Good timing: Study unravels how our brains track time

Ever hear the old adage that time flies when you're having fun? A new study by a team of UNLV researchers suggests that there's a lot of truth to the trope.

Many people think of their brains as being intrinsically synced to the human-made clocks on their electronic devices, counting time in very specific, minute-by-minute increments. But the study, published this month in the latest issue of the peer-reviewed Cell Press journal Current Biology, showed that our brains don't work that way.

By analyzing changes in brain activity patterns, the research team found that we perceive the passage of time based on the number of experiences we have -- not some kind of internal clock. What's more, increasing speed or output during an activity appears to affect how our brains perceive time.

"We tell time in our own experience by things we do, things that happen to us," said James Hyman, a UNLV associate professor of psychology and the study's senior author. "When we're still and we're bored, time goes very slowly because we're not doing anything or nothing is happening. On the contrary, when a lot of events happen, each one of those activities is advancing our brains forward. And if this is how our brains objectively tell time, then the more that we do and the more that happens to us, the faster time goes."

Methodology and Findings

The findings are based on analysis of activity in the anterior cingulate cortex (ACC), a portion of the brain important for monitoring activity and tracking experiences. To do this, rodents were tasked with using their noses to respond to a prompt 200 times.

Scientists already knew that brain patterns are similar, but slightly different, each time you do a repetitive motion, so they set out to answer: Is it possible to detect whether these slight differences in brain pattern changes correspond with doing the first versus 200th motion in series? And does the amount of time it takes to complete a series of motions impact brain wave activity?

By comparing pattern changes throughout the course of the task, researchers observed that there are indeed detectable changes in brain activity that occur as one moves from the beginning to middle to end of carrying out a task. And regardless of how slowly or quickly the animals moved, the brain patterns followed the same path. The patterns were consistent when researchers applied a machine learning-based mathematical model to predict the flow of brain activity, bolstering evidence that it's experiences -- not time, or a prescribed number of minutes, as you would measure it on a clock -- that produce changes in our neurons' activity patterns.

Hyman drove home the crux of the findings by sharing an anecdote of two factory workers tasked with making 100 widgets during their shift, with one worker completing the task in 30 minutes and the other in 90 minutes.

"The length of time it took to complete the task didn't impact the brain patterns. The brain is not a clock; it acts like a counter," Hyman explained. "Our brains register a vibe, a feeling about time. ...And what that means for our workers making widgets is that you can tell the difference between making widget No. 85 and widget No. 60, but not necessarily between No. 85 and No. 88."

But exactly "how" does the brain count? Researchers discovered that as the brain progresses through a task involving a series of motions, various small groups of firing cells begin to collaborate -- essentially passing off the task to a different group of neurons every few repetitions, similar to runners passing the baton in a relay race.

"So, the cells are working together and over time randomly align to get the job done: one cell will take a few tasks and then another takes a few tasks," Hyman said. "The cells are tracking motions and, thus, chunks of activities and time over the course of the task."

And the study's findings about our brains' perception of time applies to activities-based actions other than physical motions too.

"This is the part of the brain we use for tracking something like a conversation through dinner," Hyman said. "Think of the flow of conversation and you can recall things earlier and later in the dinner. But to pick apart one sentence from the next in your memory, it's impossible. But you know you talked about one topic at the start, another topic during dessert, and another at the end."

By observing the rodents who worked quickly, scientists also concluded that keeping up a good pace helps influence time perception: "The more we do, the faster time moves. They say that time flies when you're having fun. As opposed to having fun, maybe it should be 'time flies when you're doing a lot'."

Takeaways

While there's already a wealth of information on brain processes over very short time scales of less than a second, Hyman said that the UNLV study is groundbreaking in its examination of brain patterns and perception of time over a span of just a few minutes to hours -- "which is how we live much of our life: one hour at a time. "

"This is among the first studies looking at behavioral time scales in this particular part of the brain called the ACC, which we know is so important for our behavior and our emotions," Hyman said.

The ACC is implicated in most psychiatric and neurodegenerative disorders, and is a concentration area for mood disorders, PTSD, addiction, and anxiety. ACC function is also central to various dementias including Alzheimer's disease, which is characterized by distortions in time. The ACC has long been linked to helping humans with sequencing events or tasks such as following recipes, and the research team speculates that their findings about time perception might fall within this realm.

While the findings are a breakthrough, more research is needed. Still, Hyman said, the preliminary findings posit some potentially helpful tidbits about time perception and its likely connection to memory processes for everyday citizens' daily lives. For example, researchers speculate that it could lend insights for navigating things like school assignments or even breakups.

"If we want to remember something, we may want to slow down by studying in short bouts and take time before engaging in the next activity. Give yourself quiet times to not move," Hyman said. "Conversely, if you want to move on from something quickly, get involved in an activity right away."

Hyman said there's also a huge relationship between the ACC, emotion, and cognition. Thinking of the brain as a physical entity that one can take ownership over might help us control our subjective experiences.

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Absolute zero in the quantum computer

The absolute lowest temperature possible is -273.15 degrees Celsius. It is never possible to cool any object exactly to this temperature -- one can only approach absolute zero. This is the third law of thermodynamics.

A research team at TU Wien (Vienna) has now investigated the question: How can this law be reconciled with the rules of quantum physics? They succeeded in developing a "quantum version" of the third law of thermodynamics: Theoretically, absolute zero is attainable. But for any conceivable recipe for it, you need three ingredients: Energy, time and complexity. And only if you have an infinite amount of one of these ingredients can you reach absolute zero.

Information and thermodynamics: an apparent contradiction

When quantum particles reach absolute zero, their state is precisely known: They are guaranteed to be in the state with the lowest energy. The particles then no longer contain any information about what state they were in before. Everything that may have happened to the particle before is perfectly erased. From a quantum physics point of view, cooling and deleting information are thus closely related.

At this point, two important physical theories meet: Information theory and thermodynamics. But the two seem to contradict each other: "From information theory, we know the so-called Landauer principle. It says that a very specific minimum amount of energy is required to delete one bit of information," explains Prof. Marcus Huber from the Atomic Institute of TU Wien. Thermodynamics, however, says that you need an infinite amount of energy to cool anything down exactly to absolute zero. But if deleting information and cooling to absolute zero are the same thing -- how does that fit together?

Energy, time and complexity

The roots of the problem lie in the fact that thermodynamics was formulated in the 19th century for classical objects -- for steam engines, refrigerators or glowing pieces of coal. At that time, people had no idea about quantum theory. If we want to understand the thermodynamics of individual particles, we first have to analyse how thermodynamics and quantum physics interact -- and that is exactly what Marcus Huber and his team did.

"We quickly realised that you don't necessarily have to use infinite energy to reach absolute zero," says Marcus Huber. "It is also possible with finite energy -- but then you need an infinitely long time to do it." Up to this point, the considerations are still compatible with classical thermodynamics as we know it from textbooks. But then the team came across an additional detail of crucial importance:

"We found that quantum systems can be defined that allow the absolute ground state to be reached even at finite energy and in finite time -- none of us had expected that," says Marcus Huber. "But these special quantum systems have another important property: they are infinitely complex." So you would need infinitely precise control over infinitely many details of the quantum system -- then you could cool a quantum object to absolute zero in finite time with finite energy. In practice, of course, this is just as unattainable as infinitely high energy or infinitely long time.

Erasing data in the quantum computer

"So if you want to perfectly erase quantum information in a quantum computer, and in the process transfer a qubit to a perfectly pure ground state, then theoretically you would need an infinitely complex quantum computer that can perfectly control an infinite number of particles," says Marcus Huber. In practice, however, perfection is not necessary -- no machine is ever perfect. It is enough for a quantum computer to do its job fairly well. So the new results are not an obstacle in principle to the development of quantum computers.

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New method to synchronize devices on Earth makes use of cosmic rays

Various technologies, networks and institutions benefit from or require accurate time keeping to synchronize their activities. Current ways of synchronizing time have some drawbacks that a new proposed method seeks to address. The cosmic time synchronizer works by synchronizing devices around cosmic ray events detected by those devices. This could bring accurate timing abilities to remote sensing stations, or even underwater, places that other methods cannot serve. Early tests show promise, but the real challenge may lie in the adoption of this new technique.

Humanity is intimately connected with the idea of time. Historically, we used the cosmos itself -- stars, the sun, and the moon -- to measure time and coordinate our activities. It's fitting, then, that researchers are looking out to the cosmos again to further develop our ability to keep time. Professor Hiroyuki Tanaka from Muographix at the University of Tokyo devised and tested a way to synchronize multiple devices, so they agree upon the time, that makes use of cosmic rays from deep space. Appropriately, it's called cosmic time synchronization (CTS).

"It's relatively easy to keep time accurately these days. For example, atomic clocks have been doing this for decades now," said Tanaka. "However, these are large and expensive devices that are very easy to disrupt. This is one reason I have been working on an improved way to keep time. The other is that, related to time measurement, position measurement could also be made better. So really, CTS is a precursor to a potential replacement for GPS, but that's still a little further down the line."

The reason it's critical for devices to have a shared sense of time is that certain devices are increasingly important in many aspects of life. Computer networks responsible for financial transactions must agree upon time so that the order of transactions can be ensured. There are sensors that work in unison to observe various physical phenomena which need to agree upon time so that, for example, the origin of a particular reading can be determined. Such sensors could even potentially be part of some kind of disaster warning system.

CTS works thanks to cosmic rays from deep space that strike the atmosphere around 15 kilometers up, creating showers of particles including muons. The muons travel close to the speed of light, reaching the ground almost immediately, they can easily penetrate water or rock, and spread out as they travel to cover a few square kilometers of ground. Independent CTS-enabled devices under the same particle shower can detect the incoming muons, which will have a specific signature unique to the cosmic ray event that generated them. By sharing this information, CTS devices can confer with one another and synchronize their clocks according to when the cosmic ray event took place. The ultrahigh-energy cosmic ray strikes occur frequently enough, about a hundred times per hour over every square kilometer of Earth, for CTS devices to work together in real time.

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Key brain mechanisms for organizing memories in time

In a scientific first, researchers at the University of California, Irvine have discovered fundamental mechanisms by which the hippocampus region of the brain organizes memories into sequences and how this can be used to plan future behavior. The finding may be a critical early step toward understanding memory failures in cognitive disorders such as Alzheimer's disease and other forms of dementia.

Combining electrophysiological recording techniques in rodents with a statistical machine learning analysis of huge troves of data, the UCI researchers uncovered evidence suggesting that the hippocampal network encodes and preserves progressions of experiences to aid in decision-making. The team's work is the subject of a paper published recently in Nature Communications.

"Our brain keeps a pretty good record of when specific experiences or events occur. This ability helps us function in our daily life, but before this study, we didn't have a clear idea of the neuronal mechanisms behind these processes," said corresponding author Norbert Fortin, UCI associate professor of neurobiology and behavior. "Where it connects with everybody is that this type of memory is strongly impaired in a variety of neurological disorders or simply with aging, so we really need to know how this brain function works."

The project, which took more than three years to complete, involved experimental and data analysis phases. The researchers monitored the firing of neurons in rats' brains as they underwent a series of odor identification tests. By presenting five different smells in various sequences, the scientists were able to measure the animals' memory of the correct sequence and detect how their brains captured these sequential relationships.

"The analogy I would think about is computing," Fortin said. "If I were to stick electrodes in your brain -- we can't; that's why we use rats -- I could see which cells are firing and which ones are not firing at any given moment. That provides us with some insight into how the brain represents and computes information. When we record activity patterns in a structure, it's like we're seeing zeros and ones in a computer."

Obtained in millisecond intervals over several minutes, neuronal activity and inactivity measurements present a dynamic picture of the brain's functioning. Fortin said that he and his colleagues were, in some ways, able to "read the minds" of their subjects by viewing the "coding" of the cells -- which ones were firing and which were not -- in rapid succession.

"When you're thinking about something, it moves quickly," he said. "You're not stuck on that memory for long. Right now, it's being represented, but we can see how that changes very quickly."

Fortin knew early on that the readings of hippocampal activity would result in enormous quantities of raw data. From the beginning stages of the project, he enlisted the participation of statisticians in the Donald Bren School of Information & Computer Sciences.

"The neuroscience questions we had at the time in my lab were way too advanced for the statistical knowledge we had. That's why we needed to involve partners with data science expertise," Fortin said.

"These emerging neuroscience studies rely on data science methods because of the complexity of their data," said senior co-author Babak Shahbaba, UCI Chancellor's Fellow and professor of statistics. "Brain activities are recorded at millisecond scale, and these experiments run for more than an hour, so you can imagine how fast the amount of data grows. It gets to a point that neuroscientists need more advanced techniques to accomplish what they had imagined but weren't able to implement."

He noted that when neurons encode information such as memories, scientists can get a glimpse of that process by examining the pattern of spiking activity across all recorded neurons, known collectively as an ensemble.

"We found that we could treat these neural patterns as images, and this unlocked our ability to apply deep machine learning methods," Shahbaba said. "We analyzed the data with a convolutional neural network, which is a methodology used frequently in image processing applications such as facial recognition."

This way, the researchers were able to decode the firing of neurons to retrieve information.

"We know what the signature for odor B looks like, just as we know the ones for A, C and D," Fortin said. "Because of that, you can see when those signatures reappear at a different moment in time, such as when our subjects are anticipating something that has yet to happen. We're seeing these signatures being quickly replayed as they're thinking about the future."

Shahbaba said that the tools and methodologies developed during this project can be applied to a wide range of problems, and Fortin may extend his line of inquiry into other brain regions.

The study is an example of the power of convergence research at institutions such as UCI, Shahbaba said: "I could directly see the difference this is making for our students. Researchers in Norbert's neuroscience group are taking data science classes and can now ask some really important scientific questions they could not investigate in the past, and my own students are thinking fundamentally about the scientific method in an unprecedented way."

He added, "Through this collaboration, we are training the next generation of scientists, who have the required skills to conduct interdisciplinary research."

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Time crystals leave the lab

We have all seen crystals, whether a simple grain of salt or sugar, or an elaborate and beautiful amethyst. These crystals are made of atoms or molecules repeating in a symmetrical three-dimensional pattern called a lattice, in which atoms occupy specific points in space. By forming a periodic lattice, carbon atoms in a diamond, for example, break the symmetry of the space they sit in. Physicists call this "breaking symmetry."

Scientists have recently discovered that a similar effect can be witnessed in time. Symmetry breaking, as the name suggests, can arise only where some sort of symmetry exists. In the time domain, a cyclically changing force or energy source naturally produces a temporal pattern.

Breaking of the symmetry occurs when a system driven by such a force faces a déjà vu moment, but not with the same period as that of the force. 'Time crystals' have in the past decade been pursued as a new phase of matter, and more recently observed under elaborate experimental conditions in isolated systems. These experiments require extremely low temperatures or other rigorous conditions to minimize undesired external influences, called noise.

In order for scientists to learn more about time crystals and employ their potential in technology, they need to find ways to produce time crystalline states and keep them stable outside the laboratory.

Cutting-edge research led by UC Riverside and published this week in Nature Communications has now observed time crystals in a system that is not isolated from its ambient environment. This major achievement brings scientists one step closer to developing time crystals for use in real-world applications.

"When your experimental system has energy exchange with its surroundings, dissipation and noise work hand-in-hand to destroy the temporal order," said lead author Hossein Taheri, an assistant research professor of electrical and computer engineering in UC Riverside's Marlan and Rosemary Bourns College of Engineering. "In our photonic platform, the system strikes a balance between gain and loss to create and preserve time crystals."

The all-optical time crystal is realized using a disk-shaped magnesium fluoride glass resonator one millimeter in diameter. When bombarded by two laser beams, the researchers observed subharmonic spikes, or frequency tones between the two laser beams, that indicated breaking of temporal symmetry and creation of time crystals.

The UCR-led team utilized a technique called self-injection locking of the two lasers to the resonator to achieve robustness against environmental effects. Signatures of the temporally repeating state of this system can readily be measured in the frequency domain. The proposed platform therefore simplifies the study of this new phase of matter.

Without the need for a low temperature, the system can be moved outside a complex lab for field applications. One such application could be highly accurate measurements of time. Because frequency and time are mathematical inverses of each other, accuracy in measuring frequency enables accurate time measurement.

"We hope that this photonic system can be utilized in compact and lightweight radiofrequency sources with superior stability as well as in precision timekeeping," said Taheri.

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New type of atomic clock keeps time even more precisely

 

Clock and abstract background.

Atomic clocks are the most precise timekeepers in the world. These exquisite instruments use lasers to measure the vibrations of atoms, which oscillate at a constant frequency, like many microscopic pendulums swinging in sync. The best atomic clocks in the world keep time with such precision that, if they had been running since the beginning of the universe, they would only be off by about half a second today.

Still, they could be even more precise. If atomic clocks could more accurately measure atomic vibrations, they would be sensitive enough to detect phenomena such as dark matter and gravitational waves. With better atomic clocks, scientists could also start to answer some mind-bending questions, such as what effect gravity might have on the passage of time and whether time itself changes as the universe ages.

Now a new kind of atomic clock designed by MIT physicists may enable scientists explore such questions and possibly reveal new physics.

The researchers report in the journal Nature that they have built an atomic clock that measures not a cloud of randomly oscillating atoms, as state-of-the-art designs measure now, but instead atoms that have been quantumly entangled. The atoms are correlated in a way that is impossible according to the laws of classical physics, and that allows the scientists to measure the atoms' vibrations more accurately.

The new setup can achieve the same precision four times faster than clocks without entanglement.

"Entanglement-enhanced optical atomic clocks will have the potential to reach a better precision in one second than current state-of-the-art optical clocks," says lead author Edwin Pedrozo-Peñafiel, a postdoc in MIT's Research Laboratory of Electronics.

If state-of-the-art atomic clocks were adapted to measure entangled atoms the way the MIT team's setup does, their timing would improve such that, over the entire age of the universe, the clocks would be less than 100 milliseconds off.

The paper's other co-authors from MIT are Simone Colombo, Chi Shu, Albert Adiyatullin, Zeyang Li, Enrique Mendez, Boris Braverman, Akio Kawasaki, Saisuke Akamatsu, Yanhong Xiao, and Vladan Vuletic, the Lester Wolfe Professor of Physics.

Time limit

Since humans began tracking the passage of time, they have done so using periodic phenomena, such as the motion of the sun across the sky. Today, vibrations in atoms are the most stable periodic events that scientists can observe. Furthermore, one cesium atom will oscillate at exactly the same frequency as another cesium atom.

To keep perfect time, clocks would ideally track the oscillations of a single atom. But at that scale, an atom is so small that it behaves according to the mysterious rules of quantum mechanics: When measured, it behaves like a flipped coin that only when averaged over many flips gives the correct probabilities. This limitation is what physicists refer to as the Standard Quantum Limit.

"When you increase the number of atoms, the average given by all these atoms goes toward something that gives the correct value," says Colombo.

This is why today's atomic clocks are designed to measure a gas composed of thousands of the same type of atom, in order to get an estimate of their average oscillations. A typical atomic clock does this by first using a system of lasers to corral a gas of ultracooled atoms into a trap formed by a laser. A second, very stable laser, with a frequency close to that of the atoms' vibrations, is sent to probe the atomic oscillation and thereby keep track of time.

And yet, the Standard Quantum Limit is still at work, meaning there is still some uncertainty, even among thousands of atoms, regarding their exact individual frequencies. This is where Vuletic and his group have shown that quantum entanglement may help. In general, quantum entanglement describes a nonclassical physical state, in which atoms in a group show correlated measurement results, even though each individual atom behaves like the random toss of a coin.

The team reasoned that if atoms are entangled, their individual oscillations would tighten up around a common frequency, with less deviation than if they were not entangled. The average oscillations that an atomic clock would measure, therefore, would have a precision beyond the Standard Quantum Limit.

Entangled clocks

In their new atomic clock, Vuletic and his colleagues entangle around 350 atoms of ytterbium, which oscillates at the same very high frequency as visible light, meaning any one atom vibrates 100,000 times more often in one second than cesium. If ytterbium's oscillations can be tracked precisely, scientists can use the atoms to distinguish ever smaller intervals of time.

The group used standard techniques to cool the atoms and trap them in an optical cavity formed by two mirrors. They then sent a laser through the optical cavity, where it ping-ponged between the mirrors, interacting with the atoms thousands of times.

"It's like the light serves as a communication link between atoms," Shu explains. "The first atom that sees this light will modify the light slightly, and that light also modifies the second atom, and the third atom, and through many cycles, the atoms collectively know each other and start behaving similarly."

In this way, the researchers quantumly entangle the atoms, and then use another laser, similar to existing atomic clocks, to measure their average frequency. When the team ran a similar experiment without entangling atoms, they found that the atomic clock with entangled atoms reached a desired precision four times faster.

"You can always make the clock more accurate by measuring longer," Vuletic says. "The question is, how long do you need to reach a certain precision. Many phenomena need to be measured on fast timescales."

He says if today's state-of-the-art atomic clocks can be adapted to measure quantumly entangled atoms, they would not only keep better time, but they could help decipher signals in the universe such as dark matter and gravitational waves, and start to answer some age-old questions.

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