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Mark Mon-Williams, Geoff P Bingham; Calibration of grasp orientation (and ‘wiggle-room’ for errors in object orientation perception). Journal of Vision 2008;8(6):301. doi: 10.1167/8.6.301.
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Background. Prehension movements are controlled on the basis of visual information regarding an object's distance, size and orientation. Accuracy is maintained through calibration processes that rely on visual and/or haptic information. It is known that reach distance and grasp magnitude can each be calibrated (Mon-Williams, Coats & Bingham, 2004; Mon-Williams & Bingham, 2007) but calibration of grasp orientation has not been explored. Methods. We manipulated the felt orientation of a visually constant object to explore the role of haptic information in the calibration of grasp orientation. Participants reached-to-grasp a visible object using a pincer grip. Eighteen participants performed ten ‘virtual’ reaches before and after an adaptation stage (apparatus details in Mon-Williams & Bingham 2007). We monitored upper limb position using Optotrak markers attached to the index and thumb tip, index knuckle, inside and outside of wrist, elbow, shoulders and trunk. In the adaptation phase, three groups (n=6 per group) were subjected to distorted haptic feedback incrementally over fifty reaches (total perturbation=45°): the object's ‘pitch’ was perturbed for Group 1; ‘yaw’ for Group 2; ‘roll’ for Group 3. Results. All participants successfully grasped the object across all trials in the adaptation phase. Nonetheless, the gain of the alteration in hand orientation as a function of object orientation was low (circa 25%). To grasp the objects without altering hand orientation, participants used: (i) the full extent of their middle and distal phalanges; and (ii) deformation of the thumb and index finger's fleshy pads. Small (gain circa 33%) but reliable changes in the appropriate direction were found in the ‘virtual’ reaches following adaptation. Conclusions. Grasp orientation is subject to calibration, but with low gain. Resulting inaccuracies in object orientation perception will not lead to prehension failure: humans exploit the redundant degrees of freedom of the hand to minimise consequences of perceptual errors.
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