On the right side of your brain, neurons are firing in your frontal lobe: monitoring, sequencing and organising. Remote regions of your brain are working in parallel. Chemical reactions and electrical charges travel through neuronal circuits and across synapses, reaching the temporal lobe at the side of your head, meeting signals from the hippocampal system below.
Memory is the "most fundamental and elusive of human powers", writes Frances Yates in her book The Art of Memory. Now imagine that it all stops. Imagine amnesia.
The sun on a coastal road. Stopping to repair a puncture in the early morning light. It's 5 July 1994. The next memory that Peter Wheeler can be certain about is six weeks later. He is being driven to his parents' house near Solihull in the West Midlands and he is reciting the street names. He was a postman, and the names still come easily. He desperately needs to prove to himself that his brain is still working.
In the five years since he came off his bicycle near Bishopsbourne, south of Canterbury, fracturing his skull in three places, Peter has devoted himself to putting his past back together. Like all amnesiacs who have suffered a head injury, he will never be able to recall the accident. He has no idea how he came off his touring bike so suddenly. Police who examined the scene found no evidence of a collision or holes in the road and so the accident will always be a mystery, but Peter is determined that the rest of his life won't stay that way.
When he eventually returned to his own flat, Peter couldn't remember it, or the accumulated stuff of ordinary life: the photographs, records and books. He sat and listened to all his music, record by record, to find out what he liked - jazz and reggae - and he finally realised that the picture on the wall of his spare room was of Bob Marley.
Hung on the walls of his living room are photographs of mountain views in clip frames, mountains he has walked up. When he first saw them he couldn't remember their names and he couldn't remember being on them. It drove him to despair. For weeks he pored over maps spread across the carpet of his mother's home, cross-referencing the routes marked on them against diary entries and photo albums, talking for hours with his brother about their walking holidays together.
His memories of the weeks following the accident are still cloudy, and he still prefixes most statements about the past with, "It's all very vague." He can't remember much of his childhood: a vivid memory of being caned for smoking at school, and of going to buy his first bicycle with his dad. But the memories of his adulthood have filtered back week by week, day-by-day. They are hard-earned.
Very little is known about how the brain recovers from a head injury. Most of the improvements seem to take place in the first year, as the bruising goes down. Retrograde amnesia, the name psychologists give to the loss of memories formed prior to the illness, can gradually shrink, sometimes right up to the hour before the accident. The parts of the brain needed to store long-term memories start working again with varying degrees of efficiency. Some psychologists believe that the brain compensates by forming new pathways, but there is no solid evidence.
Relationships have proved to be one of the worst problems for Peter. A few days after he left hospital, his brother Paul took him into The Navigation, his local in the village of Lapworth, near Solihull. Although Paul had warned them, old friends took offence when Peter didn't recognise them. Months later he saw a woman drive by and smile at him. He turned to his mother and said, "I think that's my girlfriend." The woman had heard about his injuries and didn't come round anymore.
Peter's friends abandoned him, too. There would be scenes in restaurants when old friends were met with blank stares, and many of them thought he was joking or being rude. Peter's family were forever explaining to angry people that he just couldn't remember them. The people soon stopped bothering with him.
In April 1995, less than a year after the accident, Peter was on a walking holiday with Paul in the Lake District. It was raining when they reached a summit at 28,000 feet, but Peter took out his camera to snap a picture. Before his accident he hadn't much bothered about photographs, but now he was taking no chances: he wanted to be sure he could remember this. And earlier memories seeped back as he looked at the surrounding mountains. At last he believed his brother's stories of how they had climbed here many times before.
Geoffrey can't remember marrying Pam. Geoffrey knows that he married her because his best friend George has told him, and of course he remembers being married, but he has a tendency to confuse his marriage to Pam with his first marriage.
He recognised Pam from the outset. He knew that she meant something to him, and of course she's been looking after him for the last seven years. But he doesn't remember marrying her because on 1 December 1992, the driver of an articulated lorry didn't see 55-year-old quantity-surveyor Geoffrey Bailey driving home along the A505 towards Newmarket and pulled out in front of him.
Perched on the brain stem at the top of the spinal cord, the human brain has the consistency of a blancmange floating in a bowl of water. At the moment of impact Geoffrey's brain vibrated violently against the bony outcrops on the inside of his skull and twisted within itself, damaging nerve fibres and blood vessels in parts of the brain essential to memory.
Geoffrey emerged from his coma a fortnight after the accident into a world far removed from the one he had known. He had post-traumatic amnesia, a period of extreme confusion and disorientation after a severe head injury which on average lasts for two weeks. It lasted six weeks for Geoffrey. He couldn't form new long-term memories: only his working memory was functioning. He could hold seven short chunks of information, say a telephone number, for about 30 seconds.
Otherwise he constantly forgot what was going on around him. When he grew more competent it became obvious he had forgotten the last 20 years. Doctors questioned Geoffrey about his marriage, and he described in detail his first marriage while Pam stood and listened. He would never remember their honeymoon in the west of Scotland, his 50th birthday party when she took him on a gourmet break to France, or the surprise party he threw for her 40th. His memory seemed to have abdicated all responsibility for their past and, to a large extent, for his present also.
Today, Geoffrey's main problem is what neuro-psychologists call anterograde amnesia - an inability to encode new memories properly. He relies on a notice board in the kitchen which he visits throughout the day. It's his surrogate memory. For the first two years after the accident it reminded him to brush his teeth and to get dressed, but he doesn't forget those things anymore. Now it just tells him what he has to do every day and where he has to be.
Geoffrey had forgotten how to read and Pam bought him children's books and helped him relearn. We all grow angry at the fallibility of our memory, but Geoffrey lives in constant frustration at his inability to recall words. An amnesiac's problems are specific to the areas of brain damage. Injuries to the left side of the temporal lobe will affect verbal and speech memory, whilst damage to the right-hand side will affect visual- spatial or musical memory. Geoffrey doesn't read many books now, since by the time he gets to the end of a chapter he can't remember how it started, and because damage to his frontal lobe, which is vital to the sequencing of memories, means he forgets the order of the plot. In the same way, he forgets when Pam has been home and when she has been away working.
He has to concentrate hard to encode and retrieve memories and it tires him out. When he's tired his memory fails even more and he often doesn't recognise friends. But at least he has more insight into his problem than he did for the first few years. He now knows that he has a poor memory and will accept that he has forgotten things. He'll even laugh about it.
Over the last six years Geoffrey has recovered a little of the past, but has to concentrate so hard on using his memory that the present has to take priority. Still, he remembers an early childhood visit to Australia, and recently his mother gave him photographs of his children from his first marriage and he remembered things about their childhood. During the first years after the accident, Pam sat with her husband and showed him their wedding album and hundreds of other photos, talking him through their shared memories. It's difficult to tell what he has remembered and what he has learnt from Pam.
He eventually remembered someone getting drunk at his second wedding, one small detail to cling to, but he seems to have forgotten it again. A few weeks ago he went to a rugby reunion and met people from his past. He came home and talked to Pam about them as if she knew them, and was shocked when she reminded him that she wasn't in his life then. It takes its toll. "He can remember his 20s and his 30s," explains Pam. "But there's this big gap in his filing system. I don't want to fill in the gaps. I don't want to go back there. There are things I could prompt him about, memories and photographs, but I don't because it's too hard for me. It's too distressing - the fact that he can't remember our honeymoon - so I don't talk about it anymore."
Geoffrey will live with amnesia for the rest of his life, he has a routine he can work with and he's made good progress, as much as can be expected. But amnesia has ruined their marriage. Pam doesn't feel as if she's married to Geoffrey any more. The trauma of the last six years has blocked out their happy times together.
"Relationships go through different stages," she says. "What binds people together during the ups and downs are the memories. When you hit a bad patch you can remember the good times. But I can't share any good times with Geoffrey. His memory loss is a loss that I've suffered in him, it's part of him that I can never get back."
Amnesia, from the Greek for "forgetfulness", is a complex condition which bears little resemblance to the plot device so popular with screenwriters. In real life, amnesiacs might be disorientated after regaining consciousness but don't forget who they are - apart from rare forms of psychogenic memory disorder where there is no physical damage to the brain. Amnesia generally occurs when certain regions of the brain are damaged by injury, tumour, stroke or infection.
The most famous case in the UK is that of Clive Wearing, who is unable to form new memories properly after contracting the virus Herpes encephalitis. In a 1986 documentary, he says over and over again that his condition is like being dead.
The ancient Greeks would have understood. They tell of two rivers in Hades, Mnemosyne and Lethe, memory and oblivion. Whereas Mnemosyne is the daughter of heaven and earth, Lethe is the daughter of Strife. Memory gave birth to the Muses: her opposite, Oblivion, gives birth to nothingness, to a kind of death.
Professor Susan Greenfield, who gave the 1994 Royal Institution Christmas Lectures on the brain, has written: "Our character continues to adapt as we respond to, and recoil from, the incessant experiences thrown in our path. For experiences to have any lasting significance in this way, they need to be remembered. The essence of the individual thus lies in no small part with what he or she can remember." It's understandable that for some amnesiacs, recovering their past becomes an obsession.
Memory is fragile, it fails us and frustrates us. Think of the irritation of trying to recall a fact that's just beyond our grasp: now imagine doing that all day, every day.
"Memory is an extremely important component of who we are," says Michael Kopelman, a neuro-psychiatrist specialising in memory disorders at St Thomas' Hospital in London. "In very severe cases, if you lose your memory you can be left completely in limbo. Our knowledge of who we are depends upon some kind of notion of self, but feeding that are our memories of the past."
For centuries, philosophers argued about the seat of the soul. In the 2nd century BC, Galen declared it was in the clear liquid, the cerebrospinal fluid, which bathes the brain. Later, Descartes decided it was in the pineal gland. But perhaps the hippocampal system, that tiny region of the brain beneath the temporal lobe where the most important memory functions reside, is the closest we will come to locating a soul, or something close to one .
Thanks to: Dr Robin Morris, head of the neuro-psychology unit at the Institute of Psychiatry; Headway, the National Head Injuries Association on 0115-924 0800; and the Brain Injury Rehabilitation Trust on 01908 640775. The Brain and Spine Foundation has a helpline staffed by neurological nurses on freephone 0808 8081000.