(Manana Kvernadze/EyeEm/Getty Images) HUMANS Every Single Heartbeat Affects Our Perception of The World, Researchers Find PETER DOCKRILL 1 MAY 2020
While you're reading this, your heart is beating. It's pumping blood through your body, carrying vital oxygen and nutrients to help keep you alive.
We are hardly aware of this relentless, life-affirming pounding underneath our skin, even though it happens over 100,000 times every single day.
While our heartbeat escapes our notice most of the time, though, that doesn't mean it has zero effect on our powers of attention.
Quite the contrary, in fact, as researchers from the Max Planck Institute for Human Cognitive and Brain Sciences (MPI CBS) explain in a new study.
"Even though humans are mostly not aware of their heartbeats, several heartbeat-related effects have been reported to influence conscious perception," the authors write in their paper.
Last year, some of the same team explored how fluctuations in heart activity can modulate our ability to consciously perceive external stimuli.
In experiments, they found that volunteers were less likely to detect subtle electrical pulses during one phase of the heartbeat cycle, called systole.
During systole, the heart muscle contracts, pumping blood into arteries. After this happens, diastole occurs; the heart relaxes after contracting, and its chambers fills up with blood once more.
What this showed from a scientific point of view was that humans exhibit less somatosensory perception – the sensation of things throughout the body – during systole than during diastole.
In other words, we seem to perceive things less at the moment that the heart clenches and pumps blood through the body.
Directly after, though, when the heart relaxes before the next heartbeat, we can register sensations more clearly.
Why does this happen? Arguably, it's possible humans adapted this trait so that we aren't constantly disturbed by our own pulse.
"These repeating cardiac fluctuations are treated as predictable events and attenuated by the brain to minimise the likelihood of mistaking these self-generated signals as external stimuli," the researchers explain in their new paper.
Put another way: over 100,000 times a day, we feel things just a little bit less, to prevent harassment from our own heartbeats.
That's the why, hypothetical as it is. In the new research, the researchers wanted to explore how this strange phenomenon takes place.
In an experiment, 37 volunteers received mild electrical stimuli, delivered via electrode devices worn on their fingers.
During the experiment, the participants had to indicate the moments when they thought they could detect the subtle shocks being administered.
At the same time, electroencephalography (EEG) and electrocardiography (ECG) devices recorded electrical activity in the brain and heart, to try to figure out how one might be affecting the other.
As the researchers explain, conscious somatosensory perception is linked to an increased amplitude of certain markers called SEP (somatosensory-evoked potential) components, which are also associated with consciousness.
One of these markers, known as P300, seems to be suppressed during systole, which could be why, physiologically speaking, perception seems to be reduced at the moment the heart pumps blood.
The effect also seems to be stronger for people with a more stable heart rate, the results showed.
"Whether this latter effect is related to a possibly more accurate (temporal) prediction of the next heartbeat or another physiological mechanism … cannot be differentiated based on our data," the authors explain.
Another factor, called heartbeat-evoked potentials (HEP), is also involved, representing markers of cardiac interoception (awareness of the heart).
The more people were conscious of their heartbeat, the less they were aware of the electric zaps in the experiment.
"The most plausible explanation for our findings seems to be that a shift from external to internal attention, reflected by HEP amplitude increases, interferes with conscious perception of external somatosensory stimuli by decreasing the baseline firing rates within the somatosensory network," the authors explain.
There's a lot to unpack here, and a lot that remains unknown about how the heart can affect the pathways that underlie perception and consciousness. But we're getting closer to understanding them, it seems.
"While presently it is not clear which of these pathways is most relevant for heart-brain interactions, our results are consistent with the notion of the somatosensory cortex as an important relay centre for cardiac input," the researchers conclude.
"How this relay centre modulates the relationship between interoception and exteroception is an interesting topic for future research."
The findings are reported in PNAS.