Teenagers are Context Dependent

During puberty, the brain, along with the body, matures fast, but there’s a specific set of systems that mature very dramatically called the corticostriatal system.

The corticostriatal system is made of two sets of brain regions; the prefrontal cortex, and the striatum. The prefrontal cortex, essentially the entire front third of the grey matter surface of your brain. The striatum is a cluster of grey matter nuclei near the middle. The two are connected by a series of white matter loops making them able to communicate back and forth (grey matter is formed from neuron cell bodies, white matter is the axons which connect them, axons are covered in a fatty substance called myelin, which is what makes it white in colour). In teenagers, these two regions develop asynchronously, so the striatum increases in activity first, during early and mid-adolescence, and then over time, the frontal regions and the white matter loops catch up.

The corticostriatal system, overall is responsible for a huge number of functions including movement, emotional regulation, value judgements, error monitoring, planning, motivation, self-control and working memory. One specific part of the striatum called the ventral striatum (also called the nucleus accumbens) specifically responds to rewards. When you see something desirable or rewarding, or something that might lead to a reward, activity in this nucleus will increase its activity. If you then don’t get the reward you expected, there will be a little dip in its activity. The ventral striatum is connected to (among other things) parts of the frontal lobe called the orbitofrontal cortex, which calculates the value of options and the dorsolateral prefrontal cortex, which exerts self-control over behaviour.

During adolescence teenagers become impulsive. Even very risk-averse teenagers are typically more impulsive and more prone to risk taking than they were as children or as they will be as adults.

This increase in impulsivity is believed to be due to the imbalance in maturity between the more active and mature striatum and the relatively immature prefrontal cortex. This is generally called the dual systems theory of adolescence. This is a very well established models which has provided testable hypotheses which have been supported by lots of studies, including in very large samples. It seems to be a very sound model of adolescent brain development, and the effect it has on adolescent behaviour.

Impulsivity is actually composed of a number of different behaviours. One element of impulsivity which is especially important to how we think about teenagers is reward sensitivity, which is, in essence, the degree to which the ventral striatum becomes active in response to a given reward. Teens become very reward sensitive, so rewarding, exciting things seem much more rewarding, while at the same time, the frontal regions which compare those risky, rewarding options to safer ones, are still underdeveloped and can’t compensate.

Impulsivity and increased reward sensitivity is considered to be a normal feature of adolescence, but its also often viewed as a dangerous one. Increased impulsivity increases a person’s risk for drug use, binge drinking, risky driving, unprotected sex and general propensity for danger. So its easy to approach reward sensitivity as something to be ameliorated where possible. Most studies either examine impulsivity in isolation in a laboratory setting, or look at risky behaviour specifically, since they’re a health concern. Accidents, made more likely by risk-taking behaviour, are the leading cause of death in teenagers.

But a collection of studies by Dr. Eva Telzer are expanding the dual-systems model by examining how adolescent behaviour is modified by the social context it occurs in. Research by Dr. Telzer’s group has found that risk-taking behaviour in teenagers changes based on the social context it occurs in and that the modulation seems to be driven by differences in how the brain responds to social rewards.

Social rewards are exactly what they sound like, a reward derived from social approval. Its important to remember when we talk about reward and the brain that the brain only has one reward system. The ventral striatum responds to all rewards, which covers everything from food, to achieving a goal, to helping someone, to taking cocaine. And increased reward sensitivity that teenagers experience applies to all forms of rewards. But also that people can still respond differently to different kinds of rewards.

Being sensitive to social rewards means that teenagers will take more risks when they are with their friends than with their mothers, but what is most interesting about this fairly intuitive finding is that they are also find less risky choices more rewarding. Having their mother present during a task choosing between safe or risky choices meant that teenagers also had a greater response in their ventral striatum when they make that safe choice.

What makes this even more interesting is that these social effects seem to extend beyond just whose present in any given moment. Teenager who generally have a closer relationship to their family or more supportive friends also take fewer risks and find risky behaviour less rewarding. And teens who find social rewards and helping others more rewarding seem to become less impulsive and at lower risk for depression over time.

These are all fairly new results, which mostly haven’t yet been replicated in another lab, so its somewhat early to declare this an established finding, but its very interesting for two reasons. The first is that provides a much more nuanced way of looking at teenager’s behaviour. Most studies of teenaged impulsivity looks at it entirely as a risk to be ameliorated or controlled, but increased reward sensitivity, especially social reward sensitivity can also an important mechanism for teens to develop independence and adult relationships and instead of being something dangerous to be suppressed. These findings, if they bear out in later research, suggests that they can be a mechanism for improving teen’s safety and mental health, if they’re allowed an appropriate environment.

More generally, this is just an interesting paradigm. Humans are social creatures and the social context we behave in is hugely important for understanding behaviour. So the brain, which exists to take in and respond to the environment responds to huge quantities of social information. But social neuroscience like this, which attempts to study how the brain uses and responds to social information is a very young field and its very hard to do wellj. So work like this adds an important and previously lacking dimension to how we study what goes into making decisions.

References

Telzer, E. H., Fuligni, A. J., Lieberman, M. D., & Galván, A. (2014). Neural sensitivity to eudaimonic and hedonic rewards differentially predict adolescent depressive symptoms over time. Proceedings of the National Academy of Sciences of the United States of America, 111(18), 6600–5. http://doi.org/10.1073/pnas.1323014111

Telzer, E. H. (2016). Dopaminergic reward sensitivity can promote adolescent health: A new perspective on the mechanism of ventral striatum activation. Developmental Cognitive Neuroscience, 17, 57–67. http://doi.org/10.1016/j.dcn.2015.10.010

Telzer, E. H., Fuligni, A. J., Lieberman, M. D., Miernicki, M. E., & Galv??n, A. (2013). The quality of adolescents peer relationships modulates neural sensitivity to risk taking. Social Cognitive and Affective Neuroscience, 10(3), 389–398. http://doi.org/10.1093/scan/nsu064

Telzer, E. H., Ichien, N. T., & Qu, Y. (2015). Mothers know best: Redirecting adolescent reward sensitivity toward safe behavior during risk taking. Social Cognitive and Affective Neuroscience, 10(10), 1383–1391. http://doi.org/10.1093/scan/nsv026

 

 

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Super-Plums… Or Not.

Any movie worth watching is worth analyzing, so a brief scene in Captain America: Civil War involving some plums has triggered a lot of discussion plums’ potential health benefits.

So, if your memory has been damaged by years of brainwashing can you actually improve it by eating plums?

Fortunately, two Australian scientists have systematically reviewed* the research of the health effects of plums and their findings were published just two months ago.

They did identify several studies where eating plum extract or powdered plums improved memory or cognitive skills.

So is that a yes?

Well, maybe. Every single one of those studies was done in rats or mice. They didn’t find any studies showing that plums improved cognition in humans.

We do studies in animals because they can tell us a lot more about what will happen in people than a dish full of human cells or a computer model, but they aren’t perfect. When something improves cognition in a rat or a mouse it suggests that it might do the same in a human, but it doesn’t mean that it will for sure.   Studies about diet are especially difficult to generalize from, because lab rats eat precisely controlled diets with carefully measured components, while human diets are complicated and variable.

There’s a few more reasons not to get too excited about these studies. First off, none of them looked at actual plums, they used plum powder or plum extract, which is less variable in content than fresh plums, and easier to add to rat chow.

Incidentally, the researchers noted that the human studies they did find (which looked at different health effects of plums), tended to use either dried plums, or plum juice. There are very few studies on fresh plums. Dried, extracted, or juiced plums have slightly different nutrient contents than fresh ones. Some chemicals become more concentrated, and others tend to be lost during processing.

Secondly, the amounts of plums in the rat diets were comically enormous. In a lot of them, the plum component was 2-5% of their entire diet. Imagine putting all the food you eat in a day on a scale, then replacing 5% of that with dried plums and you’ll get some idea of how much plum these rats were eating. Way more plums than you would ever want to eat. Any effect that eating a normal number of plums would have would be much smaller.

Why would you do such an unrealistic study?

These studies aren’t really trying to mimic the effects of a human diet, even a very plum rich human diet at all. They’re simply asking the question can plums affect the brain conditions they were studying, at all, in any way? And when you just want to find that out its best to get the biggest effect you can. A small dose of plums might produce an effect so small that it couldn’t be detected, and that’s much less likely to happen with a giant dose. It also means that you can do a good study with fewer experimental animals, which is a good thing. This is due to a statistical concept called statistical power, in essence, the smaller an effect is the bigger the sample you need to detect it. If plums produce only a small change in brain function, you’d need to study a large group of rats to find it, but if the effect on the brain is large, you can find it with only a small number of rats.

And thirdly, these positive studies, even if they do translate to humans, are not suggesting that plums are magic brain-food. Each of the studies looked at a different condition, one found that plums extract could protect against the effects of a (very) high cholesterol diet. Another found plums improved cognition in elderly rats, and another in rats with diabetes.

What you can take from this variety of studies is that plums appear to protect the brain from various conditions that can impair it (called neuroprotection), not that they necessarily improve cognition or memory in and of themselves. And none of these studies suggest, or even look at, if they can reverse damage that has already occurred, which is really totally different.

But what about brainwashing? Could plums protect you from brainwashing?

Well, some of the sources of damage that happen when your brain is physically injured are the same as when your brain is damaged by aging or illness, so its not impossible that the same sources of neuroprotection might work too, but that would still only apply to damage that was still occurring, or had just occurred, not things that happened months or years ago.

Generalizing research from one illness to another is a little bit like generalizing between species. A finding in one case suggests that another study would be useful, but the only way to prove it, is to do the research and prove it.

 

*A review is a paper that compares and summarizes earlier studies to draw conclusions based on the totality of many studies, instead of just one. A systematic review is a review where papers are collected in a fixed way so the researchers won’t be biased in which papers they choose to include.

 

References:

Igwe, E. O., & Charlton, K. E. (2016). A Systematic Review on the Health Effects of Plums ( Prunus domestica and Prunus salicina ). Phytotherapy Research, 30, 701–731.