Finger control in the tripod grasp.

The present study aimed to determine whether grasping is based on either (1) synchronous finger movements producing stereotyped types of grasp, or (2) independently controlled finger movements producing variable final finger postures. Participants reached for and grasped sphere-shaped objects of three sizes. They were allowed to select three different grasp configurations: a "pinch" grip (thumb and index finger), a "middle" grip (thumb and middle finger) and a "tripod" grip (thumb and index plus middle finger). Object distance from the subject was varied in order to verify whether finger control and final finger postures varied according to the degree of accuracy required by target object distance. All the participants always selected the tripod grip when reaching for the large and medium size objects. The pinch grip was used by half of the participants when reaching for the small object, but only in 17% of the trials. Target object distance did not appear to influence the type of selected grip. The tripod grip was found to consist of two different components: an aperture component (opening and closing the gap between the thumb and opposition finger) and a finger separation component (increasing and decreasing the gap between the index and middle fingers). The timing of the aperture component was the same for the index and middle fingers. In contrast, the timing of the finger separation was weakly coupled with the aperture components. Moreover, the relative spatial position among the three fingers during and at the end of grasp varied according to object size. When grasping the large object, both the index finger and the middle finger were in opposition to the thumb. In contrast, when grasping the small object, the index finger was less in opposition to the thumb with respect to the middle finger. The final spatial position of the thumb relative to the starting position was independent of object size, whereas those of the index and middle fingers varied with object size. The results support the notion that grasp is accomplished by using two virtual fingers formed by the thumb and one or more other fingers that synchronously open and close on the object along the opposition space [Arbib 1990; in: Jeannerod M (ed) Attention and performance XIII: motor representation and control. Lawrence Erlbaum, Hillsdale, pp 111-138]. This suggests a degree of coupling between the control of the virtual fingers.