Contrary to existing belief, activities that stimulate the mind like reading, solving crossword puzzles etc. has the potential of not just slowing down the onset of Alzheimer's disease but also to hasten it.

A new study suggests that while such mental activity could slow down the declines in thinking and memory during normal old age, there are also chances that people addicted to these pursuits may actually see a hastening of their mental decline once dementia sets in.

Researchers at the Rush University Medical Center in Chicago suggests that there is a trade-off scenario whereby such activities keep a fit person mentally alive a little longer but once dementia sets in, the pace of degeneration also increases.

Senior study author Robert Wilson, a senior neuropsychologist at the University's Alzheimer's Disease Center, says that previous research has indicated that mentally challenging activities were helpful in warding off dementia in older people.

The study, which was published in the online edition of Neurology, tracked 1,200 older individuals for a period of 12 years during which time they assessed each person's engagement in mentally stimulating pursuits using a "cognitive activity" scale.

All participants who were selected for the study were free from dementia when they enrolled, though by the end of the time frame 614 people remained cognitively normal while 395 showed mild impairment and 148 patients reported Alzheimer's disease.

It was observed that increased cognitive activity among normal individuals seen from their ability to watch television, listen to radio and play games, meant they were less likely to experience mental decline. Each point gained on the cognitive activity scale resulted in a 52 percent fall in the rate of decline over a six year period.

However, the very opposite turned out to be true in those who went on to develop dementia. In this segment, it was observed that people who liked intellectually challenging activities showed a faster rate of decline once the illness took over. The activity scale suggested that the rate of decline accelerated by 42 percent for each point.

The study team claimed that this discrepancy could be the result of an accumulation of neurodegenerative lesions called plaques and tangles in the brain cells of dementia patients. Earlier, it was suggested that mentally stimulating work could actually prevent these lesions from forming.