The effects of symmetry on visual ensemble perception

Ensemble perception has been rigorously studied for the last two decades, and yet the exact mechanism of how the visual system forms the summary representation of the visual scene. Here we hypothesize that a global visual pattern might affect mean size judgments. Specifically, we reasoned that when the size array forms a symmetrical pattern, the mean size of that array would be overestimated because perceived numerosity is decreased for the items embedded in a symmetric pattern (Apthorp & Bell, 2015). To test this hypothesis, we measured the point of subjective equality (PSE) of mean size judgements when observers (n = 43) decided which side had a greater mean size. The pattern of standard array (16 circles, mean size = 1.4 deg) was always random. The pattern of test array (16 circles, mean size drawn from a uniform distribution of [1.2, 1.6] deg) was either random or symmetry. The duration of display was either 250 ms or 500 ms. The results show that our observers conducted the mean size judgment task reliably when the pattern of the test array was random (PSE = 1.40 deg). Critically, when the pattern of the test array was symmetrical, the PSE was shifted leftwards (1.38 deg, p = 0.01), suggesting that the perceived mean size was greater for the symmetry condition compared to the random condition. This pattern of results was stronger for the short duration compared to the long duration. Our finding suggests that the global visual pattern affects ensemble perception, and the reduced perceived number of items due to visual symmetry increases perceived mean size estimation.