Media playback is unsupported on your device Media caption The video game that's actually dementia research

The world's largest dementia research experiment, which takes the form of a video game, has indicated the ability to navigate declines throughout life.

The findings, presented at the Neuroscience 2016 conference, harnessed data from 2.4 million people who downloaded the game.

Getting lost is one of the first symptoms of Alzheimer's disease.

And the researchers at University College London believe the results could help make a dementia test.

Sea Hero Quest is a nautical adventure to save an old sailor's lost memories.

With the touch of a smartphone screen, players sail a boat round desert islands and icy oceans.

Image copyright UCL

The game anonymously records the player's sense of direction and navigational ability as they work their way through the levels.

Some require them to weave through waterways and fire a flare back home, while others challenge them to memorise a sequence of buoys and then sail round them.

Data harnessed from the flare levels is the first to have been analysed, by scientists at University College London.

And it suggests the sense of direction declines consistently after the teenage years.

Players aged 19 were 74% accurate at firing the flare back home, but accuracy fell year by year until it reached 46% at age 75.

Dr Hugo Spiers told the BBC: "What we're able to announce to the world is it does decline across the lifespan, the ability to shoot the flare back to the target - that sense of direction."

The data also suggests men have a slight better sense of direction than women and that the Nordic nations outperform the rest in the world, although it is not yet clear why.

Ideas include:

people in better health, common in Nordic countries, retain their navigational abilities longer

coastal nations create better navigators

a genetic "Viking blood" boost to navigational skills

Image copyright UCL Image caption The deeper the colour, the stronger the country's navigational ability

The point of the research is to develop a way of diagnosing dementia in its earliest stages - something not yet possible.

Becoming completely disorientated is normally rare, but is more common in people with Alzheimer's disease.

Having a record of the normal decline in the internal compass could help doctors spot patients developing Alzheimer's.

Dr Spiers added: "The value of a future test built from Sea Hero Quest is that we will be able to provide a diagnostic for Alzheimer's dementia and a tool that allows us to monitor performance in drug trials."

Hilary Evans, the chief executive of Alzheimer's Research UK, said: "Sadly, we hear all too frequently of people getting lost and being found miles away from home.

"Researchers believe that these problems with spatial navigation could form the basis of a diagnostic test for the early stages of diseases like Alzheimer's, which could add a valuable tool to a clinician's diagnostic armoury.

"For any new diagnostic tool to be effective, it must take into account natural variation in a particular skill or ability across the population."

Image copyright UCL

Using the game has provided scientists with unprecedented amounts of data from across the globe.

The data collection from 2.4 million people passing some time in the evening or on the way to work would have taken 9,400 years in the lab.

This project was funded by Deutsche Telekom and the game was designed by Glitchers.

Michael Hornberger, a professor of applied dementia research at the University of East Anglia, said: "The amount of data that has already been generated by people playing Sea Hero Quest all around the world is phenomenal and is enabling us to reveal a vital global benchmark of how people, of all ages and from all over the use spatial navigation."

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