Autism spectrum disorder (ASD) is associated with atypical social information processing, which may be partly explained by differences in Theory of Mind (ToM) network functioning and autonomic regulation. Guided by neurovisceral integration models, the present study examined whether autonomic regulation, indexed by heart rate variability (RMSSD), was associated with within-network functional connectivity of the ToM network during social processing, and whether this relationship varied across viewing conditions and autism severity. Sixty-four adults (32 autistic, 32 non-autistic) completed three fMRI runs across three conditions varying in social demand: eyes-open rest, low-social news viewing, and socially rich reality TV viewing, while cardiac and neural data were recorded simultaneously. 

Within-network ToM connectivity was calculated using pairwise correlations between selected ToM parcels, while RMSSD was extracted from cardiac recordings as an index of parasympathetic regulation. Linear mixed models showed that RMSSD was not significantly associated with ToM connectivity, and this association did not differ across conditions or as a function of autism severity; however, ToM connectivity differed significantly across viewing conditions, with the highest connectivity during rest and the lowest during reality TV viewing. These findings do not support the hypothesised brain–body coupling between autonomic regulation and ToM network connectivity during passive naturalistic viewing. Instead, they suggest that ToM connectivity is sensitive to viewing context, although this variation was not explained by individual differences in autonomic regulation or autism severity, at least as captured by averaged RMSSD during passive naturalistic viewing. Future work using more dynamic or cross-network measures of brain–body coupling may help clarify these relationships.