Magnitude estimation of tangential force applied to the fingerpad

Prior research on large-fibre skin mechanoreceptors in humans and monkeys has demonstrated their sensitivity to perpendicular skin indentation and to the rate of force application. Although some studies have examined skin afferent responses to stretch, relatively few investigations have examined the neural and perceptual correlates of shear forces applied tangentially to the skin. The present study assessed the ability of human subjects to scale different levels of tangential force applied to the distal pad of the index finger. Subjects were instructed to choose their own magnitude estimation scale. Seven force levels ranging from 0.15 to 0.70 N were delivered randomly at rates of 0.10 N/s, 0.15 N/s or 0.30 N/s. Tangential forces were produced with a smooth metal spatula coated with an adhesive to insure a shear force on the underlying skin without slip. The same procedures were also used to generate skin indentation with normal forces. The results showed that most human subjects were able to scale different magnitudes of both tangential and normal forces applied to the tip of the index finger. The rate of force change did not influence the perception of the applied forces. These results highlight the potentially important role of tangential forces in haptic perception.