Differential sensitivity to surface compliance by tactile afferents in the human finger pad

Abstract

We undertook a neurophysiological investigation of the responses of low-threshold mechanoreceptors in the human finger pad to surfaces of differing softness. Unitary recordings were made from 26 slowly adapting type I (SAI), 17 fast-adapting type I (FAI), and 9 slowly adapting type II (SAII) afferents via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, 4 N) at a constant loading and unloading rate (2 N/s) via a flat silicone disc over the center of the finger pad. Nine discs were used, which linearly increased in stiffness across the range. Population responses of the SAI afferents showed the greatest sensitivity to compliance, with a steep monotonic increase in mean firing rate with increasing stiffness (decreasing compliance) of the surface during the loading and plateau (but not unloading) phases. FAI afferents also showed a linear increase in firing during the loading but not unloading phase, although the slope was significantly lower than that of the SAI afferents at all amplitudes. Conversely, SAII afferents were influenced by object compliance only in certain conditions. Given their high density in the finger pads and their linear relationship between firing rate and object compliance during the loading and plateau phases, SAI afferents (together with FAI afferents during the loading phase) are ideally suited to contributing information on surface compliance to the overall estimation of softness, but the SAII afferents appear to play only a minor role.

Effect of Visual Information on Active Touch During Mirror Visual Feedback

“…These results suggest that assimilation of perceived roughness can occur both within and across hands. Considering that input from slowly adapting type I (SAI) and fast-adapting type I (FAI) mechanoreceptors in the finger pad is important for perceiving both roughness and hardness (Phillips et al, 1992 ; Condon et al, 2014 ), such assimilation may also occur across hands when perceiving hardness.…”

Psychophysical Investigations into the Role of Low-Threshold C Fibres in Non-Painful Affective Processing and Pain Modulation

“…Studies on compliance encoding have argued for a critical role of large-diameter fibres such as slowly adapting afferents. However, the role of affective coding in this context remains largely unexplored [ 30 , 31 ]. Previous studies [ 29 , 32 ] have conjectured about the involvement of low-threshold C mechanoafferents in affective judgments, but they did not test the contribution of C fibres by using a conduction blockade of myelinated fibres.…”

Computational Modeling Reinforces that Proprioceptive Cues May Augment Compliance Discrimination When Elasticity Is Decoupled from Radius of Curvature

“…First, our FE analyses are done in steady-state and do not take material viscoelasticity into account. This time-varying information is likely to contribute to the response characteristics of both rapid-adapting and slowly-adapting type I afferents [5, 13], and it is likely that the timing of the action potentials in the stimulus ramp contribute to compliance discrimination [5]. Second, the spherical indenter used herein does not shed any light on how distinct spatial features, such as edges, might be encoded apart from material compliance.…”

“…Third, others employ stimuli that correctly characterize compliance by elastic modulus and account for elastic-to-elastic surface contact, but only use stimuli of flat surface geometry. Finally and relatedly, single-nerve, electrophysiological recordings have recently been conducted in response to compliant stimuli [5, 6]. To fill the gap between single-unit recordings and psychophysical studies, computational models are required to decipher how a population of afferents encodes compliance.…”