Propagation Waves in Tactile Material Perception

During the exploration of surfaces with the bare finger, vibratory signals arise and propagate through the finger and hand. While research into mechanical and neural response characteristics has demonstrated that these signals carry rich information about touched objects and their properties, only little is known about the role these propagation waves play in human perception and to which extent the somatosensory system is able to collect information from afferents at more proximal locations than the skin-object surface. Using ring-block anaesthesia (lidocaine) we will temporarily inhibit haptic feedback sensations of healthy participants' index finger during interactions with 3D-printed surface probes that are systematically varied in two important material dimensions, namely their roughness and hardness (elasticity), while the participants carry out a well-established psychophysical discrimination task. The results will then be compared to a control condition without anaesthesia. An accelerometer sensor, placed on the dorsal side of the hand, will serve to simultaneously record the propagating tactile waves. Given their role in material perception, thermal cues will be monitored during the experiment with a thermometer and the hydration level of the fingertip skin will be measured regularly using a corneometer. This research will allow us to understand the role of propagation waves in material perception. It seeks to uncover some of the perceptual mechanisms that remain intact during surface discrimination of textured, compliant surfaces, while local information is temporarily inhibited. The results will have implications for how we provide feedback about material properties for sensorimotor control to this living with prosthetic limbs. It is hypothesised that propagation waves that arise during these haptic interactions contain behaviourally relevant information used for the discrimination of surface properties.