Averaging and summation of influences on visually perceived eye level between two long lines differing in pitch or roll-tilt

The presence of one or two long, dim, eccentrically-placed, parallel, pitched-from-vertical lines in darkness generates a systematic influence on the physical elevation that appears to correspond to eye level (VPEL). The influence of the line(s) in darkness is nearly as large as that produced by a complexly-structured, well-illuminated visual field (Matin L, Li W. Vis Res, 1994;34:311-330); oblique lines in a frontoparallel plane that strike the same projected orientations generate the same influences as those generated by pitched-from-vertical lines (Li W, Matin L. Perception, 1996;25:831-852). The two experiments described here examined the influence on the physical elevation of VPEL due to simultaneous viewing of two long lines of different pitch (Experiment 1) or two long lines of different obliquity in a frontoparallel plane (Experiment 2). Experiment 1 employed two long (66 degrees), simultaneously-presented, pitched-from-vertical lines in darkness on bilaterally symmetric locations at 25 degrees horizontal eccentricity, with each line at one of seven pitches in the range from -30 degrees to +30 degrees; VPELs were measured for all 49 possible pitch combinations. Experiment 2 was identically constructed, but employed oblique 2-line stimuli from a frontoparallel plane that struck the same projected orientations as did the pitched-from-vertical lines in Experiment 1. VPELs measured on four subjects in the two experiments were indistinguishable for corresponding conditions of pitch and obliquity. For a given pitch (obliquity) of one of the lines the elevation of VPEL increased linearly with the pitch (obliquity) of the second line. The VPEL for any 2-line combination is very close to the average of the VPELs for the two individual lines; a small amount of additive summation between the influences of the two lines was also found. Parallel and nonparallel 2-line stimuli appear to follow the same rules of combination. The results are clear in showing that the visual influence on VPEL is controlled by an opponent-process mechanism.