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W Li, L Matin; Separate and Combined Influences on Visually Perceived Eye Level of Visual Pitch and Height-in-the-Field of a 2-Line Stimulus . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4792.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The influence on visually perceived eye level (VPEL) of a pitched visual field consisting of only two parallel, pitched-from-vertical lines is larger if the height of the 2-line stimulus is systematically changed along with systematic variation of pitch than if stimulus height remains constant (Vision Research, 94; Perception & Psychophysics, 95). The present experiments investigate the separate effects of stimulus height and pitch on VPEL, and determine the rules of combination between them. Methods: VPELs were measured for erect Ss monocularly viewing two parallel, simultaneously-presented, pitched-from-vertical lines on opposite sides of the visual field (25º horizontal eccentricity) at each of 7 pitches (-30º to +30º) in otherwise total darkness. Expt. 1: Long Lines (65º): The horizontal pitch axis was at one of 3 heights through the midpoint of the 2-line stimulus: 22º below, 22º above, or at the observer's true eye level. Expt. 2: Short Lines (12º): The horizontal pitch axis was at one of 5 heights: 26º below, 13º below, 13ºabove, 26º above, or at the observer's true eye level. Results: The slope of the VPEL-vs-pitch function equaled approximately 0.34 and 0.20 for the long and short line cases, respectively, and each was essentially unchanged by the change in stimulus elevation. However, the variation of elevation of the 2-line stimulus produced a linear elevation of the bias of the VPEL-vs-pitch function with a total change in y-intercept of about 4.5º consequent on the 44º change in elevation of the axis of the pitched stimulus for the long-line case, and a total change of 6.7ºconsequent on the 52ºchange in elevation of the pitch axis for the short-line case. These changes in y-intercept of the VPEL-vs-pitch function were 10.2% and 12.9% of the change in elevation of the pitch axis for the long and short line conditions, respectively. Conclusions: While visual pitch is the major aspect of the visual influence on VPEL, the height of the stimulus in the visual field also plays a separate and important role (∼12%) in the egocentric perception of eye level: The influences of pitch and height-in the visual-field are linearly additive. These results account fully for the larger slope previously obtained with the pitchroom where the height change is correlated with the change in pitch than is measured with a pitched surface for which the pitch axis is maintained at a fixed height. CR: None. Support: NIH grant EY10534 and AFOSR grant F49620-94-1-0397.
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