How different types of scenes affect the Subjective Visual Vertical (SVV) and the Perceptual Upright (PU)

Different scenes contain varying cues to the direction of gravity. Do scenes with stronger cues differentially affect the ability of a scene to influence the direction of the Subjective Visual Vertical (SVV) and the Perceptual Upright (PU)? Using indoor, outdoor, natural and man-made scenes we asked participants to judge the orientation of pictures (Scene Upright: SU), viewed through a circular shroud, relative to the gravitationally defined upright. The standard deviation of these judgments was taken as an estimate of the reliability of the cues present in that scene. The SVV and PU were then measured against these scenes. The scenes in the SVV condition were tilted by +/-22.5 degrees and the SVV measured using a line. The scenes in the PU condition were tilted by +/-112.5 degrees and the PU was measure by a letter probe. The difference in orientation of the probes with the scene in these two orientations was defined as the visual effect. The man-made scenes affected the SVV more than the natural scenes. The visual effect was inversely proportional to the standard deviation with which the scene was judged as upright for the SVV but not PU. In order to be sure that the null result for the PU was not a ceiling effect we measured the SU and PU at brief exposure durations to increase the standard deviations of the SU. There was still no significant correlation between the standard deviations of the SU and the visual effect on the PU. This difference between PU and SVV suggests that the SVV may be more sensitive to global orientation information relevant to spatial orientation (as measured by Scene Upright) than the PU and that the more global spatial orientation a scene contains, the greater its effect will be on the SVV.