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Peter Tse; Smooth-pursuit eye-movements suppress motion processing. Journal of Vision 2009;9(8):409. doi: 10.1167/9.8.409.
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If retinal motion signals arising from smooth-pursuit were not discounted, smooth-pursuit eye-movements could generate spurious motion signals that generate a motion after-effect (MAE). Last year (Tse & Hsieh, VSS 2008) we showed that while the duration of the MAE experienced over stationary oriented lines increased with increasing duration and speed of smooth-pursuit, the strength of the MAE was effectively zero for vertical and horizontal bars, even though the vertical stimulus had the greatest image motion magnitude in the horizontal direction (of smooth-pursuit), presumably because motion suppression operates to discount spurious signals. The MAE was strongest for bars oriented slightly (about ten degrees) away from horizontal. We hypothesized that the motion processing system discounts component motion with less success for bars oriented at angles very close to the direction of smooth-pursuit, suggesting that filters tuned to low spatial frequency components of motion determine a global direction of motion. To inhibit spurious motion signals due to the aperture problem, motion signals derived from contours oriented at an angle close to this global direction of motion receive inhibitory feedback such that perpendicular component motions from these contours are suppressed. However, the limits of angular acuity are reached at small angles (i.e. the angle between a contour and the global direction of motion across the retina), and inhibition of perpendicular component motion of these contours is correspondingly weaker. Here we extend this work and show that radial motion appears to subjectively slow down dramatically during smooth-pursuit eye-movements. Stimuli were concentric ‘expanding’ and ‘contracting’ sinusoids. Using the method of constant stimuli, subjects equated the speed of a given stimulus expansion/contraction with and without smooth pursuit. Stimuli had to have much greater speed during smooth-pursuit to be subjectively equivalent in perceived motion magnitude to the same stimulus presented during static fixation, implying motion suppression as during saccades.
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