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.