Atypical cortical regional homogeneity observed in grapheme-colour synaesthesia and its subtypes.

Grapheme-colour synaesthesia is a phenomenon where individuals associate specific graphemes with particular colours, with two common subtypes: projector and associator. This study investigates the association between resting-state regional homogeneity (ReHo) and projector- and associator-type synaesthesia, and the behavioural significance of these ReHo abnormalities. 75 synaesthetes with varying projector and associator scores completed a diverse range of psychological measures and underwent resting-state functional MRI (rsfMRI) scans. Cortical ReHo images were extracted from preprocessed rsfMRI scans and analysed using the VertexWiseR R package. Vertex-wise ReHo analysis revealed that ReHo scores of the left caudal middle frontal negatively associated with associator-type synaesthesia scores, while ReHo scores of the left superior parietal, left superior frontal, left pars orbitalis and right precuneus negatively associated with projector scores. These ReHo clusters correlated with domains of emotional and tactile imagery (Plymouth Sensory Imagery Questionnaire) across both subtypes. Projector-score associated clusters were associated with auditory, gustatory, and visual imagery, as well as heightened avoidance behaviours. Meta-analytical image decoding analyses suggested that the clusters associated with both projector- and associator scores were associated with memory and executive function. Overall, our analyses revealed that projector and associator synaesthesia are supported by distinct yet overlapping patterns of ReHo within higher-order cognitive and perceptual brain regions.