By MEG DENNISON & MURAT YUCEL
Positive feelings are linked to brain development in teenagers, giving neuroscientists insights into why people differ in their resilience to stress and other mental health conditions later in life.
Feeling positive refers to more than just being happy. It includes things like being enthusiastic and determined, being mentally alert, having interests as well asfeelings of contentment, satisfaction and joy.
A number of studies have shown that positive emotions may play an important role in being (and becoming) resilient during times ofstress or hardship, and seem to play a role in general well-being and physical health. A proliferation of research on the role of positive emotions in resilience, known as the "positive psychology" movement,suggeststhat people who are able to experience positive emotions during difficult times, including times of extreme adversity or trauma, are more likely to bounce back or even use difficult timesto learn more about themselves and to grow as a person.
Psychologists have long known that individuals vary in the amount of positive and negative emotions they experience in everyday life. We have also learnt that people who experience extremely low positive emotions have a greater risk for a number of psychological disorders, including depression, substance use disorders and addiction, as well asless common illnesses such as schizophrenia or bipolar disorder.
Psychologists have observed that, even from the moment we are born, there are differencesin the frequency and intensity with which infants might display or feel particular emotions. Because these differences can be observed from the start of life, before the environment starts to impact on our experiences, we believe that these individual qualities are initially based on our biology, including how we are uniquely put together by our genes.
We suspect that these innate personality traits or characteristics not only shape our environment but also help to determine how our environment shapes us. Scientists are even beginning to map how our genetic coding is changed by the interaction of our genes and our environments (see box, What Is Epigenetics?)
It has long been thought that genes are fixed from birth and that the expression of genes is not affected by the environment. There is, however, evidence that the expression of our genes is less "fixed" than we thought, and that our environment and our experiences can alter how our genes work.
Since the study of epigenetics emerged in the 1940s, scientists have learned that certain environments promote the switching on or off of certain genes at certain times. These biological processes, which we call epigenetic modifications, can define how the information in genes is expressed and taken up by cells.
This research can help explain how cells carrying identical DNA differentiate into different cell types, and how they maintain thes e different cellular states. In this way, epigenetics is considered to act as a bridge between our genetic make-up and our environments, which both shape our personality and behaviour.
Most parents and caregivers raising more than one child can relate to the challenges of parenting children with very different temperaments. A child's unique temperament often brings about very different responses from their caregivers. For example, a smiling, easygoing child may bring about positive reactions from a caregiver more frequently than a shy, more withdrawn child prone to a short temper.
These findings may provide some clues as to the biological systems involved in resilience to stress, as well as, risk for psychological problems such as depression and addiction.
On the other hand, children's temperaments make them react to the same treatment by others very differently. For example, a shy child who is being ignored by their peers atschool may further withdraw into his or herself, whereas a more sociable child might seek out other company. Psychologists believe that these complex processes help to shape unique environments for every child, and scientists are beginning to take up the challenge of understanding how these processes operate at the biological level.
Because of the important role that positive emotion plays in how individuals cope with stress, over the past several decades there has been a growing interest in the brain systems underlying positive emotions. A number of regions and circuits within the brain are consistendy linked to our experience of positive emotions.
The most important network is the dopaminergic network (see box, What Is Dopamine?), which is often called the reward network. This network is related to the immediate experience of positive emotions as well as behaviourspecifically linked to getting things that we want and like.
For example, imagine it's a hot day and you start daydreaming about your favourite ice-cream. Suddenly, all you can think about is your favourite ice-cream and you can't concentrate on anything else. Soon enough you might find yourself eating ice-cream,feeling more relaxed and just enjoying the experience. There are a number of complex psychological phenomena going on here, and they are all connected to the function of dopamine within our brain.
Dopamine is one of many neurotransmitters in the human brain. A neurotransmitter is a chemical released by nerve cells to send signals to other nerve cells, and this allows neurons to communicate information.
The brain has several distinct dopamine systems, one of which plays a major role in feelings of pleasure as well as reward-motivated behaviour. Most naturally rewarding stimuli that are required for our survival as a species, such as food and sexual activity, increase the level of dopamine in the brain.
Dopamine helps to organise our behaviour so that we are more likely to get things we need to survive. For example, dopamine activity increases when we are successful at getting something we desire. This is linked to feeling good, and increases the chance we will do the same thing again.
However, dopamine activity dramatically decreases when we are unsuccessful at getting something we want. This is associated with feelings of frustration, prompting us to try a different strategy next time. In this way, dopamine helps us learn how to get what we want from our environment.
A variety of drugs of abuse also increase dopamine neuronal activity and appear to "hijack" this motivation system, which is why they are thought to be so addictive.
Our laboratory has been exploring whether the amount of positive emotions that people experience is related to brain development during adolescence. We were particularly interested to study this during adolescence because we know that the dopamine network undergoes important changes and remodelling at this time of life. We know that some areas in this brain network are getting bigger in size,some are getting smaller, and the connections between different regions are also changing to bring about more adult-like behaviour.
Furthermore, it is during adolescence that many of the emotional and psychological problems that are characterised by low positive emotion begin to emerge, so understanding how these processes vary for teenagers with different emotional experiences might help usto identify biological processes that support both resilience and risk for future mental health difficulties.
Our study examined around 90 adolescents using a Magnetic Resonance Imaging (MRI) scanner, which allowed usto measure the volume of particular brain regions within the dopamine network Twelve-year-old adolescents underwent an MRI scan and completed a questionnaire that measured their usual levels of positive emotion. At age 16 they completed another MRI scan of the same brain regions so we could map brain changes for each participant.
We found two key areas within the dopamine network that were associated with the adolescents' levels of positive emotion. The first region, the hippocampus, is an area that is associated with stress regulation, and has been consistently discussed in biological theories of depression. The other region, the caudate, is involved in learning new behaviours or actions to help us get things that we desire or like.
Interestingly it was how much the caudate volume reduced in size from age 12 to 16 years that was linked to positive emotions. We found that kids who typically experience more positive emotions tend to have greater total change, or more plasticity, in this region at this time of life.
We have predicted, but not yet shown, that these children may be more resilient to stress. There is some evidence that stress can reduce chemical factors in the brain that promote brain plasticity.
The findings are important because they suggest that adolescence may be a critical period in human development where the experience of positive emotion plays a key role in how the dopamine system is organised and prepared for life events during adulthood.
Our research may therefore provide some clues about the biological systems involved in resilience to stress, as well asrisk for psychological problems such as depression and addiction.
The development of the caudate throughout childhood and adolescence differs in a number of mental health problems, including attention deficit disorder, depression, bipolar disorder, addiction and schizophrenia. Future research is exploring possible links between these developmental differences and the exact behaviours and emotional experiences they give rise to.
For example, a recent study of children diagnosed with autism spectrum disorder in The Netherlands found that repetitive behaviour was linked to a lack of the decrease in caudate volume expected during adolescence. Repetitive behaviour indicates difficulty learning, updating and modifying behaviour based on feed- back from the environment. Children who have difficulty making links between desired goals and successful actions to achieve these goals are likely to feel high levels of frustration, and may lead to increased negative, rather than positive, emotions.
Researchers have recently become very interested in understanding the role of neural plasticity - how the brain changes and organises itself throughout development - in mental health outcomes. Consistent with our findings, preliminary evidence suggests that brains that are more plastic throughout development exhibit greater resilience to mental health difficulties.
Humans are born with largely underdeveloped brains. In fact, the human brain takes the longest of all mammals to reach maturity - around 25 years from birth. During this time the brain undergoes a number of structural and functional changes. Structural changes refer to how the shape, volume and connections between neurons change, while functional change refers to how the pattern of electrical activity in the brain changes.
These developmental changes are examples of the plasticity of our brain, and largely explain why our mental and physical abilities change so dramatically throughout our childhood and into adulthood.
Even when the brain has reached maturity there are some areas of the brain that remain plastic and can be remodelled through experiences in adulthood. However, the degree of brain plasticity in adulthood is considerably reduced compared with childhood.
While genetic factors are likely to play an important role in determining this developmental pattern, future research is particularly interested in examining the role of stress experienced early in life, when the brain is maturing rapidly, on the development of the brain across the lifespan. There is growing evidence to suggest that certain hormones that are released during periods of stress may impact upon biological systems that govern brain development. While isolated exposure to mildly stressful experiences is likely to have minimal impact, researchers are starting to explore how chronic or severe exposure to stress in early life might be linked to reduced brain plasticity throughout development.
Understanding the contribution of environmental and biological factors in determining both mental wellness and mental illness will ultimately help us to identify children who may benefit from early intervention, and help us to develop better preventative measures and treatments. There are a number of studies showing that school programs that teach children skills to increase positive emotions can reduce levels of depression and anxiety. Examples of these skills include learning how to demonstrate gratitude, to savour positive feelings and to practice mindfulness in everyday life.
Research that helps us to understand how these skills affect both behaviour and brain function, as well as which kids experience the most benefit from this type of training, are going to be key questions in the next few years.
Meg Dennison is a Research Fellow at the Australian Centre for Post Traumatic Mental Health in the Department of Psychiatry at the University of Melbourne. Murat Yucel is the Director of Monash Clinical & Imaging Neuroscience at Monash University.