Human velocity-change perceptual characteristics in passive movements of shoulder and/or elbow joint

Abstract

Focusing on humans’ velocity perceptual characteristics, this study clarified the velocity JNDs that is minimal velocity differences humans can discriminate. For this purpose, using a 2-DOF SCARA-type haptic device, we conducted an experiment, assuming a pattern in which velocities were increased from constant values to other constant ones for shoulder and/or elbow joints. In the experiment, subjects’ upper limbs were enforced to move by the device, while the subjects focused on their hand velocity change using their proprioceptive sensations. After the movements, the subjects answered whether they perceived a velocity change during the movement. Iterating this trial with various velocity difference, velocity JNDs were obtained for each of the subjects and the following two factors. The two factors to be evaluated were the joint factor and the before-acceleration velocity factor: (1) the joint factor was the joints to be moved, of which levels were set as the shoulder, the elbow, and the shoulder-and-elbow, (2) the before-acceleration velocity factor was the nearly-constant tangential velocity of hand motions before velocity change, Vbefore. As a result, it was confirmed that a linear relationship of the velocity JND against the Vbefore was confirmed for all the joint factor levels, i.e., the shoulder only, the elbow only, and both the shoulder-and-elbow. Here, it should be noted that the joint angular velocities corresponding to hand tangential velocities are greatly different between the three joint factor levels. Nevertheless, the correspondence between Vbefore and the velocity JNDs were approximately the same between the three jointfactor levels. Therefore, it is concluded that the hand velocities, not the joint angular velocities, are dominant in human velocity-change perception for passive movements in the shoulder and/or elbow joint.