How the Brain Senses Time
- Published22 Mar 2022
- Author Calli McMurray
- Source BrainFacts/SfN
Time: it flies, it drags, it runs out, it is relentless in its rush forward. Even though every second is identical in length, some feel longer or shorter than others. Waiting a few minutes for a video to buffer feels like an eternity, but a minute-long ride on a rollercoaster goes by in a flash. The situations feel so different because the brain creates its own interpretations of time.
Masamichi Hayashi is a tenure-track researcher at the Center for Information and Neural Networks in Osaka, Japan. Hayashi studies temporal perception and numerical processing, or how the brain perceives time and numbers. He spoke to BrainFacts.org about how the brain senses time.
Which brain areas keep track of time?
It depends on the length of time. My research mainly focuses on the sub-second duration, the time interval of less than one second. But some researchers are focusing on the range of minutes, hours, or days. The neural basis is totally dependent on which time range you're focused on.
Activity in the parietal cortex is associated with how accurately people can perceive the duration of time. Some research groups have reported time cells in the hippocampus, which is more related to the perception of time in the long range, like long-term memory. Neurons in the hippocampus use information about time and location to encode memories. It seems the parietal cortex plays a role in perceiving time in the current moment, while the hippocampus perceives time in the past through memories.
How do individual neurons process time?
Neurons in the parietal cortex are sensitive to specific lengths of time. That means some neurons are sensitive to 300 milliseconds, but other neurons respond the most to a stimulus with a length of 400 milliseconds. This is called duration selectivity: those neurons fire in response to a specific duration of time. The time-sensitive activity in the parietal cortex is quite similar to how visual stimuli are processed in the occipital cortex. Some neurons are tuned to a specific orientation of the stimulus; other neurons prefer a specific motion direction. These neurons fire only in response to the specific parameters they are sensitive to.
Why does time feel slow in some moments and fast in others?
The objective and subjective lengths of time are often correlated, but they can be disassociated in some cases. How you actually perceive time is totally dependent on the context of the stimuli. If you pay more attention to one kind of stimuli, then people often feel that those stimuli last longer compared to when you are not really paying attention.
In one experiment, we showed participants a gray circle for a specific length of time, 30 times. The neurons with a tuning, or preference, for that duration get worn out after repeatedly firing, which we call neural fatigue. But neurons tuned to other lengths of time keep firing normally. That imbalance changes the overall activity in the parietal cortex, skewing the perception of time. For example, if a participant repeatedly looked at the gray circle for 450 milliseconds, the neurons tuned to 450 milliseconds would fatigue. If the participant then saw the circle for a slightly longer period of time, like 500 milliseconds, their perception of the length of time would be dragged toward an even longer duration.
What is the future of temporal processing research?
Time used to be a popular topic in physics or philosophy, but the neuroscientific study of time perception is still quite new. There are many things we still need to learn. The thing my team is working on now is whether activity in the parietal cortex generates the subjective experience of time. Our current work has used neuroimaging techniques that can only establish a correlation between neural activity and time perception, not a cause and effect. We are using a technique called transcranial magnetic stimulation, where we can stimulate the parietal cortex with magnets and see if it changes the perception of time.
An even more long-term goal is to modulate the experience of time in daily life using brain stimulation or wearable devices, like a smart watch. It could be quite a useful tool to make an unpleasant task, like waiting in line, seem shorter. On the other hand, if you are having fun, you could experience the joy of a nice moment for a bit longer than the moment lasts. I think there are no days where you don’t think about time. People always think about time because they want to manage it well. But it’s actually quite difficult. So that's the long-term goal: to control our subjective feeling of time.
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