Have you ever been startled by someone suddenly talking to you when you thought you were alone? Even when they apologise for surprising you, your heart goes on pounding in your chest. You are very aware of this sensation. But what kind of experience is it, and what can it tell us about relations between the heart and the brain?
When considering the senses, we tend to think of sight and sound, taste, touch and smell. However, these are classified as exteroceptive senses, that is, they tell us something about the outside world. In contrast, interoception is a sense that informs us about our internal bodily sensations, such as the pounding of our heart, the flutter of butterflies in our stomach or feelings of hunger.
The brain represents, integrates and prioritises interoceptive information from the internal body. These are communicated through a set of distinct neural and humoural (ie, blood-borne) pathways. This sensing of internal states of the body is part of the interplay between body and brain: it maintains homeostasis, the physiological stability necessary for survival; it provides key motivational drivers such as hunger and thirst; it explicitly represents bodily sensations, such as bladder distension.
But that is not all, and herein lies the beauty of interoception, as our feelings, thoughts and perceptions are also influenced by the dynamic interaction between body and brain.
The shaping of emotional experience through the body’s internal physiology has long been recognised. The American philosopher William James argued in 1892 that the mental aspects of emotion, the ‘feeling states’, are a product of physiology. He reversed our intuitive causality, arguing that the physiological changes themselves give rise to the emotional state: our heart does not pound because we are afraid; fear arises from our pounding heart.
Contemporary experiments demonstrate the neural and mental representation of internal bodily sensations as integral for the experience of emotions; those individuals with heightened interoception tend to experience emotions with greater intensity. The anterior insula is a key brain area, processing both emotions and internal visceral signals, supporting the idea that this area is key in processing internal bodily sensations as a means to inform emotional experience. Individuals with enhanced interoception also have greater activation of the insula during interoceptive processing and enhanced grey-matter density of this area.
So what is enhanced interoception? Some people are more accurate than others at sensing their own internal bodily sensations. While most of us are perhaps aware of our pounding heart when we are startled or have just run for the bus, not everyone can accurately sense their heartbeats when at rest. Interoceptive accuracy can be tested in the lab; we monitor physiological signals and measure how accurately these can be detected.
Historically, research has focused on the heart, as these are discrete signals that can easily be quantified. For example, a typical experiment might involve the presentation of a periodic external stimulus (eg, an auditory tone) that is time-locked to the heartbeat, such that each tone (‘beep’) occurs when the heart is beating, or in between heartbeats. Participants state whether this external stimulus is synchronous or asynchronous with their own heart. An individual’s interoceptive accuracy is an index of how well they are able to do this.
Related: Soren Dreier: The Power of Inside Listening