An analysis of binocular slant contrast

When a small frontoparallel surface (a test strip) is surrounded by a large r slanted surface tan inducer), the test strip is perceived as slanted in t he direction opposite to the inducer. This has been called the depth-contra st effect, but we call it the slant-contrast effect. In nearly all demonstr ations of this effect, the inducer's slant is specified by stereoscopic sig nals; and other signals, such as the texture gradient, specify that it is f rontoparallel. We present a theory of slant estimation that determines surf ace slant via linear combination of various slant estimators; the weight of each estimator is proportional to its reliability. The theory explains sla nt contrast because the absolute slant of the inducer and the relative slan t between test strip and inducer are both estimated with greater reliabilit y than the absolute slant of the test strip. The theory predicts that slant contrast will be eliminated if the signals specifying the inducer's slant are consistent with one another. It also predicts reversed slant contrast i f the inducer's slant is specified by nonstereoscopic signals rather than b y stereo signals. These predictions were tested and confirmed in three expe riments. The first showed that slant contrast is greatly reduced when the s tereo-specified and nonstereo-specified slants of the inducer are made cons istent with one another. The second showed that slant contrast is eliminate d altogether when the stimulus consists of real planes rather than images o n a display screen. The third showed that slant contrast is reversed when t he nonstereo-specified slant of the inducer varies and the stereo-specified slant is zero. We conclude that slant contrast is a byproduct of the visua l system's reconciliation of conflicting information while it attempts to d etermine surface slant.