Tick, Tock: Your Brain’s Inner Clock

[dropcap]T[/dropcap]ime: one syllable, four letters, but one of the nebulous constructs of our world. Many bright scholars and philosophers have attempted to understand time as a primary sense, including Albert Einstein who noted that “time is an illusion,” and “the only reason for time is so that everything doesn’t happen at once”. But what is time, and how do we know it exists?

As children, telling time is one of the first lessons we are taught in school. We are told there are 12 months, or 365 days, in a year (excluding leap years, where there are 366 days in the year). Each day is comprised of 24 hours, each hour is 60 minutes, and each minute is 60 seconds. Time is presented as a rigid, unchanging, ever constant background in our lives. It exists, but not to be interacted with or manipulated.

It turns out, the ability to perceive time is not uniquely human. No plant could give you the calendar date, but plants certainly change with the seasons. No dog could tell you the time on the clock, but they are certainly aware when it’s time for their next meal. All plants and animals exhibit circadian rhythms, and respond to sensory and motor stimuli with some temporal specificity. But, humans are the only species on earth that design clocks and calendars, and that label this as “time”. How we encode such a critical part of our lives is still not fully understood, but what we do know is that the scale of time (milliseconds up to days) is represented differently in your brain.

We interact with objects in our environment through our five primary senses: taste, sight, touch, smell, and hearing. If time is a sixth sense, how do we perceive it? Neuroscientists believe that the perception of time is likely distributed across the brain rather than located in a central “time telling” brain structure. Our ability to perceive time plays a role in nearly everything we do; imagine trying to enjoy a concert if the temporal order of music disappeared, or trying to play a game of basketball if you couldn’t follow the clock or periods of the game. We are required to subconsciously detect time across many magnitudes, from hundreds of milliseconds up to hours or days, to do nearly anything that we take for granted.

Starting from the smallest magnitude, it is imperative that we can detect time within the millisecond range for much of our sensory and motor processing. Think back to the last time you watched television or went to the movies. Frames within these recordings can move as quickly as 50 or 60 frames per second, and your brain can still recognize the temporal pattern! Perceiving motion would be impossible without the brain’s ability to detect stimuli on the sub-second order. Remarkably, there are no currently known diseases that affect your ability to recognize temporal patterns of this magnitude. Many experts believe that perhaps this is because of the brain’s ability to perceive time on this millisecond range is an inherent function of the neuron with no specialized requirements. All that is required for this temporal processing is the spatial-temporal order of action potentials. As neurons communicate, they encode time.

Have you ever heard the expression, “Time flies when you’re having fun”? How we perceive time on a larger scale is susceptible to many distortions, including your emotional state. Interactions between emotion and time are a two-way street – changes in emotional state can alter your perception of time, and changes in daily timing patterns can lead to anger or stress. One possible explanation of how stress affects the perception of time is through the neurotransmitter dopamine. Dopamine is released during psychosocial stressors, and changes in dopamine are known to throw off your internal clock. One study found that increasing dopamine in the brain caused an overestimation of time (increased inner clock rate), while blocking dopamine in the brain led to an underestimation of time (slowed inner clock rate). If you have a pet that you feed at a certain time every day, chances are you fully understand how disruptions in daily timing can negatively affect them. Feeding your pet even a couple hours late can make them upset or anxious, all because of a mismatch in timing!

To understand how emotions such as stress impact your ability to perceive time, a Stanford neuroscientist, David Eagleman, and his group tested an individual’s ability to judge time during a simulated frightening event. Participants were harnessed (safely!) to a platform that was lifted off the ground, then released to experience a free fall for 2.49 seconds before landing in a net. As they were falling, they were asked to read digits that were presented extremely close together – in fact, too close to be discerned under normal circumstances. The idea was that, if a stressful situation really does cause the sense of time to slow, stimuli that were not discernible in normal circumstances would suddenly become separable. Participants were not able to discern digits at a faster speed during this fall, indicating a lack of temporal resolution distortions. However, participants did overestimate the recalled time of falling, which points to stress impacting our brain’s ability to accurately sense time.

Unlike touch, sight, smell, hearing, or taste, our ability to sense time doesn’t have a central node in the brain or single mode of action. Instead, we sense time through a broadly distributed network of neurons, which can encode time across scales ranging from milliseconds to days. Our sense of time interacts with emotions, and can be manipulated by feelings including stress.

There is no arguing that the past year has been a contentious one. It can seem like every day brings a new stressful event, and effects of this prolonged stress are manifesting in a variety of ways. If every week feels more like a year, and every year feels more like a decade, you are not alone. Between the mere amount of news many of us consume and higher baseline stress levels, our brains are tricked into thinking more time has passed than what really has. If you fall into this category – take a deep breath, step away from the news, and let your internal clock reset!

Today, it can seem like our entire lives are ruled by time, though what time is and how we perceive it remains a mystery. What we do know is that, for the most part, our brains are a remarkable inner clock!

If you want to know more about how the brain encodes time, check out Knowing Neurons’ book review of Dean Buonomano’s book Your Brain is a Time Machine.

References:

Stetson, Chess, Fiesta, Matthew P, and Eagleman, David M. Does Time Really Slow Down during a Frightening Event? PLoS One, 2007.

Mauk, Michael D., and Buonomano, Dean V. The Neural Basis of Temmporal Processing. Annu. Rev. Neurosci., 2004.

Hood, Suzanne, and Amir, Shimon. Biological Clocks and Rhythms of Anger and Aggression. Fronteirs in Behavioral Neuroscience, 2018.

Resnick, Brian. The strange reason Donald Trump’s presidency feels like an eternity. Vox, 2017. https://www.vox.com/2017/6/12/15781752/donald-trump-eternity-time-perception

Drew M.R., Fairhurst S., Malapani C., Horvitz J.C., Balsam P.D. Effects of dopamine antagonists on the timing of two intervals. Pharmacol. Biochem. Behav, 2003.

Cheng R.-K., Ali Y.M., Meck W.H. Ketamine ‘unlocks’ the reduced clock-speed effects of cocaine following extended training: evidence for dopamine-glutamate interactions in timing and time perception. Neurobiol. Learn. Mem., 2007.

Preussner, J.C., Champagne, F., Meaney, M.J., Dagher, A. Dopamine Release in Response to a Psychological Stress in Humans and Its Relationship to Early Life Maternal Care: A Positron Emission Tomography Study using [11C]Raclopride. The Journal of Neuroscience, 2004.