A point light source flickering on and off during a horizontal saccade projects a horizontal array onto the moving retina. The apparent visual direction of the tail end of the perceived (phantom) array reflects the amount of perisaccadic shift in spatiotopic coordinates that has been completed by the end of the saccade. We devised an experiment to determine the perceived location of the tail end of the phantom array.
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 degree 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.
Given that the spatial location of the first flash in the perceived (phantom) array is displaced, relative to the flashing light, in the direction of the attendant saccade (Hershberger & Jordan, 1998), and appears in this displaced location roughly 80 msec prior to the onset of the saccade (Jordan & Hershberger, 1994), it appears to be the case that both intended and actual eye-movement have effects upon perceived space, with shifts in the former preceding shifts in the latter.
Referat in der Gruppe Wahrnehmung und Psychophysik, Dienstag, 30. März 1999, 15:00, HS 21
