We don’t usually sense our heartbeat. Clearly, our brain somehow shuts down the perception of our heartbeat. How and why this happens is mostly unknown, but a new study published in The Journal of Neuroscience has unveiled some of the possible mechanisms and purposes underlying the silencing of our heartbeat.

The heart takes shape very early in embryonic development, around just two weeks after fertilization. About a week later, the heart starts beating and divides into chambers. Give it another week, and blood is already flowing through the embryo at an impressive rate of up to 160 heartbeats per minute. At this stage in embryonic development, the brain is still forming, so it’s safe to say that our heartbeat has been present since way before we could even perceive anything.

The main goal of the recent study was to unpack the effect of the so-called interoceptive (internal) signals, specifically the heartbeat, on visual awareness. Using visual awareness tasks, along with brain imaging techniques, it was studied how the heartbeat could modulate the perception of visual stimuli. The results were fascinating.

Subjects were presented with a visual stimulus consisting of a flashing yellow octagon. It was shown that the timing of the visual stimuli with respect to the heartbeat had a clear influence on visual awareness: when the visual stimulus was presented asynchronously to their heartbeat, the participants could immediately identify the target image as a yellow octagon; however, when the image was flashing at the same rhythm as their heartbeat, the participants would take longer to be aware of the yellow octagon.

The participants were then subjected to a visual discrimination task where they were presented with a flashing letter-shaped target image that was surrounded by eight similar images (distractors) displayed constantly, making the target image harder to perceive. When the target image was flashed synchronously to their heartbeat, its identification was less accurate than when it was presented asynchronously.

These findings therefore serve as evidence for a direct effect of our heartbeat on how an external visual stimulus is perceived. However, it was not the perception of the heartbeat that influenced the perception of vision since it was shown that the subjects were not aware of the synchronous rhythm.

fMRI experiments showed that there was a region in the brain, the insular cortex, that was sensitive to this cardio–visual synchrony. When the target images flashed asynchronously with the heartbeat, and the subjects perceived them easily, the insular cortex showed a normal functioning; when the flashing was in sync, on the other hand, and the subjects were less aware of the target image, the insular cortex was less active. This area of the brain is believed to enable the merging of internal and external signals, and to underlie self-awareness and bodily perception.

So, what was shown in this study was that the processing of visual stimuli that match internal cardiac signals tends to be unconsciously avoided by the brain. And it has been previously shown that the heartbeat can also affect other sensory stimuli, such as auditory, tactile, or painful stimuli. But what does that mean?

As the authors suggest, this may be due to the conflicting requirements of monitoring the heartbeat and perceiving the external world. Selectively blocking sensory information that is presented at the same rhythm as the heartbeat may be a mechanism that drives awareness away from our heartbeat, most likely to prevent internal sensations from intruding in our sensory perception of external events.

It’s as if being aware of our heartbeat would be too overwhelming to allow us to accurately notice the outside. Which actually makes sense: awareness of one’s heartbeat often occurs in the context of fear and anxiety, for example, and that is never pleasant.

References

Craig, A. (2002). How do you feel? Interoception: the sense of the physiological condition of the body Nature Reviews Neuroscience, 3 (8), 655-666 DOI: 10.1038/nrn894

Edwards, L., Ring, C., McIntyre, D., & Carroll, D. (2001). Modulation of the human nociceptive flexion reflex across the cardiac cycle Psychophysiology, 38 (4), 712-718 DOI: 10.1111/1469-8986.3840712

Karnath, H. (2005). Awareness of the Functioning of One’s Own Limbs Mediated by the Insular Cortex? Journal of Neuroscience, 25 (31), 7134-7138 DOI: 10.1523/JNEUROSCI.1590-05.2005

Macefield, V. (2003). Cardiovascular and Respiratory Modulation of Tactile Afferents in the Human Finger Pad Experimental Physiology, 88 (5), 617-625 DOI: 10.1113/eph8802548

Salomon, R., Ronchi, R., Donz, J., Bello-Ruiz, J., Herbelin, B., Martet, R., Faivre, N., Schaller, K., & Blanke, O. (2016). The Insula Mediates Access to Awareness of Visual Stimuli Presented Synchronously to the Heartbeat Journal of Neuroscience, 36 (18), 5115-5127 DOI: 10.1523/JNEUROSCI.4262-15.2016

Image via alegria2014 / Pixabay.