Prior research has shown that people’s heartbeats could sync up when engaging in the same activity, whether it’s an audience watching a live theatre performance or romantic couples mirroring each other’s actions. In a new study, researchers found that strangers’ heart rates synchronize even when they are listening to a story by themselves in isolation, but only when paying attention to the story. Furthermore, this subconscious synchronization also occurred during boring instructional videos, suggesting that there’s a cognitive function that controls the heart.

“Our heart rate is modulated by how much people are paying attention to a narrative, whether that be an auditory narrative or informative videos. Essentially, if people are paying attention to the narrative, their heart rate will change in a similar fashion to other people who are also paying attention, however if people are somehow distracted the changes in their heart rate no longer follow the changes in other people,” Jens Madsen, a postdoctoral fellow at the Parra Lab at City College of New York, told ZME Science.

“Interestingly these people watched or listened to the stories alone, so there is no social interaction, it is only the story that can drive the synchronization. So despite the fact that we have just gone through a global pandemic where we consume content by ourselves, our hearts literally still react in a very similar way.”

Madsen and Pauline Pérez, both co-first authors of the new study that appeared today in Cell Reports, were interested in exploring new ways to probe a person’s consciousness with as little intrusion as possible. Brain scans are great for this purpose, but they wondered what they could do with even simpler technology. “If you could just measure their heart rate when they listen to a story, which is equipment that is oftentimes already present in a clinic — that would be powerful,” Madsen said.

For their study, the team asked healthy volunteers to listen to the audiobook of Jules Verne’s timeless classic 20,000 Leagues Under the Sea. As they went through the story, their heart rate changed based on the story’s narrative, as measured by an electrocardiogram (EKG).

Even though each participant listened to the story by themselves and their heart rates varied, most of the participants showed increases or decreases in their heartbeat at the same points in the story. But was it because the content was emotionally engaging?

In a separate experiment, the audiobook was replaced with an educational video with no underlying emotional variations. The participants still showed synchronization in heart rates. However, this synchronization broke down when the participants were asked to watch the video for a second time while counting backward in their heads. This suggests that attention is paramount to heartbeat synchronization between people.

This point was strengthened by a third experiment during which subjects had to listen to a short children’s story. Some of the participants did so in a quiet environment, others had to deal with distractions. Those with low rates of heart synchronization also tended to score poorly on a test that asked questions about the story’s narrative.

Finally, in a fourth experiment, the researchers played a children’s audiobook to both healthy volunteers and patients in a coma or vegetative state. The patients had a significantly lower rate of heartbeat synchronization than the healthy controls. Furthermore, the patients with the highest degree of responsiveness and who had a higher heart rate synchronization had regained some consciousness six months later.

“In our finding it is surprising that the heart rate synchronizes between people in a statistically meaningful way despite the fact that part of the stimuli was instructional videos with very little emotional expression. This is not to say that emotional expression in content won’t affect our response to it, but it is not necessary for the heart rate to fluctuate similarly. We do believe that if you have an emotional expression in the stimuli, this can increase your level of attention, draw you in so to speak, and is something one could look into in future experiments. Whether the ‘heart’ follows the brain is something we are currently looking into and hopefully will have results on that published soon,” Madsen said.

The findings could prove useful in medical practice by forming the basis for techniques that measure brain function indirectly by gauging variations in heart rate. Since heart measuring devices are ubiquitous, this could be done even using simple devices like smartwatches or fitness bands.

However, the researchers note that further research is required to validate this model, this time with both EEGs and fMRIs to also look at neural activity. This is something the researchers are pursuing in their next study. Until we hear new things, these findings are an excellent example of how the mind and brain can trigger changes in our physiology.

“We are currently looking more into the mind-body connection where the heart plays an important role along with many other signals we can measure when people engage with the world,” Madsen said.