![]() ![]() Understanding crowding requires knowing where it arises in the visual system, how it is manifest in neuronal responses, and which neural computations and mechanisms are responsible. These forms of spatial integration have, alternatively, been attributed to the larger spatial receptive fields of individual V2 18 or V4 neurons 3. Spatial integration has been suggested to involve pooling or compulsory averaging of targets and distractors 14, 15, erroneous substitution of distractors for targets 16, the representation of the visual scene by a set of summary statistics 17, 18, or some combination of these 11, 16. ![]() Still others have argued that crowding is an unavoidable consequence of spatial integration, an integral component of hierarchical visual processing, which builds a representation of visual objects by combining features across space. Others have attributed crowding to fluctuations in attention 13. Some have suggested that crowding arises from interactions between the representation of targets and distractors in primary visual cortex (V1) 11, 12. Why does crowding occur? Extensive perceptual work has led to several proposals. Under crowding, targets do not disappear rather, their features become difficult to discern. ![]() Importantly, the display properties that produce strongest crowding can be different from those that produce the most potent lateral masking, in which target detectability is impaired 8– 10. For instance, crowding requires distractors to be near the target, with the critical distance for crowding scaling with target eccentricity 7. ![]() Psychophysical studies have provided a rich description of how the parameters of a visual display influence the strength of crowding 3– 6. Further, understanding crowding is important because of its central contribution to visual disorders such as dyslexia 1 and amblyopia 2. Crowding is a fundamental bottleneck in vision, and offers a powerful paradigm for understanding the factors that limit perceptual performance. The presence of these other stimuli-distractors-can severely impair our ability to identify a target object or distinguish its features, a phenomenon known as crowding. They are usually surrounded by other visual stimuli. Visual objects rarely appear in isolation. ![]()
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