Involuntary motions of the eye during monocular fixation.

Abstract

The purpose of this expt. was to measure rapid involuntary movements of the eye (physiol. nystagmus) during steady monocular fixation of a stationary object. A photographic method of recording was used. An optical lever was provided by the use of a mirror mounted in a contact lens worn by the subject. Any movement of tye eye of the subject resulted in a proportional displacement of the image of a light source on moving recording paper. Prisms properly placed in the optical system made possible the simultaneous recording of vertical and horizontal components of the motions of the eye and of the head. A fixation point at an optically infinite distance eliminated many problems associated with involuntary head movements. Records were obtained from 5 subjects. Four main "types" of movement are to be seen in the records. (a) Small rapid motions with a median extent (peak to trough) of about 17.5 sec. angle of rotation and frequencies ranging from 30 to 70 cycles per sec. (b) Slow motions of irregular frequency and extent. (c) Slow drifts in one direction or another upon which the motion mentioned above are superimposed. (d) Rapid jerks with an avg. extent of about 5.6 min. of arc occurring at irregular intervals, at times apparently compensating for the drifts. Total movement due to the combined effects of these motions over a period of 3-4 sec. was usually less than 10 min. of arc during the steadiest fixations. On the basis of the above results conclusions were drawn regarding the motions of the retinal image of the object being fixated. (a) During a fixation period of 3-4 sec. the image of the object being fixated may move across a total of 25-50 receptors. (b) The slower motions, drifts, and jerks may carry the retinal image across a dozen recptors. (c) The small rapid motions move the retinal image across 2-3 receptors at most. However, the median extent of these motions is so small that the movement of the retinal image under opt. conditions of fixation is not likely to exceed the width of one receptor. The results indicate that small rapid motions of the eye do occur during "steady" fixation, but that the magnitudes of these motions are extremely small.