Attentional effects on contrast detection in the presence of surround masks.

We studied how attention affects contrast detection performance when the target is surrounded by mask elements. In each display quadrant we presented a hexagon of six vertical Gabor patches (the 'surround'). Only one of the hexagons contained a central Gabor patch (the 'target') and the task was to report that quadrant (spatial four-alternative-forced choice). Attention was manipulated by means of a double-task paradigm: in one condition observers had to perform concurrently a central letter-discrimination task, and the contrast-detection task was then only poorly attended, while attention was fully available in the other condition. We find that under poorly attended conditions targets can be detected only when the target contrast exceeds the surround contrast (contrast popout) or when the target orientation differs from the surround orientation by more than 10-15 degrees (orientation popout). When the target orientation is similar to the surround orientation, attention can reduce the contrast detection thresholds in some cases more than four-fold, demonstrating a very strong attentional effect.