Abstract
Symmetry is a fundamental characteristic of objects and scenes, one that human observers are highly sensitive to. Moreover, observers are faster and more accurate in detecting symmetrical stimuli compared with asymmetric versions, in the left visual field but not in the right (Wilkinson & Halligan, 2002). This has been taken as evidence for a right hemisphere advantage in symmetry processing. The present study investigated whether this hemispheric advantage is affected by the complexity of the visual feature. Stimuli were familiar block shapes and random walk contour paths constructed from Gabor patches. The former are likely to tap mid-level form processing mechanisms while the latter are thought to be processed in V1. Stimuli were randomly presented 6° to the left or right of a central fixation cross, for 160ms. The observer responded as to whether the stimuli were horizontally symmetrical or asymmetrical. Fourteen participants were tested in each experiment. Experiment 1 replicated a left visual field symmetry advantage for familiar shape stimuli. Interestingly however, for contour paths a significant symmetry advantage, (faster RTs & lower error) was present in both visual fields. Experiment 2 showed that the symmetry advantage persisted for vertical and diagonal contour paths, judged relative to horizontal and oblique symmetry axes, respectively. Finally, Experiment 3 found that for paths composed of either: a) Gaussian blobs or, b) hard-edged discs, the symmetry advantage was drastically reduced. This suggests that the composition of the elements mediates performance. Our findings reveal that symmetrical contours are preferentially processed in both visual hemispheres. This holds for cardinal and oblique axes but is mediated by the spatial-frequency profile of the path elements. Taken as a whole our findings imply that symmetry is bilaterally advantaged in early visual processing while for more complex stimuli, this symmetry advantage is right hemisphere specific.
Meeting abstract presented at VSS 2013