Due to an unusual illness, Matthew creates false memories that seem as vivid as the real thing. He’s had to learn to live with a past that is as uncertain as the future.

A few months after his brain surgery, Matthew returned to work as a computer programmer. He knew it was going to be a challenge – he had to explain to his boss that he was living with a permanent brain injury.

“What actually happened at the meeting was that the employers said, ‘How can we help you, how can we make you fit back into work and get back on your feet again?’” Matthew explains. “That’s what they said. But my recollection the next day was that they were going to fire me – there was no way they could allow me back into work.”

I was really scared – I thought I can’t trust what’s actually happened - Matthew

The memory was very vivid – he says – just as believable as anything that had actually happened. Yet it was completely false. Today, Matthew knows it was one of the first signs that he was suffering from “confabulation” as a result of his brain injury. Confabulators don’t mean to lie or mislead, but some fundamental problems with the way they process memories mean they often struggle to tell fact from a fiction concocted by their unconscious mind.

The discovery was another painful blow to Matthew (whose name has been changed to preserve his privacy). “I was really scared – I thought I can’t trust what’s actually happened.”

His dilemma, although extreme, can help us all to understand the frailties of our memories, and the ways our minds construct their own versions of reality.

Today, Matthew volunteers at Headway East London, a charity that supports people with brain injury. I first came across him when he gave a talk at the Wellcome Collection in London, and keen to know more, I later interviewed him about his experiences.

He is quietly spoken and conscientious as he talks about his past, often turning to confirm details with his colleague Ben Graham, who he has known for most of the 10 years since his operation. Even before his injury, Matthew had been very ambitious and driven in the face of hardship. He was born in Birmingham in the UK, but spent most of his childhood abroad before moving to live with relatives in London at the age of 17. After around a month, however, he was kicked out of their home.

He ended up homeless before living with a Franciscan friar. Going to college in the day and working evenings and weekends to pay for his keep, he eventually earned a place at University College London to study mathematics and computer science, and graduated to find a job as a computer programmer.

It should have been time for him to reap the rewards of that hard work, but after a few months in the new job, he started noticing some strange things happening to his body: a loss of sensation in his fingertips, excruciating headaches and double vision. He often had to work all day with one eye closed.

A CT scan revealed that the problem lay at the entrance of one of the brain’s ventricles – the cavities that aid the circulation of cerebrospinal fluid around our neural tissue. In Matthew’s brain a little sac of tissue called a “colloid cyst” had grown to block the entrance of the ventricle, stopping the cerebrospinal fluid from escaping. “The pressure is building up inside this space in the brain, and the fluid starts to crush your brain against the side of the skull,” says Vaughan Bell, a neuroscientist at University College London, who discussed Matthew’s injury at the Wellcome Collection event. The expanded ventricle was also pressing on the optic nerve, leading to the double vision.

I could just remember people appearing in my field of vision and then disappearing - Matthew

The doctors performed emergency surgery, cutting a hole in the skull, around the hairline, to remove part of the cyst and drain the excess fluid. Recovering in hospital, he knew that the injury had left him with serious memory deficits. He would forget having seen people enter or leaving the room, meaning it seemed as if they had somehow teleported in front of him. “I could just remember people appearing in my field of vision and then disappearing,” he says. Bell says this may have come from damage to the “mammillary bodies” – a pair of small round nodes of tissue (hence the name) that are known to be involved with recollection.

The mind doesn’t seem to like a blank space, however, and over the course of the recovery, Matthew’s memory started creatively filling in the gaps left by this amnesia. He once sent an angry email to the neuropsychiatrist, for instance, asking why he had been asked to leave rehab. “I can assure you I'm not very well at all, there's something very wrong with me,” he told them. It was only later that he found out that he had discharged himself – the decision had been completely his own. Yet he had a clear memory that the staff had sent him away.

Discovering this tendency for confabulation was deeply unsettling; it was as if he had discovered his mind was no longer his own. “Your brain is not just a reality-creating machine,” Matthew says. “There’s a difference between the things you perceive and the things the brain creates for you to understand the world you live in.”

Often the false memories would be built around a preconceptions of the way an event would have turned out. When he had returned to work, for instance, he had worried that his bosses would not be sympathetic to his difficulties. “I knew my employers were hardnosed business people, quite harsh, very strict with work. So my brain had already put them in a specific box and expected that they were going to react in a certain way.” Thanks to his amnesia, he could not remember the details of the meeting – and so his brain had somehow filled that blank space to match those expectations.

In some ways, this constructive process could be seen as exaggeration of the ways we all remember. Whenever we try to recall the past, the brain appears to reconstruct the event, selecting the details that seem most likely to have happened. “Behind the scenes the brain is doing a lot of things in selecting and testing information,” he says. “It’s testing out how strong should those memories seem and then suppressing the ones that aren’t relevant.”

None of us do this with complete accuracy; we can accidentally pull the wrong information to mind, forming “false memories” in which we remember details that never happened. Indeed, even for the healthy mind false memories are surprisingly easy to implant. In one experiment, psychologists in New Zealand and Canada secretly doctored subjects’ photos to suggest that they had been on a hot-air balloon ride. When interviewed about the photo, 50% of the participants concocted a story about the event, innocently believing that it had actually occurred.

Some people have memories for things that are impossible – they’ll say, ‘I built a spaceship and flew around the Moon'

We are mostly correct about the important details, however, but thanks to Matthew’s brain injury, the reality check has gone awry, so a far greater number of memories are false – though he is by no means the most extreme case that Bell has come across. “Some people have memories for things that are impossible – for example, they’ll say, ‘I built a spaceship and flew around the Moon’.” One visitor to the Headway East London centre woke up from a coma convinced his girlfriend had been expecting twins. He clearly remembered seeing the ultrasound scan, and taking photos of the baby bump – yet she had never been pregnant. “I’d recall those ‘memories’ like I’d remember something from my childhood: there’s literally no distinction for me,” he says.

Matthew now keeps a diary to help him record factual details – where he was, what he ate, what people said – offering a kind of scaffolding around which he can rebuild a picture of a day’s events. Even so, he still finds that the false memories can creep in. “Quite often the confabulation happens when Matthew’s quite anxious and they take the form of what he’s worried about,” says Ben Graham, Matthew’s colleague at Headway.

When they spend time together, Matthew often checks facts with Graham. It’s a delicate task – Graham is conscious that he may accidentally sow the seeds of a false memory in the way he phrases something. “You can plant an idea – it’s something I have to be careful about.”

Despite these ongoing difficulties, Matthew claims it is not the amnesia and confabulation that bothers him as much as a constant tiredness that continues to follow the surgery years down the line.  “If the fatigue goes away then I’m happy. Then I can cope with the memory loss.”

With his prognosis still uncertain, he’s had to learn to accept the small blessings in life. When he’s got enough energy, he enjoys getting on his bike and going for long cycle rides. And although he would like to return to computer programming full-time, he’s learned not to take the future for granted and to enjoy the present moment. “Now is good. It’s all you have.”

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David Robson is BBC Future’s feature writer. He is @d_a_robson on Twitter. Olivia Howitt is BBC Future’s picture editor: @oliviahowitt. You can read more about Matthew and other brain injury survivors at Headway East London's Who Are You Now project.

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