Abstract
Recently, Burr and Ross (2008) reported a negative adaptation after-effect on the perceived numerosity of dot displays. Based on these findings, they proposed that numerosity is encoded as a basic property of scenes, like color, orientation, and other visual dimensions susceptible to adaption. Given mounting evidence that the visual system rapidly represents the mean size of sets of objects without representing individual items (e.g. Ariely, 2001), here we investigated whether a similar mean size adaptation after-effect occurs. On each trial, observers adapted to two displays of 16 dots with various mean sizes, presented simultaneously in opposite visual fields. After adaptation, two single circles replaced the dot displays, and participants judged which dot appeared larger. On average, observers perceived the test dot that appeared in the same location as the adapting display with the larger mean size as being smaller than the test dot that replaced the adapting display with the smaller mean size. This negative after-effect suggests that mean size is also represented as a basic dimension of visual scenes.