Recent studies mapping the human brain have revealed that our perception of time does not happen all at once, but rather unfolds over a series of different physical processing stages. As visual information moves from the back of the brain to the front, different groups of neurons process specific parts of the timing process, ultimately creating a subjective experience of how long an event lasts. These findings were published in the journal PLOS Biology.
For decades, researchers have mapped out the extensive network of brain regions that activate when humans estimate how much time has passed. Studies in both animals and humans have shown that specific groups of neurons respond during specific time periods.
These specialized cells are often arranged in topographical maps throughout the brain. In these maps, neurons that prefer similar lengths of time are located physically close to each other on the folded outer layer of the brain known as the cerebral cortex.
Even though researchers know where these timing regions are, they have struggled to understand exactly how they work together. It is unclear how physical features such as the duration of a light’s flashing translate into an abstract sense of the passage of time.
To piece together this puzzle, neuroscientist Valeria Centanino and colleagues Gianfranco Fortunato and Domenica Buetti from Italy’s International Institute for Advanced Studies conducted an imaging study. They wanted to track how the properties of time-tracking neurons change as the signal travels through the brain.
The researchers recruited 13 healthy volunteers to perform a visual categorization task. First, participants were trained to memorize a specific reference time of 0.5 seconds to use as a mental benchmark.
During the main experiment, volunteers observed a series of blurry, shimmering circles appearing on the screen. Each circle remained on the screen for a random amount of time between two-tenths of a second and eight-tenths of a second.
After each circle disappeared, participants pressed a button to indicate whether the shape was visible for a longer or shorter time than the internalized criterion. While the volunteers performed this task, the researchers recorded their brain activity using an ultra-high field functional magnetic resonance imaging scanner.
Functional magnetic resonance imaging is a technique that measures brain activity by detecting changes in blood flow. When certain areas of the brain work harder, they require more oxygen, and the scanner tracks the oxygen-rich blood flowing into those areas.
The scanner used in this study operates at a magnetic field strength of 7 Tesla. This is much more powerful than standard hospital scanners and allows the team to capture highly detailed images of the brain surface.
Using these detailed images, Centanino and her team modeled the behavior of individual populations of neurons. They looked for unimodal tuning, which occurs when groups of brain cells respond most strongly to certain stimuli and less strongly to others.
Researchers have discovered that the way neurons tune in to time changes depending on their location in the brain. They identified three distinct processing stages that form a hierarchy of time perception.
The first stage occurs in the visual cortex in the back of the head, where the brain first processes vision. Here, the neurons acted like simple timers collecting sensory information from the eyes.
In these visual areas, brain cells showed a strong preference for longest duration. The longer the shape remained on the screen, the more steadily its activity increased, encoding the physical length of the visual event.
The second stage occurs in the parietal and premotor cortex, located near the top and middle of the brain. In these areas, researchers observed complete topographic maps of the time.
Neurons in these intermediate areas were tuned to the full range of presented durations. Some groups of cells responded only to short flashes of light, while others responded only to moderate or long appearances.
These specialized cells are neatly organized into clusters based on priority periods. This suggests that the parietal and premotor cortex is responsible for reading out the specific duration of visual events, allowing the brain to accurately track elapsed time.
The final stage occurs in anterior regions of the brain, including the anterior insula and rostral supplementary motor area. These areas are deeply involved in complex thinking, decision-making, and self-awareness.
In these frontal areas, neurons did not represent the full range of time. Instead, they showed a strong preference for the middle of the time range, close to the 0.5 s reference time that participants remembered.
This central preference represented the boundary that participants used to decide whether a period was short or long. The researchers calculated each person’s unique subjective boundary by tracking the exact time participants switched their answer from “short” to “long.”
These frontal activities matched perfectly with these subjective boundaries. This shows that the frontal region takes raw measurements of time and transforms them into personal, abstract categories.
“Our results show that time perception is not a single process but the result of multiple processing steps distributed throughout the cerebral cortex,” the authors write. “From encoding physical duration to constructing the subjective experience of time, each stage contributes in a different way.”
To interpret the brain scan data, the research team used a mathematical approach called population receptive field modeling. This technique allows us to estimate the precise temporal preferences of neurons in small parts of the brain.
By mapping these preferences, the researchers were able to see exactly which brain folds contained neurons tuned to short moments in time, and which brain folds contained neurons tuned to long periods of time. They also assessed how these preferences are physically clustered together.
In the visual cortex, located in the back of the brain, the physical clustering of time-sensitive cells was relatively weak. However, in the parietal and frontal regions, neurons with exactly the same temporal preferences were tightly grouped together.
This strict grouping means that as the brain moves from simply seeing an event to making decisions about it, organizing time into a structured map becomes more important. The brain physically structures cells to handle the demands of classifying information.
Additionally, researchers noticed differences between the left and right sides of the brain in the motor cortex, which controls body movement. As participants used their right hand to press the response button, the motor cortex of the left hemisphere showed a distinct pattern of activation.
These motor areas consistently showed a preference for the shortest possible duration. The researchers believe this was not a precise measurement of elapsed time, but rather a byproduct of the brain preparing to make a physical movement as soon as the shape appeared.
Another surprising detail was revealed in the supplementary motor area, a part of the brain near the top of the head that helps plan movements. The researchers found that the anterior and posterior parts of this region have distinct ways of processing time.
The second half of the supplementary motor area contains cells tuned to the full range of durations, which read out time like a stopwatch. The first half contained boundary cells that helped classify time as short or long.
This division within a single brain region has been previously observed in animal studies. Its finding in the human body suggests that this particular region may serve as a central hub where real and subjective time are integrated.
Although this imaging study provides a detailed roadmap for visual time perception, it has several limitations. The study focused entirely on the cerebral cortex, the folded outer layer of the brain.
The researchers did not measure deep brain structures or activity in the cerebellum, which are known to influence processing time. Future studies should examine these deeper regions to see how they interact with cortical maps.
This experiment was also limited to visual time perception. Whether the brain uses this exact same pathway to process the duration of sound and physical contact remains an open question.
To fully understand border neurons in the frontal lobe, researchers suggest conducting experiments that test multiple different reference periods. This reveals whether the border cell’s priority physically changes when the task’s rules change.
Despite these limitations, this study provides a clearer picture of how a simple flash of light turns into a conscious experience of time. It reveals that our sense of time is collaborative, transmitted along specialized assembly lines in our heads.
The study, “Cortex-wide neuronal populations underlies discrete, categorical, and subjective representations of visual duration,” was authored by Valeria Centanino, Gianfranco Fortunato, and Domenica Bueti.
