Sensing the heart, seeing the world: Neural mechanisms of attentional allocation between interoception and exteroception

Every moment of our lives, our brains are juggling two very different kinds of information. One stream comes from the world outside: the sight of a tree, the sound of a car, the feel of wind on our skin. The other stream comes from deep inside our own bodies: the rhythm of our heartbeat, the fullness of our stomach, the tension in our muscles. We call the first kind of sensing "exteroception" and the second "interoception." While we are used to paying attention to the world around us, the signals from our own bodies are often running in the background, unnoticed. Yet, these internal signals are vital. They shape how we feel, how we make decisions, and how we keep our bodies running smoothly. The big question for scientists is how the brain decides which of these two streams to focus on when they compete for our attention. Does the brain simply turn up the volume on one while turning down the other, or does it use a more complex strategy to balance the two?

A team of researchers set out to answer this by watching the brains of healthy adults as they switched their focus between their own heartbeats and a visual task. They wanted to see if the brain uses different networks for looking inward versus looking outward, and if focusing on one actually quiets down the parts of the brain that handle the other. To do this, they designed a clever experiment where the outside world looked exactly the same, no matter what the person was asked to do. The participants lay inside a magnetic resonance imaging scanner, a machine that takes pictures of the brain's activity. Through a mirror, they saw a screen filled with a fast-moving stream of visual noise. Hidden within that noise were tiny, striped shapes that appeared and disappeared in a rhythm that matched a normal human heartbeat.

The participants were given two different jobs to perform in alternating blocks of time. In one job, they were told to count how many of those tiny visual shapes they saw. In the other job, they were told to ignore the shapes completely and instead count how many times they felt their own heart beat in their chest. Crucially, the visual stream of shapes and noise kept playing at the exact same speed and volume during both tasks. The only thing that changed was where the person was told to direct their attention. This setup allowed the researchers to isolate the brain's response to the direction of attention, rather than changes in what the eyes were seeing or how hard the task was. The results showed that the participants were equally good at both tasks, counting the visual shapes and the heartbeats with similar accuracy and confidence, which confirmed that the brain was truly just switching its focus, not struggling with one task more than the other.

When the researchers looked at the brain scans, they found that the brain used two completely different sets of neighborhoods to handle these two jobs. When people were counting the visual shapes, a network of areas at the back and top of the brain lit up. This included the visual cortex at the very back of the head, which is responsible for seeing, and a region higher up called the superior parietal lobule, which helps guide attention. Interestingly, the brain didn't just turn on the visual areas; it also turned down the activity in the parts of the visual cortex that represent the edges of our vision, while boosting the activity in the center where the shapes were. It was as if the brain put a spotlight on the center of the screen and dimmed the rest.

In contrast, when people switched to counting their heartbeats, a different set of brain regions became active. This network was centered around the insula, a deep fold of tissue on the side of the brain that acts as a hub for sensing the body's internal state. The scans showed strong activity in both the front and back parts of the insula, as well as in areas near the top of the brain that help with thinking and control. This suggests that paying attention to the body requires a dedicated system that is distinct from the one we use to look at the world.

Perhaps the most revealing finding was what happened when the brain switched gears. The study showed that when people focused on the visual shapes, the activity in the insula—the body-sensing hub—actually dropped below its normal resting level. It wasn't just that the brain wasn't using the insula; it was actively suppressing it. Similarly, when people focused on their heartbeats, the parts of the visual cortex that usually process the edges of the screen remained quiet, even though the visual noise was still there. This indicates that the brain doesn't just add attention to one thing; it actively dampens the processing of the competing source. It is a competitive process where resources are shifted by turning one system up and the other down.

The researchers also discovered a link between how well people could count their heartbeats and how their brains behaved. Those who were more accurate at counting their heartbeats showed less activity in a specific area on the right side of the brain called the inferior parietal lobule. This region is known for helping us notice things in the outside world. The finding suggests that being good at listening to your own body might depend on your ability to quiet down the parts of your brain that are constantly scanning the outside world. In other words, the best interoceptors might be those who can most effectively tune out the noise of the external environment to hear the quiet signals from within.

This study provides a clear picture of how the brain manages the delicate balance between the inner and outer worlds. It shows that attention is not a single switch but a coordinated dance of activation and suppression across different brain networks. When we look outward, we silence the internal sensors; when we look inward, we quiet the external sensors. This competitive allocation of resources ensures that we can focus on what matters most in the moment, whether that is navigating a busy street or sensing the subtle rhythm of our own life. The findings suggest that the ability to feel our bodies is not just a passive reception of signals, but an active skill that involves managing the brain's attentional resources to keep the internal and external worlds from overwhelming each other.