Interocular differences in crowding and their variation across the visual field

Abstract

Crowding occurs when an object that is visible in isolation becomes unrecognisable in clutter. Typically strongest in peripheral vision, it is thought to be a late process that occurs after binocular integration (e.g. since flankers crowd the target similarly whether presented to the same or different eyes; Kooi et al, 1994). This pattern differs markedly in amblyopia however, where crowding is strongly elevated in the fovea of the amblyopic eye and minimal in the unaffected eye. Does this interocular difference (IOD) make amblyopic crowding a distinct process, or have we simply missed the IODs of the periphery? To assess this, we measured crowding in eight directions from fixation and at two eccentricities. Observers judged the orientation of a target Gabor, presented either alone or with two abutting flankers on an iso-eccentric axis. Stimuli were presented entirely to either the left or right eye. When crowded, thresholds were 1.1-37.9 times greater than uncrowded levels, depending on visual field location. IODs were clearly present within this range, with values between 0-21.7 threshold elevation units. The direction and magnitude of these IODs varied heterogeneously across the visual field - for a given location, some observers showed significantly greater left-eye crowding, where others were more right-eye crowded. IODs did not however depend on either the dominant/sighting eye or a simple naso-temporal asymmetry. Nonetheless, they are repeatable over a period of months. Similar variations were also observed within the interference zone for crowding by measuring the above with a fixed target location and varied flanker locations. We conclude that peripheral crowding does indeed vary between the two eyes. This points to an early monocular component to crowding that bridges the gap between processes in normal vision and the more extreme variations seen in amblyopia. Meeting abstract presented at VSS 2015.