Interoception

Interoception is the collection of senses providing information to the organism about the internal state of the body. This can be both conscious and subconscious. It encompasses the brain's process of integrating signals relayed from the body into specific subregions—like the brainstem, thalamus, insula, somatosensory, and anterior cingulate cortex—allowing for a complex and highly accurate representation of the physiological state of the body. This is important for maintaining homeostatic conditions in the body and, potentially, facilitating self-awareness. Interoceptive signals are projected to the brain via a diversity of neural pathways, in particular from the lamina I of the spinal cord along the spinothalamic pathway and through the projections of the solitary nucleus, that allow for the sensory processing and prediction of internal bodily states. Misrepresentations of internal states, or a disconnect between the body's signals and the brain's interpretation and prediction of those signals, have been suggested to underlie conditions such as anxiety, depression, panic disorder, anorexia nervosa, bulimia nervosa, posttraumatic stress disorder (PTSD), obsessive compulsive disorder (OCD), attention deficit hyperactivity disorder (ADHD), alexithymia, somatic symptom disorder, and illness anxiety disorder. The contemporary definition of interoception is not synonymous with the term "visceroception". Visceroception refers to the perception of bodily signals arising specifically from the viscera: the heart, lungs, stomach, and bladder, along with other internal organs in the trunk of the body. This does not include organs like the brain and skin. Interoception encompasses visceral signaling, but more broadly relates to all physiological tissues that relay a signal to the central nervous system about the current state of the body. Interoceptive signals are transmitted to the brain via multiple pathways including the lamina I spinothalamic pathway, the classical viscerosensory pathway, the vagus nerve and glossopharyngeal nerve, chemosensory pathways in the blood, and somatosensory pathways from the skin. Interoceptive signals arise from many different physiological systems of the body. The most commonly studied system is cardiovascular interoception which is typically measured by directing attention towards the sensation of the heartbeat during various tasks. Other physiological systems integral to interoceptive processing include the respiratory system, gastrointestinal and genitourinary systems, nociceptive system, thermoregulatory system, endocrine and immune systems. Soft cutaneous touch is another sensory signal often included within the interoceptive processing system.

History and etymology

Early to mid-1900s The concept of interoception was introduced in 1906 by the Nobel Laureate Sir Charles S. Sherrington. He did not use the noun interoception, but did describe as interoceptive those receptors that are within the viscera—what are today called "visceroceptive"—and thus excluded all other receptors and information from the body, which he grouped as either exteroceptive or proprioceptive. In Sherrington's model, exteroceptive receptors were those that received information from outward stimuli, like light, touch, sound, and odor. He classified temperature and nociception as exteroceptive sensations as well, though these are now regarded as having interoceptive qualities. He further divided the internal milieu of the body by its somatic and autonomic functions. And proprioceptors were those found in skeletal tissue that control voluntary movement. For him, interoceptors (a term which has lost prevalence in modern literature) were thus confined to visceral involuntary smooth muscle (e.g. surrounding blood vessels). Further work on interoceptive processing after Sherrington was delayed for many years owing to the influential claim by John Newport Langley that the autonomic nervous system used only efferent (brain-to-body) signaling to implement its functions. By the 1950s and 1960s, many investigations of interoceptive processing had been conducted, and once it had become apparent that interoceptive receptors are present in many tissues of the body other researchers began to investigate afferent body-to-brain signals, mainly by conducting animal experiments to see if interoceptive conditioning was possible. Using principles of Pavlovian conditioning, different physiological systems in dogs were perturbed to elicit a conditioned response to food. For example, in one experiment, dogs' pelvises were distended using infusions of solution when food was presented to them. After rounds of pairing the two, salivation occurred without presenting food once the pelvis was distended. Interoceptive conditioning studies like this illustrated that interoceptive sensations may be important for learned behavior and emotion.

Mid-1900s to 2000

The increased interest in interoception during the late 1950s and the 1960s, as evidenced by the number of papers published, has been referred to as the "biofeedback blip". This was a phase during which many researchers examined humans' ability to gain control over autonomic functions as a method of treatment for various conditions. Interoception did not gain widespread popularity within the scientific community until the mid- to late-twentieth century. During that period, some researchers chose to use the terms visceroceptor and interoceptor interchangeably, in line with Sherrington's usage, others combined proprioceptive and visceroceptive information into one category—interoception—based on physiological data about the lack of differences in nerve impulses, and still others proposed that interoception includes more than just endogenous (internal) stimuli. Exactly which sensory signals could or should be classified as interoceptive remains the subject of ongoing debate. During the 1980s, psychophysiologists began to extensively examine cardiovascular interoception, introducing several different experimental tasks for studying the perception of the heartbeat: heartbeat counting, heartbeat tapping, and heartbeat detection. Psychiatrists also began to look at the effects of pharmacological stimulation on the symptoms of panic disorder. All of this further increased researchers' interest in interoception, including the development of theoretical models of the integration of interoceptive information within the body over time.

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