Reversed tilt effect for dichoptic stimulation in vertical meridian

The orientation of a test line appears shifted away from that of contextual contours. This repulsion in the orientation domain manifests itself in, for example, the tilt aftereffect, the simultaneous orientation contrast, and the Zöllner illusion. Attractive rather than repulsive interaction has been reported for collinear inducing configurations at small inducing angles. Here it is shown that the tilt induced in a monocular vertical foveal test line by a single centered inducing line is repulsive when both are in the same eye, but becomes attractive for dichoptic stimulation. This occurs for only a narrow range of test-line/inducing-line separations and varies somewhat between and, by ocularity, within observers. The phenomenon might be related to stereoscopy which involves the processing of dichoptic orientation differences around the vertical.