Space and time are closely related — not just in physics, but also in the brain.
This intimate connection becomes clearer when we take a look at how our brains form episodic memories.
Episodic memories are autobiographical memories — that is, memories about specific events that happened to someone at a specific point in time (and space).
The memory of that first kiss, or of the glass of wine you shared with your friend last week, are both examples of episodic memories. By contrast, semantic memories refer to general information and facts that our brains are capable of storing.
Episodic memories have a pronounced "where" and "when" component, and neuroscientific research shows that the brain area that processes spatial information is close to the one responsible for the experience of time.
Specifically, a new study reveals the network of brain cells that encode the subjective experience of time, and these neurons are located in a brain area adjacent to the one in which other neurons encode space.
The new study was conducted by researchers at the Kavli Institute for Systems Neuroscience in Trondheim, Norway. Albert Tsao is the lead author of the paper, which is now published in the journal Nature.
Neurons that change with time
Over a decade ago, two of the researchers who worked on the recent study — May-Britt Moser and Edvard Moser — discovered a network of neurons called grid cells that were responsible for encoding space.
This area is called the medial entorhinal cortex. In the new study, Tsao and colleagues hoped that they would find a similar network of brain cells that encodes time.
So, they set out to investigate the neurons in a brain area that is adjacent to the medial entorhinal cortex (in which grid cells were discovered). This area is called the lateral entorhinal cortex (LEC).
Initially, the researchers were looking for a pattern but struggled to find one. "The signal changed all the time," says study co-author Edvard Moser, a professor at the Norwegian University of Science and Technology, also in Trondheim, Norway.
So, the researchers hypothesized that perhaps the signal did not just change over time, but that it changed with time.
"Time [...] is always unique and changing," says Prof. Moser. "If this network was indeed coding for time, the signal would have to change with time in order to record experiences as unique memories."
So, the researchers set out to examine the activity of hundreds of LEC neurons in the brains of rodents.
Experience affects LEC time-coding signals
To do so, Tsao and colleagues recorded the neural activity of rats for hours, during which time the rodents were subjected to a range of experiments.
In one experiment, the rats ran around in a box whose walls changed color. This was repeated 12 times so that the animals could define "multiple temporal contexts" throughout the experiment.
The team examined the neuronal activity in the LEC, distinguishing between the brain activity that recorded changes in wall color from that which recorded the progression of time.
"[Neuronal] activity in the LEC clearly defined a unique temporal context for every epoch of experience on the timescale of minutes," write the authors.
The experiment's results "point to the LEC as a possible source of temporal context information necessary for episodic memory formation in the hippocampus," add the researchers.
In another experiment, the rats were free to roam through open spaces, choosing which actions to take and which spaces to explore in the pursuit of bits of chocolate. This scenario was repeated four times.
Study co-author Jørgen Sugar summarizes the findings, saying, "The uniqueness of the [neuronal] time signal during this experiment suggests that the rat had a very good record of time and temporal sequence of events throughout the 2 hours the experiment lasted."
"We were able to use the signal from the time-coding network to track exactly when in the experiment various events had occurred."
Finally, a third experiment obligated the rodents to follow a more structured path, with more limited options and fewer experiences. In this scenario, the rats had to turn either left or right in a maze, all the while searching for chocolate.
"With this activity, we saw the time-coding signal change character from unique sequences in time to a repetitive and partly overlapping pattern," Tsao explains.
"On the other hand," he continues, "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."
How LEC neurons encode experience
According to the study authors, "When animals' experiences were constrained by behavioral tasks to become similar across repeated trials, the encoding of temporal flow across trials was reduced, whereas the encoding of time relative to the start of trials was improved."
As Tsao and his colleagues conclude, "The findings suggest that populations of [LEC] neurons represent time inherently through the encoding of experience."
In other words, say the researchers, the LEC "neural clock" works by organizing experience into a precise sequence of distinct events.
"Our study reveals how the brain makes sense of time as an event is experienced [...] The network does not explicitly encode time. What we measure is rather a subjective time derived from the ongoing flow of experience."
According to the scientists, the findings suggest that by changing the activities and the experience, one can alter the time signal given by LEC neurons. This, in turn, changes how we perceive time.
Finally, the results suggest that episodic memories form by integrating spatial information from the medial entorhinal cortex with information from the LEC in the hippocampus.
This allows "the hippocampus to store a unified representation of what, where, and when."