Phenomenal and retinal influences on the perception of symmetrical forms in depth

Symmetry identification is a widely-researched area in the field of perception. Two explanations exist to explain a bias for vertical symmetry over other types of symmetry, the anatomical and the phenomenal approaches. The goal of this study was to investigate the conditions under which a phenomenal judgment of symmetry (based on the objective properties of the stimulus) is made in depth situations. By elucidating these conditions, the vertical symmetry bias can be shown to either be solely due to a symmetrical projection of a form to a vertically symmetrical nervous system (anatomical approach) or be due to a two-stage cognitive process of figural description (phenomenal approach). To accomplish this goal, 64 subjects viewed vertically symmetrical stimuli either directly in front of a screen (fronto-parallel position) or at a 65 degree angle to the screen. In addition, subjects either viewed monocularly through a pinhole (a reduced-depth cue condition) or had full view of the screen (a binocular, full-depth cue condition). The results bore out the following: The 65-degree/reduced-cue condition made more retinally-based symmetry judgments, while the 65-degree/full-cue group made correct objective judgments. Consistent with the phenomenal approach, to make a correct objective judgment the perceptual system requires depth information to place the stimulus in perspective. A key finding was the fact that a bias for vertical symmetry did not occur under the 0-degree/full-cue condition where the anatomical approach would predict one. In firm support of the phenomenal approach it appears that a bias emerges and increases with conflicting retinal and phenomenal information. Further information supporting the phenomenal approach came from the investigation whether cognitive constancy mechanisms had deteriorated in the 65-degree/reduced cue condition, but not the 65-degree/full-cue condition. This was found to be the case. A significant negative relationship was also demonstrated between shape constancy maintenance (as measured by deviations of height and width judgments from 0 to 65 degrees) and correct phenomenal judgments. This supports the two-stage figural descriptive process of the phenomenal approach, involving the detection of the retinal stimulus and its subsequent perceptual interpretation (involving depth and constancy information) is required to make a phenomenal judgment. (Abstract shortened with permission of author.