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Smooth Pursuit of a Partially Occluded Object

There has long been qualitative evidence that humans can pursue an object defined only by the motion of its parts (eg Steinbach, 1976 Vision Research 16 1371 – 1375). We explored this quantitatively using an occluded diamond stimulus (Lorenceau and Shiffrar, 1992 Vision Research 32 263 – 275). Four subjects (one naive) tracked a line-figure diamond moving along an elliptical path (0.9 Hz) either clockwise (CW) or counterclockwise (CCW) behind either an X-shaped aperture (CROSS) or two vertical rectangular apertures (BARS), which obscured the corners. Although the stimulus consisted of only four line segments (108 cd m−2), moving within a visible aperture (0.2 cd m−2) behind a foreground (38 cd m−2), it is largely perceived as a coherently moving diamond. The intersaccadic portions of eye-position traces were fitted with sinusoids. All subjects tracked object motion with considerable temporal accuracy. The mean phase lag was 5°/6° (CROSS/BARS) and the mean relative phase between the horizontal and vertical components was +95°/+92° (CW) and −85°/−75° (CCW), which is close to perfect. Furthermore, a \chi2 analysis showed that 56% of BARS trials were consistent with tracking the correct elliptical shape (p<0.05), although segment motion was purely vertical. These data disprove the main tenet of most models of pursuit: that it is a system that seeks to minimise retinal image motion through negative feedback. Rather, the main drive must be a visual signal which has already integrated spatiotemporal retinal information into an object-motion signal.