Contrast in visual selective attention: just another feature?

Neurophysiological findings and recent theorizing suggest that contrast may influence the ease of attentional selection, with high-contrast stimuli easy to select and hard to ignore. We tested this in four experiments. In Exp 1, subjects searched for a target (an “8” or a “9”) in a display of digits. In separate blocks, subjects searched a display of: (A) low-contrast digits, (B) high-contrast digits, (C) half low- and half high-contrast digits, with the target appearing among the low-contrast digits, or (D) half low- and half high-contrast digits, with the target appearing among the high-contrast digits. In conditions C and D, subjects were told the contrast of the target, potentially allowing them to select based on contrast. Subjects performed significantly better in condition C than in A (and better in condition D than in B), indicating that contrast differences between relevant and irrelevant stimuli improves search even when the irrelevant stimuli are of higher contrast. In Exps 2 and 3, subjects searched for a target among digits within half of the items, which was defined by color (red vs. green) or location, respectively. The contrast of these “relevant” and “irrelevant” sets was independently manipulated. When the relevant subset was defined by color, the search was easier whenever the “relevant” and “irrelevant” items had different contrast levels, even when the distractors were high-contrast. However, when the relevant items were in distinct locations, search was harder when the irrelevant subset was of high-contrast. This was not true, however, when the same experiment was repeated with different contrast levels presented in different blocks (Exp 4). Overall, the results suggest that selective attention to either high- or low-contrast is readily achieved, although this capability is not always utilized. The results challenge the most obvious linkage between attentional function and neurophysiological findings concerning contrast and attention.