Born to touch: Exploring social touch from a multidisciplinary perspective

Social touch is a powerful stimulus in mammalian development shaping early mother-infant bonding, social competences, cognitive functions and emotional regulation throughout the lifespan. Non-human animal research emphasizes the pervasive effects of early physical contact on healthy nervous system development and behavior. Caring maternal behavior, such as licking and grooming, profoundly impacts life-long behavioral trajectories through epigenetic effects on brain maturation. More recently, human-animal research has also started to investigate the role of social touch in human’s development. It has been shown that social touch affects neural, hormones and behavioral responses as well as psychological and subjective dimensions. Therefore, the overall aim of this thesis is to provide the achievements in the investigation of social touch focusing on infancy and caregiver-infant context, a time and a space in which social touch and its neural, behavioral, physiological and subjective dimensions impact a variety of developmental outcomes. In the last decade, even if the knowledge about social touch has been incredibly expanded, many aspects remain unexplored. While neural activation in response to social touch has been documented in infancy, the possible effect of sex on brain responsiveness remains unknown. In the first study (Chapter 2), I explored neural correlates of social touch in early infancy and potential differences between male and female infants using fMRI. A tactile stimulation that resembles affective/social touch was applied in eighteen term-born neonates aged 11-36 days during natural sleep. Results highlighted that, compared to males, females had larger blood oxygenation level dependent responses (brushing vs. rest) in bilateral orbitofrontal cortex (OFC), striatum, pons and cerebellum as well as stronger connectivity between the left OFC and posterior cingulate/cuneus. Several studies have suggested that social touch constitutes a protective factor in psychobiological development. At the same time, it has been documented how early adverse experiences are associated with epigenetic modifications of the DNA. For example, very preterm (VPT) infants exposed to high levels of procedural pain in Neonatal Intensive Care Unit often exhibit DNA methylation modifications that are predictive of less-than-optimal social stress response. To date, however, no research has examined whether social touch can mitigate the effects of adverse experiences on stress response. In the second study (Chapter 3), I therefore explored the protective role of maternal touch in VPT infants’ socio-emotional response. Twenty-nine dyads were enrolled at 3 months and participated in the Face-to-Face Still-Face paradigm to assess infants’ stress response and maternal touch. It was found that low levels of maternal touch can intensify the negative effects of SLC6A4 epigenetic changes on stress-response in 3-moths-old VPT infants. So far, most studies regarding social touch have been devoted to its correlates focusing on the infant but, more recently, research has begun to investigate the first-person perspective, that is the subjective dimension of touching and being touched. In the international landscape, only a few assessment instruments focused on the subjective perception of touch are available and in Italy, none. The third and fourth studies of this thesis (Chapter 4 and 5) aim to fill this gap contributing to the validation of two scales: the first is focused on personal touch experiences throughout the life-span (i.e., Tactile Biography Questionnaire), the second refers to maternal subjective perception of touch in mother-infant exchanges (i.e., Parental Caregivers Touch Scale). Data analysis were run on 2070 and 377 participants respectively and excellent psychometric properties were found for both scales. Finally, Chapter 6 brings together what is already known about social touch, thesis findings and future perspectives.