Classifying torque, normal force and direction using monkey afferent nerve spike rates

In this study, tactile afferents in monkey fingertips were mechanically stimulated, using a flat disc shaped probe, with several magnitudes of torque, in clockwise and anticlockwise directions. In order to prevent slip during the stimulation event, a sufficient normal force was also applied, with three different magnitudes tested. Recordings were made from afferents innervating the glabrous skin covering the entire distal segment of the finger. A Parzen window classifier was used to assess the capacity of tactile afferents to discriminate, concurrently and in real-time, the three stimulus parameters; namely, background normal force, torque magnitude and direction. Despite the potentially confounding interactions between stimulus parameters, classification accuracy was very high and was improved even further by selecting subsets of best performing afferents.