From ghosts to bats

When research on so-called interbrain synchrony emerged in the 2000s, some scientists dismissed it as parapsychology, a trippy field of the 1960s and ’70s that claimed to find evidence of ghosts, the afterlife and other wonders of the paranormal.

In 1965, for example, two ophthalmologists published in the prestigious journal Science an absurd study of 15 pairs of identical twins. Each twin, with electrodes on their scalps, was placed in a separate room and asked to blink on command. In two of the pairs, the study reported, one twin showed distinctive patterns of brain activity while the sibling was blinking in the other room. The doctors called it “extrasensory induction.”

“The paper is hilarious,” said Guillaume Dumas, a social physiologist at the University of Montreal who has studied brain-to-brain synchrony for more than a decade. In that far-out era, he said, “there were many papers with methodologically questionable conclusions claiming to demonstrate interbrain synchronization with two people.”

Since then, however, many sound studies have found brain synchronies emerging during human interactions, starting with a paper in 2002 that described how to collect and merge data from two brain scanners simultaneously as two people played a competitive game. This enabled researchers to observe how both brains were activated in response to each other. In a Science paper in 2005, this “hyperscanning” technique showed correlations of activity in two people’s brains when they played a game based on trust.

In 2010, Dr. Dumas used scalp electrodes to find that when two people spontaneously imitated each other’s hand movements, their brains showed coupled wave patterns. Importantly, there was no external metronome — like music or a turn-taking game — that spurred the pairs to “tune in” to each other; it happened naturally in the course of their social interaction.

“There’s no telepathy or spooky thing at play,” Dr. Dumas said. Interacting with someone else is complicated, requiring an ongoing feedback loop of attention, prediction and reaction. It makes sense that the brain would have some way of mapping both sides of that interaction — your behaviors as well as the other person’s — simultaneously, although scientists still know very little about how that happens.