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Leonard Matin, Wenxun Li; Gaze direction and extraretinal eye position information; Retinal orientation and eccentricity of a 1-line inducer: Separate and combined influences on visually perceived eye level (VPEL). Journal of Vision 2003;3(9):225. doi: 10.1167/3.9.225.
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Although a horizontal shift of a visual field by x deg puts the fovea at the same point as does an x deg change of horizontal gaze direction, the change in retinal orientation of an eccentrically-located line is differently affected by the two operations, significantly complicating the analysis of the extraretinal eye position signal (EEPI) for space perception (Vis. Res. 2001): Here, 3 experiments were done to disentangle the influences of EEPI and of a line's retinal orientation on the perception of elevation as measured by the visual perception of eye level (VPEL): Using the fact that the pitch of a single line in darkness induces large changes in the setting to VPEL of the elevation of a small circular target, horizontal eccentricity of the inducing line was systematically varied either alone or in conjunction with horizontal gaze direction in three different ways: Change in horizontal retinal eccentricity (from 5 deg to 25 deg) of a variably-pitched inducing line produced by shifting the horizontal eccentricity of the inducing stimulus line resulted in a 3-fold increase in the slope of the VPEL-vs-pitch function. The changes in the VPEL-vs-pitch function are much smaller when the same changes in retinal eccentricity are produced by changing horizontal gaze direction relative to the median plane along with the normally accompanying retinal orientation variation, but without changing stimulus line eccentricity relative to the median plane. However, when gaze direction is maintained in the median plane and those same retinal orientation changes are combined with changes in stimulus line eccentricity, the results are very close to those produced by changes in eccentricity of the stimulus line alone. We are able to conclude that about 3/4 of the eye position change is compensated by EEPI.
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