Effect of Stereoscopic Cues on Perceived Whiteness

It has been found that the introduction of stereoscopic cues between an induction object and a test object will reduce the magnitude of the simultaneous whiteness contrast between the two objects. The present study both verified this result and demonstrated that the reduction in contrast could not be explained as the direct result of the small directional displacements that were necessary in order to produce the stereoscopic cue. The results from the present study also support the conclusion that the perception of depth, and not the stereoscopic cue per se, was the necessary condition for the change in the magnitude of the whiteness contrast. If a large black disk is presented in a dark surround and is illuminated so that white light falls only on the disk, the disk will appear white or whitish gray. If a small object of greater luminance than the large disk is placed on the disk, the apparent whiteness of the disk decreases. This darkening of the large disk in the presence of the small object is known as the Gelb effect. The Gelb effect was long regarded as evidence for the view that knowledge of the conditions of illumination is important in the perception of whiteness, with the insertion of the small object into the beam of light providing the information necessary for this perception. Thus, the Gelb effect was usually considered to be an example of whiteness constancy and was believed to be dependent upon central rather than peripheral (retinal) factors. Stewart, however, demonstrated that the Gelb effect follows the laws of simultaneous contrast (a phenomenon usually believed to take place at a peripheral level).1 Two factors important in simultaneous contrast are the frontoparallel (directional) separation of the test object from the induction object and the size of the induction object. Stewart Received for publication November 3, 1969. The study was supported by Grant 1-R01-MH15651-02 from the U. S. Public Health Service. The authors wish to thank John Brendel for his help in collecting the data. 1 E. C. Stewart, The Gelb effect, J. exp. Psychol., 57, 1959, 235-242.