Our fond or fearful memories - that first kiss or a bump in the night - leave memory traces that we may conjure up in the remembrance of things past, complete with time, place and all the sensations of the experiences. Neuroscience call these traces memory engrams.
But are engrams conceptual, or are they a physicial network of neurons in the brain? In a new MIT study, researchers used optogenetics to show that memories really do reside in very specific brain cells, and that simply activiting a tiny fraction of brain cells can recall an entire memory - explaining, for example, how Marcel Proust could recapitulate his childhood from the aroma of a once-beloved madeleine cookie.
"We demonstrate that behavior based on high-level cognition, such as the expression of a specific memory, can be generated in a mammal by highly specific physical activation of a specific small subpopulation of brain cells, in this case by light," says Susumu Tonegawa, the Picower Professor of Biology and Neuroscience at MIT and lead author of the study reported online today in the journal Nature. "This is the rigorously designed 21st century test of Canadian neurosurgeon Wilder Penfield's early 1900s accidental observation suggesting that mind is based on matter."
In that famous surgery, Penfield treated epilepsy patients by scooping out parts of the brain where seizures originated.