Abstract

Viewing a vertical triangle-wave grating often gives rise to the perception of a corrugated surface with alternating bars that approximate a square-wave luminance profile. This square-wave illusion contains other bistable aspects. Adjacent bars appear to be offset in depth, and the corrugated surface appears to be illuminated from one side or the other. Drifting the grating to the right or left biases the direction of apparent illumination. In the present investigation, a triangle-wave with a spatial frequency of 0.25 c/d was phase-alternated (180) at various rates. The first finding was that the square-wave illusion was totally suppressed at alternation rates greater than 3 Hz. At slower rates, observers reported that all aspects of the illusion could be perceived, but a bias in the direction of apparent illumination did not appear to be linked to the occurrence of a phase shift. At very slow rates of alternation (Hz), observers reported two possible perceptual experiences: (1) alternate bars appeared to move back and forth in counter-phase; or 2) bars appeared stationary and the direction of illumination appeared to change with each phase shift.

© 1990 Optical Society of America