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Abstract

The visual system is able to adjust its contrast sensitivity to the ambient contrast level. Others have shown that this adjustment is driven by stimulus parameters in a bottom - up fashion and higher order processes by means of attention. We present evidence that contrast sensitivity is under true top - down control. We paired an absolute identification paradigm with a cueing paradigm to measure identification accuracy for four low-contrast stimuli that were intermixed with an occasional high-contrast stimulus. Foreknowledge of the presentation of the high-contrast stimulus was manipulated by presenting valid and invalid cues. In experiment 1 we found that identification and discriminability of low-contrast stimuli was impaired only when the occurrence of the high-contrast stimulus was unpredictable but not when it was predictable. Only a top - down gain-control mechanism whose functions were to maximize discriminability while protecting the system from overload was consistent with the results. Experiment 2 p
rovided further support for the idea that an important function of gain control is to protect visual mechanisms from overload.