Disease-specific roles of multidimensional apathy in linking cognitive and social functioning across neurodegenerative diseases.

BACKGROUND: Apathy is a multidimensional neuropsychiatric syndrome involving cognitive, emotional, and behavioral domains. Cognitive impairment and apathy are both associated with reduced social functioning, but how specific apathy dimensions relate cognition to social functioning across neurodegenerative diseases remains unclear. OBJECTIVE: This study compared multidimensional apathy profiles across Parkinson's disease (PD), Alzheimer's disease-like mild cognitive impairment (AD-like MCI), and normal controls (NC), and investigated whether specific apathy dimensions mediate the association between cognitive function and social functioning. METHODS: A total of 297 participants were enrolled, comprising 96 PD, 66 AD-like MCI, and 135 NC. All participants provided demographic information and underwent neuropsychological tests, a multidimensional apathy measure, and a social functioning measure. Depressive symptoms, sex, age, and education were included as covariates. Mediation analyses were conducted using PROCESS Model 4 with bootstrapping. RESULTS: A coherent triadic association among cognitive function, apathy, and social functioning was observed exclusively in the PD group. Hierarchical regression showed that cognitive and social motivation (CSM) independently predicted social functioning beyond executive function and covariates. Mediation analysis demonstrated that mental shifting was indirectly associated with social functioning through CSM (indirect effect = -.014, 95% CI [-.033, -.002]), with a non-significant direct effect. This pattern was not observed in the AD-like MCI or NC groups. CONCLUSIONS: These findings reveal a PD-specific pattern of associations among impaired mental shifting, CSM, and reduced social functioning. Its absence in AD-like MCI suggests distinct apathy mechanisms across neurodegenerative diseases and highlights CSM as a potential intervention target.