The kinematics of eating with a spoon: bringing the food to the mouth, or the mouth to the food?

In traditional reach-to-point and reach-to-grasp studies an increase in acc uracy demands typically results in a lengthening of the deceleration phase of the reach and a freezing of the more distal joints. The purpose of the p resent experiment was to examine whether similar changes in the reach kinem atics could be observed during a tool-using skill, as would be predicted fr om an effector independence perspective. Five subjects were required to eat two substances (i.e. a solid and a liquid one) that imposed different requ irements on the accuracy of the movement. The subjects transported the subs tances from the plate into the mouth. A prolonged movement duration was fou nd for the Liquid as compared to the solid substance. However, rather than being exclusively due to a lengthening of deceleration phase, the larger mo vement duration resulted from a slowing down of the whole movement. Therefo re, the skewed velocity profiles found in the traditional reach-to-grasp st udies may well be the result of the accuracy demands only impinging on the final part of the movement trajectory, rather than being a consequence of c entral, effector-independent, organising principles. In addition, under inc reased accuracy demands subjects were shown to redistribute their movement in a proximodistal direction. Movements of the distal components were reduc ed to a minimum and the involvement of trunk and head movement increased.