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Jonathan Diamond, Luc Tremblay; Visual-haptic disparity of target will modulate action-guidance strategy. Journal of Vision 2009;9(8):1112. doi: https://doi.org/10.1167/9.8.1112.
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The contribution of vision to action guidance is flexible. For example, Wolpert (2007) has demonstrated that humans integrate sensory information as a function of sensory reliability. Modalities that are more reliable are used more extensively to make a perceptual judgment than those that are less reliable (e.g. Ernst & Banks, 2002). The present study was performed to test how different visual-haptic mappings influenced grasping actions. We presented a Müller-Lyer figure (MLF) on a half-silvered mirror, such that the image mapped onto a physical object located below the mirror. This set-up gave participants the feeling that they were physically grasping the shaft of the MLF. The physical object did not always match the size of the image, which created a visual-haptic disparity. Participants performed two counterbalanced blocks of trials. In block A, the physical object was adjusted opposite to the size of the illusory percept (constant disparity block) and in block B, the visual-haptic mapping was random (variable disparity block). Participants were instructed to reach out and grasp the shaft of the MLF (kinematic variables were derived using an Optotrak Certus). It was hypothesized that grip scaling would be proportional to the size of the physical object in the constant disparity block, but would reflect the size of the illusory percept in the variable disparity block. Contrary to our hypothesis, participants' grasping behaviors were influenced by the illusion regardless of the mapping condition. That is, peak grip aperture (pGA) was smaller for the fins-in compared to the fins-out MLF configuration. Additionally, pGA decreased over the first several trials. This adaptation in pGA required fewer trials in the constant disparity condition. Thus, haptic feedback uncertainty in the variable block delayed pGA adaptations. These results suggest that visual guidance strategies are altered by inter-modal mapping certainty.
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