Exp. Brain Res. 23, 529--533 (1975) 9 by Springer-Verlag 1975 The Auber~-Fleischl Pheaomenon: A Temporal Frequency Effect on Perceived Velocity in Afferent Motion Perception* J. Dichgans, E. Wist, H. C. Diener and Th. Brandt Neurologische Universit~tsklinik mit Abteilung ffir Neurophysiologie, Freiburg (FRG) Received July 1, 1975 Summary. Apparent velocities of moving visual stimuli are known to be dif- ferent depending on whether the subject pursues the stimulus (efferently con- trolled motion perception) or whether the eye is stationary and the image moves across the retina (afferent motion perception). Afferent motion perception of a periodic pattern or a moving single object causes overestimation of velocity (magnitude estimations) as compared to smooth pursuit. This soealled Aubert- Fleischl phenomenon is shown to depend on local temporal frequency stimulation on the retina caused by the repetitive passage of contrast borders of the moving periodic pattern. This is evidenced by the fact that for a given stimulus speed the amount of overestimation is a function of the spatial frequency of the pattern (or the angular subtend of a single moving object) and that the Aubert-Fleischl phenomenon is not observed if a single edge moves. Background characteristics seem not to influence the apparent velocity during smooth pursuit. Key words: Motion perception -- Spatial frequency -- Aubert-Fleischl pheno- menon Fleischl in 1882 and Aubert in 1887 observed that an object which passes across the retina while the eyes are stationary appeared to move about twice as fast as when the object was visually pursued [1, 8]. Further observations confirmed this soealled Aubert-Fleisehl paradox [2, 6, 7, 9]. A quantitative study by KSrner and Diehgans [11], using magnitude estimations, established that the scaled speed of a horizontally moving stripe pattern during a#erent motion per- ception (i.e., with fixation on a steady point in the visual field resulting in move- ment of the stimulus across the retina) was 1.79 times greater than during "e/- ]erently controlled" motion perception (ocular pursuit of the moving stimulus and therefore no retinal image motion). In a subsequent study they found that this factor ranged between 1.6 and 1.96 depending upon stimulus field size and location [4]. * Research was supported by Deutsche Forsehungsgemeinschaft, SFB 70 (,,Hirnforschung und Sinnesphysiologie") and conducted while the second author was a "US Senior Scientist Awardee" of the Alexander yon Humboldt Stiftung on leave from the Whitely Psychology Laboratories, Franklin and Marshall College, Lancaster, Pa. USA. 37*