Glimpsing memory traces in real time

MEMORY is one of the biggest enduring mysteries of modern neuroscience, and has perhaps been researched more intensively than any other aspect of brain function. The past few decades have yielded a great deal of knowledge about the cellular and molecular mechanisms of memory, and it is now widely believed that memories are formed as a result of biochemical changes which ultimately lead to the strengthening of connections between nerve cells.

It is, however, also clear that memories are not encoded at the level of single neurons. Instead, the memory trace is thought of as a flurry of electrical activity within a scattered population of cells. Yet, very little is known about how memories are encoded and retrieved by populations of cells. Using a new large-scale recording technique, researchers from the Medical College of Georgia have now directly observed, for the first time, the population-level activity associated with encoding and retrieval of memory traces.

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Project H.M., Phase I

In February of this year, Jacopo Annese, a neuroanatomist and radiologist at the University of California, San Diego, travelled to Boston to take delivery of a brain. For Annese, collecting brains is not unusual – he is, after all, director of UCSD’s Brain Observatory, which will eventually become a comprehensive library of brains donated by people who had neurological conditions such as Alzheimer’s Disease, as well as by healthy people of all ages. This time though, the brain he collected was very special: it belonged to the amnesic patient Henry Molaison,  who for more than 50 years was known in the scientific literature, and to every neuroscience and psychology student, as H.M.

In 1953, at the age of 27, Molaison underwent a radical experimental procedure involving the surgical removal of both hippocampi. The procedure was carried out in order to alleviate the debilitating epileptic seizures he had been experiencing since childhood, and although it significantly reduced the frequency and severity of his seizures, it also left him completely unable to form new memories. Soon after the operation, Brenda Milner began a comprehensive assessment of Molaison’s memory function, which she described initially in a landmark 1957 paper, co-authored by William Scoville, who performed the operation. (At the time, Milner was doing her Ph.D. under the supervision of the neurosurgeon Wilder Penfield, who pioneered the technique used by Scoville to identify the abnormal tissue in Molaison’s brain.) In the process, she laid the foundations of neuropsychology, and in the decades that followed, Molaison became the best-known and most intensively studied case in the history of neuroscience.

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