Contact-evoked changes in EMG activity during human grasp

2215 Cutaneous receptors in the digits discharge bursts of activity on cont act with an object during human grasp. In this study, we investigated the c ontribution of this sensory activity to the responses of muscles involved i n the task. Twelve subjects performed a standardized precision grasp task w ithout the aid of vision. Electro-myographic (EMG) responses in trials when the object was present were compared with those in which the object, and h ence the associated afferent responses, were unexpectedly absent. Significa nt differences in EMG amplitude occurred in the interval 50-100 ms after co ntact in all subjects and in 33/46 of the muscles sampled. The differences emerged as early as 34 ms after contact and comprised as much as a fourfold change in EMG from 50 to 100 ms after contact with the object. Typically, EMG responses were larger when the object was present (OP), though there we re cases, particularly in the thenar muscles, in which the responses increa sed when the object was absent (OA). Local anesthesia of the thumb and inde x finger attenuated contact-evoked EMG activity in at least one muscle in a ll four subjects tested. In one subject, contact-evoked responses were abol ished completely during the anesthesia in all four muscles sampled. The res ults indicate that the sensory activity signaling contact plays a key role in regulating EMG activity during human grasp. Much of this feedback action is attributable to cutaneous receptors in the digits and probably involves both spinal and supraspinal pathways.