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Charles E. Sloane, Paul R. Schrater, Erik J. Schlicht; Reach planning and accuracy depend on task difficulty.. Journal of Vision 2004;4(8):405. doi: https://doi.org/10.1167/4.8.405.
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To successfully reach to an object, the object's physical attributes (e.g. mass, shape, orientation, friction) must be visually analyzed to plan a reach path that will put the fingers in contact with the object with appropriate placement and forces. However, the brain may not need precise values of the object' s attributes to make successful reaches. Rather, some attributes may be ignored to simplify the reach planning process. This experiment tested how reaching endpoint accuracy in different directions in space is affected by manipulations of the mass and orientation of objects. Six undergraduate students (four female and two male) with normal or corrected vision repeatedly reached for and picked up cylindrical objects of varying mass. Objects were presented at different orientations, and during the reach the positions and orientations of their thumb and forefinger were recorded. Results show that reach paths systematically varied with object mass and orientation in order to satisfy the minimal requirements needed to make a successful reach. In particular, more precise reach plans resulted when the object was more difficult to pick up with two fingers, (e.g. when the cylinder main axis was perpendicular to the direction of gravity). This finding suggests that people do not have uniform reach plans; they attempt to change their reach in a manner that maintains a high probability of task success and minimizes cost. In addition, these results implicitly show a differential use of visual information during visuomotor control, which may have consequences for perceptual-motor integration.
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