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Urs J. Kleinholdermann, Eli Brenner, Volker H. Franz, Jeroen B. J. Smeets; Grasping trapezoidal objects. Journal of Vision 2006;6(6):396. https://doi.org/10.1167/6.6.396.
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The most simple view of grasping with a precision grip assumes that humans close index finger and thumb in opposite directions around the object. For circular and rectangular objects, this will result in a perpendicular approach of each digit to the surface. Thus they move in the right direction for the application of grip force after contact. Grasping trapezoidal objects at their non-parallel surfaces in this way will result in a non-perpendicular approach to the surface. However, Smeets & Brenner (1999) assume that humans try to approach an object's surface perpendicularly in order to deal with spatial inaccuracies. We used trapezoidal objects to discriminate between these views. Twenty-three participants grasped trapezoids with angles between −20 and +20 degree in steps of 5 degree.We found a tendency to approach the objects' surfaces orthogonally. This suggests that during the grasping movement participants aimed for an optimal placement accuracy of the digits rather than for an optimal direction of grip force after contact.
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