Employing a Straight Array of Parallel Identical Lines to Study the Effect of Visual Orientation on the Visual Perception of Egocentric Space

A series of 8 experiments in 2 studies examined the influence of a peripheral linear array of parallel identical lines presented in the fronto-parallel plane on the visual perception of egocentric space. Study 1, separated and varied the orientation of the array and the orientation of the lines in the array: "array orientation" is defined as the orientation of the implicit line that connects the centers of the individual lines of the array, and "line orientation" is employed in its usual meaning as the direction of an individual line. Psychophysical measurements of visually perceived eye level (VPEL) and visually perceived vertical (VPV) were made in three experiments. Subjects monocularly viewed in darkness two simultaneously-presented inducing arrays (68-long), centered at +/-25 horizontal eccentricity, of 25 parallel equilength lines that were either 1-long or 3-long. To obtain large influences, the two inducing stimuli were bilaterally-symmetrically-oriented for VPEL trials and were parallel for VPV trials (Matin & Li, 1994b). In Experiment 1, array orientation was varied over a +/-10 range around true vertical with line orientation fixed at either horizontal or vertical. In Experiment 2, array orientation was fixed at vertical and line orientation was varied over a +10 range around true vertical. In Experiment 3, line orientation and array orientation were equal and this common orientation was varied over a +/-10 range around true vertical. The results show that VPEL and VPV changed monotonically with array orientation, but line orientation had at most a small influence. Study 2, also separated and varied the length of the array and the length of the lines in the array: "Array length" is defined as the length of the implicit line that connects the centers of the individual lines of the array, and "Line length" is employed in its usual meaning as the length of an individual line. Psychophysical measurements of VPEL were made in four experiments. Subjects monocularly viewed in darkness two bilaterally-symmetric arrays that were centered at +/-25 horizontal eccentricity. Line orientation and array orientation were separately varied over a +/-10 range around true vertical. 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. The results show that VPEL changed monotonically with both line orientation and array orientation, and that line length and array length further modulated the influence of orientation in some cases. The two studies provide evidence for the contributions of line orientation and array orientation to the perceptions of VPEL and VPV. The pattern of influences of line and array orientations suggests contributions by first and second order orientation units and their combinations across large areas of the visual periphery.