Brain’s Nerve Cells Play Key Memory Role, Scientists Find

A team of researchers at UCLA has found that individual nerve cells in the brain appear to selectively recognize specific words and faces during the processing of short-term memory.

The group’s findings, published today in the journal Nature, offer new information on the functioning of the hippocampus, a part of the brain long believed to play a crucial role in memory but whose inner workings have remained poorly understood.

“We’ve supplied (brain theorists) with the beginnings of a detailed look at what’s actually going on,” said Gary Heit, a neurophysiologist at UCLA and an author of the study, which explored ways in which the brain stores and retrieves information. “They can take that work and refine their own.”

Heit and his colleagues studied the impulses emitted by brain neurons when people were shown words or faces memorized previously. The impulses were recorded by electrodes that had been implanted in their brains for unrelated epilepsy surgery.

First, the subjects memorized 20 words. Then they were shown lists of words containing 10 of the original words and 10 new ones. The subjects were told to press a key when they saw repeated words. Meanwhile, the researchers monitored their brains.

They found increased “firing” by individual neurons in response to specific stimuli. For example, one cell showed increased response whenever the word luck appeared on the video screen; another responded aggressively to the word carve.

However, cells’ responses were not confined to single stimuli: some responded to both specific words and specific faces.

Many Cells Involved

Nor were the responses confined to single cells: 75% of the neurons for which data were collected showed a preference for at least one of 10 words tested. That means that rather than each cell memorizing a particular word, the scientists said, words are represented in the brain as a pattern involving many cells, some of which the researchers sampled.

“We can be sure that when we see a cell that fires (in response) to a particular word in a particular context, that’s not the only thing it does,” added Eric Halgren, a co-author of the study and a neuropsychologist at the Wadsworth Veterans Administration Medical Center.

Although researchers have long known that loss of the hippocampus destroys a person’s ability to form new memories, they have not known precisely how it functions. According to Heit and Halgren, two hypotheses have been discussed.

To illustrate those hypotheses, Heit used the following analogy for memory:

A person is shown a small photograph of Vincent Van Gogh’s painting, “Starry Night.” The photograph is removed and the person is led into a dark gallery. The painting is hanging somewhere on the walls. A guide, the hippocampus, helps the person find it.

Functions Like Spotlight

Under one hypothesis, the hippocampus functions as if by lighting up the entire gallery, allowing another part of the brain to locate the painting. Under the other theory, it functions like a spotlight focused directly on the painting.

The new findings appear to support the second theory, the researchers said.

“Our results seem to suggest . . . that the hippocampus is contributing specific information to the recall of memories,” Halgren said. “ . . . It is finding the missing parts between what you know and what you’re looking for.”

Halgren said the hippocampus may serve as a kind of “routing system” that receives activation from the brain’s neocortex and then helps activate other parts of the neocortex that were activated during the original experience.

Refining Theories

Researchers said the new information may be useful not only in refining theories of the hippocampus’s role in short-term memory but perhaps also in contributing to a model that might be useful in developing computers based on the brain.

“We know almost nothing about how the brain actually perceives and stores information,” said Dr. Larry Squire, a research scientist at the Veterans Administration Medical Center in La Jolla and a professor of psychiatry at UC San Diego.

“At some point, we want to get enough information to try to model on computers and develop mathematical formulations,” he said. “This kind of information is helping us understand how the brain does its fundamental jobs of perception and memory.”