Spatial and temporal tuning of motion-in-depth perception

Purpose. We used the Pulfrich stereo-phenomenon to investigate motion-in-depth. Independent control over temporal and spatial frequency content in our stimuli revealed the spatio-temporal tuning characteristics of motion-in-depth perception. Methods. Stimuli were presented to the left and right eye on a calibrated CRT display with a refresh rate of 120 Hz using a split-screen Wheatstone configuration. On each trial Ss verged on a fixation cross flanked by nonius lines as two sinusoidally oscillating sine-wave gratings were presented stereoscopically. The gratings were displayed in a Gaussian spatial envelope for 1 sec. After each presentation Ss had to indicate whether direction of motion-in-depth was clockwise or counterclockwise from a bird's eye view. Interocular phase difference between left and right sine-wave gratings was randomised (−pi/4 to +pi/4) from trial to trial to determine a discrimination threshold. Spatial frequency of the gratings (1 cpd to 4 cpd) and temporal frequency of sinusoidal motions (0.25 Hz to 5 Hz) were systematically combined in different sessions. Results. Phase thresholds for the discrimination of direction of motion-in-depth showed relative broadband temporal frequency tuning with peak sensitivities between 1 and 4 Hz. Temporal frequency tuning appeared to be independent of spatial frequency content whereas velocity tuning curves shifted systematically with increasing spatial frequency. Conclusions. Physiological evidence indicates that binocular cells in V1 typically show temporal frequency tuning whereas cells in area MT are preferably tuned to velocity. The tuning characteristics from the present experiment support the idea that binocular disparity and motion are jointly encoded in V1 before image velocity is extracted at a later cortical stage, presumably area MT