Closure and Symmetry Perception in Autism
Closure describes a perceptual grouping cue, where a closed shape (e.g. a square) stands out more than an open shape (e.g. a single line), creating a ‘pop-out’ effect. Closure is believed to involve a global processing mechanism where feedforward information is integrated with feedback from visual areas responsible for shape processing. Previous work has shown that autistic adults have a reduced closure effect, suggesting reduced visual integration in this group (Jachim et al, 2015). However, it is unknown whether this is due to the short duration of stimulus presentation used in this study, as no group differences have been observed in studies measuring different aspects of visual integration that use longer durations (Kovács et al. 1999; Hadad et al. 2010). In addition, it is still unknown whether reduced closure may affect other aspects of perception in autism that also rely on global mechanisms such as symmetry perception. During symmetry perception, symmetrical shapes are processed faster than asymmetrical shapes. Given that both closure and symmetry perception involve global processing, we investigated how symmetry perception may be affected by reduced closure in autism.
Using two experiments, we investigated the effect of stimulus duration on closure (experiment 1) and how closure affects symmetry perception (experiment 2) in autistic and non-autistic adults.
Two experiments were tested on autistics and matched non-autistic adults. Both experiments were based on a 2-IFC contour integration task, where participants had to detect a contour formed by Gabor elements, against a background of randomly oriented Gabors. In experiment 1, contour detection was compared between closed and open shapes at two durations: 150ms and 500ms. The closure effect was then calculated as the difference in performance between the two shapes, for each duration. In experiment 2, instead of detecting the contour, participants were asked to choose the most symmetric shape (symmetry detection task). Symmetry was varied by changing the angle of the sides of a square pattern, and performance was compared between closed and open shapes.
In experiment 1, we found a significant interaction of stimulusXgroup where the autistic group has less closure effect compared to the non-autistic group at both durations. This was similar across both durations with no three-way interaction with duration. For experiment 2, symmetry was significantly easier to detect with closed than the open shape for the non-autistic participants. However, symmetry detection was not significantly different between closed and open shapes in the autistic group.
Experiment 1 showed that reduced closure in autistic group does not improve with longer durations, indicating a persistent alteration in the closure effect over time. In addition, Experiment 2 showed that closure seems to provide less benefit in symmetry detection in the autistic compared to non-autistic group, suggesting that the autistic group might be using different mechanisms to process both symmetry and closure. Overall, both findings depict atypical visual processing in autism, providing knowledge on how differences in visual perception may arise.
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