Study finds the metronome running rodent brains
A steady timing reference is required by one of the leading theories of neuronal communication.
- A study of mouse neurons may have found a long-sought timing mechanism.
- If the finding carries over into humans, it may help explain the language of neurons.
- Each area of the brain may have its own metronome neurons, quietly ticking away.
That neurons communicate with each other in the human brain through the transmission of electrical signals from one to another seems clear, but there are still some big questions. One of these is that we don't know where the information in those signals actually resides. Does communication depend on the rate at which neurons fire — this is called the "rate code model" — or is it about the space between voltage spikes, AKA the "temporal code model?"
If it's the latter, it would seem that longer or shorter intervals between spikes would have to be measured by the brain in reference to some kind of steady internal timing reference. A clock or metronome, if you will. But coming from where? No such timing reference has been found in looking at the gamma rhythms of the whole human brain. However, a pair of researchers from Brown University, Christopher Moore and Hyeyoung Shin, have just identified a localized steady neuronal pulse in rodents. One of the researchers, Moore, tells WIRED, "That, right away, suggests there's something interesting going on here that we just haven't seen before. Something big is lurking in there." If this finding carries over to people, it may provide a significant clue toward answering the question of how our neurons' encode information in the electrical signals they exchange. Moore's and Shin's research was published in July in Neuron.
The tick-tock of gamma waves
Image source: artellia/Shutterstock
The electrical signals generated in our brains oscillate at different speeds, or frequencies, apparently depending on what we're doing and what mental processes those signals indicate. The fastest of these are called gamma waves, oscillating between their strongest and weakest level, or amplitudes, 38 to 42 times per second, or at a frequencies of 38-100 Hertz (Hz).
In research unsuccessfully attempting to find some kind of brain clock in humans, scientists have been measuring the aggregate of all gamma activity produced by all of the neurons in the entire brain. The new research suggests that the timing reference we've been looking for may be a far more localized phenomenon, with different brain areas having their own clocking neurons.
The pair's discovery came from Shin's examination of inhibitory neuronal responses in mice to faint touches of their whiskers. (Inhibitory neurons help keep the electrical activity of neighboring neurons under control.) The grad student was able to identify three distinct types of these neurons. One fired when she touched a whisker, and a second seemed to fire off erratically at random intervals. The third, however, was characterized by voltage spikes ticking away at a steady gamma frequency. Shin appears to have found a clock.
Response to the research
Image source: optimarc/Shutterstock
It's a very big deal if it turns out that humans have similar clock-like neurons, and it's therefore not surprising that Moore's and Shin's conclusions are being met with a mixed response. Some wonder why such neurons have not been found in humans already, and why there's no trace of them in the aggregate gamma waves that have been measured.
It's also worth noting that the gamma rhythm Shin's neurons produced doesn't match the whole-brain gamma wave frequency of her subjects. This could suggest a problem with the research, or simply that signals from these faint clocks don't travel far beyond the brain regions to which they pertain. "You have to go down to the level of local groups of neurons to really see what they're doing," says Moore. Shin herself intends to go looking at other brain regions for similar clocking neurons.
While the new research doesn't yet solve the larger mystery of neuron's electrical language, if verified in humans, it would provide compelling support for the feasibility of the temporal code model.
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The idea of 'absolute time' is an illusion. Physics and subjective experience reveal why.
- Since Einstein posited his theory of general relativity, we've understood that gravity has the power to warp space and time.
- This "time dilation" effect occurs even at small levels.
- Outside of physics, we experience distortions in how we perceive time — sometimes to a startling extent.
Physics without time<p>In his book "The Order of Time," Italian theoretical physicist Carlo Rovelli suggests that our perception of time — our sense that time is forever flowing forward — could be a highly subjective projection. After all, when you look at reality on the smallest scale (using equations of quantum gravity, at least), time vanishes.</p><p style="margin-left: 20px;">"If I observe the microscopic state of things," writes Rovelli, "then the difference between past and future vanishes … in the elementary grammar of things, there is no distinction between 'cause' and 'effect.'"</p><p>So, why do we perceive time as flowing <em>forward</em>? Rovelli notes that, although time disappears on extremely small scales, we still obviously perceive events occur sequentially in reality. In other words, we observe entropy: Order changing into disorder; an egg cracking and getting scrambled.</p><p>Rovelli says key aspects of time are described by the second law of thermodynamics, which states that heat always passes from hot to cold. This is a one-way street. For example, an ice cube melts into a hot cup of tea, never the reverse. Rovelli suggests a similar phenomenon might explain why we're only able to perceive the past and not the future.</p><p style="margin-left: 20px;">"Any time the future is definitely distinguishable from the past, there is something like heat involved," Rovelli wrote for the <a href="https://www.ft.com/content/ce6ef7b8-429a-11e8-93cf-67ac3a6482fd" target="_blank"><em>Financial Times</em></a>. "Thermodynamics traces the direction of time to something called the 'low entropy of the past', a still mysterious phenomenon on which discussions rage."</p>
The strange subjectivity of time<p>Time moves differently atop a mountain than it does on a beach. But you don't need to travel any distance at all to experience strange distortions in your perception of time. In moments of life-or-death fear, for example, your brain would release large amounts of adrenaline, which would speed up your internal clock, causing you to perceive the outside world as moving slowly.<br></p><p>Another common distortion occurs when we focus our attention in particular ways.</p><p style="margin-left: 20px;">"If you're thinking about how time is <em>currently</em> passing by, the biggest factor influencing your time perception is attention," Aaron Sackett, associate professor of marketing at the University of St. Thomas, told <em><a href="https://gizmodo.com/why-does-time-slow-down-and-speed-up-1840133782" target="_blank">Gizmodo</a></em>.<em> "</em>The more attention you give to the passage of time, the slower it tends to go. As you become distracted from time's passing—perhaps by something interesting happening nearby, or a good daydreaming session—you're more likely to lose track of time, giving you the feeling that it's slipping by more quickly than before. "Time flies when you're having fun," they say, but really, it's more like "time flies when you're thinking about other things." That's why time will also often fly by when you're definitely <em>not</em> having fun—like when you're having a heated argument or are terrified about an upcoming presentation."</p><p>One of the most mysterious ways people experience time-perception distortions is through psychedelic drugs. In an interview with <a href="https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-exploding-commonsense-notions-order-of-time-interview" target="_blank"><em>The Guardian</em></a>, Rovelli described a time he experimented with LSD.</p><p style="margin-left: 20px;">"It was an extraordinarily strong experience that touched me also intellectually," he said. "Among the strange phenomena was the sense of time stopping. Things were happening in my mind but the clock was not going ahead; the flow of time was not passing any more. It was a total subversion of the structure of reality."<br></p><p>It seems few scientists or philosophers believe time is completely an illusion.</p><p style="margin-left: 20px;">"What we call <em>time</em> is a rich, stratified concept; it has many layers," Rovelli told <em><a href="https://physicstoday.scitation.org/do/10.1063/PT.6.4.20190219a/full/" target="_blank">Physics Today</a>.</em> "Some of time's layers apply only at limited scales within limited domains. This does not make them illusions."</p>What <em>is</em> an illusion is the idea that time flows at an absolute rate. The river of time might be flowing forever forward, but it moves at different speeds, between people, and even within your own mind.
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Vanchurin interview:<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="539759cbfd8fcd5b6ebf14a3b597b3f9"><iframe type="lazy-iframe" data-runner-src="https://www.youtube.com/embed/bmyRy2-UhEE?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span>
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