INDEPENDENT EFFECTS OF LIGHTING, ORIENTATION, AND STEREOPSIS ON THE HOLLOW-FACE ILLUSION

Three experiments were conducted to investigate factors contributing t o the 'hollow face' illusion. A novel method was employed in which the distance from the mask at which the illusion became apparent or disap peared, when retreating or approaching, respectively, was taken as a m easure of the strength of the illusion. In all the experiments an effe ct of direction of observer's movement was found, demonstrating the st ability of the initial percept. Upright orientations were compared wit h inverted ones to investigate if the illusion reflects a bias towards a familiar percept. The direction of lighting was also varied. Indepe ndent main effects of orientation and lighting were found to be consis tent with preferences both for upright faces and for top lighting. How ever, inverted faces also produced the illusion to some extent, sugges ting a general preference for convexity. The role of stereopsis in res olving the illusion was tested by comparing monocular with binocular v iewing conditions. Monocular viewing conditions gave rise to shorter d istances, suggesting that the retinal disparities available with binoc ular viewing are important in disambiguating the illusion at small dis tances. The results are interpreted within the framework of a 2 1/2-di mensional sketch derived from independent modular processing of visual information.