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Wenxun Li, Ethel Matin, Jeremiah W. Bertz, Leonard Matin; A tilted frame deceives the eye and the hand. Journal of Vision 2008;8(16):18. doi: 10.1167/8.16.18.
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A roll-tilted visual frame induced a vertical line to appear roll-tilted in the opposite direction (rod-and-frame illusion). This visual illusion was measured by finding the physical roll-tilt of the line that appeared vertical—visually perceived vertical (VPV). In separate measurements, the roll-tilted visual frame was also found to induce an illusion in the felt orientation of the hand. This manual illusion was measured by setting the orientation of the hand to feel vertical in the presence of the inducing frame (manually perceived vertical—MPV), with distance of the hand from the body as a parameter. The manual illusion was large when the hand/arm was fully extended (60 cm from the body). When the hand was close to the body (in the midfrontal plane), there was no manual illusion. In a third set of measurements, subjects matched the roll-tilt of the hand to a line set by the experimenter to the subject's VPV or to VPV ± 5°. These manual/visual matches (MVM) were accurate at 60 cm—the distance at which the manual illusion was greatest. In effect, the large manual illusion at this distance tended to compensate for the visual illusion when the subject matched the orientation of the hand to the VPV. However, when the hand was close to the body, where there was no manual illusion, the manual/visual match was poor (no manual illusion to compensate for the visual illusion). The results are discussed in terms of a proximal/distal model that employs a linear weighted sum of influences from body-referenced and visual mechanisms. The model accounts for 61% of the MVM in terms of the inducer's effect on MPV. Considered in isolation, the gross VPV errors in conjunction with the accurate MVM settings with the extended arm might appear to support a theory based on perception/action dissociation. However, that theory cannot explain the dramatic effects of hand-to-body distance. The present results are very similar to those previously reported for the dimension of elevation where hand-to-body distance dependence was also found to modulate visually guided manual behavior.
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