Cognitive Dimensions in Alzheimer's Disease, Mild Cognitive Impairment, and Normal Elderly: Developing a Common Metric.

The aim of this research was to assess similarity in cognitive factor structures underlying neuropsychological test performance of elders belonging to three clinical groups: Alzheimer's disease (AD), Mild Cognitive Impairment (MCI), and normal elderly. We administered a battery of neuropsychological tests to 214 elderly participants in the groups. First, the underlying cognitive structure of a Combined-Set of AD, MCI, and Control subjects was determined by Principal Components Analysis (PCA), including quantitative relationships (loadings) between the test measures and the factors. The PCA resolved 17 neuropsychological test measures into 6 interpretable factors, accounting for 78% of the variance. This cognitive structure was compared with separate cognitive structures from an AD-Set, an MCI-Set, and a Control-Set (different individuals in each set) in additional PCA using Procrustes factor rotation. Analysis of congruence coefficients between each set and the Combined-Set by a bootstrapping statistical procedure supported the factor invariance hypothesis. These close similarities across groups in their underlying neuropsychological dimensions support the use of a common metric system (the factor structure of a Combined-Set) for measuring neuropsychological factors in all these elderly individuals.