In the spring of 1968, experimental psychologist Donald Lewis and his colleagues published a study about memory that was well before its time. The researchers first trained rats to fear a particular sound. A day later, the animals heard the same sound followed immediately by an electroconvulsive shock to the head. After getting shocked, the animals forgot their fear of the sound. The old memory was gone.

The study was remarkable for its focus on memory retrieval, rather than memory formation. See, research up until then had suggested that a memory is only unstable (and thus vulnerable to change) in the minutes or hours after it’s first created. During this period, called ‘consolidation’, the memory moves into the brain’s long-term