Determination of the Stimuli for Involuntary Drifts and Saccadic Eye Movements*

Abstract

Eye movements during monocular fixation were photographically recorded using the contact lens mirror technique. Records were taken during normal viewing and during viewing with a stabilized retinal image. Flicker was used to produce various durations of disappearance of the fixated figure under both conditions. Characteristics of the drifts and saccadic components of the eye movements were compared under these conditions. It was found that, (1) the rate of drift is the same for all conditions, (2) there are many fewer saccades during stabilized than during normal viewing, and (3) the frequency of saccades is independent of the duration of disappearance for stabilized viewing. An analysis of the characteristics of saccadic movements showed that their probability of occurrence, direction, and magnitude are dependent upon the position of the retinal image on the retina. No comparable relationship was evident for the drifts of the eye. Eye movement records taken in the dark indicate that, in the absence of visual control, the eyes are incapable of maintaining their fixation. Proprioceptive feedback, therefore, does not appear to play an important part in the fine corrective movements that serve to maintain ordinary fixation. It is concluded that the primary stimulus condition for involuntary saccadic eye movements is displacement of the retinal image on the retina, and that drift is the result of an instability of the oculomotor system.