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Daniel S McConnell, Christopher Vallee, Michelle Munevar, Roumayne Lee; Thresholds for detecting a difference between seen and felt position of the hand. Journal of Vision 2003;3(9):392. doi: 10.1167/3.9.392.
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Information about hand location is required to guide a reach to a target. Vision and proprioception are both understood to provide information about hand position. Information from these two modalities must be integrated, and the relative weighting of these two sources of information varies across many experimental conditions and tasks. Van Beers, Wolpert, and Haggard (2002) reported data consistent with the idea that these weightings coincide with the relative reliability of each source of information. Specifically, vision is more reliable at discerning differences in direction as opposed to distance, and proprioception is more reliable at discerning differences in distance as opposed to direction. In the current study, we examined this hypothesis by measuring thresholds for detection of a dissociation between the seen and felt positions of the fingertip. Two observers viewed a stereoscopically displayed virtual fingertip represented as a 1cm-diameter sphere in a virtual environment using a head-mounted display (Virtual Research V8). Finger position was monitored using an electromagnetic motion tracker (Ascension Technologies Flock of Birds). The location of the finger was passively located at any of nine locations corresponding to a 3x3 grid on a tabletop. The visually specified location of the fingertip varied randomly at either 2, 3, 4, 5 or 6cm to the left, right, front or back of the actual (felt) fingertip location, plus a twenty-first condition in which there was no dissociation between seen and felt position of the fingertip. The overall threshold for detecting the dissociation was approximately 5cm. Furthermore, thresholds were lower when the perturbation was to the left or right, as compared to in depth. Also, perturbations were lower at the nearest distances, and along the body's midline.
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