How does our brain know that “this” follows “that”? Two people meet, fall in love and live happily ever after—or sometimes not. The sequencing of events that takes place in our head—with one thing coming after another—may have something to do with so-called time cells recently discovered in the human hippocampus. The research provides evidence for how our brain knows the start and end of memories despite time gaps in the middle. As these studies continue, the work could lead to strategies for memory restoration or enhancement.
The research has focused on “episodic memory,” the ability to remember the “what, where and when” of a past experience, such as the recollection of what you did when you woke up today. It is part of an ongoing effort to identify how the organ creates such memories. A team led by Leila Reddy, a neuroscience researcher at the French National Center for Scientific Research, sought to understand how human neurons in the hippocampus represent temporal information during a sequence of learning steps to demystify the functioning of time cells in the brain. In a study published this summer in the Journal of Neuroscience, Reddy and her colleagues found that, to organize distinct moments of experience, human time cells fire at successive moments during each task. The study provided further confirmation that time cells reside in the hippocampus, a key memory processing center. They switch on as events unfold, providing a record of the flow of time in an experience. “These neurons could play an important role in how memories are represented in the brain,” Reddy says. “Understanding the mechanisms for encoding time and memory will be an important area of research.”
Hippocampi, one in each brain hemisphere
Matthew Self, a co-author of the study and a senior researcher in the department of vision and cognition at the Netherlands Institute for Neuroscience, emphasizes the importance of these hippocampal time cells’ role in encoding experiences into memory. “When we recall a memory, we are able to remember not only what happened to us but also where we were and when it happened to us,” he says. “We think that time cells may be the underlying basis for encoding when something happened.”
While researchers have known about the existence of time cells in rodent brains for decades, they were first identified in the human brain late last year by researchers at the University of Texas Southwestern Medical Center and their colleagues. To better understand these cells, Reddy and her team examined the hippocampal activity of patients with epilepsy who had electrodes implanted in their brain to evaluate a possible treatment for their condition. The subjects agreed to participate in two different experiments after their surgery.
“During the surgery, the electrodes are inserted through small holes of around two millimeters in the skull. These holes are sealed until the patients recover from the surgery and are monitored for up to two weeks with the electrodes in place in an epilepsy monitoring unit, or EMU,” Self says. “We record the hippocampal neuronal activity while the patients are performing tasks in the EMU for a period of about one week after the surgery.”
In the first experiment, the study participants were presented with a sequence of five to seven pictures of different people or scenes in a predetermined order that was repeated multiple times. A given image, say of a flower, was shown for 1.5 seconds, followed by a half-second pause and then another image—a dog, for instance. In a random 20 percent of the image intervals throughout the sessions, the parade of pictures stopped, and participants had to decide which of two images was the next correct one in the sequence before continuing. The researchers discovered that, over the course of 60 repetitions of the entire sequence, all of the time-sensitive neurons fired at specific moments in intervals between quizzes, no matter which image was shown.
Source: https://www.scientificamerican.com/
February 6, 2023
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