Saturday, September 22, 2007

Remember Clive Wearing

Every introductory course on neuroscience or psychology makes mention of H.M., a man who's medial temporal lobes (including the hippocampus and surrounding structures) were removed to treat drug-resistant epilepsy. After the operation, H.M. lost the ability to form new long-term memories, as well as most of his memory for the ten or so years before the surgery. (Ironically, H.M. has made a greater contribution to human knowledge than all but a handful of professional scientists, but he will never be able to appreciate the impact he's had.) H.M.'s case is canonical because it is the only instance where the connection between brain damage and subsequent amnesia is so clear. Indeed, no surgeon would have taken such drastic action had they known the effects, and no equivalent operation has been performed since (at least on humans). But H.M. is not alone in suffering from anterograde (can't form new memories) and graded retrograde (loss of memories of the recent past) amnesia. Korsakoff's amnesia is brought on by thiamine (vitamin B1) deficiency, primarily in alcoholics who derive a large percentage of their caloric intake from alcohol. The hippocampus can also be damaged by stroke, hypoxia, and infections.

One of the most acute known cases of amnesia is that of Clive Wearing (chronicled recently in this article by Oliver Sacks), whose temporal and frontal lobes were damaged by herpes encephalitis. Unlike H.M. and other amnesiacs, Mr. Wearing remembers nothing at all; his entire experience is restricted to the minute or two available through working memory (the short term memory which underlies active though processes, often believed to have a capacity of 7 +/- 2 "chunks"). Every time Mr. Wearing is distracted, he awakens to an entirely new and unfamiliar reality. For years, Mr. Wearing has kept a diary. Each entry contains the current time and a record of the profound realization that he is now, for the first time, alive and conscious. He then notices the previous entries. Pages of them. All making the same claim. All in his own familiar handwriting. All written by some unremembered stranger. He goes back, systematically crosses out these false entries, and underlines the current entry. The first true entry. He sets the diary down, glances out the window, and awakens for the first time.

Mr. Wearing has more to teach us than the dependency of memory formation and access on certain brain structures. His condition is not so different from our own. Consciousness is inextricably tied to the present moment. Our past and our future belong to other people. People who occupy the same body, in the same world, but who are tied together only by the hallucination of memory.