It has been proposed that the 2-D motion signals elicited by the bar endings of a barber-pole stimulus disambiguate 1-D motion signals with a variable strength which depends on depth (Shimojo et al, 1989 Vision Research 29 619 – 626): these signals would be ‘abolished’ when they are extrinsic (ie the moving grating is behind the plane of the background containing the aperture), whereas they would be given full strength when they are intrinsic (ie the plane of the grating is in front of the background). These authors have suggested that the intrinsic/extrinsic classification is an early process. However, the very long duration (2300 ms) used in their study suggests other interpretations. Therefore, we tried to test whether the barber-pole illusion could be abolished with a shorter duration when the grating had an uncrossed disparity relative to the aperture plane, as initially described in the above-mentioned study (our 30 observers had to adjust an arrow to indicate the perceived direction of the grating). In accordance with our prediction, we could not replicate their finding with a duration of 400 ms. Surprisingly, increasing the duration up to 2300 ms was not sufficient to obtain a large bias towards 1-D signals. To understand this unexpected result, we tried to isolate the relevant difference between the initial study of Shimojo et al and our. We found that the main determinant of the suppression of the barber-pole illusion was the experimental procedure: when our observers had to assess the perceived direction of the barber-pole by choosing between horizontal and vertical, as in the initial study, the results did show a much larger bias towards 1-D signals. We suggest therefore that the extrinsic/intrinsic classification is a high-level process which can be influenced by the observer's expectations.
