Grip force dynamics in the approach to a collision

This experiment investigated the prediction of load force (LF) in impulsive collisions inferred from anticipatory adjustments of grip force (GF) used to stabilise a hand-held object. Subjects used a precision grip to hold the object between thumb and index finger of their right hand and used the arm either: (1) to move the object to produce a collision by hitting the lower end of a pendulum, causing it to swing to one of three target angles, or ( 2) to hold the object still while receiving a collision produced by the exp erimenter releasing the pendulum from one of three angles. Visual feedback of the pendulum's trajectory was available in the production task only. In all conditions, subjects increased GF in advance of the collision. In recei ving the collision without advance information, subjects set GF levels to t he mid-range of the experienced forces. When subjects possessed knowledge a bout the maximum angle of pendulum swing - either because they were going t o produce it or because they were verbally informed - magnitude of the anti cipatory-GF magnitude response was scaled to the predicted LF magnitude. Fu rthermore, GF was scaled to LF with a higher gain when producing compared t o receiving the collision. This suggests that updating forward models throu gh a semantic route is not as powerful as when the updating is achieved thr ough the more direct route of dynamic exploration.