BARBER-POLE ILLUSIONS AND PLAIDS - THE INFLUENCE OF APERTURE SHAPE ONMOTION PERCEPTION

The barber-pole illusion and its influence on plaid perception were in vestigated in two experiments to test the following expectations: (i) apertures which bias the perception of grating motion in directions co nsistent with plaid direction will facilitate plaid perception, and (i i) apertures which bias the perception of grating motion in directions inconsistent with plaid direction will disrupt plaid perception. In e xperiment 1 the barber-pole illusion was measured as a function of gra ting orientation (20-degrees, 45-degrees, and 70-degrees, clockwise an d counterclockwise from horizontal), and aperture shape (vertical, hor izontal; at each of three elongations). Barber-pole illusions reported with 45-degrees gratings increased with increased aperture elongation . However, this was not found with 20-degrees and 70-degrees gratings; these were almost always reported as moving in a direction parallel t o the side of the aperture with which the gratings formed angles appro aching 90-degrees. In experiment 2 this dependence of barber-pole illu sions on the relative orientation between gratings and apertures was a lso evident with 45-degrees gratings in oblique apertures; only obliqu e directions of grating motion were reported. The influence of the sam e apertures on the separate contrast thresholds required for initial p laid coherence and initial plaid decomposition was measured. In experi ments 1 and 2, coherence thresholds were unaffected by apertures, cont rary to expectation (i). However, in both experiments expectation (ii) was confirmed; decomposition thresholds decreased in apertures which biased perceived direction of gratings towards vertical (plaid directi on), and increased in apertures which biased grating motion away from vertical. Adaptation of plaid mechanisms during measurement of decompo sition thresholds was proposed to explain the discrepancy between cohe rence and decomposition data. Taken together, the results were interpr eted as reflecting interactions between mechanisms mediating the barbe r-pole illusion and mechanisms mediating plaid perception.