Mind reading comes easily to most of us.

For all our divisions, humans are uncannily efficient at simulating another persons thoughts and beliefs. Its how you can walk a mile in someone elses shoes, know where theyre coming from, and in turn, generate empathy or predict how your actions impact others. Most of the time, we can even do this when we fundamentally disagree with the other persons point of view.

This mysterious ability to hop into someone elses headheck, even just to admit that theyre conscious beings with their own mindsis dubbed the theory of mind. Its simulation at its very best, where it allows us to connect and interact with others not just based on our own thoughts and actions, but also on our understanding of theirs. Its what lets you guess why your friend is upset on her birthday. Its behind strategy games like chess and entire disciplines such as game theory. Its what makes human society flourish or fail.

The problem? No one really knows how theory of mind works in our headsbut thats set to change.

This week, in a study with over half a dozen people, a team from Harvard Medical School and MIT recorded directly from single neurons in the forepart of their brains. For the first time, the scientists identified a special group of cells that lets us acknowledge and predict someone elses hidden beliefs. Even crazier, these neurons loyally encoded demonstrably false ideas that others may have, and beliefs that the person being studied doesnt necessarily agree with.

In other words, each of us has a smattering of brain cells dedicated to modeling another mind inside our own heads.

Until now, it wasnt clear whether or how neurons were able to perform these social cognitive computations, said study author Mohsen Jamali.

The results propelled a centuries-old debate on the nature of self and other into a new, scientifically-grounded era. But to lead author Dr. Ziv Williams, it also builds a framework to better capture the intricacies of how we model mindsand when or why it fails. Autism, for example, often leads to a breakdown in the ability to gauge social cues. People with brain injuries due to trauma can also lose that predictive superpower. And outside our own species, a model of how we model each others minds could form a powerful tool to bolster AI, providing them with an artificial theory of mind and a lot more common sense when dealing with people.

Debates over the theory of mind have roots going back to 17th-century philosophy. But modern excitement, especially in neuroscience, sparked in the early 1990s, when neuroscientists captured the inner electrical dialogue of a very special type of neurons.

Recording from the motor regions of the brains of macaque monkeys, they found a bizarre population that fired not only when the monkey waved its arm aroundsay, to grab an apple or ring a bellbut also when it watched another monkey perform the same action. Even weirder, the same neurons sparked with electrical activity when the monkey heard someone else performing the task in another room. Unlike any other known type of brain cell at the time, these mirror neurons seemed to encode for another beings actions and goals, rather than those of its own host.

Mirror neurons exploded in popularity for the next few decades. Some believed theyre the seat of empathy. Others thought theyre central to human social interaction capabilities, such as speech. One prominent pop-culture neuroscientist even went as far as saying that these cells shaped our civilization.

Yet as more sophisticated tools and techniques grew in social neuroscience, people soon realized that mirror neurons werent the end-all of empathy, language, or autism. Rather, using state-of-the-art brain imaging, scientists began honing down towards the front of the brain, sitting right behind the foreheadthe prefrontal cortexas the piece of the brain that captures anothers beliefs and thoughts.

Schooled by overpromises from mirror neurons, however, few were willing to hallmark the brain region as a supporter of theory of mind. After all, brain imaging captures the aggregated and averaged activity of thousands, if not more, neurons simultaneously. The readout is then influenced by other brain regions, painting a murky picture.

One way to sharpen it? Record from single neurons.

The new study blew people away with just that. Rather than relying on social but non-human animals, they went straight to the source: human volunteers who have electrodes implanted. These participants had already gone through brain surgery in preparation for a treatment for Parkinsons disease, and bravely signed on for the study. This allowed the team to directly record from single neurons in human brainssomething generally outside the reach of most theory of mind studies.

In all, they tapped into over 320 neurons embedded in the subjects frontal brains. As the implanted microelectrodes silently recorded the brain cells electrical activity, the team asked the participants to listen to a short story.

Take this scenario: You and Tom see a jar on the table. After Tom leaves, you move the jar to the cupboard. The listener knows that the jar is in the cupboard. But Tom doesnt. Because of theory of mind, we can reason that Tom will still think the jar is on the table.

The team then asked the listeners two seemingly simple questions. The first was where is the jar, or an objective assessment based on the listeners understanding. The second was more interestingwhere does Tom think the jar is? which probes the brains simulation of Toms mind.

Immediately, the team found a slew of neurons that surprisingly captured the distinction between internal beliefs and those of others. About 20 percent of recorded neurons reliably fired with activity when they predicted Toms belief. An even higher percentage sparked to life when Tom stated a true beliefthat is, true from his perspective. In all, the electrical activity of these neurons could predict nearly 80 percent of the time whether the listener accurately predicted Toms mental image of the jar.

To rephrase: we have neurons in our heads that encode for someone elses idea of reality, rather than whats actually true or real. This holds rather unnerving implications, in that the neurons solely reflect someone elses specific perspectiveyour perspective, or the truth, doesnt come into play.

If youre thinking oh well, these brain cells just respond to prediction, the authors have answers here too. It gets weirder.

For one, the cells that encode data for Toms ideas update to his perception of reality. When the participants heard that after Tom leaves, you move the jar to the cupboard as he watches you through the window, the same cells that encoded Toms perspective will shift gears, leading to the answer that now Tom knows the jar is in the cupboard. Your brain cells, encoding for someone elses beliefs, will update when their beliefsnot your ownupdate.

For another, the neurons also captured specific details about Toms beliefs. Using stories similar to the jar and cupboard, for example, the team found that these mind-reading neurons could encode for the items identity (a jar versus a table or vegetables), its location, color, and other characteristics. Compiling all the tests together, the team built a model with these neurons that could accurately predict another persons concept at nearly six times more than chance, regardless of the difficulty of reasoning.

Each neuron is encoding different bits of information, said Jamali. By combining the computations of all the neurons, you get a very detailed representation of the contents of anothers beliefs and an accurate prediction of whether they are true or false.

Are these predictive neurons just another mirror neuron story in the making? Many dont think so. Dr. Uta Frith, an Emeritus Professor at UCL Institute of Cognitive Neuroscience, commented, Amazing that single cells in [the prefrontal cortex]show activity during mentalizing, recapitulating findings from more blunt human brain recording instruments such as MRI. But mostly, the leap is in our methods for probing our own mindseven as they encode for someone elses. Its amazing that this type of recording can be done at all, said Frith.

Image Credit: Gerd Altmann from Pixabay

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This Is Where Empathy Lives in the Brain, and How It Works - Singularity Hub