Temporal dynamics of early light adaptation.

This study investigated two aspects of visual sensitivity to a change in light level. The first experiment examined the time course of sensitivity to a small amplitude 1-s contrast pedestal presented on a 163-td pedestal within a 115-td surround. The largest contrast pedestal was an 8% contrast change that changed the steady pedestal threshold by only 0.03 log unit. Thresholds increased by 0.6 log unit or more at both onset and offset, with a return to baseline within 100 ms. The increment and decrement thresholds showed different time courses. Increment thresholds were raised before pedestal onset and decrement thresholds were raised before pedestal offset. These data were interpreted to show that increment thresholds measured at onset and offset of a contrast pedestal accessed on-pathways and decrement thresholds accessed off-pathways. In the second experiment, we examined the time course of sensitivity using a Crawford paradigm. Observers were dark-adapted and sensitivity was measured before, during, and after a 0.5-s, 68-td pulse. Test stimuli were slightly biased in chromaticity and the observer reported the presence and the hue appearance of the test, allowing separate estimates of detection and hue identification. Thresholds increased during the pulse but showed only a minimal overshoot. The test sensitivity for detection was poorer than for hue identification in the dark, but detection and identification were similar during the pulse. These data suggest that thresholds during sizable luminance pulses are determined in the PC-pathway.