The Ebbinghaus illusion modulates visual search for size-defined targets: evidence for preattentive processing of apparent object size.

Five search experiments investigated whether the apparent size of objects is, like their retinal size, coded in preattentive vision. Observers searched for a target circle that was either larger or smaller than distractor circles, with both types of test circles surrounded by context circles modulating apparent size (i.e., the Ebbinghaus illusion). The size ratio between the test and the context circles was manipulated in such a way that the test circles were surrounded by, for example, smaller context circles (making the larger target appear even larger) or by larger context circles (making the smaller distractors appear even smaller). Under optimal conditions, the detection reaction times were independent of the number of test circles in the display, and the Ebbinghaus illusion facilitated the detection of the target even in comparison with control conditions without any context circles. This finding is consistent with preattentive, spatially parallel processing of apparent size.