Amnesia comes in distinct varieties. In “retrograde amnesia,” a movie staple, victims are unable to retrieve some or all of their past knowledge—Who am I? Why does this woman say that she’s my wife?—but they can accumulate memories for everything that they experience after the onset of the condition. In the less cinematically attractive “anterograde amnesia,” memory of the past is more or less intact, but those who suffer from it can’t lay down new memories; every person encountered every day is met for the first time. In extremely unfortunate cases, retrograde and anterograde amnesia can occur in the same individual, who is then said to suffer from “transient global amnesia,” a condition that is, thankfully, temporary. Amnesias vary in their duration, scope, and originating events: brain injury, stroke, tumors, epilepsy, electroconvulsive therapy, and psychological trauma are common causes, while drug and alcohol use, malnutrition, and chemotherapy may play a part. […]
When you asked Molaison a question, he could retain it long enough to answer; when he was eating French toast, he could remember previous mouthfuls and could see the evidence that he had started eating it. His unimpaired ability to do these sort of things illustrated a distinction, made by William James, between “primary” and “secondary” memory. Primary memory, now generally known as working memory, evidently did not depend upon the structures that Scoville had removed. The domain of working memory is a hybrid of the instantaneous present and of what James referred to as the “just past.” The experienced present has duration; it is not a point but a plateau. For those few seconds of the precisely now and the just past, the present is unarchived, accessible without conscious search. Beyond that, we have to call up the fragments of past presents. The plateau of Molaison’s working memory was between thirty and sixty seconds long—not very different from that of most people—and this was what allowed him to eat a meal, read the newspaper, solve endless crossword puzzles, and carry on a conversation. But nothing that happened on the plateau of working memory stuck, and his past presents laid down no sediments that could be dredged up by any future presents.