Inhibitory rTMS of secondary somatosensory cortex reduces intensity but not pleasantness of gentle touch

Evolutionarily conserved neural responses to affective touch in monkeys transcend consciousness and change with age

Affective touch-a slow, gentle, and pleasant form of touch-activates a different neural network than which is activated during discriminative touch in humans. Affective touch perception is enabled by specialized low-threshold mechanoreceptors in the skin with unmyelinated fibers called C tactile (CT) afferents. These CT afferents are conserved across mammalian species, including macaque monkeys. However, it is unknown whether the neural representation of affective touch is the same across species and whether affective touch's capacity to activate the hubs of the brain that compute socioaffective information requires conscious perception. Here, we used functional MRI to assess the preferential activation of neural hubs by slow (affective) vs. fast (discriminative) touch in anesthetized rhesus monkeys (Macaca mulatta). The insula, anterior cingulate cortex (ACC), amygdala, and secondary somatosensory cortex were all significantly more active during slow touch relative to fast touch, suggesting homologous activation of the interoceptive-allostatic network across primate species during affective touch. Further, we found that neural responses to affective vs. discriminative touch in the insula and ACC (the primary cortical hubs for interoceptive processing) changed significantly with age. Insula and ACC in younger animals differentiated between slow and fast touch, while activity was comparable between conditions for aged monkeys (equivalent to >70 y in humans). These results, together with prior studies establishing conserved peripheral nervous system mechanisms of affective touch transduction, suggest that neural responses to affective touch are evolutionarily conserved in monkeys, significantly impacted in old age, and do not necessitate conscious experience of touch.

Role of medial prefrontal cortex and primary somatosensory cortex in self and other-directed vicarious social touch: a TMS study

Conflicting evidence points to the contribution of several key nodes of the 'social brain' to the processing of both discriminatory and affective qualities of interpersonal touch. Whether the primary somatosensory cortex (S1) and the medial prefrontal cortex (mPFC), two brain areas vital for tactile mirroring and affective mentalizing, play a functional role in shared representations of C-tactile (CT) targeted affective touch is still a matter of debate. Here, we used offline continuous theta-burst transcranial magnetic stimulation (cTBS) to mPFC, S1 and vertex (control) prior to participants providing ratings of vicarious touch pleasantness for self and others delivered across several body sites at CT-targeted velocities. We found that S1-cTBS led to a significant increase in touch ratings to the self, with this effect being positively associated to levels of interoceptive awareness. Conversely, mPFC-cTBS reduced pleasantness ratings for touch to another person. These effects were not specific for CT-optimal (slow) stroking velocities, but rather they applied to all types of social touch. Overall, our findings challenge the causal role of the S1 and mPFC in vicarious affective touch and suggest that self- vs other-directed vicarious touch responses might crucially depend on the specific involvement of key social networks in gentle tactile interactions.

Characterizing affiliative touch in humans and its role in advancing haptic design

An emerging view in cognitive neuroscience holds that the extraction of emotional relevance from sensory experience extends beyond the centralized appraisal of sensation in associative brain regions, including frontal and medial-temporal cortices. This view holds that sensory information can be emotionally valenced from the point of contact with the world. This view is supported by recent research characterizing the human affiliative touch system, which carries signals of soft, stroking touch to the central nervous system and is mediated by dedicated C-tactile afferent receptors. This basic scientific research on the human affiliative touch system is informed by, and informs, technology design for communicating and regulating emotion through touch. Here, we review recent research on the basic biology and cognitive neuroscience of affiliative touch, its regulatory effects across the lifespan, and the factors that modulate it. We further review recent work on the design of haptic technologies, devices that stimulate the affiliative touch system, such as wearable technologies that apply the sensation of soft stroking or other skin-to-skin contact, to promote physiological regulation. We then point to future directions in interdisciplinary research aimed at both furthering scientific understanding and application of haptic technology for health and wellbeing.

Cortical Response Variation with Social and Non-Social Affective Touch Processing in the Glabrous and Hairy Skin of the Leg: A Pilot fMRI Study

Despite the crucial role of touch in social development and its importance for social interactions, there has been very little functional magnetic resonance imaging (fMRI) research on brain mechanisms underlying social touch processing. Moreover, there has been very little research on the perception of social touch in the lower extremities in humans, even though this information could expand our understanding of the mechanisms of the c-tactile system. Here, variations in the neural response to stimulation by social and non-social affective leg touch were investigated using fMRI. Participants were subjected to slow a (at 3-5 cm/s) stroking social touch (hand, skin-to-skin) and a non-social touch (peacock feather) to the hairy skin of the shin and to the glabrous skin of the foot sole. Stimulation of the glabrous skin of the foot sole, regardless of the type of stimulus, elicited a much more widespread cortical response, including structures such as the medial segment of precentral gyri, left precentral gyrus, bilateral putamen, anterior insula, left postcentral gyrus, right thalamus, and pallidum. Stimulation of the hairy skin of the shin elicited a relatively greater response in the left middle cingulate gyrus, left angular gyrus, left frontal eye field, bilateral anterior prefrontal cortex, and left frontal pole. Activation of brain structures, some of which belong to the "social brain"-the pre- and postcentral gyri bilaterally, superior and middle occipital gyri bilaterally, left middle and superior temporal gyri, right anterior cingulate gyrus and caudate, left middle and inferior frontal gyri, and left lateral ventricle area, was associated with the perception of non-social stimuli in the leg. The left medial segment of pre- and postcentral gyri, left postcentral gyrus and precuneus, bilateral parietal operculum, right planum temporale, left central operculum, and left thalamus proper showed greater activation for social tactile touch. There are regions in the cerebral cortex that responded specifically to hand and feather touch in the foot sole region. These areas included the posterior insula, precentral gyrus; putamen, pallidum and anterior insula; superior parietal cortex; transverse temporal gyrus and parietal operculum, supramarginal gyrus and planum temporale. Subjective assessment of stimulus ticklishness was related to activation of the left cuneal region. Our results make some contribution to understanding the physiology of the perception of social and non-social tactile stimuli and the CT system, including its evolution, and they have clinical impact in terms of environmental enrichment.

The Effectiveness of High-Frequency Repetitive Transcranial Magnetic Stimulation on Patients with Neuropathic Orofacial Pain: A Systematic Review of Randomized Controlled Trials

Background

Repetitive transcranial magnetic stimulation (rTMS) has been widely used in the treatment of neuropathic orofacial pain (NOP). The consistency of its therapeutic efficacy with the optimal protocol is highly debatable.

Objective

To assess the effectiveness of rTMS on pain intensity, psychological conditions, and quality of life (QOL) in individuals with NOP based on randomized controlled trials (RCTs).

Methods

We carefully screened and browsed 5 medical databases from inception to January 1, 2022. The study will be included that use of rTMS as the intervention for patients with NOP. Two researchers independently completed record retrieval, data processing, and evaluation of methodological quality. Quality and evidence were assessed using the PEDro scores and the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) system.

Results

Six RCTs with 214 participants were included in this systematic review: 2 studies were considered level 1 evidence, and 4 were considered level 2 evidence. Six studies found that high-frequency rTMS had a pain-relieving effect, while 4 studies found no improvement in psychological conditions and QOL. Quality of evidence (GRADE system) ranged from moderate to high. No significant side effects were found.

Conclusions

There is moderate-to-high evidence to prove that high-frequency rTMS is effective in reducing pain in individuals with NOP, but it has no significant positive effect on psychological conditions and QOL. High-frequency rTMS can be used as an alternative treatment for pain in individuals with NOP, but further studies will be conducted to unify treatment parameters, and the sample size will be expanded to explore its influence on psychological conditions and QOL.

The role of right supra-marginal gyrus and secondary somatosensory cortex in age-related differences in human emotional egocentricity

Emotional egocentric bias (EEB) occurs when, due to a partial failure in self-other distinction, empathy for another's emotion is influenced by our own emotional state. Recent studies have revealed a higher EEB in children, adolescents and older adults compared to young adults, but the neural correlates of this finding are largely unknown. We asked female participants (N = 95) from three different age groups (adolescents, young and older adults) to perform a well-validated EEB task in an MRI scanner. We assessed task-based changes in activity and effective connectivity as well as morphometric changes in regions of interest to pinpoint functional and structural age-related differences. Results revealed higher EEB in older compared to young adults and adolescents. Connectivity between right supramarginal gyrus (rSMG) and somatosensory cortices acted as a partial mediator between age and EEB. The findings suggest that an intact connectivity of rSMG, rather than its regional activity, with sensory-perceptual brain areas is crucial for overcoming egocentric biases of empathic judgments.

Brain circuits for pain and its treatment

[Figure: see text].

Strengths and weaknesses of affective touch studies over the lifetime: A systematic review

C-Tactile (CT) fibers are activated by slow, caress-like stimulations, and convey a specific tactile processing of hedonic and interpersonal components, defined as affective touch. Given the beneficial effects deriving from affective tactile experiences in social interactions at all ages, a systematic review of experimental studies on affective touch perception across the lifespan was performed with the aims of 1) examining whether and how affective touch has been studied in a systematic manner throughout the lifespan; 2) verifying whether the pleasantness associated to affective stimulations is found during the entire lifespan. Empirical human studies on affective touch were searched in two databases (PubMed, PsychINFO) and 112 articles were retrieved. Results indicated that most of the studies recruited participants with a mean age ranging from 18 to 40 years, whereas other age ranges came out as under-represented or not represented at all. Despite high heterogeneity across studies, affective touch was considered as a pleasant experience across the lifetime, and it was associated to specific psychophysiological patterns in infants and adults.

Relationship-specific Encoding of Social Touch in Somatosensory and Insular Cortices

Humans use touch to maintain their social relationships, and the emotional qualities of touch depend on who touches whom. However, it is not known how affective and social dimensions of touch are processed in the brain. We measured haemodynamic brain activity with functional magnetic resonance imaging (fMRI) from 19 subjects (10 males), while they were touched on their upper thigh by either their romantic partner, or an unfamiliar female or male confederate or saw the hand of one of these individuals near their upper thigh but were not touched. We used multi-voxel pattern analysis on pre-defined regions of interest to reveal areas that encode social touch in a relationship-specific manner. The accuracy of the machine learning classifier to identify actor for both feeling touch and seeing hand exceeded the chance level in the primary somatosensory cortex, while in the insular cortex accuracy was above chance level only for the touch condition. When classifying the relationship (partner or stranger), while keeping the toucher sex fixed, amygdala (AMYG), orbitofrontal cortex (OFC), and primary and secondary somatosensory cortices were able to discriminate toucher significantly above chance level. These results suggest that information on the social relationship of the toucher is processed consistently across several regions. More complex information about toucher identity is processed in the primary somatosensory and insular cortices, both of which can be considered early sensory areas.

Effects of cTBS on the Frequency-Following Response and Other Auditory Evoked Potentials

The frequency-following response (FFR) is an auditory evoked potential (AEP) that follows the periodic characteristics of a sound. Despite being a widely studied biosignal in auditory neuroscience, the neural underpinnings of the FFR are still unclear. Traditionally, FFR was associated with subcortical activity, but recent evidence suggested cortical contributions which may be dependent on the stimulus frequency. We combined electroencephalography (EEG) with an inhibitory transcranial magnetic stimulation protocol, the continuous theta burst stimulation (cTBS), to disentangle the cortical contribution to the FFR elicited to stimuli of high and low frequency. We recorded FFR to the syllable /ba/ at two fundamental frequencies (Low: 113 Hz; High: 317 Hz) in healthy participants. FFR, cortical potentials, and auditory brainstem response (ABR) were recorded before and after real and sham cTBS in the right primary auditory cortex. Results showed that cTBS did not produce a significant change in the FFR recorded, in any of the frequencies. No effect was observed in the ABR and cortical potentials, despite the latter known contributions from the auditory cortex. Possible reasons behind the negative results include compensatory mechanisms from the non-targeted areas, intraindividual variability of the cTBS effectiveness, and the particular location of our target area, the primary auditory cortex.

Damage to the right insula disrupts the perception of affective touch

Specific, peripheral C-tactile afferents contribute to the perception of tactile pleasure, but the brain areas involved in their processing remain debated. We report the first human lesion study on the perception of C-tactile touch in right hemisphere stroke patients (N = 59), revealing that right posterior and anterior insula lesions reduce tactile, contralateral and ipsilateral pleasantness sensitivity, respectively. These findings corroborate previous imaging studies regarding the role of the posterior insula in the perception of affective touch. However, our findings about the crucial role of the anterior insula for ipsilateral affective touch perception open new avenues of enquiry regarding the cortical organization of this tactile system.

Deactivation of default mode network during touch

Interpersonal touch possesses a strong affective component, which immediately evokes attention. The neural processing of such touch is moderated by specialized C-tactile nerve fibers in the periphery and results in central activation of somatosensory areas as well as regions involved in social processing, such as the superior temporal gyrus (STG). In the present functional neuroimaging investigation, we tested the hypothesis that the attention grasping effect of interpersonal touch as compared to impersonal touch is reflected in a more-pronounced deactivation of the default mode network (DMN). Using functional magnetic resonance imaging, we investigated the neural processing of interpersonal relative to impersonal touch conditions that were furthermore modulated by stroking velocity in order to target c-tactile nerve fibers to a different extent. A sample of 30 healthy participants (19 women, mean age 40.5 years) was investigated. In the impersonal touch, participants were stroked with a brush on the forearm. In the interpersonal touch condition, the experimenter performed the stroking with the palm of his hand. Interpersonal touch was rated as more pleasant and intense than impersonal touch and led to a stronger blood oxygen level dependent (BOLD) signal increase in the somatosensory cortex SII extending to the superior temporal cortex. Over all touch conditions, this activation was coupled in time to the deactivation of prominent nodes of the DMN. Although deactivation of the DMN was most pronounced for interpersonal touch conditions, the direct comparison did not show significant differences in DMN deactivation between interpersonal and impersonal touch or between different stroking velocities. We therefore conclude that all applied touch conditions deactivate the DMN and the strong connection to areas which code the contextual and social characteristics of affective touch may explain the attention grasping effect of touch.

Lack of Target Engagement Following Low-Frequency Deep Transcranial Magnetic Stimulation of the Anterior Insula

OBJECTIVE

To evaluate the safety and efficacy of low-frequency, inhibitory, deep rTMS with a novel H-coil specifically designed to stimulate the insula.

METHODS

In a randomized, crossover order, 16 healthy volunteers underwent two sessions (sham; active) of 1 Hz repetitive TMS at an intensity of 120% of individual motor threshold, over the right anterior insular cortex localized using a neuronavigation system. Before, immediately after, and one hour after rTMS, subjects performed two tasks that have previously been shown in fMRI experiments to activate insular cortex: A blink suppression task and a forced-choice risk-taking task.

RESULTS

No drop-outs or adverse events occurred. Active deep rTMS did not result in decreased urge to blink compared to sham. Similarly, no significant time × condition interaction on risk-taking behavior was found.

CONCLUSIONS

Low-frequency deep rTMS using a novel H8 coil was shown to be safe but did not affect any of the behavioral markers, also used to investigate modulation of insula activity. Our findings highlight the challenges of modulating the activity of deep brain regions with TMS. Further studies are necessary to identify effective stimulation parameters for deep targets, and to characterize the effects of deep TMS on overlying cortical regions.

Sensitivity to CT-optimal, Affective Touch Depends on Adult Attachment Style

Affective touch supports affiliative bonds and social cognition. In particular, gentle, stroking touch, which has recently been associated with the C Tactile (CT) system, is typically perceived as pleasant and prosocial. However, it remains unknown whether pre-existing models of social relating influence the perception of CT-optimal touch. In this study (N = 44 adults), we examined how individual differences in attachment styles relate to the perception of CT-optimal touch, as well as to a different modality of interoception, namely heartbeat perception. Using the gold-standard assessment of attachment (Adult Attachment Interview), we found that insecure attachment was associated with reduced pleasantness discrimination between CT-optimal vs. non-CT optimal touch. Acknowledging the different traditions in measuring attachment, we also used a well-validated self-report questionnaire that pertains to explicit representations of current close relationships. Using this measure, we found that higher scores in attachment anxiety (but not attachment avoidance) were associated with reduced pleasantness discrimination between CT-optimal vs. non-CT optimal touch. Attachment patterns (in both measures) were not related to cardiac perception accuracy. These results corroborate and extend previous literature on CT-optimal touch and its relation with affiliative bonds and social cognition. Given that attachment was not related to perceived cardiac accuracy, these findings point to the specificity of the relationship between CT-optimal touch and attachment.

Dissociable sources of erogeneity in social touch: Imagining and perceiving C-Tactile optimal touch in erogenous zones

Previous research points to two major hypotheses regarding the mechanisms by which touch can be experienced as erotogenic. The first concerns the body part to which touch is applied (erogenous zones) and the second the modality of touch (sensual touch optimal in activating C Tactile afferents). In this study, we explored for the first time the relation between those two mechanisms in actual and imagined social touch. In a first experiment, we randomly assigned “Giver” and “Receiver” roles within 19 romantic couples (20 females, 18 males, age 32.34 ± 8.71SD years) and asked the “Giver” to apply CT-optimal (3 cm/s) vs. CT-suboptimal (18 cm/s) touch on an erogenous (neck) vs. non-erogenous zone (forehead) of their partner. We then obtained ratings of pleasantness and sexual arousal from both “Receivers” and “Givers”. In a second experiment, 32 healthy females (age 25.16 ± 5.91SD years) were asked to imagine CT-optimal vs. CT-suboptimal stimulation (stroking vs. patting) and velocity (3 cm/s vs. 18 cm/s) on different erogenous vs. non-erogenous zones and rate pleasantness. While both erogenous body part and CT-optimal, sensual touch were found to increase pleasant and erotic sensations, the results showed a lack of an interaction. Furthermore, pleasantness was induced by mere imagination of touch without any tactile stimulation, and touch that was sexually arousing for the receiver was rated as more sexually arousing for the giver as well, pointing to top-down, learned expectations of sensory pleasure and erogeneity. Taken together, these studies provide the first direct evidence that while both the body location to which touch is applied and the mode of touch contribute to pleasant and erotic sensations, these two factors appear to mediate subjective pleasantness and erogeneity by, at least partly, independent mechanisms.

Balancing body ownership: Visual capture of proprioception and affectivity during vestibular stimulation

The experience of our body as our own (i.e. body ownership) involves integrating different sensory signals according to their contextual relevance (i.e. multisensory integration). Until recently, most studies of multisensory integration and body ownership concerned only vision, touch and proprioception; the role of other modalities, such as the vestibular system and interoception, has been neglected and remains poorly understood. In particular, no study to date has directly explored the combined effect of vestibular and interoceptive signals on body ownership. Here, we investigated for the first time how Galvanic Vestibular Stimulation (left, right, sham), tactile affectivity (a reclassified interoceptive modality manipulated by applying touch at C-tactile optimal versus non-optimal velocities), and their combination, influence proprioceptive and subjective measures of body ownership during a rubber hand illusion paradigm with healthy participants (N = 26). Our results show that vestibular stimulation (left GVS) significantly increased proprioceptive drift towards the rubber hand during mere visual exposure to the rubber hand. Moreover, it also enhanced participants’ proprioceptive drift towards the rubber hand during manipulations of synchronicity and affective touch. These findings suggest that the vestibular system influences multisensory integration, possibly by re-weighting both the two-way relationship between proprioception and vision, as well as the three-way relationship between proprioception, vision and affective touch. We discuss these findings in relation to current predictive coding models of multisensory integration and body ownership.

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We studied vestibular and affective contributions to body ownership.

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We stimulated the vestibular system in a Rubber Hand paradigm with affective touch.

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Right-hemisphere stimulation increased proprioceptive drift during vision of a RH.

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Applying affective touch further increased proprioceptive drift.

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Affective and vestibular signals may favour vision in multisensory integration.