Visual Search for Size-Defined Target Objects is Modulated by the Ebbinghaus Apparent-Size Illusion: Facilitatory and Inhibitory Effects of the Context Objects

Five experiments were carried out to investigate under which conditions the apparent size of objects is computed and exploited optimally in visual search for size-defined targets. Observers searched for a target test circle that was retinally larger than the distractor test circles, with both types of circles surrounded by context circles modulating the apparent size of the test circles (Ebbinghaus illusion). RTs were the faster the better test circles could be differentiated from the context circles, ie with smaller numbers of context circles, larger distances, and higher lightness (or colour) contrast between test and context circles. Apparent-size modulation had a strong influence on search RTs, resulting in faster RTs with smaller, and slower RTs with larger, context circles. A model assuming overall facilitatory effects of the apparent-size modulation and interference effects arising from decreasing test-context circle discriminability can explain the present results.

Attentional Modulation of Size Contrast

A test circle surrounded by smaller context circles appears larger if presented in isolation, whereas a test circle surrounded by large context circles is seen as smaller than in isolation. Two experiments are reported indicating that this phenomenon, the Ebbinghaus illusion, depends on whether subjects are attending to the context circles. Subjects first saw a reference circle and then a briefly presented (150 msec) test circle. Their task was to determine whether the test circle was larger or smaller than the reference. The test circle was surrounded by smaller context circles of one colour arrayed along a horizontal axis centred on the test, and larger context circles of a different colour arrayed along a vertical axis centred on the test. Subjects judged both the size of the test and the colours of either the small or large context circles. Perceived test size changed systematically, depending on which context circles were task-relevant.

Feature Analysers, Optical Illusions, and Figural Aftereffects

Simple displacement models cannot explain some aspects of optical illusions and figural aftereffects. The orientation-detector interaction model proposed by Blakemore and others is more suitable to explain many aspects of the Zöllner illusion, positive and negative illusions, the effect of gap between the inducing and test lines, and the anisotropy of illusions. If we hypothesize size detectors whose tuning width and distribution steps are proportional to logarithmic size, interactions between them explain well the fact that the Delboeuf illusion and figural aftereffects of circles are determined by the size ratio of the inducing to test circle, not by the absolute distance between the contours of these circles.