Sense of Time Can Be Skewed by Repeated Exposure to a Fixed-Duration Stimulus

By Linda Carroll

September 15, 2020

(Reuters Health) - Although we are normally very good at estimating time - say, of an eighth note or a sixteenth note - our brain's sense of time can be distorted when it is bombarded over and over again with a stimulus that is of a fixed duration, a new study suggests.

Using fMRI to scan the brains of volunteers repeatedly shown an image lasting 250 milliseconds (ms), researchers located the neurons that detect 250 ms visual inputs and found that these neurons can become "tired," leading to a slightly skewed sense of time. When the volunteers were then presented with visual inputs of 350 ms, 450 ms, 550 ms and 650 ms, they subjectively perceived the duration as longer than it was, researchers report in the Journal of Neuroscience.

The fMRI scans showed that the neurons designed to fire when they detect visual inputs of a certain duration are located in the brain's supramarginal gyrus (SMG). The scans also showed that the response of the neurons designed to detect 250 ms was depressed after the volunteers had been shown a 250 ms image 30 times.

"We found the neurocorrelates of subjective time perception," said the study's lead author, Masamichi Hayashi, a cognitive neuroscientist and researcher at the Center for Information and Neural Networks at the National Institute of Information and Communications Technology and the Graduate School of Frontier Biosciences at Osaka University, and the department of psychology at the University of California, Berkeley.

The subjective processing of time occurs in the SMG, which is located in the parietal region of the brain. That region "integrates forebrain activity," Hayashi said in a telephone interview. "It associates auditory and visual information. So, when you hear a dog barking and then see the dog, it associates those two."

To look at how a repeated stimulus might impact subjective sense of time, Hayashi and his coauthor, Richard Ivry, a professor at the University of California, Berkeley, recruited 18 healthy, right-handed volunteers whose average age was 21.

The volunteers were scanned with fMRI when they were initially presented with an image showing a gray circle for 250 ms. Then they were shown the same circle for 250 ms 30 times in a row (the adaptation), at which point fMRI scans showed the neurons sensitive to 250 ms had become fatigued and were firing less.

When the volunteers were then shown the circle for a time period slightly longer than 250, that is, 350 ms, 450 ms, 550 ms or 650 ms, and asked to compare the length of time it was on screen to an auditory stimulus, white noise in this case, the volunteers impression was that the visual stimulus was longer than the auditory one.

A longer-duration stimulus had the opposite effect. After the volunteers were shown the circle 30 times for 750 ms, the perception of time was shorter than it should have been when they viewed the circle for 350 ms, 450 ms, 550 ms or 650 ms.

So why is the effect seen when the test image duration is slightly longer or shorter than the time the volunteers had been adapted to?

"Once the participants were adapted to 250 ms, for example, neurons that are sensitive to 250 ms get fatigued the most," Hayashi explained. "However, the impact is not limited to 250 ms neurons. It also modulates the tuning property of neurons that are sensitive to slightly longer or shorter duration. As a result, of this tuning modulation, time perception is biased towards longer duration when the test stimulus is slightly longer than the adaptation duration and biased towards shorter duration when the test stimulus is slightly shorter than the adaptation duration."

The researchers were very creative in their study design, said Dean Buonomano, a professor in the departments of neurobiology and psychology at the University of California, Los Angeles.

"The study is innovative in its approach to the longstanding problem in understanding how the brain tells time and in knowing which circuits in the brain are responsible for telling time," Buonomano said. "Knowing what part of the brain is specialized to distinguish between an eighth note and a sixteenth note, for example."

The researchers cleverly used sensory adaptation to look at the question, Buonomano said. "A common example of this adaptation is the so-called waterfall illusion," he said. "If you stare at a waterfall for 30 seconds and then look at a rock beside it, it looks like the rock is going up. That's because the neurons that detect downward movement are fatigued, creating a momentary imbalance."

SOURCE: Journal of Neuroscience, online September 14, 2020.


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