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Wenxun Li, Leonard Matin; The elevation of Visually Perceived Eye Level (VPEL) is an oscillatory function of visual pitch. Journal of Vision 2007;7(9):200. doi: 10.1167/7.9.200.
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In our laboratory, a pitched-from-vertical visual field, either fully-structured and well-illuminated or consisting of only two pitched-from-vertical lines in darkness, exerts a large influence on the elevation of a target's setting to appear at visually perceived eye level (VPEL). The influence is nearlinearly related to the orientation of the visual field over the +−30° pitch range; VPEL lies above or below true eye level with the field pitched topforward (+) or topbackward (−), respectively. In naturalistic environments, however, VPEL has been reported to assimilate to the ground surface between +−7° downhill and uphill (O'shea & Ross). Since uphill corresponds to topbackward pitch, this appears opposite to the laboratory work. Here we extend the inducer's pitch range to +−120° in 10° steps around a horizontal axis at true eye level in the frontoparallel plane 63 cm from the subject's eye. The inducer consists of two parallel 104 cm x 0.7 cm lines (phosphorescent tape) at a radial distance 37 cm from the pitch axis. The elevation of the VPEL target (circular, red, 10′ dia) was presented in an erect plane 100 cm from the monocularly viewing subject, the same plane containing the inducing lines when erect (0°) and at +−25° horizontal eccentricity. We discover that the nearlinear function previously measured over the narrower pitch range is a portion of an oscillatory function over 360° with a fundamental wavelength approximately of 90° in the pitch dimension (4 cycles/360° of pitch). Over the nearlinear range (+−30°) VPEL increases from −11° (11° below true eye level) at −30° pitch to +6° at +30° pitch; a smaller second VPEL peak (+4°) occurs at −60° pitch and a second minimum (-4°) at +60° pitch. The extended function is reasonably represented as a superposition of two sinusoids. Both sets of previous results are consistent with the oscillatory function.
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