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Abstract:
During ocular pursuit of a moving target the retinal image velocity of other objects in the visual field differs from their physical velocity. To produce veridical perception of the motion of these objects, the visual system has to compensate for the eye movements. When an object is moving collinearly with the pursuit target, compensation occurs, although incompletely. This partial compensation produces illusions like the Filehne illusion and the Aubert - Fleischl phenomenon (Wertheim, 1994 Behavioral and Brain Sciences 17 293 - 355). However, whether the visual system compensates for eye movements when an object moves noncollinearly with the pursuit target is still a matter of debate. We investigated whether compensation occurs in the noncollinear case and whether it is the same as for collinear motion. We measured the perceived motion direction of a moving dot during ocular pursuit (10 deg s-1) of a horizontally moving pursuit target. The angle of the physical motion path of the dot relative to the pursuit path was varied from 0° to 360°. We found that observers made systematic errors in indicating the motion direction. The pattern of these errors was different for a dot moving with a lower (3 deg s-1) than with a higher (8 deg s-1) speed, with larger errors for the lower speed. The data can be explained by a model that assumes that compensation for eye movements is independent of speed and direction of the moving dot. Compensation is assumed to be normally distributed, with mean and standard deviation varying between observers.
