Cognitive functioning in late senescence : influences of age and health

Abstract

The overall aim of this doctoral thesis was to investigate potential age-related differences between groups of healthy very old adults from 75 to 96 years of age in various cognitive functions. An additional objective was to examine the influence of individual-difference variables within demographic (e.g., education, gender), psychometric (e.g., general cognitive status; MMSE) and biological (e.g., vitamin B12, folic acid [FA]) domains on cognitive functioning in late life. The studies were conducted within the realm of the Kungsholmen Project, which is a community-based, longitudinal study of persons aged 75 years and above, living in the parish of Kungsholmen, Stockholm. Participants were carefully screened for health, using a variety of exclusion criteria (e.g., dementia, sensory loss, psychiatric disease).

Study I and II examined the influence of age on visuospatial ability, spatial orientation, perceptual-motor speed and accuracy, and motor functions of the hands. Results indicated an age-related deterioration in visuospatial abilities spatial orientation and perceptual-motor speed. There were no age-related differences in the tests of hand-motor functioning and perceptual-motor accuracy. Visuospatial performance was higher under self-paced than under paced conditions across the entire age range. Further, results suggested that the abilities underlying time recognition may deteriorate later in the normal aging process than those underlying time drawing. Individual-difference analyses indicated that, in addition to age, visuospatial functioning and spatial orientation were related to MMSE and education in Study I. In Study II, visuospatial functioning and thyroid stimulating hormone were both related to perceptual-motor speed.

Study III and IV investigated the effects of low levels of serum vitamin B12 (B12 < 200 pmol/L) and Folic Acid (FA <11 nmol/L), and the influence of levels of thyroxin (T4) and TSH within normal ranges on cognitive performance, respectively. Tasks assessing visuospatial ability, spatial orientation, perceptual-motor speed, short-term memory, verbal fluency and episodic memory were examined. There were differential vitamin-related effects, such that subjects with low vitamin levels showed deficits in visuospatial abilities, perceptual speed, short-term memory, and letter fluency, but not in the remaining tests. In general, the effects of FA exceeded those of B12. T4 was unrelated to cognitive performance although TSH was positively related to episodic memory performance, but not to the remaining tasks. Those effects were independent of age, level of education, and depressive mood symptoms.

Finally, Study V examined the degree to which demographic variables and an index of performance IQ can predict episodic memory performance in a longitudinal study targeting optimally healthy very old adults. Subjects were assessed at baseline (T1) and after one (T2) and two (T3) years. There was a modest, but reliable, improvement of both verbal and nonverbal memory performances from T1 to T2, followed by stability from T2 to T3. No predictor variable was related to rate of change from T1 to T2, although PIQ influenced rate of change in verbal memory from T2 to T3. This effect resulted from performance pins among high PIQ subjects and performance decline among low PIQ subjects. The results suggest that changes in episodic memory in very old age may not necessarily be characterized by decline. Specifically, healthy older adults may show stability and, in some, instances, display practice-related gains in verbal episodic memory. The potential for performance gains over time may be greatest for the most cognitively able individuals.

Taken together, the studies indicated that the well-known age-related deficits in visuospatial abilities, spatial orientation, and perceptual-motor speed continue into very old age. However, episodic memory may not necessarily decline if practice opportunities are available. In addition to age, low levels of vitamin B 12, and FA were related to deficient performance in cognitive tasks in which fast and accurate processing of novel information is required, but not when pre-existing knowledge structures may be utilized. TSH was positively related to episodic memory performance, which was interpreted in terms of its potential effects on encoding and consolidation processes in episodic memory. At a general level, then, this research underscores the importance of various health-related variables in characterizing late-life cognitive functioning.