The first Alzheimer’s diseased brain I ever touched looked horrific. The cortex was shriveled, the ventricles were large, cavernous voids, and when I stained the sample I saw a galaxy of proteinaceous tangles and masses. The brain had clearly been degenerating steadily for over a decade, and it was difficult to imagine how the patient could have functioned. I was shocked to discover that, according to his charts, the patient’s dementia had only been detectable for a few years. In contrast, certain brains I analyzed appeared much more intact, yet came from patients who had suffered from severe dementia for over a decade.
These patients exemplify the dramatically different ways people can respond to neurodegenerative changes. Even when confronted with the same disease and comparable severity, people vary considerably in the extent of cognitive decline. Specifically, people with higher levels of education and occupational attainment are more successful at coping with the same amount of brain damage and degeneration.
One hypothesis that accounts for this discrepancy is the concept of cognitive reserve. The cognitive reserve hypothesis posits that people who have challenged their minds for significant portions of their lives (i.e. they didn't just start playing Sudoku at the age of 60) can compensate for neural deficits by recruiting alternate brain networks as backup or “reserve.” In support of this hypothesis, functional brain imaging shows that "high-functioning" older adults activate significantly more areas of their brains than both "low-functioning" older adults and young adults when performing certain cognitive tasks. This indicates neural compensation; the "high-functioning" old engage in alternative neural strategies in response to neural deficits or declines in cognitive abilities. Importantly, this type of compensation may be facilitated by a more flexible organization of the brain, which results from early cognitive experience.
Of course, people who did not start challenging themselves until later in life should not despair. Other requisites of compensation, such as plasticity (including the birth new neurons and enhanced signaling between existing neurons), may be improved by cognitive experience throughout life (although the earlier the better). And in a complementary aspect of cognitive reserve, people who challenge their brains throughout life may be able to protect their existing brain networks. Intellectually stimulating activities may increase the efficiency and capacity of these networks, enabling them to withstand a greater degree of age-related change while maintaining intact functioning (again, the earlier the better).
The cognitive reserve hypothesis has recently been supported by findings of Dr. Margit Bleecker, who studied the effects of lead exposure on cognitive function. The study involved 112 lead smelter workers in New Brunswick, who were divided into groups with high reading ability (12th grade or higher) and low reading ability (11th grade or lower). Reading ability is a recognized measure of cognitive reserve, and is perhaps a better metric than education and occupation (e.g. it distinguishes self-taught individuals who dropped out of school for economic reasons from people who graduated high school but are functionally illiterate). Importantly, although lead exposure has negative effects on many brain functions, well-ingrained functions like reading ability are resistant to the consequences.
Both groups had similar lead exposure, age, alcohol use, and depression levels, but those with high cognitive reserve performed 2.5 times better on cognitive tests than those with low cognitive reserve. In contrast, cognitive reserve did not protect motor speed and dexterity from the toxic effects of lead, indicating that other parts of the workers’ nervous systems were still vulnerable. These findings, published in Neurology, demonstrate that cognitive reserve protects against the cognitive effects of chronic lead exposure.
The key to cognitive reserve is not to wait until you’re in your 60s (or even 50s, 40s, 30s, or 20s, for that matter), but to challenge yourself intellectually as early and often as possible. So read, play "brain games," play soccer, and do all the other wonderfully fun and exciting things that are good for you.
UPDATE: For more information on cognitive reserve, see posts by Michael Merzenich of Posit Science and Alvaro of SharpBrains.