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Mani Nezhad, Arash Motamed, Bosco S. Tjan; Perceive the slow but pursue the fast - eye movement during shape-from-motion (SfM) with ambiguous stimuli. Journal of Vision 2005;5(8):103. doi: 10.1167/5.8.103.
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Feature correspondence across time determines the perceived 3D shape of a moving stimulus. We investigate the role of smooth-pursuit eye movement in perceiving shape-from-motion (SfM). Since a moving target is needed to engage smooth pursuit, we reason that points foveated during pursuit corresponds to fixed points on the perceived 3D shape, thus revealing the correspondence established by a subject.
Seven Ss maintained, per instruction, one of two percepts induced by a rotating ellipse during ninety 16sec epochs. The ellipse, a line drawing 20° tall with an aspect ratio of respectively 0.4, 0.7, or 0.8, was rotated 0.125 rev/s in the image plane around a point 1.7° off the center along the minor axis. Eye movement was measured using a head-eye tracker. Eye positions and velocities during smooth pursuit was extracted by imposing a speed limit of 25°/s.
The dominant percept is a tilted disc rolling circularly on an invisible frontal plane, with little sliding. This solution of the correspondence problem minimizes the total perceived stimulus motion (Rokers, Yuille & Liu 2004). We found that, when asked to maintain this dominant percept, Ss often pursued the imaginary regions in the featureless interior of the disk that has a high perceived speed. Far fewer tracking was found in the geometrically symmetric regions of the disk where the perceived speed vanished. When asked to maintain the percept of a rotating ellipse in the image plane, Ss often pursued one of the two regions on the contour where velocity component along the normal direction is the highest. In sum, Ss pursue imaginary or physical regions with higher perceived speed, while the dominant percept corresponds to an SfM solution that minimizes the perceived stimulus motion and thereby also minimizes the perceived speed of the tracked regions.
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