In a novel study, researchers focus on encoding of the brain and claim new ways to enhance memory for the people suffering from traumatic brain diseases like Alzheimer.
The findings published in PNAS and Science shows how the brain functions while encoding time and place into memories. The findings not only add to the body of fundamental research on memory but could eventually provide the basis for new treatments to combat memory loss from major diseases.
A group of neurons known as ‘time cells’ was discovered in rats, in a study done about a decade ago. The researchers found that the cells appeared vital and played a unique role in recording the events, which then correctly marked the order of what happens in episodic memory.
Bradley Lega, M.D., associate professor of neurological surgery at UTSW and senior author of the PNAS study explained that the cells, located in the brain’s hippocampus, showed a characteristic activity pattern which the animals encode while recalling events. “By firing in a reproducible sequence, they allow the brain to organize when events happen. The timing of their firing is controlled by 5 Hz brain waves, called theta oscillations, in a process known as precession,” said Lega.
Lega and his colleagues recruited volunteers from the Epilepsy Monitoring Unit at UT Southwestern’s Peter O’Donnell Jr. Brain Institute to investigate whether humans also have time cells by using a memory task that makes strong demands on time-related information.
For the research, the epilepsy patients were made to stay for several days before surgery to remove damaged parts of their brains that spark seizures. Lega told that the electrodes implanted in those patients’ brains helped their surgeons to find valuable information on the brain’s inner workings.
‘Free recall’ tasks that involved reading a list of 12 words for 30 seconds, were practiced with those 27 patients which were done following a short math problem to distract them from rehearsing the lists, and then recalling as many words from the list as possible for the next 30 seconds. The task required associating each word with a segment of time, which allowed Lega and his team to look for time cells.
What the team found was exciting and exceptional: Not only did they identify a robust population of time cells, but the firing of these cells predicted how well individuals were able to link words together in time (a phenomenon called temporal clustering). Finally, those cells appeared to exhibit phase precession in humans, as predicted.
According to Lega, “For years scientists have proposed that time cells are like the glue that holds together memories of events in our lives. This finding specifically supports that idea in an elegant way.”
In the second study, Brad Pfeiffer, PhD, assistant professor of neuroscience, led a team investigating place cells (a population of hippocampal cells in both animals and humans that records where events occur).
According to the researchers, while rats actively explore an environment, place cells further organize into ‘mini-sequences’ that represent a virtual sweep of locations ahead of the rat. These radar-like sweeps happen roughly 8-10 times per second and are thought to be a brain mechanism for predicting immediately upcoming events or outcomes.
While these ‘reverse replay’ events were known to be important for memory formation, it was unclear how the hippocampus was able to produce such sequences. Considerable work indicated that experience should strengthen forward, ‘look ahead’ sequences but weaken reverse replay events.
To determine how these backward and forward memories work together, Pfeiffer and his colleagues placed electrodes in the hippocampi of rats, allowing them to explore two different places: a square arena and a long, straight track and then analyzed the animal’s place cell activity to see how it corresponded to the locations.
Particular neurons fired as the rats wandered through the spaces, encoding information on the place. The same neurons fired in the same sequence as the rats retraced their paths, and periodically fired in reverse as they completed different legs of their journeys. However, taking a closer look at the data, the researchers found that as the rats moved through the spaces, their neurons not only exhibited forward, predictive mini-sequences, but also backward, retrospective mini-sequences. The forward and backward sequences alternated with each other, each taking only a few dozen milliseconds to complete.
“While these animals were moving forward, their brains were constantly switching between expecting what would happen next and recalling what just happened, all within fraction-of-a-second timeframes,” Pfeiffer said.
In theory, Pfeiffer said that it might be possible to hijack the system to help the brain recall where an event happened with more fidelity. Similarly, Lega added that stimulation techniques might eventually be able to mimic the precise patterning of time cells to help people more accurately remember temporal sequences of events.
(This story has been published from a wire agency feed without modifications to the text.)
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