The extrinsic/intrinsic classification of two-dimensional motion signals with barber-pole stimuli

The perceived direction of different barber-pole stimuli was assessed by ad justing an arrow on the screen. The terminator ratio (TR: number of termina tors moving along the long side divided by the number of terminators moving along the small side) was either one or three. In this latter case, the ap erture orientation was either vertical or horizontal. The grating was eithe r in the same plane as the aperture (intrinsic condition) or behind the ape rture-the frame containing the aperture had a crossed disparity relative to the grating-(extrinsic condition). A nested design with 120 observers was used for the whole study. Five grating orientations were intermingled withi n any session. With a terminator ratio of three, the results depend strongl y on the aperture's orientation. When the rectangular aperture is horizonta l, the perceived direction of an intrinsic grating is horizontal (the typic al barber-pole illusion), whereas it is only slightly biased towards orthog onal one-dimensional (1D) motion signals (Vp) in the extrinsic condition. W hen the aperture is vertical, the perceived direction in the intrinsic cond ition is largely biased toward Vp, and on average it is close to Vp in the extrinsic condition. In this latter case, however, analysing the distributi ons of responses shows that many responses do not lie around Vp but are clu stered near vertical or horizontal. This motion capture depends on the grat ing's orientation. With a terminator ratio of one, motion capture is presen t in both the extrinsic and intrinsic conditions. Moreover, a global bias t oward horizontal is observed: this horizontal bias is much larger in the ex trinsic condition. Altogether, these results suggest that binocular dispari ty alone is a weak determinant of the extrinsic/intrinsic classification of two-dimensional (2D) motion signals compared to the occlusion cues provide d by unpaired regions in binocular images. Second, truly extrinsic 2D motio n signals are not suppressed but rather actively compete against each other to capture the ID motion signals. This results in a perceptual multistabil ity which is much stronger with extrinsic signals. Finally, given the inher ent multistability of barber-pole stimuli, high-level factors can alter the strength of this competition and prime any of the 2D motion signals. (C) 1 998 Elsevier Science Ltd. All rights reserved.