As you’re reading this, your heart is beating somewhere between 60 and 100 times per minute, it’s generating electrical signals in the heart to maintain that pace, and your blood is being pushed against your artery walls with each beat. The autonomic nervous system, which regulates not just the heart but also other internal organ functions like digestion, respiration, and sexual arousal, is extremely active. Yet, we’re rarely aware of our heartbeat and often don’t consciously use it to inform our thoughts, perceptions, and emotions. But, as with other aspects of our physiology, we use our heartbeat unconsciously. And when we make it conscious, it can have a bigger impact than you may have guessed.
For the most part, people only consciously notice the beating of their heart in moments of arousal when heart rate is elevated, like when feeling anxious about public speaking or while riding a roller coaster. The rest of the time, we don’t feel our heart beating because the brain tries to actively filter out the sensation to ensure it doesn’t interfere with the perception of more relevant stimuli.
This rare, conscious awareness of the internal state of the body is called interoception. The topic is widely studied and focuses on a subset of interoception called visceroception, referring to the perception of just the ‘viscera’; the heart, lungs, stomach, and bladder. Of these, the heartbeat is the easiest to measure, with tasks like counting, synchronously tapping, or simply detecting one’s heartbeat. There is some debate about exactly the best method to measure heart beat detection and what these measures mean, like the difference between objective and subjective interoceptive measures. Regardless of the method, there’s mounting evidence that heartbeat interoception is involved in the generation of emotion and perception, as well as decision-making.
The most prominent neural correlate of heartbeat interoception is the insula, which acts as a filter to intercept sensations coming from the heart. The insula has been associated with all forms of interoception, and the prominent hypothesis is that it helps us form a general sense of self-awareness. As evidence that our hearts have a greater impact on our heads than we may realize, take three examples: emotion, visual perception, and decision-making.
It will come as no shock that subjective emotion can impact the sense of one’s heartbeat. Surely every reader of this article will have experienced top-down control of this system; a common example is increased heart rate while thinking about an emotionally stressful situation.
Likewise, research has found that interoceptive signals from the heart can impact emotional processing. Baroreceptors in the heart which fire with each beat signal the strength and frequency of heartbeats to the vagus and glossopharyngeal nerves in the brainstem, which then inform firing in classic emotion-related brain areas and create the subjective experience of emotion. The rate of baroreceptor firing can even be manipulated to inhibit the startle response and amplify threat or fear processing.
In fact, evidence for this type of bottom-up control of emotion dates back to the 1960’s when the James-Lange theory was first proposed, positing that we first experience physiological reactions to an external stimulus (e.g. increased heart rate, sweaty palms, tense muscles) and then put these symptoms together to initiate the subjective experience of emotion (e.g. fear).
As attention is a limited resource for the brain, it’s adaptive not to be aware of everything that happens to us. For example, it’s been well established that we don’t see everything—the brain filters out aspects of our visual field that aren’t relevant. This is often a top-down process, as in scenarios where attention is consciously brought to something.
There is now evidence that what we see is in fact affected by our heartbeat. For example, participants are less likely to perceive a flashing shape when it is shown synchronously with that person’s heartbeat. The tiny, jerking movements the eyes make to scan from one object to another (called microsaccades) are also linked with a person’s heartbeat, although we are not yet sure how. Additionally, neural events measured with MEG which are time-locked to heartbeats (in other words, heartbeat-evoked neural events) can predict whether a person will see a faint visual stimulus before the stimulus is ever presented.
In the same way the James-Lange theory states that we use physiological signals to generate emotion, Damasio’s somatic marker hypothesis says that we use physiology to help us make decisions. This checks out, as people with higher accuracy in perceiving their heartbeat perform better on working memory and learning tasks.
Those who are better at detecting their heartbeat also perform better on classic decision- making tasks. In fact, more activity in the right insula during these tasks is associated with better decision- making, but only for those with more accurate heartbeat perception. This means that it’s not just higher overall insula activity that mediates decision-making; only insula activity associated with heartbeat interoception helps people make better decisions!
Clinical Applications: Biofeedback
As interesting as it may be that awareness of the heart and functioning of the brain have a close relationship, this information doesn’t mean much if we can’t do something with it. Luckily, we can. Not only have researchers found that those with better heartbeat interoception make better decisions, they’ve also found ways to manipulate interoceptive ability and use it in the treatment of a remarkable range of diseases. An exciting new field of research into heart rate variability biofeedback (HRVB) has emerged not just for brain-related disorders but for a host of gastrointestinal, cardiovascular, and muscular diseases. HRVB training is simple and involves the conscious coupling of breathing with heartbeat; participants are fed back beat- by-beat heart rate data while they consciously slow their breath to match it.
We don’t yet know the exact mechanism by which HRVB works. It could be a way to restore homeostasis to the autonomic nervous system, as in the case of asthma or gastrointestinal disorders. Alternatively, it could work through modulation of the vagal afferent nerve, which would have similar effects to deep brain stimulation and looks to be a promising treatment for depression, anxiety, and sleep disorders.
Regardless of which mechanism is driving these positive outcomes, the message is clear: our hearts, and our awareness of their activity, affect how we think and feel.