Harvard University. US: Study identifies neurons that predict what another individual will do. Every day we make 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 Harvard Medical School investigators at 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 findings
appear in Cell.
“For a long time we have been interested in understanding how complex
social interactions between individuals are mediated by neurons within
the brain,” said Keren Haroush, HMS instructor in neurosurgery at Mass General and lead author of the Cell
paper. “We 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 behavior, 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 one’s own decisions and predicting what another individual will
do, Haroush and senior author Ziv Williams,
HMS associate professor of neurosurgery at Mass General, tested pairs
of monkeys in a version of the classic prisoner’s dilemma game.
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.
If both animals choose the cooperation symbol, both get an equally
large drink of juice, but if one chooses defection and the other chooses
cooperation, the defector gets the largest amount of juice and the
cooperator gets the smallest. However, if both animals choose to defect,
both get an equally small drink of juice; so deciding how to get more
juice involves predicting what the other animal will choose.
Each trial randomly alternated which animal was given the opportunity
to choose first. After both had made their choices, the monkeys could
see what each had chosen and detect how much juice each received. While
the animals were more likely to select defection versus cooperation
overall, they were less likely to cooperate if the other monkey had
defected on the previous trial. Mutual cooperation between both monkeys
increased the likelihood of cooperation on future trials.
Two versions of the trial that changed 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.
“We 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,” Williams noted. “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.
“Social interactions are complex, and here we touched on only a small
aspect of how individuals interact,” Williams added. “Our eventual hope
is to better understand how these complex, multifaceted interactions
are encoded within the human brain and use this understanding to develop
new, targeted treatment for disorders such as autism and antisocial
behavior, which are often characterized by difficulty with social
interaction.”
The study was supported by National Institutes of Health grant
5R01-HD059852, the Presidential Early Career Award for Scientists and
Engineers, and the Whitehall Foundation.
Adapted from a Mass General news release.