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Wenxun Li, Leonard Matin; Spatial induction, laterality, and homogeneity of perceived space. Journal of Vision 2008;8(6):458. doi: 10.1167/8.6.458.
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A pitched-from-vertical line in darkness systematically influences both the elevation of Visually Perceived Eye Level (VPEL) and the orientation of Visually Perceived Erect in the pitch dimension (VPE) of a visual target located in an observer's median plane. However, the influence of the inducing line on VPE for the test line does not cross the midline. Thus, while VPE increases linearly with variation of the inducer's pitch when both inducer and test line are on the same side of the midline, VPE is invariant when inducer and test line are on opposite sides of the midline. The present study investigates the possible existence of this homolateral restriction on two tasks: VPEL and Visually Perceived Vertical in the roll-tilt dimension (VPV). In otherwise complete darkness, one 77°-long luminous inducing line was pitched at either +/−5°, +/−15°, or +/−25°. The inducer and test target were located either left or right of S's median plane with horizontal eccentricity 25° and 8.3°, respectively. With a fixation point in S's median plane, and viewing distance 1m, six monocularly-viewing Ss set either a 5°-long test line to VPV or a dim 20 minarc target to VPEL. No homolateral restriction was found for either VPV or VPEL: Although both VPV and VPEL settings changed systematically with the pitch of the inducer whether inducer and test target were on the same side of the midline or on opposite sides, the VPV-vs-pitch function reversed with the inducer on the left as compared to its being on the right side of the median plane, whereas VPEL increased with increasing topforwardness in both cases. These results are consistent with earlier findings regarding influence of line orientation on VPV and VPEL, and also show that perceptual space is homogeneous for induced changes in verticality (VPV) and in elevation (VPEL).
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