Abstract
Symmetry has long been considered as an influential factor for grouping and figure-ground segregation as well as a candidate for representing shape as medial axis. A recent psychophysical study has reported the adaptation to symmetry in random dot patterns [Gheorghiu, Bell & Kingdom, VSS 2014]. We investigated whether symmetry axis is a basis for symmetry perception. Specifically, we examined whether symmetry axis is an adaptable feature in the visual system. To examine whether adaptation alters the perceptual tilt of symmetry axis, we generated a set of stimuli that consisted of mirror-symmetric arrangements of random dots. The stimuli were comprised of tens of random dots so that their symmetry axes were invisible. A pair of stimuli whose axes were tilted ±10o from the vertical was presented for adaptation. Another pair of stimuli with a distinct dot pattern/contrast was presented for test. Using a staircase procedure, we measured the apparent tilt of the symmetry axes (Tilt-After-Effect). The results showed significant TAE, with the magnitude similar to ordinary TAE observed with solid lines. Similar TAEs were observed for the adaptation with a sequence of the short presentation of distinct patterns/contrasts, indicating that the TAE was evoked by the symmetry axis but not by pattern/contrast. We also performed another set of experiment with natural contours as adapter, and observed significant TAE. These results indicate that symmetry axis is an adaptable feature in the visual system, suggesting that symmetry axis is a basis for symmetry perception. To understand the cortical mechanism underlying the adaptation, we examined the retinotopy of the TAE. Our results showed the significant TAE even when the adaptation and test stimuli were presented 90o apart across the median line, indicating the hemispherical transfer of the adaptation. These results indicate the representation of symmetry axis in a higher visual cortical area.
Meeting abstract presented at VSS 2016