Attentional modulation of crowding.

Outside the fovea, the visual system pools features of adjacent stimuli. Left or right of fixation the tilt of an almost horizontal Gabor pattern becomes difficult to classify when horizontal Gabors appear above and below it. Classification is even harder when flankers are to the left and right of the target. With all four flankers present, observers were required both to classify the target's tilt and perform a spatial frequency task on two of the four flankers. This dual task proved significantly more difficult when attention was directed to the horizontally aligned flankers. We suggest that covert attention to stimuli can increase the weights of their pooled features.