Brain Research, 222 (1981) 395-400 395 Elsevier/North-Holland Biomedical Press Interaction between forced grasping and a learned precision grip after ablation of the supplementary motor area ALLAN M. SMITH, DANIEL BOURBONNAIS and GILLES BLANCHETTE Centre de Recherche en Sciences Neurologiques, Ddpartement de Physiologie, Universitd de MontrOal, MontrOal, QuObee H3C 3J7 (Canada) (Accepted June 18th, 1981) Key words: forced grasping -- supplementary motor area -- learned motor response Monkeys were trained to compress and release a force transducer held between the thumb and forefinger. Immediately following ablation of the corltralateral supplementary motor area a grasp reflex produced a disturbance of the learned precision grip characterized by an increase in mean grasping force, an increase in the rate of prehensile force application and an inability to release the strain gauge. No change was observed in the same learned grip with the ipsilateral hand. Since the clinical observations of Adie and Critchley 1 in 1927 the generally transient appearance of a pathological grasp reflex has been associated with damage to the frontal lobes in monkeys and in man. In man, this 'forced grasping', to use the term employed by Adie and Critchley, is characterized by the fact that prehension is evoked by any tactile stimulation of the hand, whether self-initiated or not. Further- more, once grasping has occurred the patient is then unable to release the object by opening the hand. Rushworth and Denny-Brown 11 demonstrated that in monkeys after experimental lesions of almost the entire area 6, two components of force grasping could be distinguished. The first is a tactile response elicited by cutaneous stimulation of the palm and fingers, and the second is a proprioceptive phase in which stretching of the finger flexors can augment the grasping force. Local anesthesia injected into the palm to block all cutaneous sensation abolishes the appearance of forced grasping entirely. Penfield and Welch a0 suggested that forced grasping was a specific consequence of lesions restricted to the medial part of area 6 known as the supplementary motor area (SMA). Nevertheless, there have been several reports in which lesions limited to the SMA failed to produce forced grasping2,3,5, 7, and even in the frequently sited studies of Travis aS, forced grasping was not seen in all the animals with SMA lesions. In contrast, other studies in monkeys in which larger lesions included both the medial and the lateral parts of area 6 have reported consistent forced grasping when tested in the contralateral hand4,1~. Apart from the original observation by Adie and Critchley the self-initiation of forced grasping has only been studied by Goldberger 4 in monkeys with lesions of the