Storing Memories of Recent Events

Memories of recent events may be held by a small number of neurons distributed across the brain’s hippocampus, a new study suggests. Understanding how the brain stores memories will yield insights into memory problems that come with normal aging and dementia.

The hippocampus plays a critical role in memory. Much prior memory research has focused on semantic memory—remembering facts, such as famous people and landmarks. Exposure to a particular face or place becomes linked to a small number of neurons in the hippocampus; these neurons then fire when the memory is recalled. But how the brain forms episodic memories—the memories of events—isn’t well understood.

Researchers have proposed at least 3 different ways that the brain might encode episodic memories. In a localist scheme, an individual neuron would code for one memory, and each memory would be linked to the activity of one neuron. In a fully distributed scheme, each memory would be coded by a pattern of activity across many neurons. In a sparse distributed scheme, each memory would be coded by the activity of a small proportion of neurons, and each neuron would contribute to a few memories.

A research team led by Dr. Peter N. Steinmetz of the Barrow Neurological Institute in Phoenix, Arizona, and Drs. John T. Wixted and Larry R. Squire, of the University of California, San Diego, investigated how episodic memories were encoded. Their study was funded in part by NIH’s National Institute of Mental Health (NIMH) and National Institute on Deafness and Other Communications Disorders (NIDCD). It appeared online on June 16, 2014, in Proceedings of the National Academy of Sciences.

The scientists were able to explore the mechanisms of memory at the single-neuron level by studying the brains of 9 patients with severe epilepsy who were being treated at the Barrow Neurological Institute. The patients had depth electrodes implanted into several brain structures, including the hippocampus. These tiny electrodes are used to pinpoint seizure-causing brain regions for possible surgical removal. They can also be used to gather information about how individual brain cells process memories.

The patients were first asked to study 32 target words. They then took a word recognition test with 32 targets from the study list and 32 “foils” that weren’t on the list. They rated the words on an 8-point scale, from 1 when they were sure it was new to 8 when they were sure it was old. Each of the 64 items on the test was presented only once to ensure that the targets, but not the foils, were represented by an episodic memory. If the items had been presented many times, the results might simply highlight neurons that respond to long-established semantic memories, rather than to words recently studied.

Together, the patients completed a total of 18 tests. The scientists found that a small percentage of recorded neurons (less than 2%) responded to any one target. Likewise, a small percentage of targets (about 3%) evoked a strong response in any one neuron. This pattern suggests that the human hippocampus uses a sparse distributed code to store episodic memories.

“To really understand how the brain represents memory, we must understand how memory is represented by the fundamental computational units of the brain—single neurons—and their networks,” Steinmetz says. “Knowing the mechanism of memory storage and retrieval is a critical step in understanding how to better treat the dementing illnesses affecting our growing elderly population.”

—by Harrison Wein, Ph.D.