Amnesia - loss of memory - takes many forms and can be hugely distressing.

Amnesia is one of the fiction writer's favourite ploys. Typically, the victim of an assault has no memory of the incident so cannot say 'whodunit'. Better still they wake up with no recollection of who they are. Then there are those whose traumatic memories are 'suppressed' but gradually spontaneously return.

In reality, because memory is not a single simple system, amnesia is similarly complex and takes many forms. In general terms, the nature of the amnesia depends on the nature of the damage to the brain.

One of the most striking forms of amnesia is the inability to make new memories (anterograde amnesia). This often follows damage to the hippocampus, which is needed to establish new memories stored elsewhere in the brain. People may have perfect memories up to the point that they suffered the damage but can then make no new memories. Researcher Eleanor Maguire of University College London describes how people she studies can get confused if she leaves the room: they may be unable to recall what they are doing with her (see Magic Memories).

One of the most heart-rending tales is that of Clive Wearing. After a viral infection, he lost the ability to make new memories, as well as much of his past memory. In effect he 'wakes up' every few seconds, unable to remember what has just happened in his life. (His story was told in the 2005 ITV documentary 'The Man with the 7 Second Memory' and a poignant account of his relationship with his wife appeared in the 'Daily Telegraph' The man who keeps falling in love with his wife.)

Perhaps sadder still is the case of a patient known as 'HM' or 'Henry M'. Henry grew up normally enough in Hartford, Connecticut in the 1920s, but in late adolescence developed epilepsy. By the 1950s, he was enduring several seizures a week. With no other options available at the time, he underwent surgery. A significant chunk of his brain was removed – including most of his hippocampus, which at that point was not known to be important in memory making.

From that moment on, Henry was unable to add new memories. He has been studied intensively, providing new insight into many aspects of memory. He seems to have learned to cope, though worries about making friends – he forgets people minutes after seeing them. He has to be reintroduced to his doctors every morning. Most sadly, at regular intervals he relives the grief of discovering his mother has died.

Interestingly, Henry (like Clive Wearing) can still remember how to do things and can learn new skills (though cannot remember learning them). This shows how different aspects of memory are handled by different parts of the brain.

Being particularly associated with damage to the hippocampus, anterograde amnesia is rare. More common is retrograde amnesia, where memories made before a trauma cannot be recalled. The effect may be temporary, with memories gradually returning, or long term.

Head injury may cause various forms of amnesia. Impaired memory of events just before the head trauma reflects the absence of memory consolidation during the traumatic episode. Loss of memory is also a characteristic feature of neurodegenerative disorders such as Alzheimer's disease. Some (but not all) aspects of memory are impaired in old age, such as short-term memory.