Monkey Brains Provide Clues To Understanding Social Behavior
Duke University Medical Center researchers have pinpointed neurons in the brains of monkeys that may help explain how people make decisions in social situations and could aid understanding of autism.
Little is known about how the brain evaluates social information and uses this information to guide behavior. Even less is known about how this process breaks down in autism, a disease that affects more than a million Americans.
"Our prior studies described how social attention of rhesus monkeys is similar to humans -- motivated by status and sex, and sensitive to the attentive states of other individuals," said Michael Platt, Ph.D., associate professor of neurology at Duke. "Here we show that the parietal cortex, which plays a critical role in guiding attention, becomes active according to the social value of images seen and ultimately enables the behavior of the animal."
In this and previous studies, the researchers used the payment of juice rewards to gauge how much rhesus monkeys valued a view of one photograph over another. The monkeys would forego a reward of juice or "pay" to see the face of a dominant monkey, or the hindquarters of a female. They had to be paid more juice to view negative images, such as a lower-ranking monkey or a simple gray square.
Platt explained that the monkeys also chose to look longer at the female images than at the dominant males, indicating that the female images were associated with reward while the male images may have been perceived as a threat.
"This tells us that the monkeys choose to look at images that carried valuable social information for guiding behavior," Platt said. "At the same time, we monitored the activity of the neurons and noticed that the firing pattern matched the behaviors we saw."
The researchers also looked at how quickly the neurons fired and found that the firing rate increased as the monkeys saw more positive images or were given more juice. Perceived social reward, threat and the expectation of a large squirt of juice all resulted in enhanced firing by neurons in an area of the brain called the parietal cortex.
"We interpret these results to mean that the activity of the neurons provided us with information about the value of choosing one image over another -- regardless if the reward is an expected social reward or juice," Platt noted.
While the neurons provided clues into the importance of the images the monkeys were shown -- guiding the choices the monkeys made -- the neurons did not activate when the monkeys were shown the images but not required to make a choice.
"To decide between apples and oranges or between looking at an attractive female or getting a drink, the brain must evaluate these objects in a way that permits direct comparison," Platt said. "Our research found that by the time value signals have reached parietal cortex, they are already translated in a way that enables comparison and decision-making."
This work sheds light on how the normal, healthy brain reacts in a social situation calling for appropriate looking behavior, but may also explain characteristics of autism. People with autism have severe disabilities in social behavior and many have trouble looking at others, leading to problems in acquiring language and learning.
"Monkeys are motivated to look at some individuals and not at others. We demonstrated that this social motivation to look at others is mediated by neurons in the parietal cortex," Platt said. "It's possible that deficiencies in the way 'social' areas of the brain communicate with 'attention' areas of the brain may be corrupted in autism."