Modification of the Filehne illusion by conditioning visual stimuli.

During smooth pursuit eye movements made across a stationary background an illusory motion of the background is perceived (Filehne illusion). The present study was undertaken in order to test if the Filehne illusion can be influenced by information unrelated to the retinal image slip prevailing and to the eye movement being executed. The Filehne illusion was measured in eight subjects by determining the amount of external background motion required to compensate for the illusory background motion induced by 12 deg/sec rightward smooth pursuit. Using a two-alternative forced-choice method, test trials, which yielded the estimate of the Filehne illusion, were randomly interleaved with conditioning trials, in which high retinal image slip was created by background stimuli moving at a constant horizontal velocity. There was a highly reproducible monotic relationship between the size and direction of the Filehne illusion and the velocity of the background stimulus in the conditioning trials with the following extremes: large Filehne illusions with illusory motion to the right occurred for conditioning stimuli moving to the left, i.e. opposite to the direction of eye movement in the test trials, while conversely, conditioning stimuli moving to the right yielded Filehne illusions close to zero. Additional controls suggest that passive motion aftereffects are unlikely to account for the 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.