What did a man who could not form new memories teach us about memory?

Memory is not one thing: that is the lesson of a man who could not make new memories at all. In 1953 a man named Henry Molaison, later known to science only as H.M., had parts of his medial temporal lobes removed, including the front two thirds of his hippocampi, in an attempt to cure his epilepsy. The surgery partly controlled his seizures, but a severe side effect was that he became unable to form new memories. No one knew then that the hippocampi play an extremely important role in forming and remembering memories.

What he lost and what he kept taught scientists the most. After the surgery he often had a memory span of only around 30 seconds. He could recall events from childhood or young adulthood in good detail, but found it extremely difficult to recall more recent events. Researchers told him of his renown and the significance of his condition, but he was unable to internalize such facts as memories.

Yet he could still learn some things. Brenda Milner had him draw a figure while looking only at its reflection in a mirror. Each time he did it he improved, even though he had no memory of the previous attempts or of ever doing the task. Milner concluded that parts of the brain responsible for procedural implicit memory, such as the basal ganglia and cerebellum, can remember things that the conscious mind has forgotten. His case became an important contribution to how we understand memory: long-term memory is not a single store, and remembering how to do something can survive when remembering what happened is lost.