To effectively communicate with one another, we have to understand that everyone has their own unique feelings, thoughts, desires and beliefs. Without our ability to intuit the emotions and mental states of others, there would be little room for cooperation or empathy or the many other qualities that make us human.

Scientists use the term theory of mind to describe our capacity to accurately understand one another during social interaction. We aren’t exactly sure where that ability to make sense of others’ mental state comes from, but a recent study involving rhesus macaques gives us some clues about how we understand each other, reports Rockefeller University.

The study was conducted by Winrich Freiwald and postdoctoral scholar Julia Sliwa at Freiwald’s Laboratory of Neural Systems at Rockefeller University. It was published in the journal Science in May.

How the study worked

Freiwald and Sliwa worked with these Old World monkeys to see if they could gain clearer insight about how theory of mind might have developed from an evolutionary standpoint. During the research, Freiwald and Sliwa were able to identify a certain area of the macaque brain that’s exclusively dedicated to the interpretation of social interaction.

The team used fMRI technology to study which parts of the neural circuitry of the macaque brain became active when the monkeys watched a series of videos. The videos depicted images of inanimate objects — such as monkey toys — bumping into each other, monkeys interacting with the various objects, and monkeys interacting with other monkeys by engaging in activities such as playing, grooming and fighting.

Freiwald and Sliwa expected that the areas of the brain containing mirror neurons would fire when the macaques saw images of other macaques interacting and playing with inanimate objects. Mirror neurons fire when an animal sees another animal acting in a similar way or performing the same action, so this came as no surprise.

While not totally unexpected, the team also observed that mirror neurons lit up when videos were shown of macaques interacting with one another. The mirror neurons also lit up when the monkeys were shown inanimate objects colliding with other objects.

Because the mirror neuron system exists within human brains, Sliwa suggests that the mirror neuron system might play a larger role in social and non-social interactions than we might have thought.

It was also anticipated that the section of the macaque brain that exclusively responds to certain visual shapes — such as faces, bodies and objects — would respond when videos of those particular shapes were viewed. Their expectations were correct, but the study also yielded results they didn't expect.

The areas of the macaque brain that respond specifically to body recognition experienced greater activity when they monkeys were shown videos of monkeys playing with objects. The area of the brain that’s dedicated to facial recognition experienced a greater amount of activity when the macaques were shown images of monkeys interacting with each other.

But the surprises didn't end there

What Freiwald and Sliwa found most surprising was that areas of the brain that are distant from the sections dedicated to facial and body recognition also lit up when the monkeys viewed images of macaques socializing with one another. Not only that, but within this area there is a network that responds strictly to social interaction. When the macaques were shown images that featured no social interaction, that particular area of the brain didn’t light up at all.

"That was both unexpected and mind-boggling," says Freiwald. He also notes that there's no evidence from any other neural network study that has yielded such results that shows no response when the favored stimulus is absent.

The most fascinating aspect of this discovery is that the part of the macaques’ brain that's dedicated strictly to social interaction occupies the same areas of the human brain that are linked to our ability to understand the mental states of others. Sliwa suggests that this network can be seen as a sort of "evolutionary precursor" to the network responsible for human theory of mind.

All of this seems to hint that the mirrors we use to reflect upon ourselves and each other might not be that different from other primates. There's no concrete answer to how human theory of mind developed, but this research seems to offer us some good evolutionary clues.