Every day a person makes decisions based on predicting what someone else will do, from deciding whether the driver approaching an intersection will stop for the red light to determining whether a particular negotiation strategy will result in a desired outcome.
Now a study by researchers from Massachusetts General Hospital has discovered two groups of neurons that play key roles in social interactions between primates. One group is activated when deciding whether to cooperate with another individual and another group is involved in predicting what the other will do. The opensource study appears in Cell.
For a long time the medical community have been interested in understanding how complex social interactions between individuals are mediated by neurons within the brain. Now, the current study found that part of the frontal lobe called the anterior cingulate cortex plays an essential role in mediating cooperative social interactions in Rhesus monkeys. Some neurons encode the animal’s decision whether or not to cooperate with another monkey, and a separate group of neurons was activated in predicting what the other monkey would do before it had made its decision. The activity of those other-predictive neurons was uniquely affected by the social context of the interaction.
The anterior cingulate cortex is broadly connected with other brain regions known to be involved in interactive behaviour, and damage to this structure results in reduced interest in other individuals compared with inanimate objects. In fact, people with autism spectrum disorders or other conditions affecting social interactions, such as antisocial personality disorder, have been found to have abnormalities in the anterior cingulate cortex.
To better understand the role of the anterior cingulate cortex in making a person’s own decisions and predicting what another individual will do the team tested pairs of monkeys. In the game, each monkey is given a choice, in this instance which of two displayed symbols to choose, and the relationship between the two animals’ choices determines how much of a reward each will receive. In repeated trials with the monkeys sitting next to each other, the animals learn through experience that one symbol represents cooperation with the other monkey and the other represents a lack of cooperation called defection.
Two versions of the trial were used to change the social context of the experiment, one in which the monkeys were in separate rooms and the other in which a monkey played the game against a computer, significantly reduced the likelihood of cooperation and of reciprocation after previous mutual cooperation.
Measuring the activity of 353 individual neurons within the anterior cingulate cortex while the monkeys performed the trials revealed that about half were activated during the task. Of these task-responsive neurons, a quarter showed differences in activation based on the animals’ individual choice, and an even larger group, a third of those involved in the task, showed changes in activation corresponding with the as-yet unknown choice of the other monkeys. The predictions made by the activity of these neurons were as accurate as those made by an algorithm that evaluated the animals’ previous choices.
The team also found that these ‘other-predictive’ neurons were uniquely affected by the social context of the interaction and were much less active when the animals were separated, supporting the role of these neurons in anticipating another individual’s intentions or covert state of mind. In addition, temporarily disrupting the activity of the ACC during a series of trials reduced the overall likelihood of cooperation and specifically of reciprocal cooperation, which is in line with previous studies that have found ACC involvement in disorders affecting social interaction.
The team explain that social interactions are complex, and their study touches only a small aspect of how individuals interact. The team’s eventual hope is to better understand how these complex interactions are encoded within the human brain and to use this understanding to develop new, targeted treatment for disorders such as autism and antisocial behaviour, which are often characterized by difficulty with social interaction.
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