Neurophysiological Responses to Vicarious Affective Touch: An Electroencephalography (EEG) Study

Affective touch is a somatosensory modality involved in forming and consolidating rewarding social interactions and is hypothesised to be encoded by a population of cutaneous unmyelinated low threshold mechanosensory C-tactile afferents (CT). CTs are preferentially activated by a caressing type of touch delivered at speeds around 3 cm/s (CT-optimal speed) and at skin temperature. Given its social relevance, the vicarious observation of affective touch can cause high empathic resonance in the observer. However, little is known whether the observation of CT-optimal touch is processed differently in the observer's brain than at other CT-suboptimal speeds. In our study, we evaluated subjective reports of the pleasantness/willingness of observers to receive such touch while recording the electroencephalogram (EEG) as they watched touch videos at three different speeds: static, slow (CT-optimal), and fast touch. We found that CT-optimal touch was rated as more pleasant and elicited greater willingness to engage, compared to static or fast touch. CT-optimal touch also led to significantly lower beta power, reflecting greater EEG desynchronisation, compared to the other conditions. Other frequency bands, such as theta and alpha, showed increased modulation to slow and fast touch compared to static touch. However, no differences were found between slow and fast conditions in the alpha and theta bands. Our results suggest that vicarious processing of affective touch at the optimal speed for CT receptors involves broader neural resources and higher activation of somatosensory areas, highlighting the significance of this touch modality in sensory processing.