Suprathreshold spatiotemporal response characteristics of the human visual system.

The apparent contrasts of suprathreshold stationary gratings, countermodulated gratings, and homogeneous flickering fields were assessed with a contrast-matching procedure. Results show that, as stimulus amplitude is increased relative to threshold, variations in apparent contrast with spatiotemporal-frequency content become much less pronounced. In other words, the contrast-matching functions are more uniform across both spatial and temporal frequency at levels of contrast well above threshold. These data are interpreted in terms of a compensatory stage in the visual system that varies its gain characteristics according to the detectability of the stimulus.