Using Facial Electromyography to Assess Facial Muscle Reactions to Experienced and Observed Affective Touch in Humans

Summary

We describe a protocol to assess facial muscle activity in response to experienced and observed tactile stimulation using facial electromyography.

Abstract

"Affective" touch is believed to be processed in a manner distinct from discriminatory touch and to involve activation of C-tactile (CT) afferent fibers. Touch that optimally activates CT fibers is consistently rated as hedonically pleasant. Patient groups with impaired social-emotional functioning also show disordered affective touch ratings. However, relying on self-reported ratings of touch has many limitations, including recall bias and communication barriers. Here, we describe a methodological approach to study affective responses to touch via facial electromyography (EMG) that circumvents the reliance on self-report ratings. Facial EMG is an objective, quantitative, and non-invasive method to measure facial muscle activity indicative of affective responses. Responses can be assessed across healthy and patient populations without the need for verbal communication. Here, we provide two separate datasets demonstrating that CT-optimal and non-optimal touch elicit distinct facial muscle reactions. Moreover, facial EMG responses are consistent across stimulus modalities, e.g. tactile (experienced touch) and visual (observed touch). Finally, the temporal resolution of facial EMG can detect responses on timescales that supersede that of verbal reporting. Together, our data suggest that facial EMG is a suitable methodology for use in affective tactile research that can be used to supplement, or in some cases, supplant, existing measures.

Introduction

C-tactile (CT) afferents are proposed to convey the affective component of touch, which can be distinguished from the discriminative aspects of touch processed via Aβ fibers^1,2. CT-mediated affective touch is believed to play an integral role in social affiliative behaviors³, leading to the "skin as a social organ" hypothesis⁴. Physical^5,6, developmental⁷, and psychiatric^8,9 factors can influence CT-mediated to....

Protocol

This protocol is based on Mayo et al.³¹ (Experiment 1) and Ree et al.³² (Experiment 2). Ethical approval was granted by the Regional Ethical Review Board, Linköping, Sweden (Experiment 1) and the local ethical committee at the Department of Psychology, University of Oslo, Norway (Experiment 2).

1. Participant Screening and Preparation

1. Recruit participants who lack tactile or uncorrected visual disturbances and are free of any neurological or psychiatric disorder, unless a specific patient population is being recruited.
2. Ensure that participants are fully able to under....

Results

CT-optimal touch elicits distinct EMG responses compared to fast non-optimal touch across modalities
The first experiment addressed whether differential EMG reactivity could be detected in response to CT-optimal (3 cm/s) and fast non-optimal (30 cm/s) tactile stimulation that was directly experienced (Figure 3) or merely observed (Figure 2 and Figure 3)³¹........

Discussion

Here, we report on the use of facial electromyography (EMG) as a method to study affective responses to observed and experienced touch. Previously, many studies have focused on the use of self-report ratings to characterize the affective quality of touch. Touch that optimally activates CT afferents (e.g., 1-10 cm/s) is consistently rated as more pleasant than either faster or slower touch velocities¹⁰. In contrast, ratings of intensity seem to track with velocity, with faster touch velocities rate.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
4 mm Ag-AgCl sheilded reusable electrodes	Biopac	EL654
75 mm goat hair brush	IN-EX Color AB	77062	Touch application; https://www.in-exfarg.se
8 mm Ag-AgCl unsheilded reusable electrode	Biopac
Acqknowledge software	Biopac	ACK100W	Used for application of filtering steps, analysis
Adhesive collars	Biopac	ADD204
Cables	Biopac	BN-EL30-LEAD3; LEAD2	LEAD3 includes ground, LEAD2 is only bipolar recording electrodes
Electro-gel	Biopac	GEL100
EMG aplifier x 2	Biopac	BN-EMG2
El-Prep	Biopac	ELPREP	Facial exfoliant
MP160 data acqusition system	Biopac	MP160WSW
Presentation software	Neurobehavioral systems		Task presentation software