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Laurent Madelain, Richard J Krauzlis; Pursuit of the ineffable: perceptual and motor reversals during the tracking of apparent motion. Journal of Vision 2003;3(9):434. doi: 10.1167/3.9.434.
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PURPOSE. Previous studies have shown that pursuit can be guided by the perceived rather than the physical motion of a stimulus, but the temporal relationship between motion perception and pursuit is unknown. Using combined psychometric measurements and eye movement recordings during tracking of an apparent motion stimulus, we compared the timing of perceptual reversals in motion direction to the timing of pursuit reversals in smooth eye velocity. METHODS. We used an apparent motion stimulus consisting of a horizontal row of evenly spaced Kanisza illusory squares (1.44 deg2). The circular inducers rotated by 90o on each frame, so that the illusory squares appeared at the exact midpoints of the illusory squares presented in the previous frame. Each frame was presented for 66 ms, producing bi-directional apparent motion of the illusory contours (21.5 deg/s) that could be changed at will. We measured eye movements in 5 subjects asked to 1) track the motion of the illusory squares, and 2) reverse the perceived direction while continuing to track the squares. For this second experiment, we measured the timing of the volitional perceptual reversals and compared this to the time course of the reversal in tracking direction. RESULTS. Subjects could track the apparent motion without saccades. When asked to reverse the perceived direction, subjects could also produce saccade-free reversals in pursuit velocity. The time course of these reversals in pursuit velocity closely followed the psychometric measurements of the perceptual reversal. Both reversals spanned approximately 378 ms and, on average, the perceptual reversals preceded the pursuit reversals by 54 ms. CONCLUSIONS. The perceived motion of an object can drive pursuit, even when the motion is perceptually bistable and the object itself is illusory. Smooth pursuit and the perception of motion direction are in temporal register, indicating that pursuit can provide a real-time readout for the state of motion perception.
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