Control strategies in human pinch motion to detect the hardness of an object

The human pinch is a motion to perceive an object's hardness as well as a motion to hold an object. The understanding of the hardness detecting pinch motion is expected to help in developing artificial reality devices. In this study, the applied force, finger displacement and contact area were measured in the pinch motions with five different objects. The objects were transparent to allow contact area analysis, and with the same texture by putting polyvinyl film on them. Ten volunteers were required to find the harder object from the two different objects presented in sequence without visual information. The pinch motion for the first object was analyzed. The applied force, finger displacement and contact area were normalized by each maximal value. The applied force to the harder object was higher as expected. The first finding in this study was that the contact area is almost constant despite the hardness difference in the pinch motion to perceive the hardness. The next question was how the human maintains the contact area constantly without sensing the hardness in advance. The slopes of the force, the displacement and the contact area were analyzed. The calculated values showed the same tendency as the peak value. It suggests that the control of the contact area is attained without feedback mechanism. The equilibrium point hypothesis was introduced into this experiment. The imaginary displacement was calculated with the measured fingertip and the object hardness. The difference of the imaginary displacement was 29%. It appeared that human pinches object with equilibrium point tactics to perceive the hardness of the object.