Time in the brain doesn't follow the steady ticking of the world's most precise clocks. Instead, it seems to fly by at one moment and practically stand still at others. This distorted sense of time may be caused, in part, by brain cells getting tired, according to a new study.
When the brain has been exposed to the same exact time interval too many times, neurons or brain cells get overstimulated and fire less often, the study finds. However, our perception of time is complicated, and many other factors may also explain why time moves slowly sometimes and quickly at others.
We have only very recently begun to understand how our brains perceive time. It was only in 2015, that researchers found the first evidence of neurons whose activity fluctuates with our perception of time. But it wasn't clear if these neurons, found in a small brain region called the supramarginal gyrus (SMG), were keeping accurate time for the brain, or creating a subjective experience of time.
In the new study, the researchers used a "time illusion" on 18 healthy volunteers to figure it out. They hooked participants up to a functional magnetic resonance imaging (fMRI) machine that measures brain activity by detecting changes in blood flow.
The volunteers then went through an "adaptation" period, in which they were shown a grey circle on a black background for either 250 milliseconds or 750 milliseconds, 30 times in a row.
After this, the participants were shown another circle for a set period of time as a "test stimulus." They were then told to listen to white noise for a certain amount of time and asked if the test stimulus was longer or shorter than the white noise. (They used white noise as a reference because an auditory stimulus isn't affected by the visual adaptation but the visual test stimulus is.)
The researchers found that if the test stimulus was similar in length to the adaptation stimulus in duration, activity in the supramarginal gyrus decreased. In other words, neurons in that region fired less than when they were first exposed to the grey circle.
The idea is that this repetition "tire[d] out neurons," that are sensitive to that time duration, said lead author Masamichi Hayashi, a cognitive neuroscientist at the Center for Information and Neural Networks at the National Institute of Information and Communications Technology in Japan. But "other neurons that are sensitive to other durations [were] still active."
This difference in activity level distorted the participants' perception of time, he told Live Science in an email. If exposed to a stimulus longer than the duration the brain was adapted to, the participant overestimated time and if exposed to a shorter stimulus, the participant underestimated time.
This can distort our sense of time in the real world. For example, an audience at a piano concert may adapt to a musical tempo. "Your audience may feel your musical tempo subjectively slower than it actually is after being exposed to a music with a faster tempo, even if you are playing the music at the correct tempo," Hayashi said.
But "we cannot say at this point that neuron fatigue 'caused' skewed time perception because our study only showed a correlation between neuron fatigue … and distortion of subjective time," he said. "Our next step is to examine the causal relationship."
It's also possible that there are multiple mechanisms at work in the brain to create our single perception of time, he said. For example, our perception of time may be intimately related to our expectations, may be due to chemicals in the brain or even the speed at which brain cells activate one another and form a network when performing an activity, according to a previous Live Science report. "Addressing this question would be an important direction for future research," Hayashi said.
The findings were published Sept. 14 in the journal JNeurosci.
Originally published on Live Science.