Researchers have identified cells in the human brain that are responsible for episodic memories. The study has been published in the journal Proceedings of the National Academy of Sciences. NPR reports: The cells are called time cells, and they place a sort of time stamp on memories as they are being formed. That allows us to recall sequences of events or experiences in the right order. “By having time cells create this indexing across time, you can put everything together in a way that makes sense,” says Dr. Bradley Lega, the study’s senior author and a neurosurgeon at the University of Texas Southwestern Medical Center in Dallas. Time cells were discovered in rodents decades ago. But the new study is critical because “the final arbitrator is always the human brain,” says Dr. Gyorgy Buzsaki, Biggs Professor of Neuroscience at New York University. Buzsaki is not an author of the study but did edit the manuscript.
Lega and his team found the time cells by studying the brains of 27 people who were awaiting surgery for severe epilepsy. As part of their pre-surgical preparation, these patients had electrodes placed in the hippocampus and another area of the brain involved in navigation, memory and time perception. In the experiment, the patients studied sequences of 12 or 15 words that appeared on a laptop screen during a period of about 30 seconds. Then, after a break, they were asked to recall the words they had seen. Meanwhile, the researchers were measuring the activity of individual brain cells. And they found a small number that that would fire at specific times during each sequence of words.
“The time cells that we found, they are marking out discrete segments of time within this approximately 30-second window,” Lega says. These time stamps seemed to help people recall when they had seen each word, and in what order, he says. And the brain probably uses the same approach when we’re reliving an experience like falling off a bike. The results help explain why people who have damage to the hippocampus may experience odd memory problems, Buzsaki says.