June 6th, 2023
Written by: Sophie Liebergall
In Part One of our series on some of the greatest mysteries in neuroscience we talked about memory storage, sleep, dreams, and how the brain assigns functions to different regions. This week, we ask how we sense the passage of time, why the brain sometimes loses control of its electrical activity, how the brain adapts after injury, what makes the human brain unique, and, perhaps the most challenging question of all, how the brain creates conscious experience!
6. How does the brain sense the passage of time?
Even if you are sitting completely still in an acoustically sealed pitch-black room, there is one thing that you will still sense: the passage of time. And though we have a fairly solid idea of how the brain governs its daily rhythms, scientists are still on the search to discover the gears of our internal clocks that can sense the passage of time on the scale of milliseconds to minutes.
Psychological studies into the perception of time suggest that the underlying neural mechanisms are likely more complicated than the simple, regular ticking of a timer. As we have all experienced, the same interval of time can feel like the blink of an eye or an eternity. Interestingly, our perception of the passage of time seems to speed up with age.1 This altered perception may be based on the fact that one year is only 1/10th of the life of a ten-year-old, but it is 1/80th of the life of an 80-year-old. Similarly, drugs ranging from caffeine to cannabis can alter the perception of time, as has changing a person’s core body temperature.2–4 Read more about what we know about time signals in the brain in this PNK article.
7. What happens in the brain during a seizure?
Seizures, or periods of uncontrolled electrical activity in the brain, are an ancient ailment. The first record of a seizure dates all the way back to a 4000-year-old tablet from Mesopotamia!5 In the years since, neurologists have become very skilled at categorizing and controlling seizures. Modern treatments for seizures range from pills to surgically implanted electrical stimulation devices. And we know that things like genetic mutations, certain drugs, brain tumors, and infections can increase someone’s likelihood of having a seizure.6 Nevertheless, scientists still lack a clear picture of what exactly is happening at the moment that a seizing brain loses control over the activity of its neurons.
A good first step to understanding seizures may be to try to understand why the brain isn’t having a seizure all of the time. We know that the brain has some neurons whose main job is to quiet down the activity of other neurons.7 But neuroscientists are still not sure exactly how these cells sense that they need to tap on the brakes of brain activity, or why these brakes stop working during a seizure. Scientists are also trying to understand why some seizures stay close to the area where they start, whereas others quickly spread across the entirety of the brain.
There are some perplexing aspects of seizures that may give scientists further insight into what exactly is happening when they occur. For example, you may have heard that some people have seizures in response to flashing lights. Interestingly, this only occurs when the light is flashed at certain frequencies.8 In another example, children generally more prone to having seizures than adults. But scientists are still unsure as to what exactly makes the developing brain susceptible to seizures, and why many kids “grow out” of having seizures.9 You can learn more about what we think is happening in the brain during a seizure in this PNK article!
8. How does the brain adapt to injury? And why are the brains of children able to adapt much more easily than those of adults?
If someone has an injury to a certain region of their brain, they will often have difficulties with the functions that the particular region controls. But sometimes these individuals can regain the impaired functions by reassigning them to a new, uninjured region of the brain. This ability of the brain to change its structure and function in response to an injury is called neuroplasticity.10 A very dramatic example of neuroplasticity can occur after a hemispherectomy, which is a surgery in which almost a whole hemisphere (a half of the brain) is removed, usually in an effort to cure difficult-to-treat epilepsy.11 If this surgery is performed in a young child, the brain is usually able to completely reassign functions, such as language and memory, to the remaining hemisphere.
Neuroscientists are still unsure as to how the brain is able to orchestrate such large-scale neuroplasticity, and why this occurs following some injuries but not others. Additionally, neuroscientists are looking to understand why age has such a large influence on neuroplasticity. Though we know that the brains of children are much better at healing and reorganizing after an injury, the exact details of why a child’s brain is more plastic than an adult’s brain remain unclear.12 A better understanding of the factors that are unique to the brains of children could allow us to design therapies that boost neuroplasticity after brain injuries in adults as well.
9. What makes the human brain different from the brains of other animals?
We know, or at least hope, that there is a big difference between the inner lives of mice and men. But if you were to look at the neurons in a human brain and in a mouse brain under a microscope, would you be able to tell the difference? Probably not. The brains of mice and humans, as well as of chimpanzees, bats, and giraffes, are all made up of the same basic neurons that can be divided into the same major subtypes.13 So why exactly have humans emerged as the only species that use complex written and spoken language, build societies that have changed the landscape of the planet, and developed technology sophisticated enough to study our own brains?
Though there are a number of theories on what makes the human brain unique. When looking at the brain as a whole, it seems like size may matter. Humans have exceptionally large brains that are densely packed with billions of neurons. But the brain of an African Elephant has more than double the number of neurons in a human brain.14 And even if you try to scale brain size to the size of the animal, humans are still beat by tree shrews, the mammal with the largest brain-to-body size ratio.15 On a finer scale, there is evidence that the neurons in the human brain may actually have fewer ion channels, which are pores in a neuron’s membrane that are required for the neuron to generate electrical signals.16 Researchers suspect that this might make human neurons particularly energy efficient when compared to the neurons of other animals, even other primates whose neurons are otherwise very similar to our own.
10. And finally… what is the brain mechanism that underlies conscious experience?
We’ve saved perhaps the most notorious outstanding question in neuroscience for last: how do the electrical signals in our brain cells come together to create our conscious experiences? How do we experience the earthy aroma of a spring rain, or the nostalgia of flipping through old photographs, or the buttery taste of a fresh pastry? The feeling of what it is like to be us, something that philosophers and neuroscientists call phenomenal consciousness, seems different from the physical building blocks that make up the rest of our world. Even so, there is strong evidence that this phenomenal consciousness lies somewhere deep in the folds of our brains. When we injure the brain, or pump it full of drugs, or bathe it in hormones, we alter our experience of the world.
It is extremely challenging, however, for neuroscientists to study phenomenal consciousness. The only conscious experience that we can undoubtedly observe is the one playing out in our own heads. Therefore, we can’t observe or prove the conscious experience of others – we rely on others to say when they’re having a conscious experience. And because animals, and even human infants, don’t share our common language, they aren’t able to report that they’re experiencing an inner life in the way that we do. Nevertheless, this hasn’t stopped centuries of philosophers, cognitive scientists, and neuroscientists from proposing their own theories of the mechanism of consciousness. Some scientists theorize that consciousness occurs when a part of the brain broadcasts its contents to the rest of the brain. Others think that consciousness is the result of combining large amounts of information to carry out complex calculations.17 And a recently revived, but controversial theory called panpsychism even poses that every atom of the universe has a little bit of consciousness, so when all the atoms in our brain are added together it results in the complex consciousness experiences by humans.18,19
Though we have made great strides since the ancient Egyptian belief that thinking and feeling occurs in the heart, we still remain in the dark when it comes to understanding many of the basic brain functions that underlie how we experience the world. Now that we have entered the 21st century when we finally have the technology to start to chip away at some of these mysteries, neuroscience is perhaps the most exciting field to follow within biology – but, hey, we might be slightly biased over here at PennNeuroKnow!
References
1. Lemlich, R. Subjective Acceleration of Time with Aging. Percept Mot Skills 41, 235–238 (1975).
2. Stine, M. M., O’Connor, R. J., Yatko, B. R., Grunberg, N. E. & Klein, L. C. Evidence for a relationship between daily caffeine consumption and accuracy of time estimation. Hum Psychopharmacol 17, 361–367 (2002).
3. Atakan, Z., Morrison, P., Bossong, M. G., Martin-Santos, R. & Crippa, J. A. The Effect of Cannabis on Perception of Time: A Critical Review. Current Pharmaceutical Design 18, 4915–4922.
4. Feeling the Heat: Body Temperature and the Rate of Subjective Time, Revisited – J.H. Wearden, I.S. Penton-Voak, 1995. https://journals.sagepub.com/doi/abs/10.1080/14640749508401443.
5. Kaculini, C. M., Tate-Looney, A. J. & Seifi, A. The History of Epilepsy: From Ancient Mystery to Modern Misconception. Cureus 13, e13953.
6. Seizures and epilepsy in children: Clinical and laboratory diagnosis – UpToDate. https://www.uptodate.com/contents/seizures-and-epilepsy-in-children-clinical-and-laboratory-diagnosis?search=seizure%20workup&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2.
7. Dudok, B., Klein, P. M. & Soltesz, I. Toward Understanding the Diverse Roles of Perisomatic Interneurons in Epilepsy. Epilepsy Curr 22, 54–60 (2022).
8. Fisher, R. S. et al. Visually sensitive seizures: An updated review by the Epilepsy Foundation. Epilepsia 63, 739–768 (2022).
9. Holmes, G. L. & Ben-Ari, Y. The Neurobiology and Consequences of Epilepsy in the Developing Brain. Pediatr Res 49, 320–325 (2001).
10. Puderbaugh, M. & Emmady, P. D. Neuroplasticity. in StatPearls (StatPearls Publishing, 2023).
11. Vining, E. P. G. et al. Why Would You Remove Half a Brain? The Outcome of 58 Children After Hemispherectomy—The Johns Hopkins Experience: 1968 to 1996. Pediatrics 100, 163–171 (1997).
12. Sophie Su, Y., Veeravagu, A. & Grant, G. Neuroplasticity after Traumatic Brain Injury. in Translational Research in Traumatic Brain Injury (eds. Laskowitz, D. & Grant, G.) (CRC Press/Taylor and Francis Group, 2016).
13. Callaway, E. M. et al. A multimodal cell census and atlas of the mammalian primary motor cortex. Nature 598, 86–102 (2021).
14. Herculano-Houzel, S. et al. The elephant brain in numbers. Frontiers in Neuroanatomy 8, (2014).
15. Savier, E., Sedigh-Sarvestani, M., Wimmer, R. & Fitzpatrick, D. A bright future for the tree shrew in neuroscience research: Summary from the inaugural Tree Shrew Users Meeting. Zool Res 42, 478–481 (2021).
16. Beaulieu-Laroche, L. et al. Allometric rules for mammalian cortical layer 5 neuron biophysics. Nature 600, 274–278 (2021).
17. Integrated information theory: from consciousness to its physical substrate | Nature Reviews Neuroscience. https://www.nature.com/articles/nrn.2016.44.
18. Goff, P., Seager, W. & Allen-Hermanson, S. Panpsychism. in The Stanford Encyclopedia of Philosophy (ed. Zalta, E. N.) (Metaphysics Research Lab, Stanford University, 2022).
19. Seth, A. K. & Bayne, T. Theories of consciousness. Nat Rev Neurosci 23, 439–452 (2022).
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