Retinal contribution to contrast adaptation in human vision

Purpose. Previously, (ARVO 1994 1995) we described a contrast gain control, as measured by the visual evoked potential (VEP). which adapts within 700 msec. Our goal was to identify the retinal contribution to this phenomenon using the pattern electroretinogram 'ERG). Methods. Monocular pattern ERGs and VEPs were recorded simultaneously to square-wave contrastreversal checkerboards (32 mm checks; 8x8 deg field; 150 cd/m2 mean luminance, modulated at 4.22 Hz) ir three normal subjects. A stimulus repeat consisted of 16 cycles of high contrast (1 0) followed by 16 cycles of low contrast (0.25). Sixteen trials, each consisting of eight such repeats, were collected and averaged Results. Averaged responses were partitioned into sequential, nonoverlappmg epochs each 237-msec in length) and separately Fourier analyzed Each analysis epjch consisted of one cycle of pattern reversal. Following the change to h gh contrast (from 0.25 to 1 0). there is a transient increase in ERG and VEP response amplitude which lasts for 2-3 epochs. Contrast decrements (from 1.0 to 0.25) are followed by a decrease in response amplitude which also lasts for 2-3 epochs. Corresponding response phase transients are present in both measures Conclusions. Retinal adaptation to luminance contrast, as assessed by the pattern ERG. occurs within the same time interval (700 msec) as adaptation of the pattern VEP. This suggests that much of the rapid gain control observed with the VEP may be determined by the temporal dynamics of contrast adaptation in the retina.