When size matters: attention affects performance by contrast or response gain

Covert attention, the selective processing of visual information in the absence of eye movements, improves behavioral performance. Here, we show that attention, both exogenous (involuntary) and endogenous (voluntary), can affect performance by contrast or response gain changes, depending on the stimulus size and the relative size of the attention field. These two variables were manipulated in a cueing task while varying stimulus contrast. We observed a change in behavioral performance consonant with a change in contrast gain for small stimuli paired with spatial uncertainty, but a change in response gain for large stimuli presented at one location (no uncertainty) and surrounded by irrelevant flanking distracters. A complementary neuroimaging experiment revealed that observers’ attention field was wider with than without spatial uncertainty. Our results support key predictions of the normalization model of attention, and reconcile previous, seemingly contradictory, findings on the effects of visual attention.