BEHAVIOURAL RIGIDITY AS A TRANSDIAGNOSTIC MARKER OF NUCLEUS ACCUMBENS ALTERATIONS IN DEMENTIA

Behavioural rigidity, defined as a tendency to persist with inflexible patterns of thought or action, is increasingly recognised as a transdiagnostic feature across psychiatric, neurodevelopmental, and neurodegenerative disorders. However, its prevalence and neural substrates in dementia remain poorly characterised. We examined the structural and functional neural correlates of behavioural rigidity across frontotemporal dementia (FTD) and Alzheimer’s disease (AD). A total of 204 participants were included: 110 patients with FTD, 53 with AD, and 41 healthy older controls. Within the FTD group, 66 patients had behavioural variant FTD (bvFTD), 26 semantic dementia (SD), and 18 progressive non-fluent aphasia (PNFA). Behavioural rigidity was assessed using the Stereotypical and Motor Behaviour subscale of the Cambridge Behavioural Inventory–Revised (CBI-R). Voxel-based morphometry identified grey matter correlates of rigidity across the patient cohort, followed by seed-based resting-state functional connectivity analyses. Behavioural rigidity was evident across dementia syndromes, being most pronounced in bvFTD and relatively mild or absent in PNFA. Across all patients, higher rigidity scores were selectively associated with reduced grey matter intensity in the bilateral nucleus accumbens. Using the nucleus accumbens as a seed region, greater rigidity was further associated with increased functional connectivity between the left nucleus accumbens and motor-related regions, including the supplementary motor area, paracentral lobule, and precuneus. These findings identify behavioural rigidity as a transdiagnostic feature of dementia and implicate nucleus accumbens pathology as a common neural substrate, highlighting the importance of systematic clinical assessment and a potential therapeutic target.