Cerebellar tDCS and implicit theory of mind: the moderating role of vermian volume

In recent years, the cerebellum has emerged as an important contributor to social and affective domains. In particular, evidence from transcranial Direct Current Stimulation (tDCS) studies has highlighted a crucial role of the cerebellar vermis in the implicit and affective components of Theory of Mind (ToM), although polarity-dependent effects remain inconsistent. The present study combines neuromodulation before an implicit ToM task with structural MRI to determine whether individual variations in vermian volume may moderate polarity-dependent responses to stimulation. Thirty-nine adults completed the Reading the Mind in the Eyes Test, providing accuracy (ACC) as outcome. All participants underwent 3T Magnetic Resonance Imaging to quantify Gray Matter (GM) volumes of cerebellar lobules. Participants received anodal, cathodal (2 mA, 20 min) or sham tDCS over the cerebellar vermis. Behavioural testing was performed at baseline and 35 minutes post-stimulation. Linear mixed-effects models examined within- and between-group effects on ACC and tested whether vermal GM volumes moderated stimulation-related changes. No pre-post ACC differences emerged within stimulation group. However, the incorporation of structural measures in the analyses revealed a significant moderation of the group × time effect by vermian volumes. In particular, Vermis 1, 2, 3 volumes explained part of the inter-individual variance in ACC and were associated with responsiveness to anodal stimulation. The moderating role of cerebellar volumes highlights that the effects of neuromodulation on social functions may depend on individual structural features. This supports research examining additional modulatory factors, such as autonomic functioning, in which the cerebellum is strongly involved, towards personalized therapeutic protocols.