[The Week] Have you ever been startled by someone suddenly talking to you when you thought you were alone? Even when they apologize 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, 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 prioritizes interoceptive information from the internal body. These are communicated through a set of distinct neural and humoral (i.e., 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: 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 recognized. 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.