The aim of this investigation was to determine whether i) affective touch has a role in mediating beneficial social influences on resilience to depression and ii) whether affective touch acts through specific skin CT afferents to enhance central serotonin function. To develop and validate the Touch Experiences and Attitudes Questionnaire (TEAQ), 117 items about experiences and attitudes to touch were completed online by 618 participants. Principal components analysis reduced this to 57 items and 6 factors. Three factors concerned touch experienced; in social situations (CST), in intimate relationships (CIT) and during childhood (ChT) and 3 factors concerned attitude to touch; in intimate relationships (AIT), with unfamiliar people (AUT) and in Skin Care (ASkC). The shortened TEAQ was completed by a second sample of 704 participants. Confirmatory factor analysis found the 6 factor structure to be a good fit of the data, suggesting the TEAQ to be valid and reliable. Participants completed some demographic questions and some questionnaires to determine their current psychiatric symptoms, social circumstances, recent life events, childhood adversity and personality alongside the TEAQ. Currently depressed participants had lower touch scores for all factors compared to healthy controls. Remitted depressed participants had significantly lower touch scores on all factors except CST, ASkC and AIT compared to healthy controls. A multiple regression analysis found neuroticism, satisfaction with social support, recent life events, CIT and childhood adversity (CHA) to be predictive of depression, whereas extraversion, number of social supports, ChT and CST, did not significantly predict depression score. Logistic regression analysis found ChT, CHA and neuroticism to predict vulnerability to depression, but not AIT or AUT. It was concluded that CIT was the most important aspect of affective touch for promoting resilience to depression. The CNS effects of pleasant and unpleasant touch were investigated using fMRI in healthy female volunteers. It has been hypothesised that a novel class of CT afferent fibres in hairy skin encodes affective touch. Therefore, CNS responses to pleasant stroking of the forearm with stroking of the fingers were compared. No differential CNS effects of forearm stroking over finger stroking were seen. Indeed, more brain regions were activated by pleasant brush stroking of the fingers which lack CT afferents. Pleasant brush responses in left inferior frontal gyrus were attenuated by tryptophan depletion. However, the midbrain raphe was activated by unpleasant brush stroking and de-activated by pleasant and this effect was abolished by tryptophan depletion. This study found little evidence that CT afferents in hairy skin have a specific role in affective touch and serotonin cells of the raphe appear engaged by unpleasant stimuli rather than pleasant. In conclusion, the results of the questionnaire study indicated touch (hugs, kisses, stroking) in intimate relationships may promote resilience to depression whereas touch with other social contacts does not, suggesting type of affective touch to be important. It is suggested that future studies of the role of current social support and of early adversity in depression should include assessments of the correlated dimension of affective touch. The fMRI study found little evidence for a specific peripheral touch receptor encoding pleasant affective touch. The median raphe nucleus was inhibited by pleasant touch and this is in keeping with the idea that that aversive stimuli activate serotonin projections to the forebrain but not that this is strengthened by affective touch. Further investigation is required to identify CNS mechanisms of affective touch.

This essay is about the somatosensory system, which is divided into different kinds of touch. Described briefly are the proprioceptive touch, which is transported to the brain via A-alfa fibers and transmits information about e. g. limb position and movement. The cutaneous touch is the main focus and it is divided into discriminative touch and affective touch. The first corresponds to stimuli such as vibration and pressure and is transported via A-beta axons. The second, affective touch, corresponds to e.g. painful and pleasant stimuli which are transported to the brain via A-delta and C-fibers. The aim of the essay is to give an overview of the sense of touch, by doing a literature search, including a discussion of relevant neuronal correlates focusing particularly on affective touch. Moreover, the physiological aspects of touch will be presented. The sources that are used are review and original articles taken from databases such as ScienceDirect, and some articles send by the author. Some books have also been used to find more general knowledge. The conclusion for the essay is that touch is important for humans to function in everyday life. Additional, a specific receptor called C- tactile (CT) is identified to correspond to gentle touch and is suggested to have a vital role for humans in maintaining and forming social bounds. Moreover, discriminative touch is associated with activation in the primary and secondary somatosensory cortex, whereas affective touch seems to be associated with activity in the orbitofrontal cortex, cingulate cortex and the insula cortex, as well as the prefrontal cortex, which is suggested to be activated during interpersonal touch. Further, the sense touch needs to be more researched in order to understand its functions and benefits deeper.

The feeling of owning a body part is often investigated by conducting and manipulating the rubber hand illusion, a three-way integration of vision, touch, and proprioception. In the last decade, more research on the role of interoception, the sense of the body's’ internal state, in the illusion has been made. One of the studied factors has been the affective touch, a caress-like, gentle, touch that is performed at a slow specific speed (1-10 cm/sec). Affective touch activates the C tactile afferents which send interoceptive signals to the brain, specifically the insula. The present systematic review investigated the role affective touch has on the strength of the rubber hand illusion. A range of electronic databases was searched for papers reporting research findings published in English before March 20, 2020. Twelve different articles were identified, but only five papers met the inclusion criteria. This thesis looked at the results from these five different studies and compared the effect of affective touch and discriminative, regular, touch have on the rubber hand illusion to see whether there is a significant difference. The results could not show a main effect of stroking velocity, site of stimulation, or social touch, which are components of affective touch. The results was based on four different measurements, the subjective experience of the illusion, pleasantness ratings, proprioceptive drift, and temperature difference in the skin. Opposed what was hypothesized, it could not be demonstrated that affective touch would induce a stronger rubber hand illusion than discriminative touch.

The somatosensory system concerns the sense of touch. It is sectioned into various kinds of touch, such as the proprioceptive sense, providing information of sense of self and position of limbs, and the cutaneous sense, informing of the modalities of touching or being touched. The cutaneous sense is further divided into discriminative touch and affective touch. Discriminative touch is an exteroceptive sense of touch that responds to stimuli of pressure and vibration, and affective touch is an interoceptive sense of touch that corresponds to e.g. pleasant and painful stimuli, communicating information to the brain through A-delta and C-fibers. Recent studies investigates affective touch to have emotional affect on the subjective experience of touch, affecting subjective wellbeing. The aim of this thesis is to examine the sense of touch and its relevant neural correlates, focusing on affective touch and its role in subjective wellbeing and social relations. A presentation of physiological and neural aspects of touch will be held as well as a description of subjective wellbeing. The conclusion for this thesis is that affective touch appears to activate brain areas of orbitofrontal cortex, frontal polar cortice, prefrontal cortex and insula cortex, which are brain areas processing subjective wellbeing, e.g. evaluating positive and negative effect and processing emotional information and behavior. Examining correlations between affective touch and positive affect, negative affect, oxytocin release, social relations and affiliative behavior shows influence from affective touch on subjective wellbeing. A discussion of the current findings is provided, including directions for future research.

The somatosensory system concerns the sense of touch. It is sectioned into various kinds of touch, such as the proprioceptive sense, providing information of sense of self and position of limbs, and the cutaneous sense, informing of the modalities of touching or being touched. The cutaneous sense is further divided into discriminative touch and affective touch. Discriminative touch is an exteroceptive sense of touch that responds to stimuli of pressure and vibration, and affective touch is an interoceptive sense of touch that corresponds to e.g. pleasant and painful stimuli, communicating information to the brain through A-delta and C-fibers. Recent studies investigates affective touch to have emotional affect on the subjective experience of touch, affecting subjective wellbeing. The aim of this thesis is to examine the sense of touch and its relevant neural correlates, focusing on affective touch and its role in subjective wellbeing and social relations. A presentation of physiological and neural aspects of touch will be held as well as a description of subjective wellbeing. The conclusion for this thesis is that affective touch appears to activate brain areas of orbitofrontal cortex, frontal polar cortice, prefrontal cortex and insula cortex, which are brain areas processing subjective wellbeing, e.g. evaluating positive and negative effect and processing emotional information and behavior. Examining correlations between affective touch and positive affect, negative affect, oxytocin release, social relations and affiliative behavior shows influence from affective touch on subjective wellbeing. A discussion of the current findings is provided, including directions for future research.

Background: Interpersonal touch is an irreplaceable contributor to our daily social life. It is a powerful tool for communicating emotions and, at this, enables the formation and maintenance of relationships. Affective touch is one of the first interactions in parent-child-dyads and contributes positively to the cognitive and social development of children and adolescents. In recent years, a group of nerve fibres was detected and identified as a key player in the perception of affective touch. These so-called C-tactile afferents are specialized in the perception of human-to-human caress-like touch. They are optimally activated/stimulated by light and slowly stroking touch at velocities between 1 and 10 cm/s (Löken et al., 2009). Interestingly, in a standardized experiment, these tactile stimulations are likewise hedonically rated and perceived as most pleasant. C-tactile afferents project to the insular as well as somatosensory cortex at a slow speed of around 1 m/s. The insular cortex is suggested to be involved in emotional processing and integration of sensory impressions, above else. Against this background, C-tactile afferents are characterized as key players in affective touch perception and mark an antipole to discriminative touch perception and Aβ-fibres. Affective touch contributes to the cognitive and social development in children (Field, 2010) and C-tactile mediated affective touch perception is altered in individuals with Autism Spectrum Disorder (Kaiser et al., 2016). First evidence further suggests a contribution of C-tactile afferents in the perception of erotic touch (Jönsson et al., 2015). Against this background of evidence for the role of C-tactile afferents, the social-touch-hypothesis was formed (Olausson et al., 2010), stating a contribution of C-tactile afferents in the formation and maintenance of interpersonal relationships. Still, only few approaches thus far have investigated the actual influence of C-tactile mediated affective touch perception on interpersonal behaviour. Individuals seem to differ in their peripheral and central characteristics and processing of affective touch, but a method to quantify these differences is still missing. Aim: In the presented articles, we first investigated the relationship between C-tactile mediated touch perception and sexual desire and behaviour (paper 1). Next, a procedure to test the individual preference of affective touch was developed and examined (paper 2). Recent research described C-low-threshold mechanoreceptors – the equivalent of C-tactile afferents in rodents - to be arranged around certain hair follicles (Li et al., 2011). We thus hypothesized in paper 3, that hair follicle density might be related to the density of C-tactile afferents and that affective touch perception might be predicted by hair follicle density. The article describes and examines a method for quantification of C-tactile afferents in humans. Materials and Methods: Papers 1 and 3 present the results of a psychophysical study conducted on a sample of relatively young and subjectively healthy participants at the Medical Faculty of Dresden University of Technology. In paper 3, the study is compared and analysed in relation to another study as part of a cooperation together with the University of Gothenburg (Sweden). The study design involved the standardized application of different CT-optimal as well as –suboptimal stroking stimuli on the forearm using a computer-assisted robotic device (Rotary Tactile Stimulator). The participants were asked to rate these tactile stimuli concerning different hedonic qualities (pleasantness and eroticism) on visual analogue scales. Sexual Desire and sexual behaviour were determined indirectly through questionnaires. The study design further included the Cyanoacrylate Skin Stripping Method that enables sampling of the upper layer of the skin – the stratum corneum. These samples were analysed light-microscopically in order to determine the hair follicle density of the participants. Paper 2 reviews a different study which was conducted at the Center for Social and Affective Neurosciences at the University of Linköping (Sweden). In this study, the Test of Preferred-Velocity was examined and validated as a measure of individually preferred touch stimuli on a sample of young and healthy participants. Analogous to the study design described in paper 1 and 3, tactile stimuli were applied using the Rotary Tactile Stimulator as well. The Test of Preferred-Velocity includes a forced choice paradigm in which the preferred stimulus out of different pairs needs to be reported. The paper describes two different set-ups applying the test: First, the Test of Preferred Velocity was validated and tested for retest-reliability. Afterwards, a short version of the test was applied on both the forearm and the palm of the hand, where it was each validated and compared in-between. The following measures were determined in order to analyse and describe the individual sensitivity for C-tactile mediated affective touch perception: In paper 1, the erotic touch differentiation was determined as the difference of eroticism ratings between C-tactile optimal (1 cm/s) and C-tactile suboptimal (30 cm/s) tactile stimuli. Pleasant touch awareness equally reflects the sensitivity for pleasant touch in paper 3. However, this measure further considers the average level of hedonic ratings. Results: C-tactile optimal stroking stimuli at velocities between 1 and 10 cm/s were perceived as both pleasant and erotic. This confirms the results of a previous study by Jönsson et al. (2015), where a contribution of C-tactile afferents to erotic touch perception was suggested. According to the presented hypotheses, a gender-specific relation between C-tactile mediated erotic touch perception and sexual desire as well as sexual behaviour could be detected (paper 1): Within the group of female participants, erotic touch differentiation was positively correlated with the desire for sexual interaction with a partner. In the male sub-group of participants however, erotic touch differentiation was negatively related to the actual frequency of sexual interactions with a partner in the course of the last month. The Test of Preferred-Velocity was reviewed as a reliable procedure for determining the individually preferred stroking velocity of C-tactile-targeted touch, presented a high test-retest-reliability and was validated with the hedonic ratings of the participants (paper 2). The short version of the test presented equally reliable features and was related with the hedonic ratings when tested on the forearm but not the palm of the participants. Hair follicle density, however, could not be confirmed as a method to quantify C-tactile mediated affective touch perception or fibre density (paper 3). In the described extended sample of paper 3, the previously suggested and well-known rating patterns of C-tactile mediated affective touch perception were confirmed. Interestingly, a gender-specific difference in the hedonic evaluation of touch could be determined when female participants consistently rated the applied stimuli as significantly more pleasant and erotic than men. Conclusions: The presented articles confirm and extend previous assumptions on the properties and function of C-tactile afferents in humans. At this, C-tactile afferents seem to contribute significantly to the complex concepts of pleasant and erotic touch perception and have an impact on interpersonal behaviour. The results give insights into how gender might modulate C-tactile-mediated touch perception and did not find support for the hypothesis that hair follicle density and C-tactile sensitivity might be related. The Test of Preferred-Velocity however, might be used as a tool to investigate the individually preferred stroking stimulus in further studies.:Abbreviations 1 Figure legend 1 Introduction 2 INTRODUCTION TO THE SOMATOSENSORY SYSTEM: DISCRIMINATIVE AND AFFECTIVE TOUCH 2 C-TACTILE AFFERENTS – PERIPHERAL CHARACTERISTICS OF AFFECTIVE TOUCH 3 SPINAL AND CORTICAL PROCESSING OF C-TACTILE STIMULATION 5 THE “SOCIAL TOUCH HYPOTHESIS” 6 C-TACTILE MEDIATED EROTIC TOUCH PERCEPTION 7 INDIVIDUAL LEVELS OF C-TACTILE SENSITIVITY 9 HAIR FOLLICLE DENSITY AND NERVE FIBER DISTRIBUTION 11 Discussion 14 C-TACTILE AFFERENTS AND EROTIC TOUCH PERCEPTION 14 GENDER DIFFERENCES OF AFFECTIVE TOUCH PERCEPTION 15 STABILITY AND QUANTIFICATION OF AFFECTIVE TOUCH PERCEPTION: INDIVIDUAL PATTERNS OF AFFECTIVE TOUCH AND HAIR FOLLICLE DENSITY 16 LIMITATIONS 19 CONCLUSIONS 21 Zusammenfassung 22 Summary 26 References 29 Publication data 34 Annex I: C-Tactile Mediated Erotic Touch Perception Relates to Sexual Desire and Performance in a Gender-Specific Way. Annex II: The individual preferred velocity of stroking touch as a stable measurement. Annex III: The relation between human hair follicle density and touch perception. / Hintergrund: Zwischenmenschliche Berührung ist ein wichtiger Bestandteil unseres täglichen sozialen Lebens. Sie trägt wesentlich zur Entstehung und Aufrechterhaltung von Beziehungen bei und kann ein sehr kraftvolles, emotionales Kommunikationsmittel sein. In der Interaktion zwischen Eltern und Kind wirkt sich affektive Berührung positiv auf die kognitive und soziale Entwicklung der Heranwachsenden aus und ist im Allgemeinen unter anderem mit einem gesunden Körperbewusstsein und Interozeption verbunden. In den 1990er Jahren konnte eine Gruppe unmyelinisierter Nervenfasern in der behaarten Haut des Menschen identifiziert werden, welche auf die Wahrnehmung solch zwischenmenschlicher, affektiver Berührung spezialisiert sind. Diese sogenannten C-taktilen Nervenfasern werden von langsam streichelnden Berührungen (Geschwindigkeit 1 – 10 cm/s) mit nur leichtem Druck – ähnlich dem sanften Streicheln durch eine menschliche Hand - optimal aktiviert (Löken et al., 2009). Interessanterweise wird diese Stimulierung gleichermaßen in Experimenten als besonders angenehm beschrieben und empfunden. Die Stimuli werden von C-taktilen Nervenfasern mit einer langsamen Leitgeschwindigkeit (ca. 1 m/s) an den somatosensorischen Kortex, aber auch an die Inselrinde des Gehirns weitergeleitet. Letztere trägt unter anderem zu emotionaler Verarbeitung und Integration von Sinneseindrücken bei. Mit diesen Eigenschaften stellen die C-taktilen Nervenfasern einen funktionellen Gegenpol zu den Hauptakteuren der diskriminativen Berührungsempfindung, den Aβ-Fasern, dar. Affektive Berührungswahrnehmung wirkt sich positiv auf die kognitive und soziale Entwicklung von Kindern aus (Field, 2010) und C-taktil vermittelte Berührungswahrnehmung wird in Individuen mit Autismus verändert verarbeitet (Kaiser et al., 2016). Erste Ergebnisse deuten darauf hin, dass C-taktile Nervenfasern weiterhin an der Wahrnehmung erotischer Berührungen beteiligt sind (Jönsson et al., 2015). Vor diesem Hintergrund wird diese Form der taktilen Interaktion auch als „soziale Berührung“ beschrieben, die für die Formung und Aufrechterhaltung zwischenmenschlicher Beziehungen eine wesentliche Bedeutung hat (Olausson et al., 2010). Dabei haben nur wenige Studien bisher die tatsächliche Auswirkung C-taktil vermittelter affektiver Berührungswahrnehmung auf das zwischenmenschliche Verhalten untersucht. Individuen scheinen sich hinsichtlich ihrer peripheren oder zentralen Charakteristika der affektiven Berührungswahrnehmung zu unterscheiden – eine Methode zur Quantifizierung dieser Charakteristika wurde jedoch bisher noch nicht entwickelt. Fragestellung: In den hier vorgestellten Arbeiten wurde zunächst die Beziehung zwischen C-taktil vermittelter Berührungswahrnehmung und sexuellem Verhalten und Verlangen untersucht (Paper 1). In einer zweiten Veröffentlichung wird ein Testverfahren zur Bestimmung der individuell bevorzugten affektiven Berührung vorgestellt. Studien an Nagetieren haben ergeben, dass die den C-taktilen Nervenfasern entsprechenden Mechanorezeptoren um bestimmte Haarfollikel herum angefunden werden (Li et al., 2011). Daraus resultierte die Fragestellung aus Paper 3, einen Zusammenhang zwischen der Haarfollikeldichte am Unterarm und der C-taktil vermittelten Berührungswahrnehmung zu untersuchen. In der Arbeit wird auf dieser Grundlage eine Methode zur Quantifizierung C-taktiler Nervenfasern getestet. Material und Methoden: Paper 1 und 3 präsentieren die Ergebnisse einer psychophysischen Studie an jungen und subjektiv gesunden Probanden, welche an der Medizinischen Fakultät der TU Dresden durchgeführt wurde. In Paper 3 wird diese Studie im Rahmen einer Kooperation mit der Universität Göteborg mit einer dort durchgeführten ähnlichen Studie verglichen und ausgewertet. Die Teilnehmer dieser Studie haben mithilfe eines computer-assistierten Roboters standardisierte dynamische Berührungen mit sowohl C-taktil optimalen als auch suboptimalen Stimuli am Unterarm erhalten. Diese Berührungen wurden hinsichtlich verschiedener hedonischer Aspekte (Angenehmheit, Erotizität) anhand von Visuellen Analog-Skalen bewertet. Sexuelles Verhalten und sexuelles Verlangen wurden mithilfe von Fragebögen ermittelt. Mithilfe der Cyanoacrylate Skin Stripping Methode wurde den Probanden außerdem eine Probe des Stratum corneum der Unterarmhaut entnommen. Anhand dieser Proben wurde lichtmikroskopisch die Haarfollikeldichte pro cm² bestimmt. Die Ergebnisse einer weiteren Studie, welche an der Universität Linköping (Schweden) durchgeführt wurde, werden in Paper 2 aufgearbeitet. In dieser Studie wurde an einer Kohorte von jungen und gesunden Probanden ein Testverfahren (Test of Preferred Velocity) entwickelt und validiert, mithilfe dessen die individuell bevorzugten Berührungs-Stimuli bestimmt werden können. Der Test of Preferred Velocity verwendet ebenso den computer-assistierten Roboter Rotary Tactile Stimulator, mithilfe dessen in einem Forced Choice Paradigma verschiedene Berührungs-Stimuli vergleichend angeboten wurden. Die Studie beinhaltet zwei Teile: Zunächst werden Validierung und Test-Retest-Reliabilität des Testverfahrens beschrieben. Eine Kurzform der Methode wird im zweiten Teil vergleichend an der Handfläche und dem Unterarm von einer neuen Kohorte getestet und validiert. Verschiedene Werte werden für die Einschätzung der individuellen Sensitivität für C-taktil vermittelte affektive Berührung herangezogen: In Paper 1 wird die sogenannte erotic touch differentiation als Differenz zwischen den Erotizitäts-Bewertungen von C-taktil optimalen (1 cm/s) und C-taktil suboptimalen (30 cm/s) Berührungs-Stimuli ermittelt. Die pleasant touch awareness spiegelt einen ähnlichen Wert für die Sensitivität angenehmer Berührungen in Paper 3 wieder, wobei dieser zur durchschnittlichen Höhe der jeweiligen hedonischen Bewertung in Bezug gesetzt wird. Ergebnisse: In der hier vorgestellten Arbeit zeigt sich, dass C-taktil optimale Berührungs-Stimuli mit einer Geschwindigkeit zwischen 1 und 10 cm/s gleichermaßen als angenehm und erotisch empfunden werden. Dies bestätigt die Ergebnisse einer früheren Studie, welche bereits eine Beteiligung C-taktiler Nervenfasern an der Wahrnehmung erotischer Berührungen feststellen konnten (Jönsson et al., 2015). Entsprechend der vorgestellten Hypothesen konnte ein geschlechtsspezifischer Zusammenhang zwischen Werten C-taktil vermittelter erotischer Berührungswahrnehmung und sexuellem Verlangen und Verhalten beschrieben werden (Paper 1): Während unter den Probandinnen ein signifikanter Zusammenhang zwischen der erotic touch differentiation und dem Verlangen nach einer sexuellen Interaktion zu verzeichnen war, konnte in der Gruppe der männlichen Studienteilnehmer ein negativer Zusammenhang zwischen der erotic touch differentiation und der Anzahl tatsächlich stattgefundener sexueller Interaktionen im letzten Monat beobachtet werden. Der Test of Preferred Velocity zeigte sich als geeignet, um individuell bevorzugte Berührungs-Stimuli zu bestimmen und bewies eine hohe Test-Retest-Reliabilität (Paper 2). Mithilfe der hedonischen Bewertung der verwendeten Stimuli konnte der Test weiterhin validiert werden. Die Kurzversion des Testes zeigt ähnlich zuverlässige Eigenschaften und steht bei der Anwendung am Unterarm, nicht jedoch auf der Leistenhaut der Handfläche, mit den hedonischen Bewertungen in Zusammenhang. Die Haarfollikeldichte als Möglichkeit der Quantifizierung C-taktil vermittelter Berührung ließ sich nicht bestätigen (Paper 3). Anhand einer großen Stichprobe können allerdings die bisher bekannten Bewertungsmuster C-taktil vermittelter Berührungen erneut bestätigt werden. Auch hier wird ein geschlechtsspezifischer Unterschied in der hedonischen Bewertung zugunsten der Studienteilnehmerinnen beschrieben. Schlussfolgerungen: Die vorgestellten Arbeiten bestätigen und erweitern bisherige Erkenntnisse zu Eigenschaften und Funktion C-taktiler Nervenfasern im Menschen. C-taktile Nervenfasern scheinen signifikant zu dem komplexen Konzept der Wahrnehmung angenehmer und erotischer Berührungen beizutragen und sich auf das zwischenmenschliche Verhalten auszuwirken. Erstmals werden in diesen Arbeiten auch geschlechtsspezifische Eigenschaften affektiver Berührungswahrnehmung näher. Der vermutete Zusammenhang zwischen Haarfollikeldichte und affektiver Berührungswahrnehmung ließ sich nicht bestätigen. Der Test of Preferred Velocity kann jedoch zur individuellen Quantifizierung der bevorzugten Berührungs-Stimuli in weiteren Studien verwendet und ggf. weiterentwickelt werden.:Abbreviations 1 Figure legend 1 Introduction 2 INTRODUCTION TO THE SOMATOSENSORY SYSTEM: DISCRIMINATIVE AND AFFECTIVE TOUCH 2 C-TACTILE AFFERENTS – PERIPHERAL CHARACTERISTICS OF AFFECTIVE TOUCH 3 SPINAL AND CORTICAL PROCESSING OF C-TACTILE STIMULATION 5 THE “SOCIAL TOUCH HYPOTHESIS” 6 C-TACTILE MEDIATED EROTIC TOUCH PERCEPTION 7 INDIVIDUAL LEVELS OF C-TACTILE SENSITIVITY 9 HAIR FOLLICLE DENSITY AND NERVE FIBER DISTRIBUTION 11 Discussion 14 C-TACTILE AFFERENTS AND EROTIC TOUCH PERCEPTION 14 GENDER DIFFERENCES OF AFFECTIVE TOUCH PERCEPTION 15 STABILITY AND QUANTIFICATION OF AFFECTIVE TOUCH PERCEPTION: INDIVIDUAL PATTERNS OF AFFECTIVE TOUCH AND HAIR FOLLICLE DENSITY 16 LIMITATIONS 19 CONCLUSIONS 21 Zusammenfassung 22 Summary 26 References 29 Publication data 34 Annex I: C-Tactile Mediated Erotic Touch Perception Relates to Sexual Desire and Performance in a Gender-Specific Way. Annex II: The individual preferred velocity of stroking touch as a stable measurement. Annex III: The relation between human hair follicle density and touch perception.

Notre recherche vise à explorer les bases cérébrales de l'interaction sociale, par le biais notamment de l'électro-encéphalographie synchrone de plusieurs individus (hyperscanning-EEG). Cette thèse s'articule autour de trois volets théoriques, méthodologiques et expérimentaux complémentaires dans leur fonction. En premier lieu, nous proposons deux cadres conceptuels éclairant l'analyse des synchronies interindividuelles (couplage) chez l'humain. Le premier cadre s'intéresse aux conditions d'apparition du couplage, que l'on présente en considérant les principes fondamentaux qui semblent prédisposer à son émergence. Le second cadre théorique a pour but d'appréhender les rôles fonctionnels possibles du couplage. On y propose une taxonomie des processus de cognition sociale explorés au cours d'expériences sur le couplage entre sujets en interaction. En second lieu, des travaux en traitement du signal visent à développer des méthodes d'analyse adéquates pour les données produites au cours d'expérimentations en hyperscanning. On s'intéresse en particulier aux approches de séparation de source conjointe (JBSS) permettant de prendre en compte l'information inter-sujet dans la séparation. L'avantage de ces développements récents sur les méthodes classiques est démontré par une étude comparative de leurs performances sur données réelles acquises en hyperscanning-EEG. En dernier lieu, on contribue au champ des neurosciences sociales à travers une étude hyperscanning-EEG qui porte sur les effets du toucher affectif sur le couplage des activités cérébrales et physiologiques entre individus en interaction. Nous montrons que, chez des partenaires en couple, le toucher peut accroitre la dépendance des activités physiologiques, un résultat qui appuie son rôle particulier pour la communication et le support affectif au sein des relations intimes. / This research aims at exploring the neural bases of social interaction with use ofelectroencephalography acquired simultaneously from multiple individuals (hyperscanning-EEG). Wedo so by contributing on three complementary aspects at a theoretical, methodological andexperimental level. First, we provide two theoretical frameworks that can guide and inform theanalysis of interpersonal synchronies. Our first frame focuses on the necessary conditions for brain-tobraincoupling to occur. Our second frame provides a taxonomy for social cognition and situatesexisting studies in the neuroscience of social interaction. Second, we advance on the methodologicalaspects by extending the analysis framework, thereby considering and benefiting from hyperscanning-EEG data. Specifically, we focus on Joint Blind Source Separation (JBSS), a novel signal processing toolthat can take into account brain-to-brain coupling in the estimation of cortical activity. We provide acomparative study of such algorithms on real-world hyperscanning-EEG data and we demonstratetheir appropriateness for analysis of joint neural data. Finally, we contribute experimentally with ahyperscanning-EEG study in which we investigate the effect of affective touch on interpersonalcoupling during a natural interaction. We show that touch can increase coupling of physiologicalactivities between romantic partners. This results supports an instrumental role of interactive touchfor affective communication and support in close relationships.

In the last decade, neuroscience and psychology alike have paid increasing attention to the study of affective touch, which refers to the emotional and motivational facets of tactile sensation. Some aspects of affective touch have been linked to a neurophysiologically specialised system, namely the C tactile (CT) system. While the role of this system for affiliation, social bonding and communication of emotions have been investigated, less is known about the potential role of affective touch in the awareness of the body as our own, i.e. as belonging to our psychological 'self'. This thesis attempted to contribute to the knowledge on affective touch and its relation to body awareness, by exploring the potential role of this modality to the way we perceive and make sense of our body as our own. Specifically, this work aimed to advance the current state of knowledge by investigating: 1) the effect of affective touch on the sense of body ownership, which is a fundamental aspect of body awareness; 2) the relation between interoceptive modalities, originating both internally (i.e. cardiac awareness) and peripherally (i.e. affective touch), and exteroception in body awareness; 3) the effect of intranasal oxytocin on the perception of affective touch and bodily awareness; 4) the perception and social modulation of affective touch in psychiatric patients who show difficulties in body awareness, namely patients with Anorexia Nervosa, and 5) the modulating role of self-other distinction and of self-other relation in the perception of affective touch and body awareness. In a first experiment (N = 52) the rubber hand illusion paradigm was used to investigate the role played by CT-optimal, affective touch in the sense of body ownership. The results showed that slow, pleasant touch enhanced the experience of embodiment in comparison to faster, neutral touch, suggesting that affective touch might uniquely contribute to the sense of body ownership. The following study (N = 75), used a similar methodology to test whether interoceptive sensitivity as measured by a heartbeat counting task would modulate the extent to which affective touch influences the multisensory process taking place during the rubber hand illusion. The results could not confirm a systematic relation between interoceptive sensitivity and the perception of affective touch, nor its influence on body ownership. The next study (N = 41) included a double-blind, placebo-controlled, randomised, cross-over design testing the effect of intranasal oxytocin on the perception of affective touch and body ownership, as measured by means of the rubber hand illusion. There was no evidence supporting the hypothesis that intranasal oxytocin could influence the CT system as tested in this study. The next study (N = 55) applied some of the above methodologies to investigate the perception of CT-optimal touch in patients with anorexia nervosa and its emotional modulation by top-down factors. The results confirmed the hypothesis that people with anorexia nervosa show a reduced perception of affective touch compared to healthy controls, but its perception was not influenced by top-down affective modulation, in the sense that both patients and healthy controls perceived touch as more pleasant when presented concurrently with positive facial expressions compared to neutral and negative faces. Finally, the last two studies (N = 76 and 35 healthy volunteers, respectively) focused on the relationship between affective touch and body awareness in the context of social cognition. These studies used both online and offline social modulation paradigms to investigate the role of self-other distinction and of self-other relation in the perception of affective touch. The results showed that positive top-down social information can enhance the perceived pleasure of tactile stimulation. Taken together, these studies point to the central role of affective touch in body awareness and social cognition. Finally, they also pave the way for future studies examining the role of disruptions of the CT system in the development of neuropsychiatric impairments of body awareness and social cognition.