Grasp with hand and mouth: A kinematic study on healthy subjects

Neurons involved in grasp preparation with hand and mouth were previously r ecorded in the premotor cortex of monkey. The aim of the present kinematic study was to determine whether a unique planning underlies the act of grasp ing with hand and mouth in humans as well. In a set of four experiments, he althy subjects reached and grasped with the hand an object of different siz e while opening the mouth (experiments 1 and 3), or extending the other for earm (experiment 4), or the fingers of the other hand (experiment 5). In a subsequent set of three experiments, subjects grasped an object of differen t size with the mouth, while opening the fingers of the right hand (experim ents 6-8). The initial kinematics of mouth and finger opening, but not of f orearm extension, was affected by the size of the grasped object congruentl y with the size effect on initial grasp kinematics. This effect was due nei ther to visual presentation of the object, without the successive grasp mot or act (experiment 2) nor to synchronism between finger and mouth opening ( experiments 3, 7, and 8). In experiment 9 subjects grasped with the right h and an object of different size while pronouncing a syllable printed on the target. Mouth opening and sound production were affected by the grasped ob ject size. The results of the present study are discussed according to the notion that in an action each motor act is prepared before the beginning of the motor sequence. Double grasp preparation can be used for successive mo tor acts on the same object as, for example, grasping food with the hand an d ingesting it after bringing it to the mouth. We speculate that the circui ts involved in double grasp preparation might have been the neural substrat e where hand motor patterns used as primitive communication signs were tran sferred to mouth articulation system. This is in accordance with the hypoth esis that Broca's area derives phylogenetically from the monkey premotor ar ea where hand movements are controlled.