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Simon K. Rushton; Perception of egocentric direction: retinal and extra-retinal influences. Journal of Vision 2002;2(7):714. doi: https://doi.org/10.1167/2.7.714.
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© ARVO (1962-2015); The Authors (2016-present)
Perception of egocentric direction, that is the direction of an object relative to the body, is critical for visual guidance of action. What information is used to judge the direction of an object? Classically it is assumed that extra-retinal information about the orientation of the head on the shoulders, and the eye in the head, is combined with retinal object location to transform from a retinal to trunk-centric coordinate frame. However, theoretical analysis indicates that it should be possible to pick up head-centric direction directly from retinal or optic information, or that retinal information could be used to determine eye-orientation.
I report a series of experiments that bear on this matter. Observers performed a body-alignment procedure to indicate the perceived egocentric direction of an object (they turn to face a target object so that if they began walking they would end up colliding with it). Perceived egocentric direction was perturbed with displacing prisms. It is found that in a laboratory setting, prisms have less of an effect (in line with Rock et al's report of ‘immediate adaptation’) than would be expected from their optical displacement and the consequent error in extra-retinal eye-orientation signal. Typically, the effect is between 60% and 65% of that expected.
This finding is robust and holds when observers align themselves with point-lights in an otherwise dark room, or view alignment targets monocularly. In these two case potentially useful information contained in the field of vertical and horizontal disparities is absent. However, a particularly striking finding is that approximately 95% of the expected effect of the prism is observed when the experiment is performed out of doors, in an open field, with distant targets.
The consequence of these results for understanding of the perception of egocentric direction, and the interesting case of visual guidance of locomotion, will be discussed.
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