When BBC2's Top Gear presenter Richard Hammond flipped over in his jet-powered dragster while attempting to break the British land speed record, his survival was hailed as a medical miracle. The car had reached 314mph when a tyre burst and it span out of control, turning upside down so that his head was digging into the ground. Initially, it seemed certain he would die, but Hammond appeared to bounce back fast, returning home within weeks, instead of the 15 months that doctors predicted as a reasonable recovery time.

Eighteen months on, however, Hammond has revealed that his seemingly preternatural recovery has been nowhere near as straightforward as it might seem, and is far from complete. While the brain swelling may have subsided, he says, his emotions, memory and other functions have suffered more lasting damage. In a recent interview he said, "There was a lot more to fix than I thought." He now believes his entire brain will have to "rewire" itself, if it is to ever to function normally again.

He is, of course, lucky to be alive. Approximately half of all the deaths in people under 40 are caused by head injury, and around 1.4 million people a year attend A&E in the UK following head injury. Like Hammond, many of these survivors find the common memory, attention and emotional problems unsettling. Injured people may lose tolerance of noisy or stressful environments, they may have less insight and initiative, becoming more anxious and prone to depression, less motivated, more impulsive, and less able to control their anger. Hammond says that after the accident he suffered severe depression, having damaged parts of the brain to do with processing and emotional control. He still has regular meetings with a psychiatrist and struggles with short-term memory loss - recently, for instance, forgetting the pin numbers of all his bank and credit cards.

"Depression and memory loss can be a major problem," says Steve Shears, brain injury counsellor at Headway, the Brain Injury Association. In the early stages after a brain injury, the area of the brain that governs both short-term memory and self-reflection - the frontal lobe - is often compromised. This causes the injured person to behave oddly without realising it. They might, for instance, inappropriately blurt out what is on their mind, with no knowledge that they have said anything untoward.

This is because they are thinking about themselves autobiographically. Rather than interpreting their actions and words as they happen, they are effectively "remembering themselves as they were prior to the accident", says Shears. They can appear ebullient and gung-ho, thinking they have fully recovered and that nothing has changed, while everyone around them can see that this is manifestly not the case. Hammond now says that returning to Top Gear only four months after the crash was "much too early". Presumably, nobody was able to persuade him otherwise.

The next phase of recovery from a brain injury is known as the "emergent" stage, when some self-awareness returns. This is perhaps the hardest period for the injured person. "It dawns on them that they are not OK after all and this can shake their confidence. They do not stop making the mistakes, but they start to see them happening," explains Shears.

"Depression is a very big part of this self-awareness. People become aware of their cognitive problems as they experience them." There may be blank moments, memory problems, inappropriate behaviour. "People tell me how they have heard themselves say something they thought they were just thinking," he says, "and wanted the floor to open up and swallow them. People find that this new personality is difficult to handle. It eats away at their self-confidence."

Hammond describes how, after the accident, he was "prey to every single emotion that swept over me, and I couldn't deal with it". This is another common symptom, and it happens because the brain's neural pathways have been damaged.

"In a traumatic brain injury, the front of the brain, which governs self-awareness, often takes the impact," says Shears. "Shockwaves then bounce backwards and forwards around the brain, and other normal neural pathways are disrupted. Brain cells die, a bleed may deprive the brain of oxygen, there may be swelling, and since there is not much room inside the skull, this may restrict the blood supply. There is also a cascade of chemical reactions that can destroy brain tissue."

Shears likens this to a motorway. When the brain is working well, everything moves smoothly, but an injury is a bit like a lorry turning sideways across the M25 - everything stops. The clever cars then come off and use the side roads, avoiding the blockage and coming on at the next junction. "The brain tries to reorganise itself in this way. This sometimes means using new neural pathways - finding new ways to do the same things," he says.

The emotional effects of this rewiring can be overwhelming. Hammond describes how, even now, he will find himself freaking out about something, becoming scared, angry and full of self-doubt. This is because he is encountering a new emotional state, one that he no longer has a "strategy" to handle. Gradually, however, new emotional strategies are learned. "The brain becomes more and more efficient over time," says Shears.

Many people will learn to cope brilliantly after a brain injury. However, they may never fully return to their "old" selves. "It is often a question of accepting the new deal, and learning to come to terms with this new character," he says.

Thirty-nine-year-old David Morrison, who works in oil and gas industry recruitment in Aberdeen, was hit by a car in January 2003. He was in a coma for 25 days, his left side was paralysed and he had post-traumatic amnesia for three months after the accident.

"I've had some very dark moments," he says. "I've even wished that the car had hit me a bit harder. But it's like a jigsaw - you've just got to piece yourself back together, bit by bit.

"I still consider myself a work in progress. But I do feel like the same person inside now and I know I'll look back soon and think it wasn't such a bad thing after all. People say I'm more compassionate and understanding now. I'm not on top of the pile yet, but I'm getting pretty close."

· Headway, the Brain Injury Association helpline: 0808 800 2244, www.headway.org.uk