Adaptation is Faster after Luminance Decrements than after Luminance Increments,Independently of Test Pulse Polarity

As is well known, dark-adaptation in the human visual system is much slower than recovery from darkness. We show that at high photopic luminances the situation is exactly opposite. In psychophysical experiments on human subjects, we have studied detection thresholds for brief light flashes, at various delays with respect to decrement and increment steps in background luminance. Light adaptation was nearly complete within 100 ms after luminance decrements, but took much longer after luminance increments.

In an effort to determine the nature of the threshold dynamics, we have compared sensitivity after equally visible pulses or steps in the adaptation luminance. Flash detectability was initially the same in the pulse and step conditions, but recovered much faster after pulses than after increment steps. This suggests that the initial threshold elevation is caused by the temporal contrast of the background steps and pulses, whereas the residual threshold elevation after an increment step shows an incomplete luminance adaptation. We have substantiated this by manipulating the contrast of a transition between luminances: we found that these contrast manipulations affected only the initial part of the threshold curve, not later stages.

Finally, we measured detection thresholds for brief luminance decrements. For these tests with negative polarity, threshold recovery remained significantly faster after decrement than after increment steps in background luminance. Therefore, the asymmetry in adaptation dynamics that we report is indeed related to the step direction of the background luminance, and is not caused by interaction with the test polarity.