Effects of changing skin mechanics on the differential sensitivity to surface compliance by tactile afferents in the human finger pad

Abstract

It is not known how changes in skin mechanics affect the responses of cutaneous mechanoreceptors in the finger pads to compression forces. We used venous occlusion to change the stiffness of the fingers and investigated whether this influenced the firing of low-threshold mechanoreceptors to surfaces of differing stiffness. Unitary recordings were made from 10 slowly adapting type I (SAI), 10 fast adapting type I (FAI) and 9 slowly adapting type II (SAII) units via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, and 4 N) at a constant loading and unloading rate (2 N/s) via a flat 2.5-cm-diameter silicone disk over the center of the finger pad. Nine silicone disks (objects), varying in compliance, were used. Venous occlusion, produced by inflating a sphygmomanometer cuff around the upper arm to 40 ± 5 mmHg, was used to induce swelling of the fingers and increase the compliance of the finger pulp. Venous occlusion had no effect on the firing rates of the SAI afferents, nor on the slopes of the relationship between mean firing rate and object compliance at each amplitude, but did significantly reduce the slopes for the FAI afferents. Although the SAII afferents possess a poor capacity to encode changes in object compliance, mean firing rates were significantly lower during venous occlusion. The finding that venous occlusion had no effect on the firing properties of SAI afferents indicates that these afferents preserve their capacity to encode changes in object compliance, despite changes in skin mechanics.

Contact mechanics of the human finger pad under compressive loads

“…The finger pad is a particularly interesting region of glabrous skin because of its role in touch and grip, which critically depend on normal and tangential mechanical interactions [1]. In touch, for example, the tactile perception of softness is partly determined by the compressive deformation of the finger pad, because the resulting subsurface strain fields affect how slowly adapting cutaneous mechanoreceptors respond [2][3][4]. While in grip, for example, skin hydration governs the contact area that develops during compressive and tangential loading and this affects both the static friction forces [5] and the dynamics of how slippage occurs [6].…”

Computational analysis of the relationship between mechanical state and mechanoreceptor responses during scanning of a textured surface

“…Previous studies have confirmed the relationship between stimulus application and the impulse rate of SA-I and RA-I mechanoreceptors, as shown in Figure 1(a) and (c). [25][26][27] Considering this observation, it can be assumed that increases in the intensity of the stimulus will influence the number of impulses. Hence, the computational results were more strongly correlated with previous neurophysiological findings 11 under the assumption that the temporal history of von Mises stress represents the number of impulses than under the assumption that it represents the impulse rate.…”

Neuromorphic tactile sensor array based on fiber Bragg gratings to encode object qualities

“…In this study, we implemented and evaluated a model emulating fast adapting mechanoreceptors (RA1) to investigate its stiffness encoding capacity during the loading phase with constant velocity. RA1 receptors are responsive to encode the interaction with the skin during the dynamic phases of the grasp, unlike SA1 units that are more sensitive not only during the loading phase, but also during the static holding phase [27]. Two FBGs (FBG1 and FBG2 in Figure 1), each one integrated in optical fibers along the longitudinal axis of the finger, were chosen to mimic these receptors through neuromorphic processing.…”