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Leonard Matin, Wenxun Li, Ethel Matin; Actions do not escape the influence of visual illusions — even when manual behavior is accurate. Journal of Vision 2009;9(8):843. doi: 10.1167/9.8.843.
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© ARVO (1962-2015); The Authors (2016-present)
Accurate manual behavior in response to stimuli that produce errors in visual perception has often been reported. Such findings have invariably been interpreted as implying that the visual inducer has no influence on the manual behavior. We show that this assumption is incorrect in two experiments involving large egocentric visual illusions in which hand-to-body distance was systematically varied and manual behavior of the unseen hand was measured. In the first experiment, systematic changes in the physical elevation visually perceived as eye level (VPEL) were induced by changes in the pitch of a single long line and measured by the elevation of the subject's setting of a small visual target. In the second experiment, systematic changes in the frontal-plane orientation visually perceived as vertical (VPV) were induced by changes in the roll-tilt of a 2-line visual inducer and measured by the subject's setting of a short line to appear erect. For both experiments, the results showed 3 sorts of systematic effects: influences of visual inducers on the egocentric location of targets (conventional visual illusions), influences of visual inducers on the felt position of the unseen hand/arm in the absence of a visual target (manual illusions), and influences of visual inducers on manual/visual matches. The manual illusions are absent when the hand is close to the body but grow linearly with hand-to-body distance. The manual/visual matches also change linearly with hand-to-body distance. However, the matches are grossly erroneous when the hand is close to the body although the matches are veridical with the arm fully extended (the distance for which the manual illusion is greatest!). In effect, the manual illusion with the fully extended arm tended to compensate for the visual illusion. Our recently-proposed Proximal/Distal Model (VisRes2005; JOV2008) explains the apparent paradox and quantitatively fits all of the experimental results.
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