In 1953 a young man was admitted to a Montreal neurological clinic for what was later described as ‘frankly experimental’ brain surgery. The man, who has become known to the scientific world as HM, was suffering from intractable epileptic seizures, and it was agreed that the only remaining option for treatment was to remove the regions of the brain where the seizures arose. The operation involved temporal lobectomy, in which parts of both the left and the right temporal lobe were surgically excised. In one important way the operation was a success, because HM’s epileptic seizures were now controllable by drugs; but the operation also had a dramatic and wholly unexpected side-effect - HM became amnesic.