Eight, five, seven, three, one, two. If I asked you now to repeat these numbers, no doubt most of you could. If I asked you again after a long talk, you probably couldn't -- you will keep the memory for a short time only.
It seems to be the case that two quite different processes are involved in the brain in memory storage, one for the short-term -- that is about fifteen minutes to an hour -- and one for long-term memory. Many items of information find their way briefly into our short-term stores; most are discarded, and only a few find their way into the long-term store. While memories are in this short-term store, they are easily destroyed: by distraction, for instance -- do you remember the number sequence we started with? -- or by interference with the brain: by an epileptic fit, or concussion, for example. The film hero who wakes up after having been knocked out in a fight and asks "Where am I?" isn't joking; if the blow that knocked him out had been real it would have affected the electrical processes in his brain and so destroyed his store of short-term memories. But he will not have lost his store of permanent, tong-term memories -- indeed, it is extraordinarily difficult to erase them. Quite often in psychiatric treatment the psychologist tries to remove them by drugs, with electrical shock treatment, with insulin therapy, or psychoanalytic techniques, but usually with a very limited amount of success.
Indeed, when one comes to think about it, memory is perhaps one's most durable characteristic as an individual. I can lose limbs, have real organs replaced by plastic ones, alter my facial appearance with plastic surgery, but I am still "myself"--a complex of past experience, past memories, held tight and firm within my brain; only when I lose these do I cease to be myself.