On the screen, two side-by-side squares of color flicker rapidly. A study participant is tasked with pressing a button whenever one of them briefly changes hue. What they don’t know is that one square tends to change at the exact moment their heart contracts, whereas the other changes between beats. Yet their brain apparently takes note of this timing: Over hundreds of trials, it responds differently to the two squares.
That result, from a 2024 study in NeuroImage, is one of a growing number suggesting the heartbeat quietly shapes how the brain processes information. These findings also raise the uncomfortable possibility that internal rhythms frequently treated as background noise may be subtly skewing results in neuroscience experiments. Some researchers are now warning about the issue, and a paper published in April lays out guidelines for standardizing how studies should document and account for these internal rhythms.
“We have forgotten that [the brain’s] interactions with the internal world are probably as important as interactions with the external world,” says Catherine Tallon-Baudry, a cognitive neuroscientist at the École Normale Supérieure who has spent years mapping how heartbeats shape spontaneous brain activity.
Scientists have known for years that the brain tracks signals from inside the body, a phenomenon known as interoception. Every heartbeat, for example, sends a pressure wave through blood vessels and activates sensory pathways that feed back to the brain. What is newer is evidence that these signals could influence experiments that aren’t about interoception at all. “Heart function is never irrelevant to any task,” says Lisa Feldman Barrett, a neuroscientist at Northeastern University whose research links emotion, prediction, and bodily regulation.
In the NeuroImage experiment, participants’ ability to spot the color changes stayed about the same regardless of heartbeat timing. But electroencephalography (EEG) recordings from the scalp told a different story: The visual cortex responded less strongly when a color change arrived during a heart contraction. The heartbeat was apparently competing with incoming visual information for the brain’s attention, the authors concluded.
Similar issues have surfaced in brain-scanning research. Heartbeat and breathing are known to cause movements and other fluctuations that can distort functional magnetic resonance imaging data, and standard methods of correcting for these fluctuations don’t fully account for the signals they can leave across the brain. Because their patterns are organized and widespread rather than random, they can overlap with resting-state networks—the circuits that stay active when a person is awake but not engaged in a task. That means these signals could mimic, or mask, genuine differences in connectivity unless researchers model them more carefully.
The findings are a warning that neuroscientists need to reckon with heartbeat-linked effects in their data. Researchers who do control for these effects also need more standardized ways to do so, argues the April paper, published in Psychophysiology. In 132 human studies on the effects of heartbeat in EEG and magnetoencephalography studies, it found that methods varied so widely—and were so rarely reported in full—that results could not be reliably compared or reproduced. Most studies were also too small to confidently detect the effects they claimed to measure.
“Most research groups make different, sometimes unreported, choices in how they process their signals,” says study co-author Paul Steinfath, a neuroscientist at the Max Planck Institute for Human Cognitive and Brain Sciences. “To make matters worse, heartbeat-evoked responses are very prone to contamination by various artefacts,” such as movement and muscle signals that overlap in timing with the cardiac signal and must be controlled for, he adds.
The team has published a checklist for reporting research methods and a database of best practices across the field. The practical fixes are mostly straightforward: In studies of perception, randomize the timing of stimuli so they appear at various points in the cardiac cycle; routinely record heart activity and breathing during brain scans; and spell out exactly how heartbeat-linked signals were handled in analysis. Several groups say they’re already writing these requirements into their study plans.
Even those raising the alarm about heartbeat-related effects emphasize that those found so far are real but modest, and well-powered studies with many trials are unlikely to be overturned by cardiac timing alone. But paying more attention to internal rhythms might also yield new insights.
“Cardiac effects on emotion and cognition have broad relevance for clinical neuroscience,” says Sarah Garfinkel, a cognitive neuroscientist at University College London who has long studied these effects. She points to conditions such as anxiety, depression, and post-traumatic stress disorder, where the brain’s regulation of the body appears to go awry. There, she says, training people to better sense and interpret their own cardiac signals could be a treatment in itself. For a field that has long treated the beating heart as background noise, that possibility marks a big shift in perspective.