Visualizing the perisaccadic shift of spatiotopic coordinates.

A point light source flickering on and off during a horizontal saccade projects a horizontal array onto the retina. The apparent visual direction of the tail end of the perceived (phantom) array reflects the amount of perisaccadic shift of spatiotopic coordinates that has been completed by the end of the saccade. Four men, saccading 8 degrees to the right across a flashing light, judged the horizontal visual direction of the left (tail) end of the phantom array relative to the left end of a standard 8 degrees array that had projected an image onto the retina before the saccade began. On average, the left ends appeared to be aligned when the last flash in the phantom array was imaged on the retina 7.4 degrees to the right of the image of the left end of the standard array. This result implies that the shift of spatiotopic coordinates is virtually complete by the end of the saccade.