The role of implicit axes of bilateral symmetry in orientation processing

Abstract

An important role for the near-effortless perception of bilateral symmetry in human (and other) observers may well be to process the orientation of symmetrical or near-symmetrical objects which do not have explicitly delineated up-down axes. From psychophysical investigations of one- and two-dimensional tilt illusions and after-effects, as well as from orientation discrimination studies, evidence is presented here which points to the existence of neural substrates which are sensitive to real (or explicit) as well as virtual (or implicit) contours.