Visual suppression during smooth following and saccadic eye movements

Vision Res. Vol. 9. pp. Pergamon Press LETTER VISUAL SUPPRESSION Printed in Great TO THE EDITORS DURING SACCADIC (Received Britain. SMOOTH FOLLOWING AND EYE MOVEMENTS1 5 September OCULOMOTOR activities appear to be controlled by two distinct neural systems: one which regulates saccadic or version movements, and another which controls smooth following movements. The evidence for this derives from (1) the kinds of stimuli eliciting the two types of movements (WESTHEIMER, 1954; RASHBASS, 1961), (2) the nature of the eye’s response during the movements (STARK et al., 1962; ROBINSON, 1964, 1965), (3) the contribution of the two types of movements to spatial localization (FESTINGER and CANON, 1965), and (4) the differential effects of central nervous system disease on eye movements (STARR, 1967). We wished to define whether the distinction between the two oculomotor control systems also applied to the phenomenon of visual suppression that accompanies voluntary eye movements (VOLKMAN, 1962; LATOUR, 1962; ZUBER and STARK, 1966). While this phenomenon has been well studied in relationship to saccades, there has been no assessment of vision on oculomotor tasks requiring both types of eye movements. The experiments to be presented in this report measured vision during the tracking of a horizontally moving object and indicate that visual suppression is associated specifically with saccades and not with smooth following movements. The subjects were four young ladies (ages 16-20) with normal vision and eye move- ments. Their tasks were (1) to track a slowly moving visual target and (2) to press a button whenever a test flash of light appeared. The subject was seated in a darkened room 150 cm from a screen (50 cm in width), upon which the visual target was pro- jected. The target was a circle of light (approximately 1 cm in dia.) projected from a mirror galvonometer, which moved so as to swing the light sinusoidally across the screen at a frequency of 1, 2, 3, or 4 times/set. The test flash was generated by a Grass photo-stimulator, and illuminated the screen diffusely. Prior to any tracking, the flash intensity was adjusted for each subject by means of an iris diaphragm to be just visible (correct detection on at least 9 of 10 trials). The subject was then instructed to keep her eyes directed toward the visual target and to press a button, held in the palm, whenever the diffuse test flash appeared. Twenty-five light flashes were presented during a tracking run with intervals ranging from 3-10 sec. The subjects tracked the target at least two times on each of the four tracking frequencies and received a total of at least 200 test flashes. The electro-oculogram, the position of the target light, the time of the test flash, and the subject’s response were recorded on magnetic tape and then written out on an ink chart recorder with a time resolution of 1 sec/l20 mm of paper. The records were analyzed to determine whether the subject had responded to each test flash and whether This study supported by Career Development Neurological Diseases and Blindness. Award, NB 31242 from the National Institute of