Even with evidence of brain plaques associated with Alzheimer's disease, highly educated individuals manage to score higher on cognitive tests. Results from this study are published in the November issue of Archives of Neurology.
Currently there exists a popular notion - the "cognitive reserve" hypothesis - that people with greater thinking, learning, and memory capabilities can delay symptom of Alzheimer's disease even as the brain is changing. Since cognitive reserve is somewhat difficult to measure, researchers use education level as a proxy. "Adjusting for level of Alzheimer disease pathological burden determined at autopsy, greater education has been associated with better cognitive function during life," write Catherine M. Roe, Ph.D. and colleagues at the Washington University School of Medicine, St. Louis. "Education interacts with Alzheimer disease pathological burden such that a greater pathological burden is required to show an effect on cognition among persons with more education."
Between 2003 and 2008, the researchers analyzed 37 individuals diagnosed with Alzheimer's-like dementia and 161 individuals without dementia. Cognitive reserve measurements included education history as well as cognitive test scores. In addition, study participants received an injection of carbon 11-labeled Pittsburgh Compound B (PiB) prior to a 60-minute positron emission tomography (PET) brain scan. This contrast compound (PiB) has been shown to stick to beta-amyloid brain plaques that are linked to Alzheimer's disease, and thus researchers could identify the disease state in each patient.
Results of the study demonstrated that there were significant difference in the level of PiB uptake among patients with different years of education and cognitive test scores. For example, individuals with beta-amyloid plaques in the brain (organs that took up higher levels of PiB) showed increased performance on tests as education levels increased. For individuals without plaques, education was not significantly associated with cognitive scores.
"The results support the hypothesis that cognitive reserve influences the association between Alzheimer disease pathological burden and cognition," conclude Roes and colleagues. "Based on autopsy data, there may be a ceiling effect when extensive beta-amyloid pathological burden is present as in late-stage dementia of the Alzheimer type. Presumably, as the Alzheimer disease pathological burden increases, a greater proportion of highly educated participants reaches the threshold for dementia and the initial advantage provided by cognitive reserve decreases. Longitudinal imaging of beta-amyloid pathology in vivo will soon allow us to determine whether these inferences from cross-sectional studies are accurate."