Dr Sallie Baxendale is a Consultant Clinical Neuropsychologist with 29 years of experience working as a clinician in the NHS. She has strong clinical and research interests in the impact of drugs on cognitive function and brain development. In this article she examines the capabilities of the teenage brain and how puberty blockers may impact brain development.
Neuroscience: Decision Making and The Teenage Brain
Although the legal system assigns the arbitrary age of 18 years to the onset of adulthood, sophisticated neuroimaging studies demonstrate that the human brain doesn’t actually stop developing until we reach our mid-twenties, a fact that is beginning to be recognised in the criminal justice systems both in the UK and the USA (1,2). The last abilities to fully develop within the human brain are our ‘executive functions’; these are complex and include self-restraint, emotional control, the ability to appreciate nuance, the ability to plan, prioritise and strategise to achieve long term goals and the ability to weigh up information to make decisions.
Recent brain imaging studies have found that teen brains work differently from those of younger children and adults when it comes to decision making (3). As Dr Joseph Campellone, a Neurologist from the University of Rochester explains,
“adults think with the prefrontal cortex, the brain’s rational part. This is the part of the brain that responds to situations with good judgment and an awareness of long-term consequences. However, teens process information with the amygdala. This is the emotional part. In teens’ brains, the connections between the emotional part of the brain and the decision-making center are still developing—and not always at the same rate. That’s why when teens have overwhelming emotional input, they can’t explain later what they were thinking. They weren’t thinking as much as they were feeling.” (4)
In addition to prioritising feelings over facts, brain imaging studies have also shown us that teenagers assess risk in different ways from both younger children and adults (5). Many of the choices teenagers make carry consequences that unfold over the course of their adult lives. The ability to accurately ‘value’ an outcome in the future, attached to a decision taken today requires a complex network to develop within the brain (cortiostriatal connectivity), a process that isn’t complete until adulthood. Whilst these connections are being established, teenagers are not good at judging the ‘value’ of likely outcomes in the future, particularly if they are far in the future and outside their experience to date.
Researchers have begun to explore the impact of context on decision making in teenagers, particularly when they are in a state of heightened arousal or excitement, and in situations involving their peers. Studies have shown that in exciting situations where consequences are immediate and direct, adolescents are less influenced by their explicit knowledge of the probabilities of potential negative outcomes, and are more willing to take risks to obtain potential rewards.
The influence of peers is another uniquely powerful factor that impacts decision making in teenagers. Adolescence is a period of life in which peer relationships become increasingly important. This appears to be hard wired. One brain imaging study found that merely being observed by a peer was sufficient to induce uniquely high levels of physiological arousal in adolescents and modulation of the corticostriatal valuation systems (6). Another study found that young adolescents are more strongly influenced by other teenagers than adults when it comes to the assessment of risk. For early adolescents, the opinions of other teenagers about risk matter more than the opinions of adults (7).
Neuroscience has much to tell us about the adolescent brain and the way in which teenagers understand the world and make decisions. This knowledge must be integrated into the provision of any medical treatments which may have a long term, irrevocable impact on their lives as adults.
Neuroscience: Puberty Blockers and The Teenage Brain
As you might expect for such a complex organ, the human brain develops in a remarkable way. Most of our neurons (the brain cells that send electrical signals to each other to enable us to sense the world around us and think and feel and move) are in place by the time we are born. Cognitive development isn’t just a question of connecting up the cells we are born with; it is also important to get rid of the connections we don’t need anymore, a process called pruning. In normal brain development, the establishment of new connections is synchronised with the pruning of connections that are no longer required. The chemical signals that govern these processes are released in stages and target different regions of the brain at different ages, resulting in a predictable sequence of development throughout childhood and adolescence and well into the third decade of life.
Neuroscientists have developed the concept of “Windows of opportunity” to describe the age sensitive periods in children’s lives that represent the optimum periods for the development of specific functions. See Figure 1.
Figure 1: Developmental ‘Windows of Opportunity’ in childhood.
These are the times when specific functions develop at an incredible rate. If children are deprived of the appropriate environment at these critical times, the window of opportunity closes and they are often left with lifelong difficulties in that area. It’s not possible to just ‘pause’ development. For example, the window of opportunity for sight is open from birth, but closes relatively early. If babies were blind-folded or brought up in the pitch dark for the first few years of their life they would not be able to develop normal sight even if the blind fold was removed or the lights were turned on when they were toddlers. The window of opportunity will have already closed. Anyone who tries to learn a new foreign language as an adult is doing so once the ‘language window’ has closed. They may be able to master the new language but they will always speak it with an accent, unlike bilingual children who acquire the language when the ‘window’ is open. There is a reason we have a national reputation for being very poor at second languages: most children in the UK only begin to learn a second language at secondary school when the language window is all but closed.
Adolescence has been described as the second window of opportunity in neurodevelopment (8). See Figure 2.
Puberty doesn’t just involve the development of secondary sex characteristics. It’s a whole system change that has profound and irreversible effects on the brain as well as the body. It’s the time where the brain decides which connections are important to keep and which can be let go. The pruning that occurs in our teenage years helps the brain transition from childhood to adulthood. At present, we just don’t know the impact of puberty blockers on this critical stage of neurodevelopment and subsequent lifelong brain function in adulthood. The ‘windows of opportunity’ model indicates that rather than a whole system pause, some parts of brain development may continue regardless, but without the expected hormonal regulation the process may well be disrupted. One theory of why pruning is so prolific in adolescence is that maintaining redundant connections between the brain cells uses nutrients which may be needed elsewhere in the body during growth and sexual maturation. If sexual maturation is artificially ‘blocked’ what happens to this vital pruning process? The answer is that we just don’t know. These drugs were not developed to block normal puberty. If they had been, they would have undergone extremely rigorous testing with respect to their impact on cognitive development, to ensure that anyone choosing to take them was able to make an informed decision, aware of all of the potential advantages and side effects.
To find out more about how the teenage brain develops, interested readers are encouraged to look at the proceedings from the Unicef sumposium on ‘The Adolescent Brain’ held in 2016. See https://www.unicef-irc.org/publications/933-the-adolescent-brain-a-second-window-of-opportunity-a-compendium.html
This article is available to download here:
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- Somerville LH. The teenage brain: Sensitivity to social evaluation. Current Directions in Psychological Science. 2013;22:129–135.
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- Balvin, Nikola; Banati, Prerna (2017). The Adolescent Brain: A second window of opportunity – A compendium, Miscellanea UNICEF Office of Research – Innocenti, Florence.