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Jenny J. Naji, Tom C.A. Freeman; Pursuit eye-movements disambiguate depth order in an ambiguous motion display. Journal of Vision 2002;2(7):167. doi: 10.1167/2.7.167.
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Motion gradients are important cues for depth perception but can be modified by eye-movements. For example, a translating corrugated surface can be specified by combining relative motion (shear) and translation, both of which are needed to determine depth order correctly. During pursuit, however, the eye movement cancels retinal translation so an extra-retinal estimate of translation is required to recover depth order accurately. We examined depth judgements in three conditions to investigate whether extra-retinal signals are used in this way. In condition 1, the eye was stationary and shear combined with retinal translation. In condition 2 the translation was removed. In condition 3, a fixation point moved at the same velocity as the retinal translation. Assuming accurate eye movements, conditions 2 and 3 are retinally equivalent. Stimuli were composed of sparse dot patterns shown on the black background of a screen viewed monocularly in the dark. The shear depicted one cycle of a sinusoidal depth corrugation. Four observers judged whether the top corrugation was concave or convex and whether the stimulus appeared flat or not. In conditions 1 and 3, %concave responses were sigmoidally related to shear. In condition 2, the sigmoidal relationship disappeared. In all three conditions, %flat responses peaked at 0 shear. Depth order was therefore perceived ambiguously in condition 2. In experiment 2, the three conditions were presented separately for 60s. Naive observers indicated if the stimulus appeared concave, convex or flat. Mean number of reversals was 12.55 in condition 2 but 0.28 and 0.16 in conditions 1 and 3, respectively. Moreover, the time spent reporting concavity and convexity was similar in condition 2 but highly-skewed in favour of the correct depth order in conditions 1 and 3. The experiments suggest that extra-retinal estimates of translation are needed to determine depth order correctly during an eye movement.
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