Quantity and Quality Matter: Different Neuroanatomical Substrates of Apathy in Alzheimer’s Disease and Behavioural Variant Frontotemporal Dementia †

Background: Apathy is almost ubiquitous across neurodegenerative diseases and can be a general model for understanding neuropsychiatric symptoms in dementia. Methods: We assessed apathy via Starkstein’s Apathy Scale—caregiver version (SAS-C) in patients with Alzheimer’s disease (AD) and behavioural variant frontotemporal dementia (bvFTD). Neuropsychological and structural neuroimaging data were also collected. Images were processed using the FreeSurfer program, and cortical thickness data were acquired for 68 brain regions. Results: Patients with bvFTD had statistically higher levels of apathy than those with AD. The multivariate linear regression model found that the left entorhinal cortex (lEC) was the only region statistically associated with apathy in the AD group (F(1,31) = 5.17; p = 0.030; R2 = 0.527), whereas, for bvFTD, the right lateral orbitofrontal cortex achieved significant association with apathy (F(1,30) = 5.69; p = 0.009; R2 = 0.804). Conclusions: These results demonstrate that apathy is associated with multiple brain regions, reinforcing its multidimensionality and specific profiles.