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Robert Volcic, Fulvio Domini; Adaptation of reach-to-point movements changes reach-to-grasp actions. Journal of Vision 2013;13(9):338. doi: 10.1167/13.9.338.
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© ARVO (1962-2015); The Authors (2016-present)
Positioning the fingers on the opposite sides of an object is essential for a stable grasp. Most commonly, the thumb contacts the object on the visible part, whereas the other fingers enclose it on the occluded part. Given this functional asymmetry, we hypothesized that the different fingers play different roles in guiding the hand during reach-to-grasp actions. In the present study, we investigated if and how visuomotor adaptation of reach-to-point movements performed with the index finger or the thumb impacts grasping. Subjects were asked to reach for and grasp virtual objects (precision grip along the sagittal axis) without seeing their hand. Objects were random-dot elliptic cylinders with varying relative depth rendered in stereo and presented at different viewing distances with consistent vergence and accommodative information. Virtual instead of real objects were used to avoid proprioceptive/tactile feedback. The grasping trials block was always preceded by a block of reach-to-point movements towards a virtual rod positioned at a random viewing distance. Two factors were varied: (a) pointing finger ͨ2; subjects pointed with either the index finger or the thumb; (b) visuomotor congruence ͨ2; the visual feedback about the finger position either corresponded to the actual finger tip position (Congruent) or it was displaced farther in depth (Incongruent). Grasping behavior was measured after each of these four conditions. We observed a general transfer of visuomotor adaptation from pointing to grasping movements. The grasping hand was positioned at a closer distance after the incongruent than after the congruent visuomotor exposure. However, we found differential effects after index and thumb reach-to-point movements: visuomotor adaptation of the thumb affected grasping actions more strongly. These results provide evidence for a special role of the thumb in guiding the hand during reach-to-grasp actions and have implications on models for the control of reaching and grasping.
Meeting abstract presented at VSS 2013
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