Thumb invariance during prehension movement: effects of object orientation

The aim of the present study was to examine the contribution of the thumb a nd index finger during the task of reaching to grasp a cylinder positioned at different orientations. To this end an axis was defined between a marker positioned on the subjects' wrist and the target. For each frame the perpe ndicular distances of the thumb and index Finger from this axis were determ ined. The perpendicular distance was greater for the index finger than the thumb, confirming a relative stability of the thumb during natural prehensi on and supporting the notion of the thumb as a guide for the transport comp onent of reaching. Further, index finger perpendicular distance was varied according to object orientation. When the object was positioned at an angle that requires hand pronation, the perpendicular distance for the index fin ger was the greatest. It is concluded that changes in the index finger dist ance are necessary to allow the thumb to maintain stability in order to pro vide appropriate movement guidance. NeuroReport 12:2185-2187 (C) 2001 Lippi ncott Williams & Wilkins.