“Time cells” are a group of neurons that play a unique role in recording events and marking the order of what happens in episodic memory. These cells were discovered in rats a decade ago, and are located in the brain’s hippocampus. It has shown a characteristic activity pattern while the animals were encoding and recalling events. The timing of their firing was by 5 Hz brain waves, called theta oscillations, and this process is known as precession. Two new studies by researchers to shed light on whether humans also use time cells were carried out. This would give us an insight about time-related information.
The first study involved epilepsy patients being asked to stay for several days before surgery, and damaged parts of their brains that spark seizures were removed. Electrodes implanted in these patients’ brains helped in precise identification of the seizure foci, and also provided information on the brain’s inner workings. The tasks given included reading a list of 12 words for 30 seconds, then doing a short math problem, which was only to distract them from rehearsing the lists, and recalling as many words possible from the list for the next 30 seconds. This task requires association of each word with a segment of time. A robust population of time cells were identified, and the firing of these cells predicted the ability of individuals to link words together in time. This is called temporal clustering. These cells exhibit phase precession in humans.
In the second study, the research team was investigating place cells. These are located in the hippocampus in both animals and humans, and they record where events occur. When animals travel on a path they’ve been on before, neurons will fire in sequence to locate different points along the path, similar to time cells firing in the order of temporal events. In addition, place cells are further organized into “mini-sequences”, that represent a virtual sweep of locations ahead, which happen roughly 8-10 times per second. This could be a brain mechanism for predicting immediate upcoming events or outcomes.
Place cells would often reactivate in long sequences when there is a pause in movement, that appeared to replay prior experience in the reverse. These “reverse replay” events are important for memory formation. Experience strengthens forward, “look ahead” sequences but weakens reverse replay events. To determine how these backward and forward memories work together, electrodes were placed in the hippocampi of rats, and they were allowed to explore two different places – a square arena and a long, straight track. They placed wells with chocolate milk at various places along the path to encourage them to move through these spaces. Place cell activity was then analyzed to see how it corresponded to their locations. Particular neurons fired, encoding information on place. As the rats moved through these spaces, their neurons exhibited forward mini-sequences, as well as retrospective mini-sequences. These alternated with each other, taking only a few dozen milliseconds to complete. In summary, their brains were constantly switching between expecting what would happen next and recalling what just happened, all within milliseconds.
Researchers are currently studying what inputs these cells are receiving from other parts of the brain that enable them to act in these forward or reverse patterns. It might be possible to hijack this system to help the brain recall where an event happened, and stimulation techniques might eventually be able to mimic this precise patterning of time cells to help people remember temporal sequences of events more accurately.
Reference: ScienceMag
Also Have a Look at: Experiencing childhood trauma accelerates ageing of body and brain
Author
