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Boris M Sheliga, Frederick A Miles; Perceived slant influences vergence responses during horizontal gaze shifts across a surface. Journal of Vision 2003;3(9):699. doi: 10.1167/3.9.699.
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A flat surface in the frontal plane appears slanted about a vertical axis when the image in one eye is vertically compressed/expanded relative to the image in the other eye (induced effect). Such changes in the vertical magnification of one image also reduce or enhance the perceived slants induced by horizontal compression/expansion of the other image (geometric effect). We asked if these changes in perceived slant caused by changes in vertical magnification influence the vergence eye movements associated with horizontal transfers of gaze between targets located on a surface plane. We recorded the horizontal eye movements of 4 human subjects (Ss) viewing a random dot pattern on a tangent screen in a dichoptic viewing arrangement. Ss shifted their gaze horizontally across the pattern, which was compressed horizontally at one eye by 3-6%. The patterns were extinguished as soon as the Ss initiated the gaze shift (open-loop responses). In half the trials, prior to the gaze shift, Ss were additionally required to vertically compress/expand the pattern at the other eye until the binocular image appeared to be fronto-parallel (“nulling”). We found that “nulling” reduced the vergence accompanying the horizontal transfers of gaze, on average, by 37% (SD ±8%, p<0.001). This reduced vergence was not simply due to degradation of the binocular depth signal(s) by the sometimes considerable vertical disparities: In a separate experiment, we showed that if these same vertical compressions/expansions were applied but with the opposite sign (“anti-nulling”), then gaze shifts were accompanied by an increase in vergence (21±6%, p<0.001). We conclude that perceived slant can influence the planned changes in vergence that accompany gaze shifts across a surface.
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