Modulation of perceived contrast in the brightness comparison of asynchronous stimuli

Comparative judgment is a crucial task in ecological settings, as well as in many experimental studies about basic aspects of perceptual processes. It has long been known that sequential comparison is prone to order effects. This phenomenon has received little attention and has often been discounted as a type of response bias. In the present study, we investigated brightness discrimination of two brief (100 ms) spatially disjoint luminance stimuli. In the first and second experiments, stimuli were presented against a dark background with a stimulus onset asynchrony (SOA) from 0 to 200 ms, in a paradigm controlling for response bias. In the third experiment, stimuli were presented against a bright background. We demonstrate that the time interval between stimuli modulates and even inverts their perceived brightness difference, enhancing the second stimulus relative to the first. When the background is brighter than the target stimuli, the sign of the effect is inverted, suggesting that the underlying mechanism operates on contrast rather than brightness. The magnitude of this effect is shown to depend on SOA and average luminance level of the target stimuli. Hypotheses in terms of neural and attentional dynamics are proposed.