The Effect of Environmental Pitch on Perceived Optic Slant and Eye Level: Lines vs Dots

Abstract

Visually perceived eye level (VPEL) has been shown to be strongly affected by the pitch of the visible environment (Stoper and Cohen, 1989 Perception & Psychophysics46 469 – 475), even if this environment consists of only two luminous lines pitched from the vertical (Matin and Li, 1992 Journal of Experimental Psychology: Human Perception and Performance18 257 – 289). Here, two luminous vertical lines or 32 randomly distributed luminous dots were mounted on a plane that was viewed monocularly and was pitched (slanted in the pitch dimension) 30° forward or backward from the vertical. In addition to measuring the VPEL, we measured the perceived optic slant (rather than the perceived geographic slant) of this plane by requiring each of our ten subjects to set a target to the visually perceived near point (VPNP) of the plane. We found that, for the lines, VPNP shifted 50% and VPEL shifted 26% of the physical pitch of the plane. For the dots, VPNP shifted 28% but VPEL shifted only 8%. The effect of the dots on VPEL was weaker than might have been predicted by their effect on VPNP, which was used to indicate perceived optic slant. The weakness of the effect of the dots on VPEL implies that changes in VPEL result from a direct effect of the stimuli on VPEL, rather than one mediated by the perceived optic slant of the plane. The non-zero effect of the dots shows that pitched from vertical line segments are not necessary to shift VPEL.