May 16th, 2023
Written by: Margaret Gardner
Puberty. For many of us, it’s a time of crushes, stinky armpits, and being sat down by the gym teacher to watch a video about “our changing bodies”. But why do those videos never mention changes in the brain? While this topic is difficult to study, many scientists have found evidence that the brain’s structure and function change over the course of puberty. Understanding puberty’s impact on the brain may be important for mental health, since the number of children and teens diagnosed with a mental illness increases during this time1. So, cram into the auditorium and roll your eyes at your friends, because we’re going to talk about your brain on puberty: what we know, why it matters, and how much we still have to learn.
What is puberty?
Puberty refers to a series of physical changes that happen during late childhood or early adolescence when the body starts producing a lot more sex hormones, such as estrogen, testosterone, and dehydroepiandrosterone (DHEA). These hormones act on cells throughout the body and, over time, produce “sex characteristics”: body hair, getting one’s period, voice changes, and everything else that needs to happen for a person to be able to reproduce2,3. Generally speaking, how much of each sex hormone the body creates depends on a person’s sex, which is usually either female (people with two X chromosomes) or male (people with one X and one Y chromosome). For this reason, puberty’s effects are different in females and males3*. However, there’s a lot of variation both within and between sexes. How old someone is when they start puberty, how high their hormone levels are, and how long it takes them to complete puberty vary between individuals.
*Quick note on the topic of sex differences: while puberty’s effects on the brain often do appear to vary by sex, it’s important to keep in mind that male and female brains are much more similar than they are different4. While studying these differences may be important for understanding male’s and female’s different risks for neurological and psychiatric diseases, they do not mean that people of any sex have either “abnormal” or “better” brains!
Why study it?
This variability in puberty’s starting age and total duration are what make it important to study. If puberty lasted from age 11 to age 16 in every single child, one could simply study the effect of being a 13-year-old girl on the brain or body and get a very accurate picture of what was going on. However, a real group of 13-year-old girls will vary a lot in terms of what phase of puberty they’re in; for instance, some will have gotten their period already while some won’t. This also means that any effects puberty and its hormones have on the brain or body are distinct from the effects of aging. Again, to return to our example, all the girls in our group have had 13 years for their brains to learn, grow, and be affected by everything besides pubertal hormones that changes as kids grow up. However, because the girls are in different stages of puberty, some will have higher levels of estrogen that may cause additional changes to their brains. Some of these changes are visible in the brain’s structure, which influence how it functions, while others can be seen by measuring brain activity. In short, while age and puberty are clearly related, each has a unique and important impact on brain development in childhood.
Puberty and Brain Structure
Though much remains to be learned, studies have found evidence that puberty affects the brain’s structure above and beyond the effects of aging. Many scientists use mice and rats to model children’s brain development across puberty, since puberty in rodents happens much more quickly than in humans and, unlike with children, researchers can make sure all the animals grow up in the exact same environment. Studies of these animal models show that sex hormones alter the number of neurons in and the connections between certain brain regions5,6. Some of these effects vary by sex and lead to some of the sex differences in adult rodent brains7. Other scientists have used magnetic resonance imaging (MRI) scans to study human participant’s brain structure as they go through puberty. Some of these studies show puberty is associated with volume reduction in many brain areas, including those involved in complex thought coordination and, in females, regions related to emotional processing2. Though “volume reduction” may sound bad, it’s a normal part of brain development wherein your brain gets rid of unnecessary connections, allowing the whole system to run more efficiently. This means that as puberty progresses, it may trigger brain regions that coordinate thoughts and feelings to mature into their “adult” form2. Puberty is also associated with increases in white matter – the brain’s insulation for passing electrical signals – which could help brain regions communicate more effectively2. These MRI results typically vary by sex and/or brain region, suggesting that the different hormones involved in puberty affect some parts of the brain but not others. However, results are often inconsistent across studies, meaning more, thoughtful research is needed to understand which effects are real.
Puberty and Brain Function
There is evidence that puberty is directly responsible for some changes in brain function seen over the course of adolescence. Brain function is often measured using a special kind of MRI technology called functional MRI (fMRI). fMRI visualizes which parts of the brain are most active by mapping what areas are using the most oxygen and, therefore, the most energy, while subjects do things like play games, make decisions, or just rest. Several fMRI studies show that pubertal development – specifically increased testosterone – is associated with more brain activity when subjects gets a reward7. One study even showed that this increased activation is associated with risk-taking, suggesting that as children go through puberty, good things feel more rewarding, and therefore kids are more willing to take risks to get them7. So, when you hear someone talk about all the “stupid, risky things” they did as a teenager, there’s a good chance that puberty was at least partially to blame. The evidence for pubertal effects in emotional or social processing, however, are more mixed; some studies show that development and increasing hormone levels are associated with greater brain activation to emotional stimuli, while others have found reduced responses that are similar to adult patterns of activation2,7. These mixed findings may reflect differences in the studies’ methods, or they may get at a more central paradox of adolescence whereby teenagers are both viewed as “overly emotional” and also becoming mature adults. At its core, growing up involves a lot of complicated physical, emotional, and neurological changes that together make for a messy transition into adulthood. Therefore, while existing studies and theories of human development suggest that puberty does impact brain function, more studies are needed to understand how.
What’s left to learn?
Unfortunately, there’s still a lot we aren’t certain about when it comes to puberty and the brain. Of all the neuroscience studies that measure adolescents, most only look for the effects of age on the brain while just a tiny fraction measure the effects of puberty. As mentioned above, puberty’s effects happen alongside and interact with the brain development that happens as you age, making one hard to distinguish from the other. In addition to the biological effects of puberty, this period of adolescence often comes with many social changes as kids start to see themselves differently or be treated differently by others. Since the brain is affected by one’s environment, these changes in someone’s everyday life can also influence brain development in ways that are tricky to separate from the direct effects of pubertal hormones. There are also several ways to measure how far along someone is in pubertal development, usually involving an individuals’ hormone levels or an assessment of their physical development. All methods have pros and cons, and the fact that different studies measure puberty differently may contribute to inconsistencies in their results. Overall, although both animal and human studies do point to puberty having an impact on brain structure and function, we’re only just starting to uncover what they are. While more and more scientists are looking into this new and exciting field, it will probably take some time before neuroscience becomes a part of middle school health class.
References
1. Cole KM, Wei SM, Martinez PE, et al. The NIMH Intramural Longitudinal Study of the Endocrine and Neurobiological Events Accompanying Puberty: Protocol and Rationale for. NeuroImage. 2021;234:117970. doi:10.1016/J.NEUROIMAGE.2021.117970
2. Vijayakumar N, Op de Macks Z, Shirtcliff EA, Pfeifer JH. Puberty and the human brain: Insights into adolescent development. Neurosci Biobehav Rev. 2018;92:417-436. doi:10.1016/J.NEUBIOREV.2018.06.004
3. Normal puberty – UpToDate. Accessed May 7, 2023. https://www.uptodate.com/contents/normal-puberty?search=puberty&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1#H22627003
4. DeCasien AR, Guma E, Liu S, Raznahan A. Sex differences in the human brain: a roadmap for more careful analysis and interpretation of a biological reality. Biol Sex Differ 2022 131. 2022;13(1):1-21. doi:10.1186/S13293-022-00448-W
5. Juraska JM, Sisk CL, DonCarlos LL. Sexual differentiation of the adolescent rodent brain: Hormonal influences and developmental mechanisms. Horm Behav. 2013;64(2):203-210. doi:10.1016/j.yhbeh.2013.05.010
6. Juraska JM, Willing J. Pubertal onset as a critical transition for neural development and cognition. Brain Res. 2017;1654(Pt B):87-94. doi:10.1016/J.BRAINRES.2016.04.012
7. Goddings AL, Beltz A, Peper JS, Crone EA, Braams BR. Understanding the Role of Puberty in Structural and Functional Development of the Adolescent Brain. J Res Adolesc. 2019;29(1):32-53. doi:10.1111/JORA.12408
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