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Abstract

When a drifting grating is viewed through a stationary aperture, the global position of the aperture is displaced in the direction of local motion (Ramachandran \& Anstis, 1990, Perception, 19, 611-616). The purpose of the current study was to assess whether such displacement continues to occur when observers actively control the global position of the aperture. We created a simple game in which observes where given a birds-eye-view of a curving pathway along which they had to guide a target object. The target object was a gabor patch with a spatial frequency of 1 cycle per degree and an extent of approximately 2.5 degrees visual angle. The pathway was scrolled downwards to create the impression that the object was moving upwards along the path at a constant velocity. The vertical position of the target object was fixed at the centre of the screen, and a joystick was used to adjust the horizontal position so that the aperture was always centred on the pathway. In separate blocks we varied the speed of local motion in the aperture from 0 to 3 cycles per second, in steps of 0.5 cycles. When the grating was stationary, observes were able to guide the target object along the path with virtually no errors. As the speed of local motion increased errors also increased, reaching an asymptote of 27 min arc at 1.5 cycles per second. These results suggest that active control of an object cannot overcome the perceptual displacements induced by the drifting grating.