Episodic memory (sometimes called implicit memory) is the ability to recall an event from the past, no matter whether that episode occurred within the last hour or months back. Remembering where you parked your car is one example of episodic memory. Recalling where you were when you heard Philip Seymour Hoffman died is another. Every hour, then, our brains create countless memories in rapid-fire succession — how exactly does your brain do that?

Researchers at the Dignity Health Barrow Neurological Institute and UC San Diego School of Medicine say the brain commits the details of each recollection to a small portion of individual cells distributed across the hippocampus. These new findings illuminate what happens in our brains on the cellular level whenever we remember. "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," said Dr. Peter N. Steinmetz, program director of neuroengineering at Barrow and senior author of the study.

Competing Theories

Scientists have long understood our ability to form episodic memories depends on the hippocampus, but they remain uncertain about the individual cells involved in creating such memories. On one side of this debate, scientists hypothesize each memory might be represented by a single neuron (brain cell) in the hippocampus (referred to as localist coding). Other scientists hold to the opposite view, theorizing each memory might be represented by a unique pattern of activity across all neurons of the hippocampus (referred to as distributed coding). Of course, there are other models envisioning an intermediate scheme, what might be called sparse distributed coding, whereby individual neurons participate in the coding of a few memories each while an individual memory is coded by a particular fraction of hippocampal neurons. This would be like passing a baton through one end of a crowd to another by feeding it through some and not every hand.

To investigate these competing theories, a team of researchers led by Steinmetz began by recruiting nine epilepsy patients. The nine patients had undergone an operation in which a surgeon implanted electrodes in their brain as a way to monitor the patterns of their seizures. Because these implants can record activity at the level of single neurons, epilepsy patients who have undergone this surgery are sometimes asked to participate in neuroscience research projects.

Next, Steinmetz and his colleagues designed an experiment to shed light on what happens to individual neurons inside the hippocampus during an act of episodic memory. Patients first memorized a list of words on a computer screen and then they viewed a second, longer list that contained those words as well as others. They were asked to identify words they had seen earlier, and to indicate how well they remembered them. Meanwhile, the scientists observed how individual neurons fired in their brains, noting how differences in cell-firing activity between words seen on the first list and those not on the list would clearly indicate which cells in the hippocampus were storing and conveying memories.

Sparse and Distributed

What did the researchers observe? Recently viewed words were stored in a distributed fashion throughout the hippocampus. A small fraction of cells, about two percent, responded to any one word, while a small fraction of words, about three percent, produced a strong change in firing in these cells. In short, the findings were in line with a sparse distributed coding theory.

"Intuitively, one might expect to find that any neuron that responds to one item from the list would also respond to the other items from the list, but our results did not look anything like that,” said Dr. John T. Wixted, professor of psychology at UC San Diego. Although only a small fraction of cells coded memory for any one word, the scientists said the absolute number of cells coding memory for each word was still large — hundreds of thousands at least. The loss of any one cell, then, would have a negligible impact on your ability to remember specific words recently seen.

Going forward, Wixted, Steinmetz and their colleagues hope to figure out how the human brain forms memories of places and objects, which cells are involved, and how those cells are affected by illness or disease. For their very next study, the team will attempt to determine whether similar coding is involved in memories of pictures of people and landmarks as in memories of words.

Source: Wixted JT, Squire LR, Jang Y, et al. Sparse and Distributed Coding of Episodic Memory in Neurons of the Human Hippocampus. PNAS. 2014.