Hand-use and tool-use in grasping control.

The goal of this study was to elucidate the underlying mechanisms of hand and tool grasping control. We assumed that there is a single principle-governing grasping control irrespective of its effectors and that the degree of prior experience of the effector determines the smoothness of aperture control. Eight participants performed a reach-to-grasp task with four different effectors: index finger and thumb, middle finger and thumb, chopsticks, and a scissor-like tool. Although we employed different effectors with large mechanical variations and different degrees of prior use, maximum grip aperture was scaled as a function of object size and appeared at almost the same timing in all four types of grasping movements. Moreover, reaching time did not substantially differ among grasping conditions. However, plateau duration of the aperture profile differed by effector. Plateau duration was the longest in the unfamiliar scissor-like tool grasping condition. There was no difference between the unfamiliar hand-use grasp with the thumb and the middle finger and the familiar tool-grasp with chopsticks. The familiar hand-use grasp with the thumb and the index finger had the shortest plateau duration. These results supported the idea that there is an effector-independent continuity between hand-use and tool-use in motor control as a function of prior degree of use, rather than the conventionally assumed dichotomy between them.