Perceptual consequences of ocular lens overshoot during saccadic eye movements.

In a previous paper we compared eye globe records of saccadic eye movements (recorded with a scleral eye coil) with lens reflection records of the same eye movements (recorded with a dual-Purkinje-image eyetracker); we found evidence for considerable dynamic deviations between the two during and immediately after saccades. We ascribed these deviations to the movements of the eye's lens relative to the optical axis of the eye. This paper quantifies a predicted psychophysical effect of lens displacements during and after saccades. Two small targets, one above the other, were flashed for 2 msec in total darkness, the bottom one exactly at the end of the saccade, the top one 30 msec later. The first target appears deviated horizontally relative to the other, in a direction opposite to the saccade. Magnitude of the relative mislocalization can be up to 0.03 deg for each degree of saccadic eye movement. The result shows that the position of the visual image on the retina is affected both by position of the globe and by deviations of the lens from its normal location.