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Adam Shavit, Wenxun Li, Leonard Matin; The influences of array orientation and of line orientation on visually perceived eye level (VPEL) are modulated by line length and array length. Journal of Vision 2008;8(6):457. doi: https://doi.org/10.1167/8.6.457.
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The elevation of visually perceived eye level (VPEL) is influenced by linear arrays of parallel equal-length straight lines (Shavit, et al., 2005). We examined this influence by varying the line orientation and the independently-manipulable array orientation. “Array orientation” is defined as the orientation of the implicit line that connects the centers of the individual lines of the array; “line orientation” is employed in its usual meaning as the direction of an individual line. Psychophysical measurements of VPEL were made in four experiments employing two otherwise-identical bilaterally-symmetric arrays that were centered at ±25°; horizontal eccentricity, and were presented together in a frontoparallel plane to the right eye of the observer. Line orientation and array orientation were separately varied, employing orientation values equal to −10°; (ccw), −5°;, 0°;, +5°;, or +10°; (cw); line lengths were 3°;, 9°;, or 15°;. Spacing between the centers of the individual lines of an array was always 3°;. Array lengths were 6°;, 12°; or 18°; with variation produced by changing the number of lines in the array. VPEL changed monotonically with both line orientation and array orientation. Off-vertical orientations (line or array) shifted VPEL away from true eye level (TEL); vertical orientations (line or array) shifted VPEL toward TEL. Line length and array length modulated the influence of orientation: The influence of line orientation increased with line length; the influence of array orientation increased with array length. The influences of array orientation and line orientation were found to summate. In summary, the net influence of an array of parallel lines on the elevation of VPEL depends on the orientation and length of the array as well as on the orientation and length of the individual lines.
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