Dissociable effects of attention and crowding on orientation averaging.

It has been proposed that visual crowding-the breakdown in recognition that occurs when objects are presented in cluttered scenes-reflects a limit imposed by visual attention. We examined this idea in the context of an orientation averaging task, having subjects judge the mean orientation of a set of oriented signal elements either in isolation, or "crowded" by nearby randomly oriented elements. In some conditions, subjects also had to perform an attentionally demanding secondary task. By measuring performance at different levels of signal orientation variability, we show that crowding increases subjects' local uncertainty (about the orientation of individual elements) but that diverting attention reduces their global efficiency (the effective number of elements they can average over). Furthermore, performance with the same stimulus-sequence, presented multiple times, reveals that crowding does not induce more stimulus-independent variability (as would be predicted by some accounts based on attention). We conclude that crowding and attentional load have dissociable perceptual consequences for orientation averaging, suggesting distinct neural mechanisms for both. For the task we examined, attention can modulate the effects of crowding by changing the efficiency with which information is analyzed by the visual system but since crowding changes local uncertainty, not efficiency, crowding does not reflect an attentional limit.