Can external life experiences - such as exposure to violence, childhood abuse or bullying - change the physical properties of our brains? What about depictions of violence in the media - such as on TV or in video games?
Last month, Medical News Today investigated what the adult health consequences - both psychological and physical - of childhood bullying may be. Among the adverse effects linked with a history of being bullied, our feature briefly touched on some intriguing findings concerning physiological changes linked to bullying.
These included a 2014 study into the long-term health effects of bullying that posited bullying as a kind of "toxic stress," measurable by abnormal levels of C-reactive protein, which last into adulthood.
However, other studies have gone further - assessing the physiological impact that not only physical, but verbal bullying may have on brain development.
The notion that an experience external to the body - not something we have ingested, that has been affected by disease or damaged through physical injury - can measurably change the physical properties of an organ as intrinsic to our functioning as the brain is revelatory. But can we prove cause and effect?
Tracy Vaillancourt, a psychologist at the University of Ottawa, Canada, who has conducted a range of studies into the emotional and psychological effects of bullying - as well as the neurobiological impact of bullying - finds it frustrating the media, public and policy makers are more inclined to pay attention to research on the subject if researchers can demonstrate biological damage.
"When I show that something is biological, it makes headlines," she told The Boston Globe. "For some reason I think humans are more compelled to believe biological evidence than someone saying, 'Oh I'm depressed. I don't feel good about this.' I'm hoping that that is a policy changer."
Bullying and the brain
In a 2008 study, Vaillancourt found that while bullied boys have higher levels of the stress hormone cortisol than their non-bullied peers, bullied girls have much lower levels of cortisol compared with their peers.
She also found that bullied teens score less well on tests of verbal memory than their peers, suggesting that the abnormal cortisol levels may be killing neurons in the hippocampus, leading to memory problems.
As part of an ongoing, long-term study, Vaillancourt has been following teenagers - some of whom have a history of being bullied by their peers - and assessing their cognitive functioning every 6 months. Vaillancourt is also using magnetic resonance imaging (MRI) to scan the brains of the teens for evidence of damage to the hippocampus.
In a previous study, neuroscientist Martin Teicher scanned the brains of 63 young adults, as part of a study into verbal victimization.
Teicher found that, among the subjects who reported being the victims of verbal bullying, there were abnormalities in the corpus callosum. This region of the brain consists of a bundle of fibers connecting the brain's left and right hemispheres and is known to be important in visual processing and memory.
The neurons in the corpus callosums of the bullied subjects were found to have less of the myelin coating that boosts communication between brain cells.
It has been suggested that these brain abnormalities might make it difficult for victims to process what is happening around them and respond appropriately.
It might also explain some of the cognitive impairments associated with being bullied - poor memory, attention and concentration - and could even contribute to the anxiety, depression and suicidal thoughts experienced by many victims.
Reduced grey matter in people maltreated as children
The impact of childhood maltreatment, more generally, on neurobiology has been explored in several studies. Previously, the results of neuroimaging studies in abused children have been considered to be inconsistent.
However, in June of this year, an international study published in the American Journal of Psychiatry claimed to have strong evidence of an association between childhood maltreatment - defined by the World Health Organization (WHO) as physical or emotional ill-treatment, sexual abuse, neglect or exploitation resulting in harm to the child's health, survival, development or dignity - and volume of cerebral grey matter.
"Childhood maltreatment acts as a severe stressor that produces a cascade of physiological and neurobiological changes that lead to enduring alterations in the brain structure," says author Joaquim Radua.
Radua's study used a neuroimaging technique called voxel based morphometric (VBM) to compare the brains of 56 children and 275 adults who had a history of childhood maltreatment with 56 children and 306 adults who had no history of maltreatment.
The researchers found that the participants who had been exposed to maltreatment exhibited significantly smaller volumes of grey matter in several brain regions.
"Deficits in the right orbitofrontal-temporal-limbic and left inferior frontal regions remained in a subgroup analysis of unmedicated participants, indicating that these abnormalities were not related to medication but to maltreatment," says Radua.
In addition, abnormalities in grey matter volume in the left post-central gyrus were only found in adults who had been exposed to maltreatment as children. The most consistent grey matter abnormalities in victims of maltreatment were found in the ventrolateral prefrontal and limbic-temporal regions, which show signs of late development.
As with Teicher's findings of myelin deficits among bullied teens, Radua's team believes that this late development following maltreatment could explain why people with a history of child abuse sometimes exhibit cognitive impairment.
Children exposed to family violence show same brain activity as soldiers in combat
A 2011 study from researchers at University College London in the UK was the first to use functional MRI (fMRI) to investigate the neurobiological impact of physical abuse and domestic violence on children.
Startlingly, the study demonstrated that children exposed to family violence have the same patterns of brain activity as soldiers exposed to combat.
In the study, the brain scans of 20 children who had been exposed to documented violence - and had all been referred to social services - at home were compared with 23 matched peers who had not been exposed to violence in their families. The average age of the participants was 12 years old.
While in the scanner, the children were shown pictures of male and female faces exhibiting sad, calm or angry expressions - the children were not required to identify the emotion, but simply whether the face was male or female.
However, when the children were shown the angry faces, the fMRI scans registered increased activity in the anterior insula and amygdala of children that had been exposed to violence. Both of these brain regions are associated with threat detection, and previous studies of combat veterans have shown the same heightened activation in the anterior insula and amygdala.
What this suggests, said the researchers, is that both maltreated children and soldiers have adapted to be "hyper-aware" of danger in their environment.
However, both of the hyper-activated brain regions are also associated with anxiety disorders, so this might explain why abused children have an increased risk of anxiety problems in later life. As lead author Dr. Eamon McCrory explained:
"All the children studied were healthy and none were suffering from a mental health problem. What we have shown is that exposure to family violence is associated with altered brain functioning in the absence of psychiatric symptoms and that these alterations may represent an underlying neural risk factor. We suggest these changes may be adaptive for the child in the short term but may increase longer term risk."
However, exposure to violence in the family or at school are not the only experiences that neuroscientists have suggested may influence brain development. The effects of exposure to violence on TV and in video games on brain development has been a recurring area of interest.
White matter and violence on TV
This year, a study from the Indiana University School of Medicine found that young adult men who watch more violence on TV exhibit less mature brain development and poorer executive functioning - the ability to make decisions, reason and solve problems - than peers who watch less violent TV shows.
While the amount of TV viewing overall was not found to be linked to performance on executive function tests undertaken by the study participants - 65 healthy males with normal IQs, aged 18-29 - viewing of violent TV was.
"We found that the more violent TV viewing a participant reported, the worse they performed on tasks of attention and cognitive control," said author Tom A. Hummer, PhD.
What is more, when Hummer's team looked at MRI scans of participants who watched a lot of violent TV, they found physiological abnormalities:
"When we looked at the brain scans of young men with higher violent television exposure, there was less volume of white matter connecting the frontal and parietal lobes, which can be a sign of less maturity in brain development."
White matter insulates nerves that connect different brain regions. Some of these connections - such as between the frontal and parietal lobes - are believed to be important for executive functioning.
Normally, the amount of white matter in the brain increases, gradually making more connections, up until about the age of 30.
Although Hummer's study excluded regular players of video games - to avoid confounding evidence on the relationship of white matter volume and TV violence - a 2011 study looked specifically at the influence of violent video games on the brain.
In this study, 22 healthy men aged 18-29 who were not regular players of violent video games were randomized into two groups. One group was required to play a shooting game for 10 hours at home for 1 week but instructed not to play at all the second week. The second group did not play any violent video games throughout the 2-week period.
All of the participants underwent fMRI at the beginning of the study, with follow-up sessions after the first and second weeks.
The researchers reported that after 1 week of playing the shooting game, participants exhibited less activation in the left inferior frontal lobe during an "emotional interference task" and less activation in the anterior cingulate cortex during a "cognitive inhibition counting task," compared with their results at the start of the study and the results of the control group.
"These brain regions are important for controlling emotion and aggressive behavior," said author Dr. Yang Wang, from the Indiana University School of Medicine in Indianapolis.
However, after a week without playing the shooting game, these changes to the executive regions of the brain were shown to diminish.
Proving cause and effect is challenging
Although Dr. Wang's study demonstrates a specific change in brain activity that occurs following exposure to violence (in video game form), in many of the other studies this spotlight feature has looked at, it is difficult to prove cause and effect.
For instance, in the violent TV study, the researchers were unable to prove whether it was because of the violent TV that study participants had lower volumes of white matter, or whether the participants responded favorably to the violent TV because they had lower volumes of white matter.
As author Tom Hummer explained, additional research is needed:
"With this study we could not isolate whether people with poor executive function are drawn to programs with more violence or if the content of the TV viewing is responsible for affecting the brain's development over a period of time."
"There is still much that neuroscientists need to sort out," admitted science writer Emily Anthes, in an article on the relationship of bullying and neurobiology. "It remains difficult to thoroughly disentangle cause and effect: it's possible, for instance, that kids with certain hormonal levels or brain characteristics are more likely, for whatever reason, to be bullied in the first place."
This question has perhaps been more extensively investigated in animal models than in human subjects. For instance, a recent study looking at the interaction of sleep and learning found that sleeping after a learning task promoted the growth of dendritic spines - connectors that pass information between synapses - in the brains of mice.
Even more interestingly, mice that learned to run forward on a spinning rod in the learning task exhibited spines growing on different dendritic branches to another group of mice that learned to run backward on the rod.
Other studies in rats have found that, following exposure to intimidation by a larger rat, the production of neurons is damaged in the brains of bullied rats. In a 2007 paper published in The Journal of Neuroscience, the researchers reported that an unusually high percentage of neurons in the bullied rats would die before maturing.
But further research is required before scientists can say whether results such as these might apply to humans.
As complex and mysterious an organ as the human brain is, neuroscientists hope that the results of their studies will provide new targets for possible interventions in patients whose mental health problems may be related to victimization or exposure to violence, whether as children or adults.