Seeing touch in the somatosensory cortex: a TMS study of the visual perception of touch

Multisensory integration and motor resonance in the primary motor cortex

Humans are endowed with a motor system that resonates to speech sounds, but whether concurrent visual information from lip movements can improve speech perception at a motor level through multisensory integration mechanisms remains unknown. Therefore, the aim of the study was to explore behavioral and neurophysiological correlates of multisensory influences on motor resonance in speech perception. Motor-evoked potentials (MEPs), by single pulse transcranial magnetic stimulation (TMS) applied over the left lip muscle (orbicularis oris) representation in the primary motor cortex, were recorded in healthy participants during the presentation of syllables in unimodal (visual or auditory) or multisensory (audio-visual) congruent or incongruent conditions. At the behavioral level, subjects showed better syllable identification in the congruent audio-visual condition as compared to the unimodal conditions, hence showing a multisensory enhancement effect. Accordingly, at the neurophysiological level, increased MEPs amplitudes were found in the congruent audio-visual condition, as compared to the unimodal ones. Incongruent audio-visual syllables resulting in illusory percepts did not increase corticospinal excitability, which in fact was comparable to that induced by the real perception of the same syllable. In conclusion, seeing and hearing congruent bilabial syllables increases the excitability of the lip representation in the primary motor cortex, hence documenting that multisensory integration can facilitate speech processing by influencing motor resonance. These findings highlight the modulation role of multisensory processing showing that it can boost speech perception and that multisensory interactions occur not only within higher-order regions, but also within primary motor areas, as shown by corticospinal excitability changes.

Infants' facial electromyographic responses to the sight of emotional interpersonal touch

Adult studies have shown that observed interpersonal touch provides crucial information about others' emotional states. Yet, despite the unique communicative function of touch during development, very little is known about infants' sensitivity to the emotional valence of observed touches. To investigate this issue, we measured facial electromyographic (EMG) activity in response to positive (caress) and negative (scratches) observed touches in a sample of 11-month-old infants. Facial EMG activity was measured over the zygomaticus major (ZM) and corrugator supercilii muscles, respectively involved in positive (i.e., smiling) and negative (i.e., frowning) facial expressions. Results have shown distinct activations of the ZM during the observation of scratches and caresses. In particular, significantly greater activation of the ZM (smiling muscle) emerged specifically in response to the observation of caresses compared to scratches. Our finding suggests that, in infancy, observed affective touches can evoke emotional facial reactions.

How material sensory properties and individual differences influence the haptic aesthetic appeal of visually presented stimuli

Touch plays a crucial role for humans. Despite its centrality in sensory experiences, the field of haptic aesthetics is underexplored. So far, existing research has revealed that preferences in the haptic domain are related to stimulus properties and the Gestalt laws of grouping. Additionally, haptic aesthetics is influenced by top-down processes, e.g., stimulus familiarity, and is likely to be modulated by personality and expertise. To further our understanding of these influences on haptic aesthetic appraisal, the current study investigated the imagined haptic aesthetic appeal of visually presented material surfaces, considering the role of haptic expertise, Need for touch, personality traits. The results revealed a positive influence of familiarity, simplicity, smoothness, warmth, lightness, dryness, slipperiness and a negative influence of complexity on individuals' aesthetic responses. While the study failed to support the predicted influence of Need for touch and haptic expertise on aesthetic responses, results did reveal an influence of openness to experience, conscientiousness and neuroticism. Despite the limitations related to the indirect stimuli presentation (vision only), the findings contribute to the relatively unexplored role of bottom-up and top-down features in haptic aesthetics that might be incorporated into the design of consumers' products to better meet their preferences.

To touch or to be touched? comparing appraisal of vicarious execution and reception of interpersonal touch

Unmyelinated C-Tactile (CT) fibres are activated by caress-like touch, eliciting a pleasant feeling that decreases for static and faster stroking. Previous studies documented this effect also for vicarious touch, hypothesising simulation mechanisms driving the perception and appreciation of observed interpersonal touch. Notably, less is known about appreciation of vicarious execution of touch, that is as referred to the one giving gentle touch. To address this issue, 53 healthy participants were asked to view and rate a series of videoclips displaying an individual being touched by another on hairy (i.e., hand dorsum) or glabrous (i.e., palm) skin sites, with touch being delivered at CT-optimal (5 cm/s) or non-CT optimal velocities (0 cm/s or 30 cm/s). Following the observation of each clip, participants were asked to rate self-referred desirability and model-referred pleasantness of vicarious touch for both executer (toucher-referred) and receiver (touchee-referred). Consistent with the CT fibres properties, for both self-referred desirability and model-referred pleasantness judgements of vicarious touch execution and reception, participants provided higher ratings for vicarious touch delivered at CT-optimal than other velocities, and when observed CT-optimal touch was delivered to the hand-dorsum compared to the palm. However, higher ratings were attributed to vicarious reception compared to execution of CT-optimal touch. Notably, individual differences in interoceptive trusting and attitude to interpersonal touch were positively correlated with, respectively, toucher- and touchee-related overall appraisal ratings of touch. These findings suggest that the appreciation of both toucher- and touchee-referred vicarious touch is specifically attuned to CT-optimal touch, even though they might rely on different neurocognitive mechanisms to understand affective information conveyed by interpersonal tactile interactions.

Levodopa improved different motor symptoms in patients with Parkinson's disease by reducing the functional connectivity of specific thalamic subregions

BACKGROUND

The thalamus is an important relay station for the motor circuit of human. Levodopa can reverse the clinical manifestations by modulating the function of motor circuits, but its detailed mechanisms are still not fully understood. We aimed to explore (1) the mechanism by which levodopa modulates the functional connectivity (FC) in the subregions of the thalamus; (2) the relationship between the changed FC and the improvement of motor symptoms in Parkinson's disease (PD) patients.

METHODS

Resting-state functional MRI was used to scan 36 PD patients and 37 healthy controls. The FC between the subregions in the thalamus and the whole brain was measured and compared under different medication states of PD patients. The correlation between the improvement of motor symptoms and changes in FC in the thalamus subregions was examined.

RESULTS

The PD on state exhibited decreased FC between the right pre-motor thalamus and the right postcentral gyrus, as well as the right lateral pre-frontal thalamus and the right postcentral gyrus. These decreases were positively correlated with the improvement of resting tremor. The PD on state also exhibited decreased FC between the left lateral pre-frontal thalamus and right paracentral lobule, which was positively correlated with the improvement of bradykinesia.

CONCLUSIONS

This study demonstrates that levodopa treats PD by decreasing the FC between the thalamus subregions and pre/post-central cortex. Our results provide a basis for further exploration of the functional activity of thalamic subregions and offer new insights into the precision treatment in PD patients.

Effect of material properties on emotion: a virtual reality study

Introduction

Designers know that part of the appreciation of a product comes from the properties of its materials. These materials define the object's appearance and produce emotional reactions that can influence the act of purchase. Although known and observed as important, the affective level of a material remains difficult to assess. While many studies have been conducted regarding material colors, here we focus on two material properties that drive how light is reflected by the object: its metalness and smoothness. In this context, this work aims to study the influence of these properties on the induced emotional response.

Method

We conducted a perceptual user study in virtual reality, allowing participants to visualize and manipulate a neutral object - a mug. We generated 16 material effects by varying it metalness and smoothness characteristics. The emotional reactions produced by the 16 mugs were evaluated on a panel of 29 people using James Russel's circumplex model, for an emotional measurement through two dimensions: arousal (from low to high) and valence (from negative to positive). This scale, used here through VR users' declarative statements allowed us to order their emotional preferences between all the virtual mugs.

Result

Statistical results show significant positive effects of both metalness and smoothness on arousal and valence. Using image processing features, we show that this positive effect is linked to the increasing strength (i.e., sharpness and contrast) of the specular reflections induced by these material properties.

Discussion

The present work is the first to establish this strong relationship between specular reflections induced by material properties and aroused emotions.

S1 represents multisensory contexts and somatotopic locations within and outside the bounds of the cortical homunculus

Recent literature suggests that tactile events are represented in the primary somatosensory cortex (S1) beyond its long-established topography; in addition, the extent to which S1 is modulated by vision remains unclear. To better characterize S1, human electrophysiological data were recorded during touches to the forearm or finger. Conditions included visually observed physical touches, physical touches without vision, and visual touches without physical contact. Two major findings emerge from this dataset. First, vision strongly modulates S1 area 1, but only if there is a physical element to the touch, suggesting that passive touch observation is insufficient to elicit neural responses. Second, despite recording in a putative arm area of S1, neural activity represents both arm and finger stimuli during physical touches. Arm touches are encoded more strongly and specifically, supporting the idea that S1 encodes tactile events primarily through its topographic organization but also more generally, encompassing other areas of the body.

Associations between digital media use and brain surface structural measures in preschool-aged children

The American Academy of Pediatrics recommends limits on digital media use ("screen time"), citing cognitive-behavioral risks. Media use in early childhood is ubiquitous, though few imaging-based studies have been conducted to quantify impacts on brain development. Cortical morphology changes dynamically from infancy through adulthood and is associated with cognitive-behavioral abilities. The current study involved 52 children who completed MRI and cognitive testing at a single visit. The MRI protocol included a high-resolution T1-weighted anatomical scan. The child's parent completed the ScreenQ composite measure of media use. MRI measures included cortical thickness (CT) and sulcal depth (SD) across the cerebrum. ScreenQ was applied as a predictor of CT and SD first in whole-brain regression analyses and then for regions of interest (ROIs) identified in a prior study of screen time involving adolescents, controlling for sex, age and maternal education. Higher ScreenQ scores were correlated with lower CT in right-lateralized occipital, parietal, temporal and fusiform areas, and also lower SD in right-lateralized inferior temporal/fusiform areas, with substantially greater statistical significance in ROI-based analyses. These areas support primary visual and higher-order processing and align with prior findings in adolescents. While differences in visual areas likely reflect maturation, those in higher-order areas may suggest under-development, though further studies are needed.

Photographs of Actions: What Makes Them Special Cues to Social Perception

I have reviewed studies on neural responses to pictured actions in the action observation network (AON) and the cognitive functions of these responses. Based on this review, I have analyzed the specific representational characteristics of action photographs. There has been consensus that AON responses provide viewers with knowledge of observed or pictured actions, but there has been controversy about the properties of this knowledge. Is this knowledge causally provided by AON activities or is it dependent on conceptual processing? What elements of actions does it refer to, and how generalized or specific is it? The answers to these questions have come from studies that used transcranial magnetic stimulation (TMS) to stimulate motor or somatosensory cortices. In conjunction with electromyography (EMG), TMS allows researchers to examine changes of the excitability in the corticospinal tract and muscles of people viewing pictured actions. The timing of these changes and muscle specificity enable inferences to be drawn about the cognitive products of processing pictured actions in the AON. Based on a review of studies using TMS and other neuroscience methods, I have proposed a novel hypothetical account that describes the characteristics of action photographs that make them effective cues to social perception. This account includes predictions that can be tested experimentally.

Paired associative stimulations: Novel tools for interacting with sensory and motor cortical plasticity

In the early 2000s, a novel non-invasive brain stimulation protocol, the paired associative stimulation (PAS), was introduced, allowing to induce and investigate Hebbian associative plasticity within the humans' motor system, with patterns resembling spike-timing-dependent plasticity properties found in cellular models. Since this evidence, PAS efficacy has been proved in healthy, and to a lesser extent, in clinical populations. Recently, novel 'modified' protocols targeting sensorimotor and crossmodal networks appeared in the literature. In the present work, we have reviewed recent advances using these 'modified' PAS protocols targeting sensory and motor cortical networks. To better categorize them, we propose a novel classification according to the nature of the peripheral and cortical stimulations (i.e., within-system, cross-systems, and cortico-cortical PAS). For each protocol of the categories mentioned above, we describe and discuss their main features, how they have been used to study and promote brain plasticity, and their advantages and disadvantages. Overall, current evidence suggests that these novel non-invasive brain stimulation protocols represent very promising tools to study the plastic properties of humans' sensorimotor and crossmodal networks, both in the healthy and in the damaged central nervous system.

Neural time course of pain observation in infancy

Perception of pain in others is of great evolutionary significance for the development of human empathy. However, infants' sensitivity to others' painful experiences has not been investigated so far. Here, we explored the neural time course of infants' processing of others' pain by measuring event-related brain potentials (ERPs) while 6-month-old infants observed a painful tactile stimulation directed towards the eye and a neutral tactile stimulation on the eyebrow. We analyzed both the Negative Central (Nc) and the later Late Positive Potential (LPP) ERP components, indexing respectively attention allocation and cognitive evaluation of perceptual stimuli. Results showed that observing painful touch elicits a mid-latency Nc (300-500 ms) over the right fronto-central site, which is greater in amplitude as compared to neutral touch. A divergent activity was also visible in the centro-parietal early (550-750 ms) and late (800-1000 ms) LPP, showing increased amplitudes in response to neutral compared to painful touch. The cognitive evaluation of painful stimuli, reflected by the LPP, might thus not be fully developed at 6 months of age, as adults typically show a larger LPP in response to painful as compared to neutral stimuli. Overall, infants show early attentional attuning to others' pain. This early sensitivity to others' painful tactile experiences might form a prerequisite for the development of human empathy.

Cross-modal involvement of the primary somatosensory cortex in visual working memory: A repetitive TMS study

Recent literature suggests that the primary somatosensory cortex (S1), once thought to be a low-level area only modality-specific, is also involved in higher-level, cross-modal, cognitive functions. In particular, electrophysiological studies have highlighted that the cross-modal activation of this area may also extend to visual Working Memory (WM), being part of a mnemonic network specific for the temporary storage and manipulation of visual information concerning bodies and body-related actions. However, the causal recruitment of S1 in the WM network remains speculation. In the present study, by taking advantage of repetitive Transcranial Magnetic Stimulation (rTMS), we look for causal evidence that S1 is implicated in the retention of visual stimuli that are salient for this cortical area. To this purpose, in a first experiment, high-frequency (10 Hz) rTMS was delivered over S1 of the right hemisphere, and over two control sites, the right lateral occipital cortex (LOC) and the right dorsolateral prefrontal cortex (dlPFC), during the maintenance phase of a high-load delayed match-to-sample task in which body-related visual stimuli (non-symbolic hand gestures) have to be retained. In a second experiment, the specificity of S1 recruitment was deepened by using a version of the delayed match-to-sample task in which visual stimuli depict geometrical shapes (non-body related stimuli). Results show that rTMS perturbation of S1 activity leads to an enhancement of participants' performance that is selective for body-related visual stimuli; instead, the stimulation of the right LOC and dlPFC does not affect the temporary storage of body-related visual stimuli. These findings suggest that S1 may be recruited in visual WM when information to store (and recall) is salient for this area, corroborating models which suggest the existence of a dedicated mnemonic system for body-related information in which also somatosensory cortices play a key role, likely thanks to their cross-modal (visuo-tactile) properties.

Effects of online repetitive transcranial magnetic stimulation (rTMS) on cognitive processing: A meta-analysis and recommendations for future studies

Online repetitive transcranial magnetic stimulation (rTMS), applied while subjects are performing a task, is widely used to disrupt brain regions underlying cognition. However, online rTMS has also induced "paradoxical enhancement". Given the rapid proliferation of this approach, it is crucial to develop a better understanding of how online stimulation influences cognition, and the optimal parameters to achieve desired effects. To accomplish this goal, a quantitative meta-analysis was performed with random-effects models fitted to reaction time (RT) and accuracy data. The final dataset included 126 studies published between 1998 and 2016, with 244 total effects for reaction times, and 202 for accuracy. Meta-analytically, rTMS at 10 Hz and 20 Hz disrupted accuracy for attention, executive, language, memory, motor, and perception domains, while no effects were found with 1 Hz or 5 Hz. Stimulation applied at and 10 and 20 Hz slowed down RTs in attention and perception tasks. No performance enhancement was found. Meta-regression analysis showed that fMRI-guided targeting and short inter-trial intervals are associated with increased disruptive effects with rTMS.

Somatosensory cortical representation of the body size

The knowledge of the size of our own body parts is essential for accurately moving in space and efficiently interact with objects. A distorted perceptual representation of the body size often represents a core diagnostic criterion for some psychopathological conditions. The metric representation of the body was shown to depend on somatosensory afferences: local deafferentation indeed causes a perceptual distortion of the size of the anesthetized body part. A specular effect can be induced by altering the cortical map of body parts in the primary somatosensory cortex. Indeed, the present study demonstrates, in healthy adult participants, that repetitive Transcranial Magnetic Stimulation to the somatosensory cortical map of the hand in both hemispheres causes a perceptual distortion (i.e., an overestimation) of the size of the participants' own hand (Experiments 1-3), which does not involve other body parts (i.e., the foot, Experiment 2). Instead, the stimulation of the inferior parietal lobule of both hemispheres does not affect the perception of the own body size (Experiment 4). These results highlight the role of the primary somatosensory cortex in the building up and updating of the metric of body parts: somatosensory cortical activity not only shapes our somatosensation, it also affects how we perceive the dimension of our body.

Affective judgement of social touch on a hand associated with hand embodiment

Social touch constitutes a critical component of human interactions. A gentle tap on the hand, for instance, can sometimes create emotional bonding and reduce interpersonal distance in social interactions. Evidence of tactile empathy suggests that touch can be experienced through both physical sensation and observation, yet vicarious perception of observed touch on an object as a function of the object’s conceptual representation (e.g., Is this object identified as mine? Does this object feel like part of me?) remains less explored. Here we examined the affective judgement of social touch when the illusory sense of ownership over a dummy hand was manipulated through the rubber-hand illusion. When the same social touch was performed on either the real or the dummy hand, we found a similar sense of perceived pleasantness between the felt and observed touch, but only when the dummy hand was embodied; when it was not, the perceived pleasantness of the observed touch was lesser (an “embodiment effect”; Experiment 1). In addition, we found that the embodiment effect associated with the observed touch was insensitive to the way in which embodiment was manipulated (Experiment 2), and that this effect was specific to social but not neutral touch (Experiment 3). Taken together, our findings suggest a role of embodiment in the affective component of observed social touch and contribute to our understanding of tactile empathy for objects.

Sensorimotor uncertainty modulates corticospinal excitability during skilled object manipulation

Sensorimotor memory built through previous hand-object interactions allows subjects to plan grasp forces. The memory-based mechanism is particularly effective when contact points on the object do not change across multiple manipulations, thus allowing subjects to generate the same forces in a feedforward fashion. However, allowing subjects to choose where to grasp an object causes trial-to-trial variability in fingertip positioning, suggesting a decreased ability to predict where the object will be grasped. In this scenario, subjects modulate forces on a trial-to-trial basis as a function of fingertip positioning. We suggested that this fingertip force-to-position modulation could be implemented by transforming feedback of digit placement into an accurate distribution of fingertip forces. Thus, decreasing certainty of fingertip position on an object would cause a shift from predominantly memory- to feedback-based force control mechanisms. To gain further insight into these sensorimotor transformation mechanisms, we asked subjects to grasp and lift an object with an asymmetrical center of mass while preventing it from tilting. To isolate the effect of digit placement uncertainty, we designed two experimental conditions that differed in terms of predictability of fingertip position but had similar average fingertip positioning and force distribution. We measured corticospinal excitability to probe possible changes in sensorimotor processing associated with digit placement uncertainty. We found a differential effect of sensorimotor uncertainty after but not before object contact. Our results suggest that sensorimotor integration is rapidly tuned after object contact based on different processing demands for memory versus feedback mechanisms underlying the control of manipulative forces. NEW & NOTEWORTHY The relative contribution of predictive and feedback mechanisms for scaling digit forces to position during dexterous manipulation depends on the predictability of where the object will be grasped. We found that corticospinal excitability shortly after contact was sensitive to digit position predictability. This supports the proposition that distinct sensorimotor integration processes are engaged, depending on the role of feedback about digit placement versus sensorimotor memory in controlling manipulative forces.

The role of the inferior frontal gyrus in vicarious social touch: A transcranial direct current stimulation (tDCS) study

The neural mechanisms facilitating the experience of vicarious social touch are largely unknown. The right inferior frontal gyrus (rIFG) has been suggested as part of a simulation observation-execution neural network that plays a key role in the perception of tactile stimuli. Considering that vicarious social touch involves vicarious sharing of emotions, we hypothesized that emotional empathy, i.e., the ability to feel what another individual is feeling, modulates the neural responses to vicarious touch. To examine the role of the rIFG in vicarious touch and its modulation by levels of emotional empathy, we used anodal transcranial direct current stimulation (tDCS) on forty participants who observed photos depicting social touch, nonsocial touch or no touch during tDCS or sham stimulation. The results show that while participants with high levels of emotional empathy exhibited no change in ratings of vicarious social touch, participants with low levels of emotional empathy rate human touch as more emotional following anodal stimulation of the rIFG than following sham stimulation. These findings indicate that emotional responses to vicarious social touch are associated with rIFG activity and are modulated by levels of emotional empathy. This result has major therapeutic potential for individuals with low empathic abilities, such as those with ASD.

The multidimensional representational space of observed socio-affective touch experiences

Observed touch interactions provide useful information on how others communicate with the external world. Previous studies revealed shared neural circuits between the direct experience and the passive observation of simple touch, such as being stroked/slapped. Here, we investigate the complexity of the neural representations underlying the understanding of others' socio-affective touch interactions. Importantly, we use a recently developed touch database that contains a larger range of more complex social and non-social touch interactions. Participants judged affective aspects of each touch event and were scanned while watching the same videos. Using correlational multivariate pattern analysis methods, we obtained neural similarity matrices in 18 regions of interest from five different networks: somatosensory, pain, the theory of mind, visual and motor regions. Among them, four networks except motor cortex represent the social nature of the touch, whereas fine-detailed affective information is reflected in more targeted areas such as social brain regions and somatosensory cortex. Lastly, individual social touch preference at the behavioral level was correlated with the involvement of somatosensory areas on representing affective information, suggesting that individuals with higher social touch preference exhibit stronger vicarious emotional responses to others' social touch experiences. Together, these results highlight the overall complexity and the individual modulation of the distributed neural representations underlying the processing of observed socio-affective touch.

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∙Neural bases of observed touch are investigated with touch videos and MVPA.

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∙Social touch evokes stronger activation in the theory of mind (ToM) network.

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∙The ToM network represents affective meanings of observed social touch events.

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∙Affective representations of observed touch are present in somatosensory areas.

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∙Affective representations in S1 relate to individual's attitude towards touch.

Socio-affective touch expression database

Socio-affective touch communication conveys a vast amount of information about emotions and intentions in social contexts. In spite of the complexity of the socio-affective touch expressions we use daily, previous studies addressed only a few aspects of social touch mainly focusing on hedonics, such as stroking, leaving a wide range of social touch behaviour unexplored. To overcome this limit, we present the Socio-Affective Touch Expression Database (SATED), which includes a large range of dynamic interpersonal socio-affective touch events varying in valence and arousal. The original database contained 26 different social touch expressions each performed by three actor pairs. To validate each touch expression, we conducted two behavioural experiments investigating perceived naturalness and affective values. Based on the rated naturalness and valence, 13 socio-affective touch expressions along with 12 corresponding non-social touch events were selected as a complete set, achieving 75 video clips in total. Moreover, we quantified motion energy for each touch expression to investigate its intrinsic correlations with perceived affective values and its similarity among actor- and action-pairs. As a result, the touch expression database is not only systematically defined and well-controlled, but also spontaneous and natural, while eliciting clear affective responses. This database will allow a fine-grained investigation of complex interpersonal socio-affective touch in the realm of social psychology and neuroscience along with potential application areas in affective computing and neighbouring fields.

Studying and modifying brain function with non-invasive brain stimulation

In the past three decades, our understanding of brain-behavior relationships has been significantly shaped by research using non-invasive brain stimulation (NIBS) techniques. These methods allow non-invasive and safe modulation of neural processes in the healthy brain, enabling researchers to directly study how experimentally altered neural activity causally affects behavior. This unique property of NIBS methods has, on the one hand, led to groundbreaking findings on the brain basis of various aspects of behavior and has raised interest in possible clinical and practical applications of these methods. On the other hand, it has also triggered increasingly critical debates about the properties and possible limitations of these methods. In this review, we discuss these issues, clarify the challenges associated with the use of currently available NIBS techniques for basic research and practical applications, and provide recommendations for studies using NIBS techniques to establish brain-behavior relationships.

Primary somatosensory cortex necessary for the perception of weight from other people's action: A continuous theta-burst TMS experiment

The presence of a network of areas in the parietal and premotor cortices, which are active both during action execution and observation, suggests that we might understand the actions of other people by activating those motor programs for making similar actions. Although neurophysiological and imaging studies show an involvement of the somatosensory cortex (SI) during action observation and execution, it is unclear whether SI is essential for understanding the somatosensory aspects of observed actions. To address this issue, we used off-line transcranial magnetic continuous theta-burst stimulation (cTBS) just before a weight judgment task. Participants observed the right hand of an actor lifting a box and estimated its relative weight. In counterbalanced sessions, we delivered sham and active cTBS over the hand region of the left SI and, to test anatomical specificity, over the left motor cortex (M1) and the left superior parietal lobule (SPL). Active cTBS over SI, but not over M1 or SPL, impaired task performance relative to sham cTBS. Moreover, active cTBS delivered over SI just before participants were asked to evaluate the weight of a bouncing ball did not alter performance compared to sham cTBS. These findings indicate that SI is critical for extracting somatosensory features (heavy/light) from observed action kinematics and suggest a prominent role of SI in action understanding.

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TMS over the somatosensory cortex disrupts performance on a weight judgment task.

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Disruption is specific for judgements based on observed human actions.

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No TMS effect is found for judgements based on observed non-human motion.

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No effect is found when TMS is administered over nearby frontal and parietal region.

Inter-Individual Differences in Vicarious Tactile Perception: a View Across the Lifespan in Typical and Atypical Populations

Touch is our most interpersonal sense, and so it stands to reason that we represent not only our own bodily experiences, but also those felt by others. This review will summarise brain and behavioural research on vicarious tactile perception (mirror touch). Specifically, we will focus on vicarious touch across the lifespan in typical and atypical groups, and will identify the knowledge gaps that are in urgent need of filling by examining what is known about how individuals differ within and between typical and atypical groups.

Dissecting neural circuits for multisensory integration and crossmodal processing

We rely on rich and complex sensory information to perceive and understand our environment. Our multisensory experience of the world depends on the brain's remarkable ability to combine signals across sensory systems. Behavioural, neurophysiological and neuroimaging experiments have established principles of multisensory integration and candidate neural mechanisms. Here we review how targeted manipulation of neural activity using invasive and non-invasive neuromodulation techniques have advanced our understanding of multisensory processing. Neuromodulation studies have provided detailed characterizations of brain networks causally involved in multisensory integration. Despite substantial progress, important questions regarding multisensory networks remain unanswered. Critically, experimental approaches will need to be combined with theory in order to understand how distributed activity across multisensory networks collectively supports perception.

Encoding of Touch Intensity But Not Pleasantness in Human Primary Somatosensory Cortex

Growing interest in affective touch has delineated a neural network that bypasses primary somatosensory cortex (S1). Several recent studies, however, have cast doubt on the segregation of touch discrimination and affect, suggesting that S1 also encodes affective qualities. We used functional magnetic resonance imaging (fMRI) and repetitive transcranial magnetic stimulation (rTMS) to examine the role of S1 in processing touch intensity and pleasantness. Twenty-six healthy human adults rated brushing on the hand during fMRI. Intensity ratings significantly predicted activation in S1, whereas pleasantness ratings predicted activation only in the anterior cingulate cortex. Nineteen subjects also received inhibitory rTMS over right hemisphere S1 and the vertex (control). After S1 rTMS, but not after vertex rTMS, sensory discrimination was reduced and subjects with reduced sensory discrimination rated touch as more intense. In contrast, rTMS did not alter ratings of touch pleasantness. Our findings support divergent neural processing of touch intensity and pleasantness, with affective touch encoded outside of S1.

SIGNIFICANCE STATEMENT

Growing interest in affective touch has identified a neural network that bypasses primary somatosensory cortex (S1). Several recent studies, however, cast doubt on the separation of touch discrimination and affect. We used functional magnetic resonance imaging and repetitive transcranial magnetic stimulation to demonstrate the representation of touch discrimination and intensity in S1, but the representation of pleasantness in the anterior cingulate cortex, not S1. Our findings support divergent neural processing of touch intensity and pleasantness, with affective touch encoded outside of S1. Our study contributes to growing delineation of the affective touch system, a crucial step in understanding its dysregulation in numerous clinical conditions such as autism, eating disorders, depression, and chronic pain.

Primary somatosensory contribution to action observation brain activity-combining fMRI and cTBS

Traditionally the mirror neuron system (MNS) only includes premotor and posterior parietal cortices. However, somatosensory cortices, BA1/2 in particular, are also activated during action execution and observation. Here, we examine whether BA1/2 and the parietofrontal MNS integrate information by using functional magnetic resonance imaging (fMRI)-guided continuous theta-burst stimulation (cTBS) to perturb BA1/2. Measuring brain activity using fMRI while participants are under the influence of cTBS shows local cTBS effects in BA1/2 varied, with some participants showing decreases and others increases in the BOLD response to viewing actions vs control stimuli. We show how measuring cTBS effects using fMRI can harness this variance using a whole-brain regression. This analysis identifies brain regions exchanging action-specific information with BA1/2 by mapping voxels away from the coil with cTBS-induced, action-observation-specific BOLD contrast changes that mirror those under the coil. This reveals BA1/2 exchanges action-specific information with premotor, posterior parietal and temporal nodes of the MNS during action observation. Although anatomical connections between BA1/2 and these regions are well known, this is the first demonstration that these connections carry action-specific signals during observation and hence, that BA1/2 plays a causal role in the human MNS.

ALE meta-analysis reveals dissociable networks for affective and discriminative aspects of touch

Emotionally-laden tactile stimulation-such as a caress on the skin or the feel of velvet-may represent a functionally distinct domain of touch, underpinned by specific cortical pathways. In order to determine whether, and to what extent, cortical functional neuroanatomy supports a distinction between affective and discriminative touch, an activation likelihood estimate (ALE) meta-analysis was performed. This meta-analysis statistically mapped reported functional magnetic resonance imaging (fMRI) activations from 17 published affective touch studies in which tactile stimulation was associated with positive subjective evaluation (n = 291, 34 experimental contrasts). A separate ALE meta-analysis mapped regions most likely to be activated by tactile stimulation during detection and discrimination tasks (n = 1,075, 91 experimental contrasts). These meta-analyses revealed dissociable regions for affective and discriminative touch, with posterior insula (PI) more likely to be activated for affective touch, and primary somatosensory cortices (SI) more likely to be activated for discriminative touch. Secondary somatosensory cortex had a high likelihood of engagement by both affective and discriminative touch. Further, meta-analytic connectivity (MCAM) analyses investigated network-level co-activation likelihoods independent of task or stimulus, across a range of domains and paradigms. Affective-related PI and discriminative-related SI regions co-activated with different networks, implicated in dissociable functions, but sharing somatosensory co-activations. Taken together, these meta-analytic findings suggest that affective and discriminative touch are dissociable both on the regional and network levels. However, their degree of shared activation likelihood in somatosensory cortices indicates that this dissociation reflects functional biases within tactile processing networks, rather than functionally and anatomically distinct pathways.

Locomotor adaptation is modulated by observing the actions of others

Observing the motor actions of another person could facilitate compensatory motor behavior in the passive observer. Here we explored whether action observation alone can induce automatic locomotor adaptation in humans. To explore this possibility, we used the "broken escalator" paradigm. Conventionally this involves stepping upon a stationary sled after having previously experienced it actually moving (Moving trials). This history of motion produces a locomotor aftereffect when subsequently stepping onto a stationary sled. We found that viewing an actor perform the Moving trials was sufficient to generate a locomotor aftereffect in the observer, the size of which was significantly correlated with the size of the movement (postural sway) observed. Crucially, the effect is specific to watching the task being performed, as no motor adaptation occurs after simply viewing the sled move in isolation. These findings demonstrate that locomotor adaptation in humans can be driven purely by action observation, with the brain adapting motor plans in response to the size of the observed individual's motion. This mechanism may be mediated by a mirror neuron system that automatically adapts behavior to minimize movement errors and improve motor skills through social cues, although further neurophysiological studies are required to support this theory. These data suggest that merely observing the gait of another person in a challenging environment is sufficient to generate appropriate postural countermeasures, implying the existence of an automatic mechanism for adapting locomotor behavior.

Common coding and dynamic interactions between observed, imagined, and experienced motor and somatosensory activity

Motor imagery and perception - considered generally as forms of motor simulation - share overlapping neural representations with motor production. While much research has focused on the extent of this "common coding," less attention has been paid to how these overlapping representations interact. How do imagined, observed, or produced actions influence one another, and how do we maintain control over our perception and behavior? In the first part of this review we describe interactions between motor production and motor simulation, and explore apparent regulatory mechanisms that balance these processes. Next, we consider the somatosensory system. Numerous studies now support a "sensory mirror system" comprised of neural representations activated by either afferent sensation or vicarious sensation. In the second part of this review we summarize evidence for shared representations of sensation and sensory simulation (including imagery and observed sensation), and suggest that similar interactions and regulation of simulation occur in the somatosensory domain as in the motor domain. We suggest that both motor and somatosensory simulations are flexibly regulated to support simulations congruent with our sensorimotor experience and goals and suppress or separate the influence of those that are not. These regulatory mechanisms are frequently revealed by cases of brain injury but can also be employed to facilitate sensorimotor rehabilitation.

Seeing is not feeling: posterior parietal but not somatosensory cortex engagement during touch observation

Observing touch has been reported to elicit activation in human primary and secondary somatosensory cortices and is suggested to underlie our ability to interpret other's behavior and potentially empathy. However, despite these reports, there are a large number of inconsistencies in terms of the precise topography of activation, the extent of hemispheric lateralization, and what aspects of the stimulus are necessary to drive responses. To address these issues, we investigated the localization and functional properties of regions responsive to observed touch in a large group of participants (n = 40). Surprisingly, even with a lenient contrast of hand brushing versus brushing alone, we did not find any selective activation for observed touch in the hand regions of somatosensory cortex but rather in superior and inferior portions of neighboring posterior parietal cortex, predominantly in the left hemisphere. These regions in the posterior parietal cortex required the presence of both brush and hand to elicit strong responses and showed some selectivity for the form of the object or agent of touch. Furthermore, the inferior parietal region showed nonspecific tactile and motor responses, suggesting some similarity to area PFG in the monkey. Collectively, our findings challenge the automatic engagement of somatosensory cortex when observing touch, suggest mislocalization in previous studies, and instead highlight the role of posterior parietal cortex.

Synchronization to auditory and visual rhythms in hearing and deaf individuals

A striking asymmetry in human sensorimotor processing is that humans synchronize movements to rhythmic sound with far greater precision than to temporally equivalent visual stimuli (e.g., to an auditory vs. a flashing visual metronome). Traditionally, this finding is thought to reflect a fundamental difference in auditory vs. visual processing, i.e., superior temporal processing by the auditory system and/or privileged coupling between the auditory and motor systems. It is unclear whether this asymmetry is an inevitable consequence of brain organization or whether it can be modified (or even eliminated) by stimulus characteristics or by experience. With respect to stimulus characteristics, we found that a moving, colliding visual stimulus (a silent image of a bouncing ball with a distinct collision point on the floor) was able to drive synchronization nearly as accurately as sound in hearing participants. To study the role of experience, we compared synchronization to flashing metronomes in hearing and profoundly deaf individuals. Deaf individuals performed better than hearing individuals when synchronizing with visual flashes, suggesting that cross-modal plasticity enhances the ability to synchronize with temporally discrete visual stimuli. Furthermore, when deaf (but not hearing) individuals synchronized with the bouncing ball, their tapping patterns suggest that visual timing may access higher-order beat perception mechanisms for deaf individuals. These results indicate that the auditory advantage in rhythmic synchronization is more experience- and stimulus-dependent than has been previously reported.

Sensory motor mechanisms unify psychology: the embodiment of culture

Sensorimotor mechanisms can unify explanations at cognitive, social, and cultural levels. As an example, we review how anticipated motor effort is used by individuals and groups to judge distance: the greater the anticipated effort the greater the perceived distance. Anticipated motor effort can also be used to understand cultural differences. People with interdependent self- construals interact almost exclusively with in-group members, and hence there is little opportunity to tune their sensorimotor systems for interaction with out-group members. The result is that interactions with out-group members are expected to be difficult and out-group members are perceived as literally more distant. In two experiments we show (a) interdependent Americans, compared to independent Americans, see American confederates (in-group) as closer; (b) interdependent Arabs, compared to independent Arabs, perceive Arab confederates (in- group) as closer, whereas interdependent Americans perceive Arab confederates (out-group) as farther. These results demonstrate how the same embodied mechanism can seamlessly contribute to explanations at the cognitive, social, and cultural levels.

Perturbing the action observation network during perception and categorization of actions' goals and grips: state-dependency and virtual lesion TMS effects

Watching others grasping and using objects activates an action observation network (AON), including inferior frontal (IFC), anterior intraparietal (AIP), and somatosensory cortices (S1). Yet, causal evidence of the differential involvement of such AON sensorimotor nodes in representing high- and low-level action components (i.e., end-goals and grip type) is meager. To address this issue, we used transcranial magnetic stimulation-adaptation (TMS-A) during 2 novel action perception tasks. Participants were shown adapting movies displaying a demonstrator performing goal-directed actions with a tool, using either power or precision grips. They were then asked to match the end-goal (Goal-recognition task) or the grip (Grip-recognition task) of actions shown in test pictures to the adapting movies. TMS was administered over IFC, AIP, or S1 during presentation of test pictures. Virtual lesion-like effects were found in the Grip-recognition task where IFC stimulation induced a general performance decrease, suggesting a critical role of IFC in perceiving grips. In the Goal-recognition task, IFC and S1 stimulation differently affected the processing of "adapted" and "nonadapted" goals. These "state-dependent" effects suggest that the overall goal of seen actions is encoded into functionally distinct and spatially overlapping neural populations in IFC-S1 and such encoding is critical for recognizing and understanding end-goals.

Distributed neural networks of tactile working memory

Microelectrode recordings of cortical activity in primates performing working memory tasks reveal some cortical neurons exhibiting sustained or graded persistent elevations in firing rate during the period in which sensory information is actively maintained in short-term memory. These neurons are called "memory cells". Imaging and transcranial magnetic stimulation studies indicate that memory cells may arise from distributed cortical networks. Depending on the sensory modality of the memorandum in working memory tasks, neurons exhibiting memory-correlated patterns of firing have been detected in different association cortices including prefrontal cortex, and primary sensory cortices as well. Here we elaborate on neurophysiological experiments that lead to our understanding of the neuromechanisms of working memory, and mainly discuss findings on widely distributed cortical networks involved in tactile working memory.

Vicarious motor activation during action perception: beyond correlational evidence

Neurophysiological and imaging studies have shown that seeing the actions of other individuals brings about the vicarious activation of motor regions involved in performing the same actions. While this suggests a simulative mechanism mediating the perception of others' actions, one cannot use such evidence to make inferences about the functional significance of vicarious activations. Indeed, a central aim in social neuroscience is to comprehend how vicarious activations allow the understanding of other people's behavior, and this requires to use stimulation or lesion methods to establish causal links from brain activity to cognitive functions. In the present work, we review studies investigating the effects of transient manipulations of brain activity or stable lesions in the motor system on individuals' ability to perceive and understand the actions of others. We conclude there is now compelling evidence that neural activity in the motor system is critical for such cognitive ability. More research using causal methods, however, is needed in order to disclose the limits and the conditions under which vicarious activations are required to perceive and understand actions of others as well as their emotions and somatic feelings.

Understanding others' feelings: the role of the right primary somatosensory cortex in encoding the affective valence of others' touch

Brain imaging studies in humans have shown the existence of a shared somatosensory representation in the primary somatosensory cortex (S1), putatively involved in understanding others' sensations (Keysers et al., 2010); however, the role of S1 in such a high-level process is still unknown. To ascertain the causal involvement of S1, and its possible hemispheric lateralization, in encoding the affective valence of emotional scenes, depicting, or not, a tactile event, we gave to healthy participants a picture-based affective go/no-go task and low-frequency repetitive transcranial magnetic stimulation (rTMS). The dorsolateral prefrontal cortex (DLPFC) was chosen as control site. rTMS over the right, but not the left, S1 selectively increased the participants' latencies in the affective go/no-go task, but only when the affective state was conveyed by touch; intriguingly, this interfering effect was associated with the empathic ability to adopt the subjective perspective of others. The left, not the right, DLPFC is also involved in affective go/no-go performance, but regardless of the sight of touch, and independently of empathic abilities. This novel evidence demonstrates the crossmodal role of right S1 in encoding the pleasant and aversive consequences of others' sensations evoked by touch.

The functional architecture of S1 during touch observation described with 7 T fMRI

Recent studies indicate that the primary somatosensory cortex (S1) is active not only when touch is physically perceived but also when it is merely observed to be experienced by another person. This social responsivity of S1 has important implications for our understanding of S1 functioning. However, S1 activity during touch observation has not been characterized in great detail to date. We focused on two features of the S1 functional architecture during touch observation, namely the topographical arrangement of index and middle finger receptive fields (RFs), and their dynamic shrinkage during concurrent activation. Both features have important implications for human behavior. We conducted two fMRI studies at 7 T, one where touch was physically perceived, and one where touch was observed. In the two experiments, participants either had their index finger and/or middle finger stimulated using paintbrushes, or just observed similar touch events on video. Our data show that observing and physically experiencing touch elicits overlapping activity changes in S1. In addition, observing touch to the index finger or the middle finger alone evoked topographically arranged activation foci in S1. Importantly, when co-activated, the index and middle finger RFs not only shrank during physical touch perception, but also during touch observation. Our data, therefore, indicate a similarity between the functional architecture of S1 during touch observation and physical touch perception with respect to single-digit topography and RF shrinkage. These results may allow the tentative conclusion that even primary somatosensory experiences, such as physical touch perception, can be shared amongst individuals.

Transcranial magnetic stimulation over human secondary somatosensory cortex disrupts perception of pain intensity

Pain is a complex sensory experience resulting from the activity of a network of brain regions. However, the functional contribution of individual regions in this network remains poorly understood. We delivered single-pulse transcranial magnetic stimulation (TMS) to the contralateral primary somatosensory cortex (S1), secondary somatosensory cortex (S2) and vertex (control site) 120 msec after selective stimulation of nociceptive afferents using neodymium:yttrium–aluminium–perovskite (Nd:YAP) laser pulses causing painful sensations. Participants were required to judge either the intensity (medium/high) or the spatial location (proximal/distal) of the stimulus in a two-alternative forced choice paradigm. When TMS pulses were delivered over S2, participants' ability to judge pain intensity was disrupted, as compared to S1 and vertex (control) stimulation. Signal-detection analysis demonstrated a loss of sensitivity to stimulation intensity, rather than a shift in perceived pain level or response bias. We did not find any effect of TMS on the ability to localise nociceptive stimuli on the skin. The novel finding that TMS over S2 can disrupt perception of pain intensity suggests a causal role for S2 in encoding of pain intensity.

Neural time-course of the observation of human and non-human object touch

Recent functional magnetic resonance imaging studies have reported activation of primary and secondary somatosensory cortices when participants observe another person or object being touched. In this study, we used event-related potentials to examine the nature and time-course of the neural mechanisms associated with the observation of humans and non-human objects being touched. Participants were presented with short video clips of a human arm or a non-human cylindrical object being touched by an object, compared with an object moving in front of the arms or cylinders without touching them. Touch vs non-touch effects were observed in the amplitudes of the N100 and N250 components, as well as a late slow wave component (500-600 ms), measured from electrodes over primary somatosensory cortex. Human vs non-human stimulus effects were reflected in the latencies of the N100, P170 and N250 components recorded over somatosensory cortex, as well as the temporal-parietal visual-perceptual N170 and N250 components. These findings suggest that human and non-human touch observation are associated with somatosensory processing at both an early sensory-perceptual stage and a relatively late cognitive stage, both preceding and following the perceptual encoding of the humanness of stimuli that typically occurs in extrastriate visual areas.

Visual perception of bodily interactions in the primary somatosensory cortex

Brain imaging studies in humans have revealed the existence of a visuo-tactile system, which matches observed touch with felt touch. In this system, the primary somatosensory cortex (SI) appears to play a causal role in the visual processing of tactile events. Whether this visuo-tactile mechanism for touch in SI applies to the sight of 'any' touch, or whether it is restricted to the domain of body-related tactile experiences remains unresolved. To address this issue, repetitive transcranial magnetic stimulation (rTMS) was used to determine whether activity in SI is strictly related to the visual processing of human body-part interactions, or is also involved in processing the contact between inanimate objects, or between human body-parts and objects. The results show that rTMS over SI selectively impaired the processing of a contralateral visual stimulus depicting a human body-part being touched by a human agent, while it did not affect the visual perception of contact between objects, or between human body-parts and objects. Correlation analysis shows that this effect was associated with the intensity and embodiment of the observed touched. This result suggests that SI is more suited to represent social touch, contributing to our understanding of the effect of interpersonal tactile interactions between people.