Speed of Processing and Depression Affect Function in Older Adults with Mild Cognitive Impairment

Objectives: To evaluate the effect of depression and cognition on function in older adults with amnestic and nonamnestic mild cognitive impairment (aMCI and non-aMCI). Design: The study uses baseline data from the National Alzheimer's Coordinating Center. Setting: Data were collected at multiple Alzheimer's Disease Centers in the United States. Participants: The sample included a total of 3,117 individuals with MCI, mean age = 7437 years, SD: 9.37 (aMCI, n = 2,488; non-aMCI, n = 629). Measurements: The 10-item Pfeffer Functional Activities Questionnaire assessed function. Results: Depressive symptoms (Geriatric Depression Scale), memory impairment (Logical Memory II), and processing speed decrements (Digit Symbol Substitution Test) were significantly associated with functional impairment (p < 0.001). Processing speed partially mediated the effect of depression on function and fully mediated the effect of executive dysfunction on function (p < 0.001) in the total MCI and aMCI subsample, while in the non-aMCI subsample, processing speed mediated the effect of executive function but not the effect of depression (p = 0.20) on function. Conclusions: The findings show that processing speed is central to the effect that depression and executive dysfunction have on functional impairment in cognitively impaired older adults. Future studies are needed to better understand the physiologic underpinnings in age-related and disease-specific decrements in processing speed, and to address the problems in the assessment of processing speed in clinical samples.