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Benjamin R. Kunz, J. Scott Lauritzen, William B. Thompson, Sarah H. Creem-Regehr; Visual capture may influence body-based judgments of object extent. Journal of Vision 2009;9(8):717. doi: 10.1167/9.8.717.
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© ARVO (1962-2015); The Authors (2016-present)
The “mirror illusion” shows that when viewing one hand's reflection in a mirror while the other hand is hidden behind the mirror, the perceived location of the hidden hand will be strongly influenced by the visual information from the mirror reflection. Holmes, Crozier and Spence (2004) demonstrated the effect of this visual capture of hand position on a spatial localization task in which visual information influenced reaching movements towards a target when there was conflict between vision and proprioception. Our goal was to test whether visual capture with the mirror illusion would influence judgments about the extent of an object made with the hands. In this experiment, participants viewed their visible hand and its reflection in a mirror after the unseen hand was positioned at one of four locations on a tabletop. Because the visible hand was positioned fourteen cm from the mirror, the unseen hand appeared to be fourteen centimeters from the mirror. After viewing the visible hand and its reflection while simultaneously performing simple finger movements with both hands, participants viewed a block and had to move their unseen hand to a position that would allow them to grasp the block between their two hands. Movements of the unseen hand relative to the visible hand were biased by the visual information, reflecting errors in moved hand position that increased as the visual-proprioceptive conflict increased. Because this experiment aligned the object with the center of the body and required the movement of only one hand relative to the end of the object, the finding does not provide definitive evidence for an effect of visual capture on judgments of extent. Additional experiments are necessary which will dissociate object extent and egocentric location by displacing the object relative to the observer.
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