Have you ever wondered how your brain keeps track of time? Humans learnt how by observing mice.
Did you know that your brain neatly records event markers such as time and space? These are called episodic memories and they help you recall and remember events.
Episodic memories can be lost with damage or disease to specific brain regions, as with Alzheimer’s disease—a condition in which memories of when and where experiences have happened can degrade over time.
Episodic memories are made in the medial temporal lobe. This brain area includes the hippocampus and medial entorhinal cortex. Brain cells (neurons) that record spatial information have been found in both brain regions. Neurons involved in tracking time have also been found in the hippocampus, but less is known about the role of the medial entorhinal cortex in time keeping.
Researching spatial and time keeping memory
A team led by Drs. James Heys and Daniel Dombeck at Northwestern University carried out a series of studies in mice to investigate whether brain cells in this brain region also help track time. This US-based National Institute of Mental Health (NIMH) supported research published their results in the November 2018 issue of Nature Neuroscience.
The researchers set up an experiment where mice ran on a treadmill, surrounded by a virtual reality screen, displaying a virtual track as they ran. The researchers then tracked neuron activity in the medial entorhinal cortex by using a brain imaging technique.
As the mice made their way through the track, they came upon an invisible door they couldn’t run past. If they did, they had to start the track over. To open the door, they had to wait at least 6 seconds without moving. If they moved before 6 seconds, they had to start the wait time over. Once the mice figured out how long to sit still to open the door, they could complete the track and receive their reward.
The researchers observed that:
- When the animals were running through the track, the specific set of cells responsible for recording spatial information were active.
- Once the mice stopped at the invisible door, those cells turned off and a different set of cells became active.
These new cells appeared to keep track of how long the mice waited for the door to open. The ensemble of these timing cells predicted how long the mice waited in the trials. Even when mice were given a different virtual track to run through, many of the same neurons responded to the same spatial or time components across the different tracks.
This suggests that there are two separate sets of brain circuits in the medial entorhinal cortex that are specifically attuned to record either time or spatial information.
“Patients with Alzheimer’s disease notably forget when things happened in time,” says Dr James Heys, a co-researcher of the study. “Perhaps this is because they are losing some of the basic functions of the entorhinal cortex, which is one of the first brain regions affected by the disease.”
This finding might lead to new early-detection tests for Alzheimer’s, Dr Daniel Dombeck adds. “We could start asking people to judge how much time has elapsed or ask them to navigate a virtual reality environment—essentially having a human do a ‘door stop’ task.”
- Your brain records events and keeps track of information such as time and space.
- Researchers discovered a set of brain cells in the medial entorhinal cortex known to contain spatial encoding (space and time tracking) neurons likely contribute to encoding spatial aspects of episodic memories.
- The study suggests that there are separate brain circuits for recording information about time and place when forming memories.
To know more about the role the hippocampus plays in memory formation and spatial navigation, watch this video:
References: Evidence for a subcircuit in medial entorhinal cortex representing elapsed time during immobility. Heys JG, Dombeck DA. Nat Neurosci.2018 Nov;21(11):1574-1582. doi: 10.1038/s41593-018-0252-8. Epub 2018 Oct 22. PMID: 30349104.
Title Image Credit: 95C from Pixabay