Judging the orientation of human faces: Effects induced by varying the illumination

How do people estimate the orientation of other people's faces? We observed that two images of a face seen from the same orientation, but illuminated from different angles, appeared to have different orientations. The first experiment was designed to document and quantify this phenomenon with respect to the average orientation of the face. The images were rendered on a black background that made it impossible to discriminate the shadowed facial parts from the background. We determined the physical orientation shift necessary to compensate for the illumination-induced effect. Results showed that the measured illumination-induced apparent orientaion shift (IAOS) correlates positively with the average orientation of the face and reaches values of up to 9°. This correlation implies that the mechanism is not based on local surface attitude judgements. We propose a model in which the symmetry plane of the face is detected, and then a comparison is made between the visible parts on both sides of this plane. The effect of the shadow occluding parts of the faces would then be responsible for the apparent orientation shift. To test this hypothesis we repeated the first experiment using a background colour that allowed subjects to perceive the true outline of the faces. We found that the IAOS was reduced to values of less than 2° and no longer depended on the average orientation of the faces. The results imply that orientation may be judged by comparing the size of the visible parts of the left and right halves of the face