Video games are the biggest form of entertainment in the world, but sometimes they bleed into people's lives offline in surprising and disturbing ways.

Christian Dines' hands were twitching. As though he were still gripping his video game controller, about to make a killer move. But the game was switched off and his hands were free. The US-based sustainability advisor had also noticed how, when he glanced at objects in his room, he felt an urge to absorb or "collect" them, like weapons or power-ups in his game.

He swallowed hard. "I thought, 'what the hell is this?' It was something I'd never experienced before as a gamer," he says. After a week of playing the same game maybe two or three hours a day, Dines' virtual experience was spilling over, disturbingly, into reality.

"It all only lasted a couple of days, but the effect was disorientating," he recalls. "It's unnerving to be distracted in some way by a screen when you're no longer in front of it."

Dines had, it seems, experienced something called Game Transfer Phenomenon, a condition in which the physical world and video games bleed together. While this might not be very common, for gamers who experience it, the condition can be extremely unsettling. And potentially even dangerous.

The term Game Transfer Phenomenon, or GTP, was first coined by Angelica Ortiz de Gortari, a psychologist at the University of Bergen in Norway. She first proposed the concept a decade ago while working on her doctoral thesis under the supervision of Mark Griffiths, head of Nottingham Trent University's International Gaming Research Unit. Ortiz de Gortari was motivated by her own experience of GTP. One day, she was walking around her local supermarket and realised that she was imagining peering at products on the shelves through a rifle scope.

"I thought, 'Wow! This is interesting'," she recalls. "A phenomenon that changes your perception by encouraging you to see objects through the lens of the game you're playing," she says, adding that her response had felt involuntary, leaving her with serious questions about what it meant.

But what exactly is GTP? Ortiz de Gortari suggests that one could compare it to potentially more common experiences such as ear-worms, in which you spend days trying to get a catchy song out of your head. Or when images from a television show you binge-watched keep popping up in your mind's eye. With GTP, though, the intensity is arguably dialled up, says Ortiz de Gortari. Not least because gaming activates brain areas associated with control inhibition – the ability, or not, to control one's thoughts and behaviour rather than acting on impulse. This can also occur while passively watching television, but to a lesser degree than while gaming.

Ortiz de Gortari's studies suggest that GTP induces distress and dysfunction for around half of those gamers who say they have experienced it, with confusion, hyper-vigilance and irrationality among the symptoms. For others the only notable response may be a feeling of embarrassment that their game-play has spilled over into "real life".

One study participant she interviewed reported seeing health indicator bars like those in the role-playing game World of Warcraft floating above their companions' heads. Another spoke of lapses in concentration after not being able to stop "seeing" images from a game. Others said colours in the real world seemed transformed, and began to mimic the colours of a game world they had recently played in. While such effects are usually transitory, GTP can provoke a startling array of spontaneous or involuntary effects, according to Ortiz de Gortari's research.

While rare, these effects can even take the form of involuntary physical actions and behaviours. A game could end up shaping the way one interacts with real world objects or people. For example, a walk down a supermarket aisle could be experienced through the lens of gameplay, with the player perhaps "shooting" at products or people, possibly with a corresponding involuntary physical action in the hands as though working console controls.

In total, Ortiz de Gortari has recorded GTP experiences among gamers related to more than 400........

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