Edvard Moser, the Nobel-prize wining professor at the Norwegian University of Science and Technology, post-doctoral researchers Albert Tsao and Jørgen Sugar believe these 'neural clock' cells record time by organising our experiences into an orderly sequence of events.
This would mean we are hard-wired to experience time subjectively, explaining why at times the hours seem to fly by and at others they drag on seemingly without end.
"Our study reveals how the brain makes sense of time as an event is experienced,” Tsao, who spent twelve years researching the cells, said in a press release.
"The network does not explicitly encode time. What we measure is rather a subjective time derived from the ongoing flow of experience."
Albert Tsao has been investigating the lateral entorhinal cortex (LEC) since 2007.
Over the last two years, Tsao's colleague Jørgen Sugar has taken on his research, setting rats loose in mazes and encouraging them to explore in the hunt for small pieces of chocolate. The signals in the LEC were recorded with a scanner mounted on the rats' heads.
In one experiment a rat was introduced to a wide range of experiences and options for action. It was free to run around, investigate and chase bits of chocolate while visiting a series of open space environments.
“The uniqueness of the time signal during this experiment suggests that the rat had a very good record of time and temporal sequence of events throughout the two hours the experiment lasted,” Sugar said.
“We were able to use the signal from the time-coding network to track exactly when in the experiment various events had occurred.”
In a second experiment, the rat was trained to chase after bits of chocolate while turning left or right in a constrained figure-of-eight maze, a much more structured environment.
“With this activity, we saw the time-coding signal change character from unique sequences in time to a repetitive and partly overlapping pattern,“ Tsao added.
“On the other hand, the time signal became more precise and predictable during the repetitive task. The data suggest that the rat had a refined understanding of temporality during each lap, but a poor understanding of time from lap to lap and from the start to end throughout the experiment.”
Marco the rat exploring.
According to Moser, living organisms have developed multiple biological clocks, with some timekeepers, like the circadian clock, set to external processes such as daylight. Others, like the hippocampal time cells, use a domino-like chain to track time spans of up to 10 seconds precisely.
The area that Moser and his colleagues believe records experienced time sits in the lateral entorhinal cortex (LEC), right next to the medial entorhinal cortex (MEC), the part of the brain that provides our understanding of space.
In 2005, Moser and his wife May-Britt Moser discovered grid cells in the MEC which map our environment at different scales by dividing space into hexagonal units.
In 2014, The Mosers shared the Nobel Prize in Physiology or Medicine with their colleague and mentor John O'Keefe at University College London for discovering the cells that make up the brain's positioning system.
In 2007, inspired by the Mosers' discovery of spatially coding grid cells, then-Kavli Institute PhD candidate Albert Tsao set out to understand the lateral entorhinal cortex.
“I was hoping to find a similar key operating cell that would reveal the functional identity of this neural network”, Tsao said.
It has since taken more than a decade for him to realise that the constant changes to the signals in the network reflected the passage of time.
On Twitter, May-Britt Moser commended Tsao for his "brilliant, curious mind".
“Time is a non-equilibrial process. It is always unique and changing,” Professor Edvard Moser explained. “If this network was indeed coding for time, the signal would have to change with time in order to record experiences as unique memories.”
According to Moser, the discovery that we understand time primarily as a sequence of events means we can make it speed up or slow down by the decisions we make.
"If we change what we are doing, we also change our experience of time,” he told NRK. “It's not about absolute time, clock time, but about our subjective perception of time.”
The Mosers are Co-Directors of the Kavli Institute for Systems Neuroscience, and run the Moser Lab together where they supervise and guide promising PhD candidates and post docs.
They divorced in 2016, but their close and mutually beneficial research collaboration and friendship remains.