Abstract
While there is abundant psychophysical and neurophysiological evidence for an extra-retinal pursuit signal disambiguating the perception of depth from motion parallax, we note two conditions producing unambiguous depth where the eyes remain stationary within the orbit. The common feature of both is that the oculomotor system presumably generates an internal pursuit signal to countermand either translational vestibular ocular response or optokinetic response (OKR). Here we employ the transducer-corrected version of the Motion/Pursuit Ratio (Nawrot et al., VSS, 2011), along with the perceived depth of motion parallax stimuli, to determine the magnitude of the internal pursuit signal generated to maintain fixation against a large OKR inducing background. Observers performed comparisons of perceived depth magnitude between motion parallax and binocular stereopsis stimuli. A Stereographics Z-screen provided ocular separation for stereo stimuli and monocular viewing of the parallax stimuli. Both stimuli remained stationary on the monitor with fixation enforced with an eye-tracker. Parallax stimuli had a 41 x 25 deg OKR-evoking background of square-wave grating (0.25 cyc/deg) translating leftward or rightward at 5.5 or 11 deg/sec. Stereo stimuli had a stationary grey background. Parallax stimuli were quantified by maximum translation velocity of the stimulus dots (dθ: 0.09 - 0.55 deg/sec). Stereo stimuli had a range of disparities (1.5 – 15 min). For each dθ value, a point of subjective equality (PSE) was estimated from the psychometric function, giving the amount of binocular disparity producing the equivalent magnitude of perceived depth from motion parallax. Depth from motion parallax was unambiguous suggesting that an internal pursuit signal was being generated to countermand the OKR. However, the two different OKR background velocities generated the same perceived depth magnitude for each of the dθ values. This result suggests that the two different OKR background stimulus velocities evoked identical internal pursuit signals.