A PERCEPTUAL ANALYSIS OF VISCOSITY

The perception of viscosity was studied using the contralateral limb-m atching procedure in which subjects adjusted the viscosity of a motor connected to the wrist of one (matching) arm until it was perceived to be the same as that of the motor attached to the other (reference) ar m. Two servo-controlled electromagnetic linear motors with computer-co ntrolled viscosity were used to present viscosities ranging in amplitu de from 2 to 1024 N . s/m to 11 subjects. Ten different viscosities we re matched by subjects, and there were ten repetitions of each stimulu s amplitude. The psychophysical function relating the reference to mat ching viscosity was linear (99% variance accounted for), and the accur acy with which the viscosities were matched (slope of 0.88) paralleled that reported previously for force, limb position and stiffness. The Weber fraction for viscosity was 0.34, which is 50% larger than that m easured for stiffness and over twice that reported for force. An analy sis of the movements and forces generated to perceive the reference vi scosity revealed that subjects did not vary the amplitude of the movem ents and typically made very small excursions of the forearm, but that both the velocity of the movements and force changed significantly as a function of the reference viscosity. These findings were interprete d as indicating that the human proprioceptive system is capable of int egrating information regarding force and movement velocity so as to pe rceive the viscosity of a mechanical system connected to the limbs, bu t that its sensitivity to changes in viscosity is much less than would be predicted from its capacity to detect variations in muscle force a nd limb movement.