Bilateral symmetry is highly salient visual feature. The visual system seems to have efficient mechanism to perceive symmetry. However it is not fully understood what type of visual information is used for symmetry perception. The present study investigated the temporal characteristics of symmetry perception. In Experiment 1, the minimum time for symmetry perception was assessed by a backward masking experiment. A bilaterally symmetric dot pattern (40 dots for 13ms) was followed by a random dot mask (160 dots for 160 ms) with various stimulus onset asynchronies (SOAs). Results showed that symmetry detection required 50 ms to be completed, during which had to be free from physical stimulation. We hypothesized that symmetry perception can occur based on visible persistence of stimulus elements. To test this hypothesis, we employed a temporal integration task, where subjects had to integrate two asymmetric patterns over time to detect symmetry (Experiment 2). A symmetric dot pattern was divided into two asymmetric patterns so that each half contained half the number of dots. The halves were presented with various SOAs. Symmetry was detected successfully with SOAs up to 70 ms. This result strongly supports that symmetry perception depends on visible persistence of stimulus elements and does not require a common onset/offset of the elements. Corroborating the visible-persistence hypothesis further, we observed the inverse-intensity effect of visible persistence. Interestingly, however, we also found that a smaller number of dots leaded to better performance in the integration task (Experiment 3), but not in the masking task (Experiment 4), implying an additional effect of temporal synchrony of symmetric elements. We conclude that visible persistence can be the basis of symmetry perception but the temporal synchrony (common onset/offset) of elements is important for efficient symmetry perception.
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