Perceptual slant induced through optical contact.

Stimuli simulating a corridor with a pole inside it were presented. The positions of the ends of the pole within the ceiling and ground of the corridor were the optical contacts of concern. The main hypothesis was that a difference between the simulated distances of these optical contacts may influence the apparent slant in depth of the pole. Two experiments were conducted, with 30 and 20 participants (university students). The tasks were a choice between three alternative responses (upward, downward, or no apparent slant) in the first experiment and matching by adjustment in the second. The results supported the hypothesized dependence of apparent slant on differences in optical contacts. The results also revealed a dependence of apparent slant on the interaction between the optical tilt of the pole and its position to the right or left of the vertical median through the stimulus. Comparisons between both effects showed that they combine in a cumulative way and that the former is weightier than the latter. Further findings were a privileged association between null optical tilt of the pole and the no apparent slant response and a bias (for poles with nonnull optical tilt) in favor of the upward apparent slant response.