Mechanisms underlying the anisotropy of stereoscopic tilt perception

There is a marked anisotropy in the perception of stereoscopic tilt: vertical gradients of horizontal disparity are more easily perceived than horizontal gradients. This could be explained if orientation disparity (the orientation difference in the two eyes' views of the same line) were one of the cues used to determine tilt, since orientation disparities are in general larger for vertical gradients. We show here that a marked anisotropy in tilt perception is present even with stereograms which contain equally strong orientation disparity cues for horizontal and vertical gradients. This implies that there must be other mechanisms for stereoscopic tilt perception, or further processing steps in the use of orientation disparity, which are anisotropic in their mode of action.