Phase shifts and the square-wave illusion.

When observers view a vertical triangle-wave luminance profile, they often report a square-wave illusion with a depth component, resembling a corrugated surface. Alternate bars seem to be in front of or behind adjacent bars and the surface appears to be illuminated from the right or left. These perspectives alternate with continuous viewing. One explanation for this illusion stems from the notion of instability among phase-selective mechanisms. Two experiments (1 and 3) were designed to determine whether systematic phase shifts introduced between the fundamental and the odd harmonics of the waveform would lead to a systematic bias of the illusion. The results indicated that a significant bias occurred when a phase shift as small as 9 deg was introduced, and that the bias from the phase shifts was more powerful than previous reports of drift-induced bias. There was a highly significant effect of direction of phase shift and the corresponding perceived direction of illumination. Another experiment (2) was designed to determine if illusional cues within the phase-shifted profiles aided phase discrimination. The results indicated that experienced subjects, presumably using cues within the profiles, discriminated between the stimuli significantly better than did naive subjects. These data support the role of phase in the square-wave illusion, but they also raise questions about the role of contrast changes in local regions of the stimulus.