Previous studies had suggested that engaging in cognitively challenging activities may help prevent dementia, as well as slow the progression of dementia and Alzheimer's in in older people. To test this, Wilson and his co-workers tracked almost 1,200 older individuals over nearly 12 years.
The researchers assessed each person's engagement in cognitively challenging activities using a 5-point "cogntive activity" scale.
At the time of study enrollment, all of the participants were free of dementia; by the study's end, 614 people were cognitively normal, 395 showed mild cognitive impairment, and 148 had Alzheimer's disease.
The team found that increased cognitive activity among normal individuals, such as reading, playing games, watching TV, listening to the radio, and going to museums -- meant that they were less likely to experience cognitive decline over several years.
For each gained point on the cognitive activity scale, the rate of mental decline fell by 52% over 6 years.
However, the opposite was true for those who went on to develop dementia. People who had loved mentally challenging activities actually showed a quicker mental decline after dementia started. In fact, the rate of cognitive decline accelerated by 42% for each point on the cognitive activity scale.
"We think there's a trade-off," said senior study author Robert Wilson of Rush University Medical Center in Chicago. Keeping mentally active means that there is "a little more time during which the person is cognitively competent and independent and a little less time in a disabled and dependent state" once dementia does set in, said Wilson, who is senior neuropsychologist at Rush's Alzheimer's Disease Center.The researchers believe that this discrepancy may be explained by the accumulation of neurodegenerative lesions called plaques and tangles in the brains of dementia patients.
Previous work has suggested that mentally stimulating exercises do not actually prevent these lesions from accumulating. Instead, they allow individuals to remain relatively cognitively normal for a while longer, even in the presence of those lesions.
However, once the plaques and tangles accumulate to a certain threshold, high cognitive activity can no longer prevent symptoms of dementia, and the behavioral signs of the disease appear.
Because people can behave normally for years -- even while brain lesions are appearing -- at the point at which they're first diagnosed with dementia, a person with a history of cognitive activity actually has more plaques and tangles in their brain than a person who wasn't so cognitively active, Wilson believes.
"The person who has a history of being cognitively active actually has more of the pathology in their brain, and so really has more severe disease," he theorized. "That's why they decline more rapidly from that point on."
According to the researchers, the results suggest that mental exercises help prevent the onset of dementia, but only if they're started before signs of cognitive impairment appear -- after that point, the brain is probably too damaged for such interventions to make a difference.
The research is published online Sept. 1 in Neurology.
SOURCE: Sept. 14, 2010 , Neurology