The Structural Role of the Fingernail in Force Perception (2010)
Undergraduates: Trevor Brothers, Alicia Mullis
Faculty Advisor: Mark Hollins
Department: Psychology & Neuroscience
The role of the human fingernail in sensory estimations of fingertip force was investigated in a series of three experiments. In the first experiment, psychophysical data was collected on the ability of human participants to discriminate forces actively applied to a stationary surface using the pad of the thumb. The resulting Just Noticeable Difference (JND) curve showed that participants are able to accurately judge high levels of force application even as the structural deformation of the fingerpad ceases to respond to additional force application. In the second experiment, participants’ thumbnails were submerged in water, increasing their nail flexibility and reducing the forces imparted to mechanoreceptors located in the bed of the nail. This manipulation caused a sensory illusion of decreased force intensity, which was measured through biased performance in a contralateral hand-matching paradigm. In the third experiment, the ability of the water manipulation to increase nail flexibility was confirmed by the use of strain gauges attached to the surface of the thumbnail. The amount of nail flexibility present in the control (dry) condition was also found to be significantly, negatively correlated with accurate force discrimination. Together these results suggest that interactions between SA-II nail units and the fingernail are responsible for producing the sensory information necessary to accurately discriminate forces applied with the fingertips.