Objects are almost always seen in spatial proximity to other objects, whose shapes, sizes, and colors may influence the perception of corresponding properties of the target object. People seldom perceive an object in isolation. This assimilation account fits well with previous explanations of one-dimensional illusions of linear extent (e.g., the Müller-Lyer and Baldwin illusions), which are extended to account for the present two-dimensional illusion of shape. We suggest that the CSI is caused by edge assimilation modulated by similarity between the target and inducer arising from population coding of edge positions. Experiments 3 through 5 examine the influence of relative inducer size and target size on illusion magnitude. Experiments 1 and 2 show that the CSI magnitude varies systematically with factors known to influence grouping strength between configural elements, including proximity, good continuation, positional alignment, lightness similarity, hue similarity, and common fate. The target is selectively elongated in a direction consistent with the extension of the larger configuration that includes both target and inducer. Abstract A new illusion, called the configural shape illusion (CSI), is described in which the apparent shape of an object (the “target”) is systematically distorted by the presence of an adjacent shape (the “inducer”) that is distinct from, but perceptually grouped with, the target.

Circles in rectangle illusion