A Force-Controlled Indenter to Elicit a Tactile Afferent Response that Accommodates for the Skin's Viscoelastic Relaxation and Differing Thickness

Tactile afferents elicit neural biopotentials upon deformation of the skin’s surface. To study the underlying input-output relationship, typically the stimulus input is displacement, though force may be more naturalistic. Force control is advantageous because equal pressure can be delivered to specimens that vary in skin thickness and it can counterbalance the skin’s viscoelastic relaxation. However, current force-controlled indenters approach an afferent’s receptive field at an angle so as to create an unequal stress distribution and base their feedback control indirectly on motor performance as opposed to directly measuring probe force. The proposed research seeks to address these issues by designing, building, and evaluating a single-axis indenter. The indenter consists of hardware (motion controller coupled with a mechanical sled and custom designed electronics) and software (control algorithm and user interface). The control algorithm utilizes a sequential, pre-planned trajectory and then a real-time feedback technique to counterbalance the three phases of skin relaxation that accompany the early hold (< .19 sec), intermediate hold (>~0.19 sec and ~1.5 sec) of the stimulus. The graphical user interface (Python software) traverses a user intuitively through a series of experimental tasks. With collaborators at Columbia University, we conducted two experiments. The results of the experiments indicate the indenter 1) can clamp an achieved force in skins of differing thickness and 2) evoke neurophysiological responses of slowly adapting afferents whereby the firing rate drop between the early and late phases of the sustained hold is less during force control, leaving mostly neural adaptation.