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John Greenwood, Peter Bex, Steven Dakin; Position and orientation are bound in crowding. Journal of Vision 2010;10(7):1344. doi: 10.1167/10.7.1344.
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© ARVO (1962-2015); The Authors (2016-present)
The recognition of complex objects, like letters, requires the encoding of multiple visual attributes, such as orientation and position, and their binding into features. While the encoding of individual attributes is impaired by clutter, a process known as crowding, it is unclear whether crowding is driven by interactions between these attributes or between features (i.e. conjunctions of attributes). To test this idea, we investigated the interaction between crowding effects on two feature attributes: position and orientation. Stimuli were crosses composed of two near-orthogonal lines, presented 15deg. in the upper visual field. When crowded, the target cross was flanked to the left and right by flanker crosses (with 2.25deg. separation). Observers were required to judge either the orientation (clockwise/counterclockwise) or position (up/down relative to the stimulus centre) of the near-horizontal target feature. We first confirm the effects of crowding on the discrimination of orientation and position separately. For each feature attribute, crowding induces both threshold elevation (i.e. impaired discrimination) and a systematic bias in target identification towards the identity of the flanker features (i.e. responses are biased towards either the flanker feature-positions or orientations). We next examined the interaction between these crowding effects by requiring conjoint judgements of both the orientation and position of the near-horizontal target feature. If crowding occurred independently for each feature attribute, errors in target identification should be equally likely in either orientation or position, with a lower probability of combined errors for both attributes. In contrast, when targets and flankers differed in both orientation and position, errors were significantly more likely to occur for both attributes simultaneously (matching flanker identities) than for either attribute in isolation. This suggests that crowding takes place either after, or during, binding of the position and orientation of features within objects.
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