Research sheds new light on how the brain encodes time and place into memory

Two studies, led by UT Southwestern researchers, shed new light on how the brain encodes time and place into memory.Recently published survey results PNAS And ScienceIn addition to adding to the body of basic research on memory, it may ultimately provide the basis for new therapies to combat amnesia due to conditions such as traumatic brain injury and Alzheimer’s disease.

About 10 years ago, a group of neurons known as “time cells” was discovered in rats. These cells appear to play a unique role in recording when an event occurs, allowing the brain to correctly mark the order of what happens in episodic memory.

Located in the hippocampus of the brain, these cells exhibit characteristic activity patterns while the animal encodes and recalls the event, and is an associate professor of neurosurgery at UTSW. PNAS Survey.

By firing in a reproducible order, it allows the brain to organize when an event occurs, says Lega. The timing of their firing is controlled by a 5 Hz brain wave called theta vibration, a process known as precession.

Lega used a memory task, which demands time-related information, to investigate whether humans also have time cells. Lega and his colleagues recruited volunteers from the Epilepsy Monitoring Unit at the Peter O’Donnell Jr. Brain Institute in UT Southwestern. Here, epilepsy patients stay for several days before surgery to remove the damaged part of the brain that causes the seizures.

Electrodes implanted in the brains of these patients help surgeons pinpoint the lesions of seizures and also provide valuable information about the inner workings of the brain, says Lega.

While recording electrical activity from the hippocampus of the brains of 27 volunteers, researchers read them a list of 12 words for 30 seconds and perform short math problems to distract them from the list rehearsals. I had them perform a “free recall” task, including that. Remember as many words as possible from the list for the next 30 seconds.

For this task, you need to associate each word with a segment of time (the list it was in). This allowed Lega and his team to look for a time cell. What the team found was exciting. In addition to identifying a robust population of time cells, firing these cells predicted how well an individual could link words in time (a phenomenon called time clustering). Finally, these cells appear to exhibit phase progression in humans, as expected.

For years, scientists have suggested that time cells are like glue that holds memories of events in our lives. This discovery specifically supports the idea in an elegant way. “

Bradley Lega, MD, Southwestern Medical Center, University of Texas, Principal Author and Associate Professor of Neurosurgery

In the second study of ScienceDr. Brad Pfeiffer, an assistant professor of neuroscience, led a team investigating place cells, a population of both animal and human hippocampal cells that record where the event occurred.

Researchers have long said that when an animal travels through a previous pathway, neurons that encode different locations along the pathway fire in sequence, much like cells fire in the order of temporal events. I knew it for a while, Pfeiffer explains.

In addition, place cells are further organized into “mini-sequences” while the rat is actively exploring the environment, representing a virtual sweep of the location in front of the rat. These radar-like sweeps occur about 8-10 times per second and are thought to be the brain’s mechanism for predicting immediate events and outcomes.

Prior to this study, place cells were often known to reactivate in long sequences when rats stopped running, which appeared to reverse the rat’s previous experience.

These “reverse regeneration” events were known to be important for memory formation, but it was unclear how the hippocampus could generate such sequences. Indeed, considerable research has shown that experience enhances the positive “look-ahead” sequence, but the reverse replay event needs to be weakened.

To determine how these posterior and anterior memories work together, Pfeiffer and his colleagues placed electrodes on the hippocampus of the rat in two different locations: a square arena and a long straight track. I made it possible to search.

To encourage them to move through these spaces, they placed wells with chocolate milk in various places. Next, we analyzed the activity of animal place cells to see how they corresponded to the animal’s place.

When the rat wandered through these spaces, a specific neuron fired and encoded information about its location. These same neurons fired in the same order that the rat followed the path, and periodically reversed as they completed different sections of the journey.

However, upon closer examination of the data, the researchers discovered something new. As the rat moved through these spaces, the neurons showed not only anterior predictive minisequences, but also posterior retrospective minisequences. The forward and reverse sequences alternate with each other, and each takes only a few tens of milliseconds to complete.

“While these animals were moving forward, their brains were constantly switching between predicting what would happen next and remembering what happened now. All for a moment. Within the time frame, “says Pfeiffer.

Pfeiffer and his team are currently studying how these cells receive inputs from other parts of the brain and act in these forward or reverse patterns. In theory, he says, it might be possible to hijack this system so that the brain can more faithfully remember where the event happened.

Similarly, Lega adds that stimulus techniques could eventually mimic the exact pattern of time cells, allowing people to more accurately remember the temporal sequence of events.

“In the last few decades, new discoveries of memory have exploded,” he added. “The distance between the basic discoveries of animals and how they can help people is now much shorter.”