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Joe Anderson, Geoffrey Bingham; Evidence for the use of a binocular Tau-dot strategy in visually guided reaching. Journal of Vision 2007;7(9):158. doi: https://doi.org/10.1167/7.9.158.
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In this work, we propose a novel control strategy for the visual guidance of targeted reaching based on the use of binocular disparity information. The disparity information that we propose is different from that previously proposed (Gray & Regan, 2004). We claim that individuals with binocular vision can use a disparity-nulling strategy combined with information about the rate of change of disparity to control velocity changes of a reach and bring the hand to the target. Although disparity-nulling strategies for the control of visually-guided reaching have been proposed before (Bingham et al, 2001); none of these proposals included a control mechanism for the timing of the movement. The accurate control of movement timing via control over velocity is paramount for successful human action. In this model, velocity can be directly controlled using a purely optical, binocular tau-dot strategy. Using this strategy, an individual need only hold the value of disparity defined tau-dot at ∼−.5 in order to achieve soft contact of the hand at the target. We isolated the use of binocular disparity in a manually controlled distance matching task to test this hypothesis. Eight participants were asked to match in the dark the distance of two point-lights in depth by reaching and moving one light along a track to the distance of a second, target light. The trajectories of these reaching movements were recorded using a Mini-bird motion measurement system. Disparity tau-dot values were computed using three methods. Tau-dot values were computed by analyzing the viewing geometry and kinematics, by taking the derivative of tau, and using the slope of a line fit to tau values. All three methods produced equivalent measures of tau-dot. Results show that participants use a constant tau-dot strategy beginning just before the decelerative portion of the reach and ending just before target contact.
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