Cortical mechanism for the visual guidance of hand grasping movements in the monkey - A reversible inactivation study

Picking up an object requires two basic motor operations: reaching and gras ping. Neurophysiological studies in monkeys have suggested that the visuomo tor transformations necessary for these two operations are carried out by s eparate parietofrontal circuits and that, for grasping, a key role is playe d by a specific sector of the ventral premotor cortex: area F5, The aim of the present study was to test the validity of this hypothesis by reversibly inactivating area F5 in monkeys trained to grasp objects of different shap e, size and orientation. In separate sessions, the hand field of the primar y motor cortex (area F1 or area 4) was also reversibly inactivated. The res ults showed that after inactivation of area F5 buried in the bank of the ar cuate sulcus (the F5 sector where visuomotor neurones responding to object presentation are located), the hand shaping preceding grasping was markedly impaired and the hand posture was not appropriate for the object size and shape. The monkeys were eventually able to grasp the objects, but only afte r a series of corrections made under tactile control. With small inactivati ons the deficits concerned the contralesional hand, with larger inactivatio ns the ipsilateral hand as well. In addition, there were signs of periperso nal neglect in the hemispace contralateral to the inactivation site. Follow ing inactivation of area F5 lying on the cortical convexity (the F5 sector where visuomotor neurones responding to action observation, 'mirror neurone s', are found) only a motor slowing was observed, the hand shaping being pr eserved. The inactivation of the hand field of area F1 produced a severe pa ralysis of contralateral finger movements with hypotonia, The results of th is study indicate the crucial role of the ventral premotor cortex in visuom otor transformations for grasping movements. More generally, they provide s trong support for the notion that distal and proximal movement organization relies upon distinct cortical circuits. Clinical data on distal movement d eficits in humans are reexamined in the light of the present findings.