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Cognitive “speed training” can reduce the risk of a dementia diagnosis by 25 per cent – that’s according to results from the world’s first randomised controlled trial of any intervention against the condition.

“There was a lot of scepticism about whether or not brain training interventions were beneficial, and to me, [our study] answers the question that they are,” says at Johns Hopkins University School of Medicine in Baltimore, Maryland.

Brain training has generated controversy for years. Early enthusiasm waned after several brain-training companies that had promised protection against cognitive decline .

In 2014, almost 70 scientists signed an saying that there was no conclusive evidence that brain training produced changes that had real-world relevance or promoted brain health. Months later, another signed by more than 100 scientists countered their arguments.

Now, a 20-year study of 2832 people aged 65 and older suggests specific exercises may offer benefits.

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The participants were randomly assigned to one of three intervention groups or to a control group. One group engaged in speed training, using a computer-based task called Double Decision, which briefly displays a car and a road sign within a scene before they disappear. Participants must then recall which car appeared and where the sign was located. The task is adaptive, becoming harder as performance improves.

The other two groups took part in memory or reasoning training, learning strategies designed to improve those skills.

The participants completed two 60-75-minute sessions per week for five weeks. About half of those in each group were then randomly assigned to receive booster sessions – four additional 1-hour sessions at the end of the first year, and another four at the end of the third year.

Twenty years later, the researchers assessed US Medicare claims data to determine how many of the participants had been diagnosed with dementia. They found that those who completed speed training with booster sessions had a 25 per cent lower risk of diagnosis with Alzheimer’s or a related dementia compared with the control group. No other group – including speed training without boosters – showed a significant change in risk. “The size of the effect is really quite astonishing,” says Albert.

“The analysis seems rigorous,” says at the Karolinska Institute in Stockholm, Sweden. “It is impressive to have a 20-year follow-up, and reducing the risk score for dementia is an impressive and important result.”

at Weill Cornell Medicine in New York state cautions that the team measured many outcomes over the 20-year period, including mental health. “The more outcomes that are examined and the more statistical tests that are conducted, the greater the likelihood that at least one result will look meaningful, even if the intervention itself had no true impact,” he says. “This does not mean the findings are wrong, but it does mean they should be interpreted cautiously.”

Why speed training might work remains unclear. One possibility is its reliance on implicit learning, which occurs without conscious awareness. “We know that changes that occur from this kind of learning are very long-lasting,” says Albert. What’s more, although the duration of the training was relatively modest, it was demanding. “You really have to pay attention, and it gets harder if you do it well,” she says.

There are plenty of examples of short experiences that drive long-lasting changes in the brain, says at McGill University in Montreal, Canada. For example, a single car crash can induce a lifelong fear of driving.

He suggests that speed training may build brain reserve – a kind of cognitive buffer against damage. If you assume cognition diminishes at a particular threshold of damage, then a brain with more neurons and connections is going to succumb later.

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Albert adds that altered brain connectivity might also help people divide their attention more effectively, making it easier to navigate daily life as they age. This could then reduce isolation, encourage greater activity or increase social engagement – things known to contribute to long-term brain health.

The authors also claim that the results for the booster group might reflect speed training having a dose-dependent effect. at Ontario Tech University says that future work should focus on this relationship: “What is the optimal amount of training?”

The take-home message, says at Boston University, is “not that everyone should go into their windowless basement and start doing speed training games on their computer”. But activities that use implicit learning may be beneficial in delaying the effects of Alzheimer’s disease. “Learning a new sport, vocation or craft is likely to produce long-lasting [beneficial] changes in the brain, in addition to any enjoyment you derive from engaging in these activities.”