Individual differences in perceived elevation and verticality: evidence of a common visual process

Thel frontoparallel orientation of a long peripheral line influences two visual norms, elevation, also called the visual perception of eye level (VPEL), and orientation in the frontoparallel plane, called visually perceived vertical (VPV). However, VPEL and VPV are distinct in that different integration rules describe the combinatorial effects of two lines symmetrically located on opposite sides of the median plane. Nevertheless, we propose that the same orientation-sensitive process underlies the two discriminations. We measured the two norms while we manipulated visual orientation with 1-line and 2-line stimuli (on opposite sides of the median plane), then modeled the large and significant effect of line orientation on VPEL and VPV settings as linear averages of signals from vision and from non-visual, body-referenced, vestibular and proprioceptive mechanisms. Significant correlations are evident between observers (n = 30) in the effect of visual orientation on both VPEL and VPV, and in the baseline measures (dark value, intercept) on both norms. The latter egocentric bias is further discussed in the context of the operation of the body-referenced mechanism across different egocentric discriminations for an individual subject. Given the evidence for different integration rules, the pattern of individual co-variation implies the existence of a single, shared visual orientation process that feeds to separate integration processes.