Mentally stimulating activities may somehow initially boost the brain's ability to function relatively normally despite lesions in the brain related with dementia. (CBC)

Mental activity such as crossword puzzles, reading or listening to the radio may slow the decline of cognitive skills initially, but speed up dementia later in old age, according to new research.

The research suggests that mental activities tend to compress the time period that a person spends with dementia — delaying its start and then speeding up its progress.

The study was published Wednesday in the online issue of Neurology, the medical journal of the American Academy of Neurology.

"Our results suggest that the benefit of delaying the initial signs of cognitive decline may come at the cost of more rapid dementia progression later on, but the question is why does this happen?" study author Robert S. Wilson of Rush University Medical Center in Chicago said in a press release.

Mentally stimulating activities may somehow boost the brain's ability to function relatively normally despite lesions in the brain related with dementia, the study found.

However, once they are diagnosed with dementia, people who are more mentally active are likely to have more dementia-related brain changes compared to those without this mental stimulation. As a result, those who are mentally active may experience a faster rate of decline once dementia begins.

"This reduces the overall amount of time that a person may suffer from dementia," Wilson said.

Researchers evaluated the mental activities of 1,157 people age 65 or older who did not have dementia at the start of the 12-year study. People scored points (on a five-point scale) for how often they participated in mental activities such as listening to the radio, watching television, reading, playing games and going to a museum.

The more points scored, the more often people participated in mentally stimulating exercises.

During the next six years, the study found that the rate of cognitive decline in people without cognitive impairment was reduced by 52 per cent for each point on the cognitive activity scale.

For people with Alzheimer's disease, the average rate of decline per year increased by 42 per cent for each point on the cognitive activity scale.