MODIFICATION OF THE FILEHNE ILLUSION BY CONDITIONING VISUAL-STIMULI

During smooth pursuit eye movements made across a stationary backgroun d an illusory motion of the background is perceived (Filehne illusion) . The present study was undertaken in order to test if the Filehne ill usion can be influenced by information unrelated to the retinal image slip prevailing and to the eye movement being executed. The Filehne il lusion was measured in eight subjects by determining the amount of ext ernal background motion required to compensate for the illusory backgr ound motion induced by 12 deg/sec rightward smooth pursuit. Using a tw o-alternative forced-choice method, test trials, which yielded the est imate of the Filehne illusion, were randomly interleaved with conditio ning trials, in which high retinal image slip was created by backgroun d stimuli moving at a constant horizontal velocity. There was a highly reproducible monotic relationship between the size and direction of t he Filehne illusion and the velocity of the background stimulus in the conditioning trials with the following extremes: large Filehne illusi ons with illusory motion to the right occurred for conditioning stimul i moving to the left, i.e. opposite to the direction of eye movement i n the test trials, while conversely, conditioning stimuli moving to th e right yielded Filehne illusions close to zero. Additional controls s uggest that passive motion aftereffects are unlikely to account for th e modulation of the Filehne illusion by the conditioning stimulus. We hypothesize that this modification might reflect the dynamic character of the networks elaborating spatial constancy.