhanced to 125% when tested parallel and to 150% when tested orthogonal to adaptation. CONCLUSIONS VEP results confirm and extend previous findings and fit well with single-cell recordings. The PERG findings suggest that retinal contrast adaptation occurs and mainly operates in the temporal domain, comparable to rapid gain-control findings in cats and primates.