Afterimage movement during saccades in the dark.

The spatial values of retinal coordinates are "recalibrated" to the "egocentric" coordinates during and after a saccade within a fraction of a second. We measured the time constant of this retinal coordinate transformation by means of an afterimage technique: our ten subjects performed "auditory" horizontal saccades in total darkness (0.2-4.5 saccades/sec). At a saccade frequency below 1 saccade/sec, the subjects observed saccadic displacement of the foveal afterimage, but the afterimage seemed to arrive at its final position more slowly than the center of gaze (state 1). At saccade frequencies above 1.5 saccades/sec, the perceived amplitude of afterimage displacement decreased with increased saccade frequency (state 2). Above 2 saccades/sec all subjects perceived two stationary afterimages simultaneously at the saccadic end-position (state 3). A further increase in saccade frequency reduced the distance between the two afterimages till only one stationary afterimage was seen in a mid-position between the two auditory targets at a saccade frequency above 3.2-3.5 saccades/sec (state 4). Saccade amplitude remained constant within the frequency range between 0.2 and 4.5 saccades/sec. A one-step or two-step linear model was applied to simulate the experimental data, indicating that the spatial coordinates shift more slowly than the saccadic eye movements.