Previous research has shown that when the targets of successive visual searches have features in common, response times are shorter. However, the nature of the representation underlying this priming and how priming is affected by the task remain uncertain. In four experiments, subjects searched for an odd-sized target and reported its orientation. The color of the items was irrelevant to the task. When target size was repeated from the previous trial, repetition of target color speeded the response. However, when target size was different from that in the previous trial, repetition of target color slowed responses, rather than speeding them. Our results suggest that these priming phenomena reflect the same automatic mechanism as the priming of pop-out reported by Maljkovic and Nakayama (1994). However, the crossover interaction between repetition of one feature and another rules out Maljkovic and Nakayama's (1994) theory of independent potentiation of distinct feature representations. Instead, we suggest that the priming pattern results from contact with an episodic memory representation of the previous trial.

in one of two ways: (a) by selecting one feature value in one dimension (e.g., selecting the color red) or (b) by iteratively combining the output of (a) with a preexisting Boolean map via the Boolean operations of intersection and union. Boolean map theory offers a unified interpretation of a wide variety of visual attention phenomena usually treated in separate literatures. In so doing, it also illuminates the neglected phenomena of attention to structure.

In order to understand how attention affects visual processing, we investigated the degree to which attention effects can be accounted for by increases in the contrast gain of the contrast response function, CRF (represented by an increase in effective contrast) vs. increases in the response gain (represented by an overall amplification of response). To this end, we used a dual-task paradigm to compare psychophysical "threshold vs. pedestal contrast" (TvC) curves obtained under conditions of full- vs. poor-attention. The attention effect, defined as the ratio of thresholds for poor- vs. full-attention conditions, was roughly four-fold at a pedestal contrast of 0% (i.e., at detection threshold) and there was a significant decrease in attention effect with increasing pedestal contrast, from approximately ten-fold at the lowest non-zero pedestal contrast tested (0.25%) to three-fold at the highest pedestal contrast tested (64%). These findings are consistent with the existence of both contrast gain effects of attention (needed to account for the substantial attention effect at detection threshold and the decrease in attention effect with increasing pedestal contrast) as well as response gain effects of attention (needed to account for the fact that attention was beneficial across all pedestal contrasts-rather than harmful at some contrasts, as a pure contrast gain model would predict). The results of a model fitting Naka-Rushton CRF equations to the TvC data also support this conclusion. Here we found a two-fold increase in contrast gain and a five-fold increase in response gain in the CRF for the full-attention, as compared to the poor-attention, condition. Because pure contrast gain effects, on the order of two-fold, have been observed at early stages of visual processing (for example in areas V4 and MT), our psychophysical results suggest a hybrid model of attention; contrast gain control at an early stage of visual processing, followed by response gain control at a later stage.

When a visual search task is very difficult (as when a small feature difference defines the target), even detection of a unique element may be substantially slowed by increases in display set size. This has been attributed to the influence of attentional capacity limits. We examined the influence of attentional capacity limits on three kinds of search task: difficult feature search (with a subtle featural difference), difficult conjunction search, and spatial-configuration search. In all 3 tasks, each trial contained sixteen items, divided into two eight-item sets. The two sets were presented either successively or simultaneously. Comparison of accuracy in successive versus simultaneous presentations revealed that attentional capacity limitations are present only in the case of spatialconfiguration search. While the other two types of task were inefficient (as reflected in steep search slopes), no capacity limitations were evident. We conclude that the difficulty of a visual search task affects search efficiency but does not necessarily introduce attentional capacity limits. q

Momentary awareness of a visual scene is very limited; however, this limitation has not been formally characterized. We test the hypothesis that awareness reflects a surprisingly impoverished data structure called a labeled Boolean map, defined as a linkage of just one feature value per dimension (for example, the color is green and the motion is rightward) with a spatial pattern. Features compete with each other, whereas multiple locations form a spatial pattern and thus do not compete. Perception of the colors of two objects was significantly improved by successive compared with simultaneous presentation, whereas perception of their locations was not. Moreover, advance information about which objects are relevant aided perception of colors much more than perception of locations. Both results support the Boolean map hypothesis.