Stereoscopic and contrast-defined motion in human vision

There is considerable evidence for the existence of a specialized mechanism in human vision for detecting moving contrast modulations and some evidence for a mechanism for detecting moving stereoscopic depth modulations. It is unclear whether a single second–order motion mechanism detects both types of stimulus or whether they are detected separately. We show that sensitivity to stereo–defined motion resembles that to contrast–defined motion in two important ways. First, when a missing–fundamental disparity waveform is moved in steps of 0.25 cycles, its perceived direction tends to reverse. This is a property of both luminance–defined and contrast–defined motion and is consistent with independent detection of motion at different spatial scales. Second, thresholds for detecting the direction of a smoothly drifting sinusoidal disparity modulation are much higher than those for detecting its orientation. This is a property of contrast–modulated gratings but not luminance–modulated gratings, for which the two thresholds are normally identical. The results suggest that stereo–defined and contrast–defined motion stimuli are detected either by a common mechanism or by separate mechanisms sharing a common principle of operation.