Brain scans of around 500 Scottish septuagenarians confirmed a link between smoking and an acceleration of age-related thinning of the cortex - the outer layer of grey matter. But the researchers also pointed, for the first time, to potential for recovery after quitting.

The cortex of ex-smokers in the group "seems to have partially recovered for each year without smoking," the multinational research team wrote in the journal 'Molecular Psychiatry'.

But they warned ,"Although partial recovery seems possible, it can be a long process."

According to many studies that have linked cigarette smoking with cognitive decline and dementia, and some also with brain degeneration.

"Evidence suggests that smokers have, on average, slightly poorer global cognitive functioning in later life, as well as lower mean scores on several cognitive domains such as cognitive flexibility and memory," researchers said.

However, it has never been shown whether the effects may be reversible.

The team used people who had participated in the Scottish Mental Survey as school children in 1947, when their cognitive function was tested.

The survivors underwent MRI (magnetic resonance imaging) scans again in 2007, and the results of 504 of them were analysed.

There were 36 current smokers, 223 ex-smokers and 245 who had never smoked in the group, which had an average age of 73, said the study paper.

And surprisingly, there was no difference between their ages or childhood IQ.

Analysis of the scans showed that current smokers had a generally thinner cortex than those who had never smoked, the study said.

"It took roughly 25 years without smoking for differences in cortical thickness to no longer be observed between ex-smokers and those that never smoked," the authors wrote in the journal.

Smokers need to be informed that cigarettes are associated with accelerated cortical thinning, a biomarker of cognitive ageing, the study revealed.

"Importantly, cortical thinning can persist for many years after smoking cessation," researchers said.

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