TACTILE INPUT OF THE HAND AND THE CONTROL OF REACHING TO GRASP MOVEMENTS

The role of tactile information of the hand in the control of reaching to grasp movements was investigated. The kinematics of both reaching (or transport) and grasp components were studied in healthy subjects i n two experimental conditions, In one condition (control condition) su bjects were required to reach and grasp an object that could have two sizes and that could be located at two distances from the viewer. In t he other condition (anaesthesia condition) the same movements were exe cuted, but anaesthesia was provided to the subjects' fingertips. In bo th conditions vision of the hand was prevented during movement, Anaest hesia affected mainly the kinematics of the first phase of grasping, t hat is, the finger-opening phase. This phase was lengthened and maxima l finger aperture increased. In contrast, the duration of thr successi ve phase (finger-closure) was poorly modified. The reaching component was also impaired by anaesthesia. Although the total extent of hand pa th and the spatial relations between the finger aperture and closure p hases did not change between the two conditional hand path variability increased. This occurred during transport deceleration phase and afte r the increase in variability of finger path. In addition, the whole m ovement was slowed down. The results; of the present experiment sugges t that tactile signals at tile beginning and at the end of movement ca n be used to compute grasp time and to optimise grasp temporal paramet ers. Alternatively, signals from tactile receptors can be involved in encoding the position sense of the fingers. When this input is lacking , the control of grasp and in particular that of finger-opening phase call be impaired, Finally, the effect of the grasp impairment on the r eaching component supports the notion that the coordination between re aching and grasping involves the whole temporal course of the two comp onents.