Ogle's induced-size effect refers to the percept of slant elicited by a difference in vertical size between the left and right half images of a stereoscopic display. The effect is not readily predicted by the geometry of the situation and has been of considerable interest in the stereoscopic literature. Rogers and Koenderink (Nature, 322: 62-63) demonstrated that modulation of the vertical size of a monocular image during lateral head motion produces the impression of a surface slanted in depth - a motion-parallax analogue of the induced-size effect. We investigated motion parallax analogues of the induced-size and induced-shear effects further and compared them with the corresponding stereoscopic versions. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar

analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller than the geometric effects. For both stereopsis and motion parallax, relative slant and inclination were more pronounced when the stimulus contained discontinuities in disparity/velocity gradient than for continuous disparity/flow fields. The results have important implications for the processing of disparity and optic flow fields.

The support of the McDonnell-Pew Centre for Cognitive Neuroscience is greatly appreciated.