“Cognitive reserve refers to a lifetime cognitive experience, influencing the number of neurons and synapses that survive into adult life,” says Dorina Cadar, a senior lecturer in cognitive epidemiology and dementia at the UK’s University of Sussex.

The term defines the mind and brain’s resistance to brain damage “by engaging increased neural networks”. The amount of cognitive reserve we build up varies and is influenced by what Cadar calls “preventive interventions”.

These include cultural and social engagement, reading, doing crossword puzzles, visiting art galleries, clubs and cinemas, and volunteering. These add to our cognitive reserve and “brain reservoir”.

Building cognitive reserve is increasingly thought to play a key role in dementia prevention. The theory is that accumulated knowledge and experience may provide increased resilience against later cognitive disorders, including Alzheimer’s disease.

A bigger or better cognitive reserve increases the brain’s resilience and flexibility, and might extend cognitive function even in the face of dementia, helping sufferers manage and mask cognitive decline.

It is thought that the “reserve” allows the brain to use different neural networks in healthier parts of the brain.

If you think of dementia as roadblocks in the brain where messages can’t get where they need to go, a healthy cognitive reserve means they have the wherewithal to reroute and get there in the end.

Merrill advocates the deliberate pursuit of things that build cognitive reserve to keep a brain sharp, and demand concentration or collaboration.

Like any other part of our bodies, we need to use our brains, he says.

“The phrase ‘use it or lose it’ is a basic biological fact that holds as true for our brains as for our muscles or bones.”

While the loss of muscle can be seen and skeletal health can be measured using bone density scans, he says, it’s only recently that we became aware the same reality was evident in our brains.

“Disuse atrophy” applies to cognitive decline the same way it does to sedentary muscles.

Brain imaging shows that learning and “interested immersion” contribute towards building psychological and physiological lift, in the preservation of brain volumes and preventing atrophy of memory centres.

When they aren’t used enough, brain cells begin to lose healthy synapses, and if there’s enough disuse, they’ll die, Merrill says.

Using our brains – thinking, reading, solving difficult challenges – is key, says Jenna Najar at Sweden’s University of Gothenburg. She was part of a large extended study that examined cognitive and physical activity among women in midlife to identify risk of dementia later.

“We found that women who were cognitively active (playing an instrument, singing, reading or writing a book, doing crosswords) had a reduced risk of Alzheimer’s disease (regardless of physical activity), compared to women who did not engage in cognitive activity at all.

“Women who were physically active (from either regular exercise or intensive exercise) had a reduced risk of vascular forms of dementia (regardless of how cognitively active they were) compared to the women who were totally inactive.”

Dr David Robinson, a geriatrician in Ireland, says novelty is important.

“When we’re young, everything is new and our brains grow. But as we get older, we tend to do the same things over and over. So go somewhere new – go to a different town or cafe. Visit a museum,” he recommends.

Or, as Cadar suggests, an art gallery.

I look at my mum now and know that her cognitive reserve was drained by many things – decades of depression isolated her; she withdrew, there was none of the all-important brain-stimulation that comes from socialising.

A loss of confidence meant she didn’t go anywhere or try anything new when she developed dementia.

Later, a stroke left her with pure alexia – acquired reading impairment – which meant she could no longer read or do the cryptic crossword puzzles she’d loved.

Maybe she would have succumbed to dementia anyway, but perhaps if she had been able to lead a more engaged life, and kept reading, she would not have developed it until much later, or the symptoms would not have handicapped her as early.