Vision Rrs. Vol. 24, No. IO. pp. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1181-1187, 1984 0042.6989,‘84 $3.00 + 0.00 Printed in Great Britain Perf+mon Press Ltd OPTOKINETIC AND VECTION RESPONSES TO APPARENT MOTION IN MAN C. M. SCHOR, V. LAKSHMINARAYANAN and V. NARAYAN University of California, School of Optometry, Berkeley, CA 94720, U.S.A. (Received 26 September 1983;final revision received 27 March 1984) Abstract-Apparent motion was investigated as a stimulus for optokinetic nystagmus (OKN) and self-motion perception (vection). Apparent motion was stimulated by stroboscopically illuminating vertical stripes on the interior of a large drum that rotated about the observer at 20, 40 and 60 deg/sec. We determined threshold stroboscopic frequencies (f) for the appearance of smooth continuous apparent motion and measured responses of pursuit, OKN, optokinetic after nystagmus (OKAN) and vection, to stroboscopic frequencies at, above and belowf. Pursuit occurred for all of these stimuli. However OKN, OKAN and vection only occurred for frequencies equal to or greater than the threshold for continuous apparent motion. Our results suggest that pursuit can occur as a response to apparent motion generated by both small and large image displacements, while OKN and vection are responses to apparent motion generated by small image displacements only. These results suggest that different aRerent sources are utilized for the control of pursuit and of the slow phase of OKN. Apparent motion Vection Pursuit Optokinetic nystagmus Optokinetic aftemystagmus INTRODUCTION Although pursuit tracking eye movements and the slow phase of optokinetic nystagmus (OKN) have similar dynamic properties, they may not be con- trolled by the same processes. Several observations suggest that separate mechanisms underlie the con- trol of pursuit eye movements and the slow phase of OKN. Prolonged stimulation of OKN results in a continuation or aftereffect of OKN in darkness (OKAN) (Cohen et al., 1977) whereas prolonged stimulation of pursuit tracking movements results in a smaller drift bias in darkness (pursuit after nys- tagmus) (Muratore and Zee, 1979) or no aftereffect at all (Lisberger et al., 1981). In addition ablation studies in monkey indicate that removal of the flocculus of the cerebellum selectively disrupts pursuit eye movements but not optokinetic nystagmus (Zee et a/., 1978). Afferent stimuli for the control of pursuit eye movements and OKN slow phase may also be com- pared. Both types of eye movements occur as responses to continuous motion of the retinal image and to correlates of eye movements such as outflow that contribute to perceived motion (Young, 1977). Examples of the latter are pursuit tracking of after- images (Young, 1977) and Sigma OKN which is OKN stimulated by stroboscopic stationary patterns (Adler et al., 1981). Both types of eye movements also respond to stroboscopic motion (Westheimer, 1954; Rock et al., 1964; Tauber et al., 1960; Morgan and Turnbull 1978; Ter Braak, 1972; Wolfe, Held and Bauer, 1980). Recent studies suggest that separate processes un- derlie stroboscopic motion stimulated with large and small image displacements (Braddick, 1973, 1974, 1980; Ramachandran and Gregory, 1978; Anstis, 1978). Previous studies of pursuit tracking and OKN have not classified their stroboscopic stimuli accord- ing to the magnitude of image displacement. In this study we have compared pursuit tracking and OKN slow phase responses to apparent motion generated by small and large image displacements. We have observed that these two types of eye movement respond differently to stroboscopic motion. Only the pursuit system responds to large image displacements whereas both OKN and pursuit systems respond to small image displacements. We also observed that circular vection (Dichgans and Brandt, 1978), a sen- sation of self-motion that is evoked by optokinetic stimuli, could not be stimulated with large image displacements. These observations suggest that different afferent sources are utilized for the control of pursuit tracking eye movements and the slow phase of OKN. METHODS Each of 5 subjects was seated at the center of a 4 ft wide vertical cylindrical drum whose inside surface was black with half inch wide (1.2 deg) white vertical stripes at 20 deg intervals. A bidirectional d.c. motor rotated the drum either clockwise or counter- clockwise at velocities of 20, 40 and 60 deg/sec. The interior of the drum was illuminated stroboscopically by a Grass PS-2 stimulator mounted on top of the drum, with a flash duration of approximately 10 microseconds at frequencies ranging from 5 to 60 Hz. Eye movements were recorded with an infrared d.c. monitor (Gulf Western SGVH/Z). For experiments in 1181