Abstract
In the Barber-Pole illusion (BPI), a drifting 1D texture is viewed through an elongated aperture and the perceived direction of motion is generally along the orientation of the aperture rather than the Fourier component (i.e., orthogonal to the 1D texture). According to the motion-path-integration theory, the BPI results from a greater path-motion energy along the aperture’s elongated orientation. Alternatively, the end-stop theory rather suggests that the perceived direction of motion is driven by the processing of line-ends along the contours of the aperture. Although there is good evidence supporting the end-stop theory for foveal viewing, its applicability for peripheral viewing (>10 degrees) has been questioned. The current study investigated the underlying cause of the BPI under peripheral viewing conditions (20 degrees) by systematically manipulating the shape of the aperture, which was an elongated parallelogram. When the long edges of the aperture were not parallel to the drifting texture, the perceived direction of motion was close to the orientation of the long edges (as typically perceived in the BPI) and the orientation of the short edges had little impact on the perceived direction of motion. However, when the long edges were parallel to the drifting texture (resulting in no motion along these edges), the perceived direction of motion was close to the orientation of the short edges. The fact that a small change in the contour orientation of the short edges had little impact on the global shape of the aperture, but drastically affected the perceived direction of motion, suggests that the BPI can be mainly driven by the processing of line-ends along the aperture contour even under peripheral viewing conditions. These results suggests that the BPI viewed peripherally does not depend on the global shape of the aperture per se, but rather on the contours of the aperture.