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Arthur J. Lugtigheid, Eli Brenner, Andrew E. Welchman; Extra-retinal signals affect the perceived speed of 3D motion. Journal of Vision 2011;11(11):323. doi: https://doi.org/10.1167/11.11.323.
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When tracking an object moving in depth, eye vergence changes to minimise the object's absolute disparity. It is widely held that extra-retinal cues about changes in the orientation of the eye do not support perceptual estimates of motion-in-depth. However, we have shown (Welchman et al., 2009, Vis. Res., 49, 782) that extra-retinal signals support judgments of the sign of motion-in-depth (approaching vs. receding). Here we ask whether extra-retinal signals affect judgments of 3D speed. Observers (n = 6) fixated a small target surrounded by a large background. We induced (perceptually unnoticeable) vergence pursuit movements by continuously varying the lateral position of the left and right eyes' images in counter-phase following a sinusoid (frequency 1/4 Hz; amplitude 34 arcmin). On each trial, the target's disparity relative to the background changed at one of five rates, so that the target was seen as approaching at different speeds. Observers judged whether the speed of the target was faster or slower than the mean of the stimulus set. We determined psychometric functions in four conditions: when the eyes were moving to (a) converge or (b) diverge, and when the eyes were nearly static in (c) near or (d) far vergence positions. We found a shift between the psychometric functions of 30% in the rate of change in relative disparity when the eyes were moving, such that an approaching target was seen as faster during convergence and slower during divergence. In contrast, we observed no shifts in the psychometric functions when the eyes were nearly static. This demonstrates that extra-retinal signals support judgments of 3D motion magnitude as well as of sign. Specifically, we show that 3D speed judgments are affected by extra-retinal signals about changes in eye orientation, but are unaffected by extra-retinal signals relating to the static orientation of the eyes.
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