Human Systems Integration Division @ NASA Ames

Visually perceived eye level (VPEL) has been shown to be strongly affected by the pitch of the visible environment (Stoper and Cohen, 1989, Perception and Psychophysics, 46:469-475 ), even if this environment consists only of two luminous lines pitched from the vertical ( Matin and Li, 1992, JEP:HP&P, 18:257-289). 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 deg. forward or backward from the vertical. In addition to measuring VPEL, we measured the perceived optic slant (rather than the perceived geographic slant) of this plane by requiring each of our 10 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 this effect with the dots implies that changes in VPEL result from a direct effect of stimuli, rather than one mediated by the perceived pitch of the plane. The non-zero effect of the dots shows that pitched from vertical line segments are not necessary to shift VPEL.