A recent brain-mapping study has revealed that our experience of time is not instantaneous but is constructed through several distinct physical processing stages. As visual data flows from the rear of the brain toward the front, different neuron populations tackle specific parts of the timing computation, ultimately synthesizing our subjective sense of an event’s duration. These findings were detailed in the journal PLOS Biology.
For decades, scientists have been charting the extensive network of brain regions that light up when people judge how much time has elapsed. Studies involving both animals and humans have demonstrated that specific neuronal groups are tuned to particular intervals of time.
These specialized cells often manifest as topographic maps spread across the brain. On these maps, neurons that prefer similar durations are physically situated near one another on the brain’s wrinkled outer layer, known as the cortex.
Although the locations of these temporal zones have been identified, researchers have struggled to grasp precisely how these areas communicate. It was unclear how a physical attribute, such as the length of a light flash, translates into the abstract sensation of time passing.
To piece together this puzzle, neuroscientist Valeria Consani and her colleagues Gianfranco Fortunato and Domenica Bueti, from the International School for Advanced Studies in Italy, conducted an investigation utilizing neuroimaging techniques. They aimed to trace how the properties of time-tracking neurons shift as signals propagate throughout the brain.
Thirteen healthy volunteers participated in the study, engaging in a visual categorization task. First, participants were trained to internalize a specific reference duration of half a second, which they were to use as a mental yardstick.
During the main experiment, volunteers viewed a series of blurry, flickering circles displayed on a screen. Each circle remained visible for a random period ranging from two-tenths of a second up to eight-tenths of a second.
After each circle vanished, participants pressed a button to indicate whether the shape had been on screen longer or shorter than their internal reference. As the volunteers performed this task, researchers monitored their brain activity using an ultra-high-field MRI scanner.
fMRI is a technology that gauges brain activity by detecting changes in blood flow. When a brain region works harder, it demands more oxygen, and the scanner tracks the surge of oxygen-rich blood to that area.
This study employed a scanner with a magnetic field strength of seven Tesla. This is significantly more powerful than typical hospital machines, allowing the team to capture exceptionally detailed images of the brain’s surface.
Using these high-detail images, Consani and her team modeled the behavior of distinct neuronal populations. They were searching for unimodal tuning, which occurs when a set of brain cells responds most strongly to a single, specific stimulus and less so to everything else.
The investigators discovered that the manner in which neurons encode time varies depending on their location within the brain. They identified three distinct processing stages that constitute a hierarchy of time perception.
The initial stage occurs in the visual occipital areas, situated at the back of the head, where the brain first processes visual input. Here, neurons functioned like simple chronometers, gathering sensory input from the eyes.
In these visual regions, the brain cells displayed a clear preference for the longest durations. Their activity steadily ramped up as the shape stayed on screen, effectively encoding the physical length of the visual event.
The second stage takes place in the parietal and premotor areas, located in the upper and middle parts of the brain. In these regions, researchers observed a complete topographic map of time.
Neurons in these middle areas were tuned across the entire spectrum of presented durations. Certain cell groups responded only to brief flashes, while others were tuned exclusively to medium or long presentations.
These specialized cells were neatly organized into clusters based on their preferred duration. This suggests that the parietal and premotor areas are responsible for reading out the specific duration of the visual event, enabling the brain to accurately track how much time has just passed.
The final stage occurs in the frontal regions of the brain, which encompass the anterior insular cortex and the rostral supplementary motor area. These areas are heavily involved in complex thought, decision-making, and self-awareness.
In these frontal regions, the neurons did not represent the full temporal range. Instead, they exhibited a distinct preference for the middle of the time span, which aligned closely with the half-second reference duration participants had memorized.
This central preference represented the boundary participants used to classify a duration as either short or long. By tracking the precise moment participants switched their responses from “shorter” to “longer,” the researchers calculated each individual’s unique subjective boundary.
Activity in these frontal areas correlated perfectly with these subjective boundaries. This indicates that the frontal regions take the raw time measurement and transform it into a personal, abstract categorization.
“Our results suggest that time perception is not a single process, but the outcome of manifold processing stages distributed across the entire cerebral cortex,” the authors noted. “Each stage contributes differently, from encoding physical duration to shaping the subjective experience of time.”
To interpret the brain scan data, the research team employed a mathematical technique known as population receptive field modeling. This method allowed them to precisely estimate the temporal preferences of neurons within tiny patches of brain tissue.
By mapping these preferences, the team could pinpoint exactly which brain folds housed neurons tuned to shorter intervals and which held those tuned to longer ones. They also assessed how these preferences clustered together physically.
In the visual areas toward the back of the brain, the physical clustering of time-sensitive cells was relatively weak. However, in the parietal and frontal regions, neurons sharing identical temporal preferences were grouped very tightly.
This dense clustering implies that organizing time into structured maps becomes increasingly crucial as the brain transitions from simply perceiving an event to making a decision based on it. The brain physically arranges its cells to meet the demands of information categorization.
Furthermore, the researchers observed an asymmetry between the left and right sides of the brain in motor areas responsible for physical movements. Since participants used their right hand to press response buttons, the motor regions in the left hemisphere showed distinct activity patterns.
These motor areas consistently displayed a preference for the shortest possible intervals. The researchers posit this was a byproduct of the brain being primed to execute a physical movement immediately after the stimulus appeared, rather than a genuine measure of elapsed time.
Another fascinating detail emerged in the supplementary motor area—a brain region located on the top of the head involved in planning movements. The researchers found a clear segregation in how the anterior and posterior parts of this area processed temporal information.
The posterior half of the supplementary motor area contained cells tuned to the full range of durations, reading time much like a stopwatch. The anterior half housed boundary cells that helped classify time as either short or long.
Such decoupling within a single brain region has been seen before in animal studies. Finding this phenomenon in humans suggests this specific area might serve as a crucial nexus where actual and subjective time converge.
While this neuroimaging study provides an intricate map of visual time perception, it has several limitations. The investigation focused entirely on the cerebral cortex, which is the brain’s convoluted outer layer.
The research team did not measure activity in deeper brain structures or the cerebellum, which are known to also play roles in time processing. Future studies will need to explore these deeper areas to understand how they integrate with the cortical maps.
The experiment was also restricted to visual time perception. Whether the brain employs the same pathway to process the duration of sounds or physical touches remains an open question.
To fully map the role of the frontal lobe’s perceptual boundary neurons, the researchers suggest conducting experiments using several different reference signal durations. This would reveal whether the boundary-defining cells physically shift their preferences when the task rules are altered.
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