Interoception

Interoception refers to the sense of the physiological condition of the body^[1]. This type of perception involves the processing of signals originating from within the body, providing a continuous update on the body's internal state^[2]. It helps us feel sensations such as hunger, thirst, temperature, heart rate, respiration, and pain, among other internal processes^[3].

Several pathways convey interoceptive information to the brain.

The Spinothalamic Pathway is a major conduit for interoceptive information^[4]. Sensory fibers carry input from various internal organs and other tissues to the spinal cord, which then travels up to the brain via the spinothalamic tract^[5]. This pathway is important for the perception of pain and temperature^[6].

The spinothalamic tract consists of two primary tracts: the lateral spinothalamic tract, which carries pain and temperature information, and the anterior spinothalamic tract, which carries crude touch and pressure sensations^[7]. The neurons that form these tracts originate from different Rexed laminae in the dorsal horn of the spinal cord^[8].

• Lamina I, or the Posteromarginal nucleus, is the most superficial lamina in the dorsal horn^[9]. It contains neurons that respond to noxious (i.e., potentially harmful) stimuli, heat, and cold^[10]. These neurons send their axons into the lateral spinothalamic tract^[11].

• Lamina II, or the Substantia Gelatinosa, contains neurons that receive input from unmyelinated C-fibers and play a significant role in modulating the transmission of pain signals^[12].

• Lamina V receives various types of sensory input, including noxious, thermal, and visceral stimuli^[13]. Many neurons in this lamina contribute to the spinothalamic tract^[14].

The Vagus Nerve (Cranial Nerve X) is another crucial pathway for interoceptive information^[16]. It is involved in the transmission of a broad range of signals from the body to the brain, including signals from the heart, lungs, and gastrointestinal tract^[17]. These signals can influence a wide range of bodily functions, including heart rate, breathing, and digestion^[18].

The Glossopharyngeal Nerve (Cranial Nerve IX) carries afferent sensory information from the body to the brain, especially from the carotid body and sinus, which monitor blood oxygen and pressure levels^[19].

The Trigeminal Nerve (Cranial Nerve V) carries sensory information from the face and head, which, while not strictly "interoceptive," contributes to our internal sense of self and can impact our internal state^[20].

The solitary nucleus, also known as the nucleus of the solitary tract (NTS), is a structure located in the medulla oblongata in the brainstem^[21]. It is an important center for visceral and taste information processing and serves as a major relay station for interoceptive information^[22]. The NTS receives visceral sensory input primarily from the vagus (X) and glossopharyngeal (IX) nerves, and taste information from the facial (VII), glossopharyngeal (IX), and vagus (X) nerves^[23]. This input can include a wide variety of information about the physiological state of the body's internal organs, such as heart rate, blood pressure, blood oxygen levels, digestive status, and more^[24].

After processing, the NTS sends this information to other parts of the brain for further integration^[26].

Once interoceptive signals reach the brain, they are processed by several different brain areas, including the insular cortex, the anterior cingulate cortex, and the somatosensory cortex, as well as the hypothalamus and brainstem, which play a critical role in maintaining homeostasis^[27]. These regions work together to integrate interoceptive information and help us make sense of our internal bodily states^[28].

The key regions involved are listed below:

1. Insular Cortex: This is considered the primary region for interoceptive processing. The insular cortex receives interoceptive information from the thalamus and other areas and integrates this information to generate a conscious awareness of the body's internal state. It plays a crucial role in emotions, empathy, and self-awareness^[29].

2. Anterior Cingulate Cortex (ACC): The ACC is involved in processing emotional and motivational aspects of interoception. It plays a key role in linking interoceptive information with emotional responses^[30].

3. Somatosensory Cortex: While primarily associated with processing external sensory inputs, parts of the somatosensory cortex, particularly the posterior regions, also process interoceptive signals^[31].

4. Thalamus: As the brain's main sensory relay station, the thalamus receives interoceptive information from the body via the spinal cord and cranial nerves and forwards this information to the insular cortex and other areas^[32].

5. Hypothalamus: As a central player in maintaining homeostasis, the hypothalamus receives interoceptive inputs and uses this information to regulate a wide range of bodily functions, from temperature to hunger and thirst^[33].

6. Amygdala: The amygdala plays a role in the emotional aspects of interoception, contributing to the emotional response to different bodily states^[34].

7. Prefrontal Cortex: This area, particularly the ventromedial region, integrates interoceptive information with other cognitive processes, contributing to decision making, self-regulation, and other higher cognitive functions^[35].

Interoception serves several vital functions. Its primary function is to maintain homeostasis - the stable state of the body's internal environment that is necessary for survival. It does so by allowing the brain to monitor and respond to changes in the body's internal states. For instance, if you're dehydrated, your brain will recognise the signs through interoception and create the sensation of thirst, prompting you to drink water^[36].

Interoception is also fundamental to our emotional experience. It provides the physiological basis for feelings, moods, and subjective wellbeing. For example, if you feel your heart racing, you might interpret that as anxiety or excitement depending on the context^[37].

The relationship between emotion and interoception is rooted in a theory known as the James-Lange theory of emotion, which suggests that our emotional experiences are largely a result of the physiological reactions that occur in response to life events. In essence, the brain interprets the physical reactions as emotions^[38].

Interoception plays a key role in this process. Our emotional responses are often linked with changes in our physiological state. When you experience a strong emotion, your body reacts in certain ways (e.g., heart rate increases when you're anxious). These bodily changes are monitored through interoception, and this interoceptive information helps shape your emotional experiences^[39].

Therefore, interoception provides a physiological and neural basis for the concept of 'feeling' an emotion. Some studies have shown that people who have a high degree of interoceptive awareness are better at recognising their own emotions^[40].

However, interoception isn't just about recognising our own emotions. Research suggests that it may also play a part in empathy, or understanding the emotions of others. Some researchers propose that understanding another person's emotions involves simulating their bodily state in our own bodies, a process that would involve interoception^[41].

Finally, abnormal interoceptive processing is thought to contribute to certain psychiatric conditions, such as anxiety disorders, depression, eating disorders, and autism spectrum disorder^[42].