The nature of memory has intrigued thinkers for centuries. From the 1930s onwards, it became clear in the most dramatic way that memories were physically represented in the brain, they were formed by networks of cells. Scientists called this physical basis of memory the engram.
One of the first signs of the reality of the engram came from the work of the Montreal neurosurgeon Wilder Penfiel, who carried out hundreds of brain operations with the aim of relieving chronic, debilitating epilepsy. To identify which part of the brain to remove, Penfield gently stimulated his conscious patients with electric currents carried by delicate electrodes. This procedure revealed something rather eerie: sometimes the stimulation led the patient to relive very precise events.
These experiences were vivid and detailed, like a waking dream. Often the patients heard sounds—a piano being played, someone singing a well-known song, or a telephone conversation between two family ...
