Inside Neuroscience: Scientists Investigate Underpinnings of Social Behavior
Humans are very social creatures. While positive social experiences can boost health and mood, unstable or negative social environments can lead to stress and increased risk for mood disorders.
During a press conference at Neuroscience 2013, researchers described the ways the brain evaluates interactions with others, copes with sudden changes in social hierarchies, and responds to social stress. The event was moderated by Larry Young, director of the Center for Translational Social Neuroscience at Emory University in Atlanta.
Brain May Adapt To Size of Social Network
Sociability likely gave our ancestors an evolutionary advantage over competitors and prey, allowing them to survive and thrive in communities. While previous studies point to the role of the frontal lobes in social behaviors, recent studies suggest other brain regions are also engaged during social behaviors.
Press conference presenter Maryann Noonan of the University of Oxford and McGill University wanted to know whether variations in the size of an individual’s social network were related to gray matter size and connectivity in the brain. She and her colleagues scanned the brains of 18 people with varying sized social networks and found that brain regions including the anterior cingulate cortex (ACC), posterior cingulate cortex, and amygdala — key cortical areas thought to be involved in such complex social thoughts as theory of mind — were larger in size and displayed increased connectivity in people who reported having larger social networks. The study also showed that some of the brain regions that were bigger in people with larger social networks were also more strongly connected with the default mode network (DMN).
Scans of macaque monkeys housed in large and small social groups of differing sizes revealed similar differences, with animals with greater social networks showing more gray matter in the ACC, mid-superior temporal cortex, and rostral prefrontal cortex, and greater connectivity between some of these brain regions and with the DMN.
“Areas in the brain that are important for social cognition and behavior are bigger and better connected in both species,” Noonan said. “These studies suggest that the brain adapts to [each individual’s] social environment.”
Stimulating Cingulate Cortex Decreases Cooperative Behavior
To successfully navigate social settings, individuals must not only consider how decisions might affect them personally but also how others might react to them.
Curious about the neuronal basis for social decision-making, press conference presenter Keren Haroush of Harvard Medical School and her colleagues paired monkeys together for a game in which the animals had to decide whether to work together to enhance their mutual reward or against each other to enhance their personal reward. As the monkeys played the game, Haroush measured the activity of cells in the ACC, a brain region known to be important for decision-making and reward anticipation.
Recordings from hundreds of neurons in the ACC revealed that neurons are divided into distinct populations, with some neurons signaling the action a monkey planned to take and others predicting the opponent monkey’s concurrent or upcoming response. The researchers next evaluated whether electrical stimulation of the ACC would influence the monkeys’ choices. They found that animals that had cooperated with partners on preceding trials became less likely to cooperate after electrical stimulation of the ACC, “effectively abolishing the established positive interaction with their opponent,” Haroush said.
According to Haroush, the study “suggests that the cingulate is involved in mutually beneficial social interactions between individuals.”
Social Competition Activates Reward Networks
An individual’s ability to sense where they stack up against potential rivals in social settings and adapt to the social hierarchy is critical to success in social settings. Press conference presenter Romain Ligneul, a graduate student in the laboratory of Jean-Claude Dreher at the National Center for Scientific Research in Bron, France, described his research examining how people come to understand social rank. In one study, Ligneul and Dreher asked healthy male participants to compete against three different players in a computerized decision-making task while undergoing a functional magnetic resonance imaging scan. During the game, participants were unaware that their challengers were computer-generated (representing three levels of difficulty).
As participants learned how they stacked up against their opponents through the frequency of their wins and losses, the researchers measured their brain activity to track the dynamics of social learning and the emerging social hierarchy. Analysis revealed that social competition activates brain networks involved in reward learning, even when there is no tangible reward at stake in the competition.
Social Instability May Better Equip Animals for Novel Social Situations
In humans, socially unstable settings are associated with persistent stress and increased incidence of mental illness. To better understand how the brain responds to social instability, press conference presenter Maya Opendak, a graduate student in the laboratory of Elizabeth Gould at Princeton University, and her colleagues allowed two groups of rats to form stable hierarchies before switching the dominant animals between the two communities.
The change led to increased aggression between the animals and significant changes in hierarchy, with dominants losing their position to previous subordinates. Subsequent behavioral tests revealed that while the rats from disrupted social environments behaved differently in social environments, they performed similarly to controls on cognitive tasks and appeared less anxious.
Analysis of the hippocampi of the animals revealed that animals that experienced the disrupted social setting had fewer new neurons and stem cells than animals living in standard cages.
Reduced generation of neurons in the hippocampus is often accompanied by cognitive impairments, Opendak explained. However, “these data suggest that animals with experience in more dynamic social environments, such as one with a disruption, may be better equipped for dealing with novel social situations,” she said.
Too Much Stress May Trigger Brain Mechanisms of Natural Resiliency
While exposure to prolonged stress can lead some animals to become depressed and socially avoidant, others become more resilient in coping with stress. To better understand the neurophysiological basis of stress resilience, press conference presenter Allyson Friedman of Icahn School of Medicine at Mount Sinai and her colleagues measured the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) of mice that were susceptible to stress of social defeat and mice that were resilient. Previous studies show that these neurons are hyperactive in mice expressing depressive-like symptoms, and this hyperactivity is associated with an increase in ion channels called H-channels.
Recordings from DA neurons in the VTA of stress-susceptible mice revealed a hyperactive firing pattern and increased H-channel current after social defeat stress. In contrast, the DA neurons in the VTA of resilient mice exhibited controlled firing after the social stress. It is noteworthy that this controlled firing was accompanied by an even larger increase in the H-channel current compared with susceptible mice.
To understand how DA neurons in the VTA of resilient mice are able to achieve the stable firing pattern despite larger hyperpolarization-activated current, Friedman and her colleagues examined K+ channels, which are known to decrease the hyperactivity of cells. The DA neurons in the VTA of resilient mice exhibited increased K+ channel current, suggesting “there is a unique stress-induced balancing homeostatic mechanism underlying resilient behavior,” according to Friedman.
When the researchers increased the H-channel current in the susceptible mice by infusing the drug lamotrigine into the VTA, they were able to trigger increased K+ current, normalizing the firing pattern rate in the DA neurons in the VTA and reducing the depressive-like symptoms in the mice.
“Our findings suggest that the resilient brain remains stable through the heightened use of ion channels,” Friedman said. “By promoting and triggering homeostatic function, we may be able to come up with novel therapeutic strategies not only to reverse depressive symptoms but to also push people toward resilience.”
Understanding the biological and cognitive factors that determine how we relate to others has the potential to answer big questions about the complex relationship between biology and behavior.