Individual differences in behavioural and physiological responses to affective touch

This thesis examines how individual differences in social traits relate to behavioural and physiological responses to affective touch. Over the last few decades the functionality of C-Tactile afferents (CTs) have been investigated, with researchers positing that CTs function to signal the rewarding value of social tactile interactions. Here, by exploring the relationship between trait sociability and affective touch perception this social touch hypothesis is explored. In the first three studies, the role of sociability on the vicarious experience of affective touch was investigated. In study one; the aim was to determine how trait sociability affected an individual’s vicarious experience of affective and discriminative touch. Here, individuals with the lowest number of autistic traits and theoretically the highest sociability were found to show the greatest sensitivity in their affective ratings of different velocities of touch, resulting in a significant quadratic relationship between non-CT-optimal and CT- optimal stimuli at CT-innervated locations. In study two the aim was investigate the vicarious experience of touch in young children. Children both with a diagnosis of ASD (here theorised to be one extremity of trait sociability) and without ASD observed the same videos depicting social touch. It was found that young children (aged 7-12) did not show the typically observed vicarious preference for CT-optimal velocity touch. Furthermore, there was no difference between children with an autism spectrum disorder (ASD) and typically developing children in their affective ratings of the observed touch. Study three again took the same sample of videos and used facial EMG to see whether the explicitly rated vicarious preference for CT-optimal over non-CT-optimal touch could be detected implicitly. It was found that observation of CT-optimal social tactile interactions did not elicit the same affective responses that have previously been reported in response to directly felt touch. This finding is perhaps consistent with the rather weak affective response elicited by touch in comparison to pain, for example. The fact that self-reported levels of empathetic concern correlated negatively with corrugator muscle activity, indicative of negative affect, in response to touch on CT innervated sites suggests individual differences in implicit affective response to touch are present. In study four, the aim was to determine whether individual differences in trait sociability affected implicit affective responses to first-hand experience of touch. Consistently, participants will low levels of autistic traits (high trait sociability) showed greater zygomaticus activity, indicative of positive affect, during evaluation of the touch they received than those with high levels of autistic traits. Stroking touch elicited little activity in the Corrugator, indicative of negative affect, in either group. Finally, study 5 used EEG to determine how the cortical activity related to fast conducting A-beta stimulation compared to later activity in response to slow conducting CTs. Specifically an ultra-late potential (ULP) was measured for CT-optimal stimuli. Furthermore, 30cm/s, which generates greater A-beta stimulation than 3cm /s, elicited a significantly greater p300 orienting response. Furthermore, there was a significant difference in the ULP peak amplitude between individuals with high and low levels of autistic traits suggesting differential patterns of activity. Taken together, these studies suggest that touch, including touch targeted to optimally activate CTs is indeed processed differently, both physiologically and behaviourally, by individuals with different levels of autistic traits, whether directly felt or vicariously experienced. It is hypothesised that these differences s reflect variation in sensitivity to the rewarding value of social stimuli. These studies provide some of the first evidence that individual differences in stable personality traits are associated with differential responses to social / affective touch.