Earlier theories of memory could only hypothesize that such recollections were stored by time cells, or “neurons in the hippocampus and entorhinal cortex that fire at specific moments within a cognitive task or experience”.
If you mentally transport yourself back to a birthday party from your childhood, you will likely recall movie-like details such as running in the grass with your friends, opening colorfully wrapped presents or blowing out candles on a vivid cake. These cinematic recollections are known as episodic memories.
Earlier theories of memory could only hypothesize that such recollections were stored by time cells, or “neurons in the hippocampus and entorhinal cortex that fire at specific moments within a cognitive task or experience”. As of late October, however, The Proceedings of the National Academy of Sciences (PNAS) published an article that confirms the existence of time cells in the human brain for the first time.
Previously, time cells had only been confirmed to exist in rodents. A 2016 article published in The Journal of Neuroscience presented research on time cells in rodents, focusing on hippocampal area CA3. Anatomically, the hippocampus is separated into four areas: CA1, CA2, CA3 and CA4.
The former study involved collecting neural activity data from 11 rats who were placed in a rectangular maze and tasked with foraging for water. Throughout the study, the rats were given as much food as they needed but did not have access to water outside of the maze.
Initially, the Long-Evans rats were given free rein of the maze, allowed to explore and locate the areas that contained water ports. On later days, rats were introduced to water-reward based treadmills that gradually increased in speed and duration.
With eventual use of physiological recording via electrode implants, the researchers were able to confirm the existence of time cells in the rats’ brains. “In the memory demanding task, CA3 cells exhibited robust temporal modulation similar to the pattern of time cell activity in CA1, and the same populations of cells also exhibited typical place coding patterns in the same task. Furthermore, the temporal and spatial coding patterns of CA1 and CA3 were equivalently robust when animals performed a simplified version of the task that made no demands on working memory,” quotes the research.
Today, the new PNAS published research has confirmed that these unique, time-stamped cells support episodic memory in humans, as well. “By having time cells create this indexing across time, you can put everything together in a way that makes sense,” said Dr. Bradley Lega, neurosurgeon at University of Texas Southwestern Medical Center in Dallas and senior author of the research article.
The study gathered microelectrode recordings from 27 participants with severe epilepsy, each performing a free recall task. Patients were asked to memorize common nouns shown to them on a laptop screen in 1.6-s encoding intervals. Some sessions involved lists of 12 words, and lasted 30 seconds, while others involved longer lists of 15 words that were shown for a duration of 40 seconds.
Then, patients were given an arithmetic task that was meant to distract them, following which the researchers asked them to freely bring to mind the words they had been shown.
“We told participants how many words they would see on each list and how long they would have to recall words following each list. We did not explicitly communicate the duration of encoding lists to the participants; however, it is reasonable to assume they developed an expectation from the information shared and from experience on the practice lists completed before formal testing began,” confirms the study.
On average, patients were able to complete an average of 15.3 item lists and recalled the earlier list items with greater accuracy. Patients also generally listed the terms in the order they were displayed. The researchers were able to conclude that time cells are activated at specific moments during recall.
Lega continued to say that “The time cells that we found, they are marking out discrete segments of time within this approximately 30-second window.”
The implications of the new research are significant, bringing formerly unproven aspects of the workings of episodic memory to light. Vivid recollections of memory are a vibrant aspect of human existence. Whereas place cells are known to fire at specific locations, time cells have now been proven to consistently fire at specific moments throughout a sustained interval.
“The existence of time cells in rodents with properties analogous to place cells sparked significant interest among both human and rodent memory researchers because time cells offer a unifying mechanism for the representation of both spatial and temporal context. The demonstration of time cells in humans lends critical support to prominent models of memory positing that temporal coding mechanisms are central to the process of associative memory,” informs the research.
Perhaps you’ll now think of time cells next time you recollect that early birthday memory.
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