Behavioural rigidity as a transdiagnostic marker of nucleus accumbens dysfunction in dementia

Behavioural rigidity, the tendency to persist with inflexible patterns of thoughts or actions, is increasingly recognised as a transdiagnostic symptom across psychiatric, neurodevelopmental, and neurodegenerative disorders. Empirical studies exploring the prevalence and underlying neural mechanisms of behavioural rigidity in dementia, however, are lacking. This cross-sectional study sought to delineate the structural and functional neural correlates of behavioural rigidity using a transdiagnostic approach looking across the frontotemporal lobar degeneration (FTLD) spectrum and Alzheimer's disease. A total of 204 participants were recruited, including 110 frontotemporal dementia (FTD), 53 Alzheimer's disease (AD), and 41 healthy older control participants. Within the FTD group, 66 cases were diagnosed with clinically probable behavioural variant FTD (bvFTD), 26 presented with semantic dementia (SD), and 18 cases had progressive non-fluent aphasia (PNFA). Behavioural rigidity was assessed using the Stereotypical and Motor Behaviour subscale of the Cambridge Behavioural Inventory-Revised. Voxel-based morphometry (VBM) was performed to identify grey matter regions associated with behavioural rigidity transdiagnostically, the results of which informed subsequent seed-based voxel-wise functional connectivity analyses. All imaging analyses were adjusted for relevant demographic and technical covariates. Statistical thresholds were set at voxel-level p < 0.001 (uncorrected) and cluster-level p < 0.05 (FDR-corrected). Our main finding was that behavioural rigidity is pervasive across dementia subtypes, ranging from most pronounced in bvFTD to milder/relatively absent in PNFA, relative to Controls. Whole-brain VBM across the entire patient sample revealed a significant negative association between behavioural rigidity and grey matter intensity exclusively in the bilateral nucleus accumbens. Using the bilateral nucleus accumbens as seeds, resting-state functional connectivity analysis showed that higher levels of behavioural rigidity were associated with stronger connectivity between the left nucleus accumbens and the left supplementary motor area, paracentral lobule, and precuneus. This is the first study, to our knowledge, to examine the neural substrates of behavioural rigidity across FTLD syndromes transdiagnostically using structural and functional neuroimaging approaches. Our findings reveal a gradation of rigid and repetitive behaviours, most apparent in bvFTD, which in turn reflects pathological disruption of the nucleus accumbens. Taken together, our findings highlight the need to consider repetitive and rigid behaviours as a transdiagnostic feature in neurodegenerative disorders, and one which indexes underlying nucleus accumbens pathology. More broadly, this study underscores the importance of screening for rigid and repetitive behaviours in the clinic and identifies the nucleus accumbens as a promising neural target to ameliorate these symptoms.