Jeremiah W. Bertz, Wenxun Li, Leonard Matin; The influences of visual pitch on visually perceived eye level, visually perceived pitch, felt head orientation, and felt hand orientation. Journal of Vision 2005;5(8):882. doi: 10.1167/5.8.882.
A pitched environment exerts a large systematic influence on the visually perceived elevation of objects within the visual field; however, reports by observers of changes in the perceived orientation of their own bodies relative to gravity have been very rare. We now present measurements showing that systematic changes in felt orientation of the head and of the hand are general. Our investigation was conducted in a well-illuminated, complexly structured pitchroom (1.52 m × 1.82 m × 1.52 m) with binocular viewing of the pitchroom at a viewing distance of 1 m. Ten participants made four types of settings while binocularly viewing the pitchroom at six orientations from −30° to +20°: (1) setting the elevation of a small luminous target to appear at eye level (VPEL); (2) setting the pitch of a 30 cm × 30 cm surface (80 cm in front of the observer) to appear erect, i.e. unpitched; (3) setting the pitch of the head to feel erect; (4) manually setting the pitch of an unseen plane surface laterally located within the observer's midfrontal plane to feel erect. All four settings changed systematically with physical pitch. The average slopes of the functions relative to physical pitch are: (1) VPEL, 0.69; (2) visual pitch, 0.87; (3) head pitch, 0.10; (4) hand pitch, 0.11. The influences on felt head pitch and felt hand pitch are 14% and 16% of the VPEL setting, respectively. These changes in head and hand pitch suggest that changes in felt body orientation contribute to changes in VPEL. Taking into consideration the average head and hand pitches calculated in this experiment, an average VPEL-vs-pitch slope of 0.59 is directly accounted for by an uncompensated change in retinal orientation of the pitched visual environment. This pattern is consistent with the earlier view that the pitch-induced change in VPEL is a consequence of a linear weighted sum of inputs from the visual field and from a body-referenced mechanism that makes use of extraretinal information.