The temporal course of recovery from brief (sub-second) adaptations to spatial contrast.

Visual adaptation is a critical and ubiquitous mechanism that occurs for any stimulus feature and involves a continuous adjustment of the neuronal contrast gain. These adjustments prevent our visual system from dropping in sensitivity for the prevailing ranges of stimulus features that are processed at a given time. In addition to the classical adaptation, which arises over several seconds to minutes, a number of psychophysical, electrophysiological and interference studies have documented a much faster form of adaptation occurring with motion stimuli. This faster adaptation operates on a sub-second scale. In the present study, we investigated whether a fast form of adaptation also exists for spatial contrast and whether its characteristics (e.g., dependence on the duration of adaptation, time course of recovery) are similar to the classical, slower contrast adaptation. We found that a fast form of adaptation does exist and is maximal at intervals of 16-50 ms after the offset of the adapting stimulus. Similar to what previous studies have found regarding the classical contrast adaptation, the initial threshold elevation of this study did not depend on the duration of the adapting stimulus, but only on its contrast. Our results showed that the function which best describes the decay of brief adaptations to high-contrast stimuli was a double exponential decay function, whereas the best function for describing adaptation to low-contrast stimuli was a single exponential decay function with a very fast recovery rate. Thus, adapting contrast influences both the threshold elevation, which rises with increasing adapting contrast, and the time course of recovery from adaptation. Overall, our data suggest the presence of a mechanism that is similar to the classical contrast adaptation involved in longer adaptations, but it operates over much shorter timescales.