Cerebral correlates of cognitive aging: Gray-white-matter differentiation in the medial temporal lobes, and fluid versus crystallized abilities

We investigated the relationship between age, structural properties of selected cerebral regions, and cognitive performance in healthy adults, 18 to 78 years old. Spin-lattice relaxation time (T1), measured by nuclear magnetic resonance, was used to describe the structural composition of the brain tissue. Temporal lobe white-matter T1 showed age-related prolongation best described by a quadratic polynomial. There was a significant cubic trend in the association of hippocampal (gray-matter) T1 with age. In the examined regions of the medial temporal lobes, normally observed differentiation between gray- and white-matter T1 diminished linearly with age and dis-appeared almost completely in the elderly. Age and the ratio of gray-to white-matter T1 accounted for 53% of the variance in a measure of fluid intelligence (Cattell Culture Fair Test); the unique contributions of age and of gray-white-matter T1 ratio were 23% and 3%, respectively. The largest share of the variance in fluid intelligence (27%) was explained by the common influence of age and gray-white-matter T1 ratio. The same set of variables explained no significant proportion of the variance in crystallized intelligence. The possible mechanisms underlying age-related changes in gray-white-matter differentiation, their relationship to age-related selective deterioration of cognitive functions, and the implications of the findings for research on biological markers of aging are discussed.