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Celia Litovsky, Feitong Yang, Zheng Ma, Jonathan Flombaum, Michael McCloskey; Visually guided unimanual and bimanual reaching rely on different cognitive mechanisms: Evidence from optic ataxia. Journal of Vision 2018;18(10):69. doi: https://doi.org/10.1167/18.10.69.
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Although most visually guided actions require coordination of both limbs, the vast majority of research on reaching focuses on reaching with a single hand. In order to investigate whether unimanual (single-hand) and bimanual (two-hand) reaching rely on different cognitive mechanisms, we tested optic ataxic patient MDK on a series of tasks of unimanual and bimanual reaching to peripheral visual targets. Targets were presented in MDK's visual periphery and he immediately pointed either to the left side of each target with the left hand, the right side of each target with the right hand, or to both sides of each target with both hands simultaneously. If bimanual reaching involves simply performing two unimanual reaches simultaneously, we would expect no difference in performance on bimanual and unimanual reaching. In fact, however, we observed significantly better performance in bimanual than in unimanual reaching. In unimanual reaching MDK's points with either hand typically undershot the target by more than half its eccentricity (p < 0.001); that is, he systematically pointed far too close to fixation. However, MDK's bimanual points to these same targets showed significantly less undershooting (p < 0.001). The dissociation between MDK's unimanual and bimanual reaching behavior indicates that unimanual and bimanual pointing may involve different cognitive mechanisms. We suggest that bimanual, but not unimanual, pointing involves using the distance between two targets as a basis for determining the distance between the pointing hands. MDK's relatively accurate representation of the distance between two peripheral targets allows him to point more accurately (i.e., with less fixation-bias) for bimanual compared to unimanual reaching. This dissociation between unimanual and bimanual reaching may therefore arise because of additional computation of the distance between two targets during bimanual, but not unimanual, reaching.
Meeting abstract presented at VSS 2018
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