Object and Head Orientation Effects on Symmetry Perception Defined by Shape from Shading

Abstract

A grey disc which exhibits a vertical gradient of shading usually appears convex if lighter above and concave if lighter below. This phenomenon was investigated by Howard et al (1990) who varied both the shading axis relative to gravity and the orientation of the head. Their results indicated that head-centric or retinocentric coordinates determined the depth effect rather than gravitational axes. However, several possible problems with their study were noted, not the least of which was the possible intrusion of response rather than perceptual factors in the task they used. Here, we attempted to use an indirect measure of the perception of depth from shading; rather than asking subjects whether discs looked convex or concave, we constructed ensembles of shaded discs which, in terms of depth from shading, were or were not bilaterally symmetrical about an horizontal axis. These stimulus displays were briefly flashed to prohibit the intrusion of conscious assumptions about direction of light sources. Subjects were never asked whether any discs looked concave or convex, merely whether the set of discs was or was not depth symmetrical. Results were generally consistent with those of Howard et al and supported the conclusion that depth from shading is largely a low-level and automatic mechanism.

The gender-specific face aftereffect is based in retinotopic not spatiotopic coordinates across several natural image transformations

“…Although some studies showed a contribution from gravitational coordinates (Nicholls, Smith, Mattingley, & Bradshaw, 2006) most of them showed stronger dependence of perception and aftereffects on retinotopic coordinates (Friederici & Levelt, 1990; Mast, Ganis, Christie, & Kosslyn, 2003; Wenderoth & Hickey, 1993). The face inversion effect also generally follows the retinotopic coordinate system (Lobmaier & Mast, 2007).…”

Cross-cultural effects on the assumed light source direction: Evidence from English and Hebrew readers

“…Research into the cerebral basis of shape from shading has suggested that the estimation of the light source direction depends on a low level mechanism, within early visual areas (Gerardin, Kourtzi, & Mamassian, 2010;Humphrey et al, 1997;Mamassian, Jentzsch, Bacon, & Schweinberger, 2003). Behavioral data have offered some confirming evidence to this idea, for example, demonstrating that the light source is predominantly represented in retinal or head-centric coordinates (Howard, Bergstrom, & Ohmi, 1989;Kleffner & Ramachandran, 1992;Wenderoth & Hickey, 1993) but gravitational influences on shape judgments have also been found (Adams, 2008;Yonas, Kuskowski, & Sternfels, 1979).…”

Orientation tuning of shapefrom shading

“…The assumption continued to hold until the shading axis was rotated 60°with respect to the median plane of the subject's head. Wenderoth and Hickey (1993), who employed a task in which subjects' reaction times and accuracies in determining symmetry of shaded displays were measured, found a gradual reduction in performance as the orientation ofshading moved from orthogonal to the median plane of the head to parallel.…”