March 24, 2003 -
Age-related changes in the brain -- the appearance, starting around age
60, of "white-matter lesions" among the brain's message-carrying
axons -- significantly affect cognitive function in old age. White-matter
lesions are small bright patches that show up on magnetic resonance
imaging (MRI) of the brain. What's more, hypertension may account for some
of this cognitive impact.
A full report on these
relationships appears in the March issue of Psychology and Aging, which is
published by the
American
Psychological Association
(APA).
Psychologists want to
find the factors that contribute to individual differences in cognitive
functioning among the elderly, because, says lead researcher Ian Deary,
Ph.D., "People who retain their cognitive function in old age tend to
have higher quality of life and live longer."
However, researchers
have been stymied by the lack of data on the childhood cognitive
performance of elderly individuals. Without that data, it is hard to tell
whether individual differences are due to aging or existed all along.
Luckily, Deary, from the University of Edinburgh, and his colleagues at
the University of Aberdeen, discovered that on June 1, 1932, Scotland gave
its 11-year-olds a validated cognitive test. With its results, the authors
gained a measure of early-life cognitive ability for people who were in
their late 70s at the time of the study.
Deary and his co-authors
used local health registers to track down healthy living men and women who
took the Scottish Mental Survey of 1932. Of the 427 possible matches, they
contacted 327 people chosen at random; 83 of those people took part in a
brain imaging study.
Testing took place in
1999, when most participants were 78 years old. Participants took four
different cognitive tests, examining nonverbal reasoning, memory and
learning, processing speed, and executive function. They also underwent
magnetic resonance imaging (MRI) of their brains to allow researchers to
assess the extent of white-matter lesions, which are like little scars in
the brain.
The amount of brain
white-matter abnormalities made a significant contribution to general
cognitive ability differences in old age, independent of prior ability. In
other words, if "Mary" tested better than "Billy" at
age 11, they didn't necessarily test the same way at age 78. An elderly
Mary might still have tested better, but the gap could have widened,
narrowed or reversed --- and the differences in their white-matter lesions
would matter more than differences in their earlier ability. In old age,
the amount of white-matter lesions contributed 14.4 percent of the
variance in cognitive scores; early IQ scores contributed 13.7 percent of
the variance.
What's more, these two
predictors of cognitive performance in old age were independent; they
didn't consistently affect scores in the same way. That is, after taking
into account people's mental ability in youth, these researchers establish
a factor that contributes significantly to people's cognitive function in
healthy old age.
Although white-matter
lesions are viewed as a normal part of aging, and are found in people with
no dementia or other neurocognitive disorders, they are linked with other
health problems. In this study, hypertension accounted for a small but
significant amount of variance both in white-matter lesion scores and in
general cognitive scores in old age. This finding builds on other recent
evidence that white-matter abnormalities may be related to circulatory
problems (including hypertension, diabetes, heart disease and
cardiovascular risk factors).
Given the role played by
white-matter abnormalities in cognitive performance, "Avoiding risk
factors for [them] or preventing their accumulation may ameliorate
age-related cognitive decrements," say the authors. "The
understanding of the functional neurobiology of brain aging will be
enhanced by the discovery of interactions among etiological factors."
In a side note, Deary
and his colleagues observe that, "the search for the causes of
intelligence differences in youth is relevant to research on aging because
much variance from youth persists into old age."
