Two-dimensional motion of the retinal image during monocular fixation.

A new modification of the contact lens technique was employed to record simultaneously vertical and horizontal components of eye movements. These were trigonometrically transformed to yield information about retinal image motion parallel to eight retinal meridia. It was found that eye movements occur predominantly in a small range of directions, which differs from subject to subject. Correlational and other statistical analyses of drift and saccadic movements led to the following conclusions about the nature of visual fixation: (1) Saccades compensate for the displacement of the retinal image away from some optical locus, but their probability of occurrence may increase with time since the last saccade, rather than with displacement. (2) The error signal to which saccades respond is probably generated 0.10 sec prior to their occurrence. (3) Parallel to those meridia where saccade compensation is poor, compensation by drifts becomes appreciable.