WEDNESDAY, Sept. 1 (HealthDay News) -- Reading, crossword puzzles and other mentally stimulating activities have pros and cons when it comes to Alzheimer's disease, new research suggests.

In line with prior research, the study finds that such mental activity may slow declines in thinking and memory during normal old age.

But folks who loved these pursuits actually displayed a hastening of their mental decline once symptoms of dementia began to set in, the researchers say.

"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 findings were published online Sept. 1 in Neurology.

Previous work has suggested that engaging in cognitively challenging activities may help ward off the appearance of dementia in older people. To test this, Wilson and his co-workers tracked almost 1,200 older individuals over nearly 12 years.

The team assessed each person's engagement in mentally stimulating pursuits 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 researchers found that increased cognitive activity among normal individuals -- things such as listening to the radio, watching television, reading, playing games and going to museums -- meant that they were less likely to experience cognitive decline over several years.

Specifically, for each gained point on the cognitive activity scale, the rate of mental decline fell by 52 percent over 6 years.

But the opposite was true for those who did go on to develop dementia -- in that case, people who had loved mentally challenging activities actually showed a quicker mental decline after the illness took over. In fact, the rate of decline accelerated by 42 percent for each point on the cognitive activity scale, the researchers report.

Wilson and his colleagues 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 authors, 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 results do suggest that mental exercises help stave off dementia but then increase mental decline after dementia onset," said Charles Hall, professor of neurology at Albert Einstein College of Medicine, New York City. He cautioned that it remains possible that some unknown factor still connects mental activity with these effects.

To be sure that there is a direct causal relationship between mental exercises and the effects the authors found, "it's really important that we do intervention studies to test this hypothesis," Hall said.

More information

Find out much more about Alzheimer' at the Alzheimer's Association.