Crowding

Crowding (or visual crowding) is a perceptual phenomenon where the recognition of objects presented away from the fovea is impaired by the presence of other neighbouring objects (sometimes called "flankers"). It has been suggested that crowding occurs due to mandatory integration of the crowded objects by a texture-processing neural mechanism, but there are several competing theories about the underlying mechanisms. It is considered a kind of grouping since it is "a form of integration over space as target features are spuriously combined with flanker features."

Crowding has long been thought to be predominantly a characteristic of peripheral vision. Yet, while it is indeed particularly prominent there, it is present in the whole of the visual field, with only its spatial extent varying (governed by Bouma's law; see below). In particular, crowding is of utmost importance in foveal vision, overriding the importance of visual acuity in pattern recognition and reading where crowding represents the bottleneck in processing.

Crowding is prominently present in amblyopia and has been first mentioned in that context and studied quantitatively there. Crowding deficits have further been found in neuropsychiatric disorders such as schizophrenia and autism and may have clinical implications in these disorders. It is also suggested that head injuries can cause a crowding effect. Normally sighted children up to the age of about eight years further have more pronounced crowding than adults, and this may be the reason for larger print in children's books.