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

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 touch processing. Thus, establishing an objective measure to quantify affective reactions to CT-relevant touch is critical to allow for comparisons across populations.

In recent years, much insight has been gained regarding the characteristics of CT afferents. These unmyelinated afferents demonstrate an inverted U-shaped firing frequency, with velocities of 1-10 cm/s ("CT-optimal") eliciting the greatest frequency and both greater ("fast non-optimal") or lesser ("slow non-optimal") velocities eliciting reduced firing¹⁰. CT firing frequency correlates with self-reported ratings of touch "pleasantness", producing a similar inverted U-shaped curve in pleasantness ratings¹⁰. Moreover, CT-afferents also respond most robustly to stimuli close to skin temperature¹¹. These fibers also show distinct conduction speeds. The unmyelinated CT afferents are slower² and thus the volley of afferent input to the cortex shows a temporal lag when compared to the speed of the faster, myelinated Aβ fibers^1,12. Affective and discriminative touch can also be distinguished on a neural level. While both types of touch activate overlapping somatosensory areas, affective touch is more likely to activate the posterior insula, while discriminative touch activates sensorimotor areas^13,14,15,16. This activation pattern is consistent whether the touch is directly experienced or merely observed¹⁷, suggesting that affective touch is not just a "bottom-up" process driven by physical activation of CT afferents, but also involves "top-down" integration of multimodal sensory processing.

Situations in which CT processing is deficient or otherwise atypical has also provided insight into the functional significance of these afferents. In a unique patient group with a heritable mutation affecting the nerve growth factor β gene, there is a reduction in the density of thin and unmyelinated nerve fibers, including CT afferents. Compared to healthy controls, these patients report touch at CT-optimal velocities as less pleasant⁵. The converse scenario is also true; patients who lack myelinated Aβ fibers are able to retain a faint sensation of pleasant touch carried by the still intact CT afferents⁶. Abnormal affective touch processing is not just confined to instances of physical changes in CT-afferents. Across patient and healthy populations, those higher on the spectrum of autistic traits reported reduced pleasantness ratings of touch⁸. Psychiatric patients also demonstrate reduced hedonic ratings of affective touch, with a history of childhood maltreatment as one of the most consistent predictors of dysregulated affective touch awareness⁸. Dysregulation in the CT-based affective touch system in anorexia nervosa has also been reported⁹. Thus, both physical and psychological factors can influence affective touch processing, and as such, it is imperative to establish methodologies that can be applied to all individuals in an equitable and comparable manner.

Insights into normo-typical and dysregulated affective processing have the opportunity to provide a more nuanced picture of many patient groups. However, one potential limitation of affective touch research is the necessity of self-reported ratings. At times, self-report can be unreliable¹⁸ and subject to recall bias¹⁹. Inquiries of self-report can psychologically remove a participant from the current setting, limiting the ecological validity of the responses and removing them temporally from the experience²⁰. Moreover, self-report relies on a firm understanding of language and semantics, making cross-cultural and developmentally diverse (e.g. infant and toddler-aged individuals) comparisons challenging. For instance, individuals with an autism spectrum diagnosis frequently show distinct behavioral responses to touch²¹, but can also have difficulties in communicating verbally²². Thus, finding non-invasive methods to measure responses to touch that circumvent a reliance on self-report may translate, at least, to a better understanding of the mechanisms of affective touch, and at most, novel insights into dysregulation of social processing in patient populations.

Facial electromyography (EMG) is a suitable candidate to objectively assess affective responses to touch. It has been used to measure valence-specific reactions to visual²³, audio-visual²⁴, olfactory²⁵, and gustatory²⁶ stimuli. Facial EMG is a safe and non-invasive method consisting of surface electrodes that adhere to the face²⁷. These surface electrodes record facial muscle activity continuously in real-time with time scale sensitivity in the tens of milliseconds. Of particular interest is the corrugator supercilii ("corrugator"), which is activated when furrowing the brow and relaxes during a smile. As a result, corrugator activity has a linear relationship with affective valence, with increased response to negative stimuli and decreased activity in response to positive stimuli²⁸. In addition, the zygomaticus major ("zygomatic") is the muscle activated as the corners of the mouth pull up into a smile. The zygomatic displays a "J-shaped" activation pattern with positive stimuli eliciting the greatest response, and the most negative stimuli eliciting a greater response than neutral stimuli²⁸. Facial EMG recordings of these muscles can even be observed when stimuli are presented outside conscious awareness or when individuals are explicitly trying to suppress their reactions^29,30. Importantly, facial EMG can be used alone or in combination with self-report ratings or other physiological recordings. Thus, it is an ideal methodology to assess affective reactions to tactile stimulation^31,32.

In sum, facial EMG can be combined with self-report ratings to determine how CT-optimal tactile stimulation influences facial muscle activity as a potential indicator of affective response. One can take advantage of the velocity-dependent firing frequency of CTs to apply touch at CT-optimal and non-optimal velocities, and touch can be applied both to the CT-rich arm and the putatively CT-lacking palm. Comparisons can be made across modalities to determine whether affective responses to touch require direct stimulation or can be elicited via mere observation, suggestive of shared processing across sensory modalities. Finally, upon establishing facial EMG as a suitable methodology to study affective reactions to affective touch, researchers can then explore how affective touch processing may be influenced by various interventions (e.g., drug administration; stress exposure), how it changes throughout development⁷, how it is influenced by the relationship of the interactants³³, and whether it is dysregulated in clinical populations⁸.