Interoception
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Interoception is contemporarily defined as the collection of
senses A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...
perceiving the internal state of the body. This can be both conscious and unconscious. It encompasses the brain's process of integrating signals relayed from the body into specific subregions—like the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...
,
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
, insula,
somatosensory In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position ( proprioception), and pain. It ...
, and
anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involve ...
—allowing for a nuanced representation of the physiological state of the body. This is important for maintaining
homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism an ...
conditions in the body and, potentially, facilitating
self-awareness In philosophy of self, self-awareness is the experience of one's own personality or individuality. It is not to be confused with consciousness in the sense of qualia. While consciousness is being aware of one's environment and body and lifest ...
. Interoceptive signals are projected to the brain via a diversity of
neural pathway In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to ...
s, in particular from the lamina I of the spinal cord along the spinothalamic pathway and through the projections of the
solitary nucleus In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, (SN or NTS) is a series of purely sensory nuclei (clusters of nerve cell bodies) forming a vertical column ...
, 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 Anxiety is an emotion which is characterized by an unpleasant state of inner turmoil and includes feelings of dread over anticipated events. Anxiety is different than fear in that the former is defined as the anticipation of a future threat wh ...
, depression, panic disorder,
anorexia nervosa Anorexia nervosa, often referred to simply as anorexia, is an eating disorder characterized by low weight, food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thin. ''Anorexia'' is a term of Gre ...
,
bulimia nervosa Bulimia nervosa, also known as simply bulimia, is an eating disorder characterized by binge eating followed by purging or fasting, and excessive concern with body shape and weight. The aim of this activity is to expel the body of calories eaten ...
,
posttraumatic stress disorder Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threat ...
(PTSD),
obsessive compulsive disorder Obsession may refer to: Psychology * Celebrity worship syndrome, obsessive addictive disorder to a celebrity's personal and professional life * Fixation (psychology), a persistent attachment to an object or idea * Idée fixe (psychology), a pr ...
(OCD),
attention deficit hyperactivity disorder Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inap ...
(ADHD),
alexithymia Alexithymia is a personality trait characterized by the inability to identify and describe emotions experienced by oneself. The core characteristic of alexithymia is marked dysfunction in emotional awareness, social attachment, and interpersonal ...
, 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 The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
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 The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
and
glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus 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 The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies g ...
, gastrointestinal and
genitourinary system The genitourinary system, or urogenital system, are the organs of the reproductive system and the urinary system. These are grouped together because of their proximity to each other, their common embryological origin and the use of common path ...
s, nociceptive system,
thermoregulatory Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
system,
endocrine The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...
and
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
s. 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 Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
and
nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an org ...
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 Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
(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 The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies internal organs, smooth muscle and glands. The autonomic nervous system is a control system t ...
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 Biofeedback is the process of gaining greater awareness of many physiological functions of one's own body by using electronic or other instruments, and with a goal of being able to manipulate the body's systems at will. Humans conduct biofeed ...
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 An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ca ...
, and still others proposed that interoception includes more than just
endogenous Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, ...
(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.
Psychiatrist A psychiatrist is a physician who specializes in psychiatry, the branch of medicine devoted to the diagnosis, prevention, study, and treatment of mental disorders. Psychiatrists are physicians and evaluate patients to determine whether their sy ...
s 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.


2000 and on

The twenty-first century has seen a tremendous increase in publications on the topic of interoception, and to a recognition of its multifaceted nature . This has led to the emergence of different ideas about interoception. One contemporary definition widens the concept to encompass "the skin and all that is underneath the skin" and the perception and function of bodily activity to more fully understand psychosomatic processes. In a similar vein, neuroanatomists hoping to find the anatomical basis of interoceptive functioning have stated the existence of a
homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism an ...
pathway from the body to the brain that represents "the physiological status of all tissues in the body," and that this mapping onto the brain provides an individual with subjective feeling states that are critical for human
emotion Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definitio ...
and
self-awareness In philosophy of self, self-awareness is the experience of one's own personality or individuality. It is not to be confused with consciousness in the sense of qualia. While consciousness is being aware of one's environment and body and lifest ...
. For example, interoception is the fundament of the modern view on allostasis and
allostatic load Allostatic load is "the wear and tear on the body" which accumulates as an individual is exposed to repeated or chronic stress. The term was coined by Bruce McEwen and Eliot Stellar in 1993. It represents the physiological consequences of chron ...
. The regulatory model of allostasis claims that the brain's primary role as an organ is the predictive regulation of internal sensations. Predictive regulation is the brain's ability to anticipate needs and prepare to fulfill them before they arise. In this model, the brain is responsible for efficient regulation of its internal milieu. Interoception is sometimes generally referred to as "the perception of internal body states" although there are many interoceptive processes that are not consciously perceived. Importantly, interoception is made possible through a process of "integrating the information coming from inside the body into the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
". This definition deviates from Sherrington's original proposition, but exemplifies the dynamic and widening breadth of interoception as a concept in modern literature.


Facets of interoception

Although interoception as a term has more recently gained increased popularity, different aspects of it have been studied since the 1950s. These include the features of attention, detection, magnitude, discrimination, accuracy, sensibility, and self-reporting. Despite not using the word ''interoception'' specifically, many publications in the
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
and medical fields have focused on understanding interoceptive information processing in different organ systems. Attention describes the ability to observe sensations within the body, it can be directed voluntarily in a "
top down "Top Down" is a song by American hip hop record producer and recording artist Swizz Beatz, included as the eighth track from his debut studio album ''One Man Band Man'' (2007). "Top Down" contains samples of swirls and riotous bursts of 1970s-so ...
" manner or it can be attracted involuntarily in a "bottom up" manner. Detection reflects the presence or absence of a conscious report of interoceptive stimuli, like a heartbeat or growling stomach. Magnitude is the intensity of the stimulus, or how strongly the stimuli is felt. Discrimination describes the ability to localize interoceptive stimuli in the body to specific organs and differentiate them from other bodily stimuli that also occur, like distinguishing between a heart which is beating hard from an upset stomach. Accuracy (or sensitivity) refers to how precisely and correctly an individual can monitor specific interoceptive processes. Self-reporting is itself multifaceted. It describes the ability to reflect on interoceptive experiences occurring over different periods of time, make judgments about them, and describe them. Brain-body interactions can also be studied using neuroimaging techniques to map functional interactions between brain and peripheral signals. Although all of these components of interoception have been studied since the mid-twentieth century, they have not been brought together under the umbrella term "interoception" until more recently. The term "interoceptive awareness" is also frequently used to encompass any (or all) of the different interoception features that are accessible to conscious self-report. This multifaceted approach offers a unified way of looking at interoceptive functioning and its different features, it clarifies the definition of interoception itself, and it informs structured ways of assessing interoceptive experiences in an individual.


Interoceptive physiology


Cardiovascular system

Cardiac interoception has been widely studied as a method of evaluating interoceptive sensation. This is done using different tasks including heartbeat counting, heartbeat tapping, heartbeat detection and heartbeat attention tasks. Heartbeat counting tasks ask participants to count the number of felt heartbeats during short time periods. Their reported count is then compared with the actual count obtained with an electrocardiogram. This measures the participant's attention to his or her own heartbeat, the accuracy with which that is perceived, and the ability of the participant to report that measurement; however, results can be influenced by the participant's preexisting knowledge of his or her heart rate and an insensitivity to heart rate change. Heartbeat detection tasks work by providing a participant with a musical tone which is played simultaneously or non-simultaneously with one's heartbeat, asking the participant to report whether it is simultaneous or not with the tones. Heartbeat detection is commonly used because of its ability discern an individual's performance above chance levels, so-called "good detectors". However, such detection rates among participants for this task are usually only 35%. It also measures the participant's attention, detection, discrimination, accuracy and self-report of the interoceptive process. Heartbeat attention tasks are the most minimalistic, and involve simply the top-down direction of attention towards an interoceptive sensation such as the heartbeat, breath, or stomach. Most perceptions of heartbeat sensations usually occur during a time of homeostatic perturbation, such as when the state of the body changes from external or internal influences such as physical exertion or elevated arousal states (e.g., riding a roller coaster, watching a scary movie, public speaking anxiety, or having a panic attack). For this reason, cardiac interoception is also sometimes studied by inducing perturbations of bodily state. This can be done pharmacologically using adrenaline-like drugs, such as isoproterenol, which mimic activation of the
sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...
, resulting in increased heart rate and respiration rate, similar to the "fight-or-flight" response. This approach provides a physiological basis for understanding psychiatric and
neurological disorder A neurological disorder is any disorder of the nervous system. Structural, biochemical or electrical abnormalities in the brain, spinal cord or other nerves can result in a range of symptoms. Examples of symptoms include paralysis, muscle weakn ...
s that are characterized by heightened
sympathetic nervous system The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of t ...
activity.


Respiratory and chemoreceptive system

Respiratory perception can differ from other interoceptive physiological symptoms because of an individual's ability to exert voluntary control over the system with controlled breathing or breathing exercises. This system is often measured using restrictive breathing loads and/or inhalation, which are designed to mimic labored breathing sensations. Dyspnea, or difficulty breathing, is a commonly felt sensation associated with panic attacks; however, due to the voluntary control of breathing, this domain of interoception usually requires implementation of much more elaborate experimental controls to quantify in comparison to cardiac interoception.


Gastrointestinal and genitourinary systems

Common interoceptive sensations related to the gastrointestinal and genitourinary systems are hunger and fullness. These are
homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism an ...
signals that tell an individual when to eat and when to stop eating. The dorsal mid-insula appears to be integral in taste processing during gastrointestinal interoceptive attention tasks. Rectal and bladder distensions are used as a method to perturb the homeostatic environment of the gastrointestinal and genitourinary systems, using placement of
balloon catheter A balloon catheter is a type of "soft" catheter with an inflatable "balloon" at its tip which is used during a catheterization procedure to enlarge a narrow opening or passage within the body. The deflated balloon catheter is positioned, then infl ...
s which can be inflated to achieve different stimulus intensities. Associative fear learning paradigms have been used to study how innocuous signals might lead to abnormal states of gastrointestinal hypersensitivity and
anxiety Anxiety is an emotion which is characterized by an unpleasant state of inner turmoil and includes feelings of dread over anticipated events. Anxiety is different than fear in that the former is defined as the anticipation of a future threat wh ...
.
Biofeedback Biofeedback is the process of gaining greater awareness of many physiological functions of one's own body by using electronic or other instruments, and with a goal of being able to manipulate the body's systems at will. Humans conduct biofeed ...
therapy has been used for individuals with impaired gastrointestinal interoception, showing positive outcomes for some patients.


Nociceptive system

Nociception Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the Somatosensory system, sensory nervous system's process of encoding Noxious stimulus, noxious stimuli. It deals with a series of events and processes required for an org ...
refers to the receiving and processing of pain inducing stimuli by the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
. Functional brain imaging studies during painful stimulation of the skin with heated probes, during mechanical compression, and electric shock have suggested that the insular cortex is prominently activated during pain processing. Thus while pain was once thought of as an exteroceptive sensation, based on functional imaging and anatomical evidence it is now understood that it has an interoceptive component.


Thermoregulatory system

Temperature and pain are thought to be represented as "feelings" of coolness and warmness and pleasantness or unpleasantness in the brain. These sensory and affective characteristics of
thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperatur ...
may motivate certain behavioral responses depending on the state of the body (for example, moving away from a source of heat to a cooler space). Such perturbations in the internal
homeostatic In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism an ...
environment of an organism are thought to be key aspects of a motivational process giving rise to emotional states, and have been proposed to be represented principally by the insular cortex as feelings. These feelings then influence drives when the
anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involve ...
is activated.


Endocrine and immune systems

The
endocrine The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...
and immune systems are necessary body systems that aid in allostasis and homeostatic control. Imbalances in these systems, along with other genetic and social factors, may be linked to interoceptive dysregulation in depression. These increased allostatic changes may cause a hyperawareness of interoceptive signaling and a hypo-awareness of exteroceptive signaling in depression patients.


Affective touch

Affective touch refers to the stimulation of slow, unmyelinated C tactile afferents. This is accompanied by a sense of pleasantness, and has been likened to other interoceptive modalities like thermoregulation and nociception because of the similarities in anatomical function. Soft touch activates the insula rather than the somatosensory cortex, indicating that it has an affective importance absent in Aβ fibers. Since soft touch utilizes a separate pathway, it may have a social relevancy, allowing the body to separate the "noise" of outward stimuli from stimuli that evokes an affective feeling.


Neuroanatomical pathways

Multiple neural pathways relay information integral to interoceptive processing from the body to the brain. these include the lamina I spinothalamic pathway, the visceroceptive pathway, and the somatosensory pathway.


Lamina I spinothalamic pathway

The lamina I spinothalamic pathway is commonly known for carrying information to the brain about temperature and pain, but it has been suggested to more broadly relay all information about the homeostatic condition of the body. * Afferent signals enter the spinal cord at the superficial layer of the dorsal horn * Second order neurons cross the midline of the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
and project up the opposite side, synapsing on the nucleus of the solitary tract,
parabrachial nucleus The parabrachial nuclei, also known as the parabrachial complex, are a group of nuclei in the dorsolateral pons that surrounds the superior cerebellar peduncle as it enters the brainstem from the cerebellum. They are named from the Latin term f ...
, and periaqueductal gray in the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...
* Third order neurons in the ventral posteromedial nucleus in the thalamus relay the signal to the dorsal posterior insula * The signal is re-represented on either the right or left side


Visceroceptive pathway

The visceroceptive pathway relays information about visceral organs to the brain. * Afferent signals from the
vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and righ ...
enter the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...
making synaptic connections with the nucleus of the solitary tract and
parabrachial nucleus The parabrachial nuclei, also known as the parabrachial complex, are a group of nuclei in the dorsolateral pons that surrounds the superior cerebellar peduncle as it enters the brainstem from the cerebellum. They are named from the Latin term f ...
* The signal is relayed to the ventromedial basal nucleus of the thalamus * Third order neurons send the signal to the posterior insula


Somatosensory pathway

The somatosensory pathway relays information about proprioception and discriminative touch to the brain through different receptors in the skin. * Afferent signals from the
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, a ...
s or proprioceptors enter the spinal cord at the dorsal root ganglia * Second order neurons cross the midline in the medulla, projecting up the opposite side and synapse on third order neurons in the
ventral posterior lateral nucleus The ventral posterolateral nucleus (VPL) is a nucleus of the thalamus. Together with the ventral posteromedial nucleus (VPM), ventral posterior inferior nucleus (VPI) and ventromedial posterior nucleus (VMpo), it constitutes the ventral posterior ...
or ventromedial posterior nucleus of the thalamus * Third order neurons in the thalamus relay the signal to the
primary somatosensory cortex In neuroanatomy, the primary somatosensory cortex is located in the postcentral gyrus of the brain's parietal lobe, and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and pa ...
in the brain


Cortical processing of interoception


Thalamus

The thalamus receives signals from sympathetic and parasympathetic afferents during interoceptive processing. The ventromedial posterior nucleus (VMpo) is a subregion of the thalamus which receives sympathetic information from lamina I spinothalamic neurons. The human VMpo is much larger than that of primates and sub-primates and is important for processing of nociceptive, thermoregulatory, and visceral sensations. The ventromedial basal nucleus (VMb) receives parasympathetic information from
visceral In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...
and gustatory systems.


Insular cortex

The insula is critically involved in the processing, integration, and cortical representation of visceral and interoceptive information. Lamina I spinothalamic and vagal afferents project via the brainstem and thalamus to the posterior and mid dorsal insula respectively. From there, information travels to the posterior and mid-insula, which combines visceral and somatosensory information. The insula is also activated during a variety of exteroceptive and affective tasks. The insula is considered to be a "hub" region because it has an extremely high number of connections with other brain areas, suggesting it may be important for an integration of lower-level physiological information and salience.


Anterior insular cortex

The anterior insular cortex (AIC) is involved in the representation of "cognitive feelings" which arise from the moment-to-moment integration of homeostatic information from the body. These feelings engender self-awareness by creating a sentient being (someone able to feel and perceive) aware of bodily and cognitive processing. Essentially all subjective bodily feelings are associated with activation in the anterior insula: interoceptive feelings come to awareness in the anterior insula. PET studies of cool feelings revealed that activiation that correlated with the subjective ratings of cool temperatures on the hand appeared first in the mid-insula, and then it was strongest in the right anterior insula and orbitofrontal cortex. Feelings from the body of heat pain or C-tactile affective touch also produce activation in the posterior, mid-, and anterior insula, and the strongest activation associated with these subjective feelings is in the anterior insula. One study of heat pain explicitly showed that activation in the dorsal posterior insula correlated with objective painful heat intensity, whereas activation in the right anterior insula correlated with subjective pain intensity ratings. Similarly, subjective ratings of flavors and tastes correlates strongly with activation in the left anterior insula. The available evidence supports the interpretation that feelings from the body is engendered in the anterior insula.


Cytoarchitecture and granulation

The insula contains three major subregions defined by the presence or absence of a granule cell layer: granular, dysgranular (slightly granulated) agranular. Each of these portions of the insular cortex are important for different levels of functional connectivity. Information from the thalamus is projected to all three regions. Those with increased granulation are considered to be capable of receiving sensory input.


Anterior cingulate cortex

The
anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involve ...
(ACC) plays a significant role in
motivation Motivation is the reason for which humans and other animals initiate, continue, or terminate a behavior at a given time. Motivational states are commonly understood as forces acting within the agent that create a disposition to engage in goal-dire ...
and the creation of
emotion Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definitio ...
. An emotion can be seen as comprising both a feeling and a motivation based on that feeling. According to one view, the "feeling" is represented in the insula, while the "motivation" is represented in the ACC. Many interoceptive tasks activate the insula and ACC together, specifically tasks that elicit strong aversive feeling states like pain.


Somatosensory cortex

The sensory motor cortex provides an alternative pathway for sensing interoceptive stimuli. Although not following the conventional pathway for interoceptive awareness, skin afferents which project to the primary and secondary somatosensory cortices provide the brain with information regarding bodily information. This area of the brain is commonly engaged by gastrointestinal distension and nociceptive stimulation, but it likely plays a role in representing other interoceptive sensations as well. In one study, a patient with bilateral insula and ACC damage was given isoproterenol as a method of exciting the
cardiovascular system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
. Despite damage to putative interoceptive areas of the brain, the patient was able to perceive his heartbeat with similar accuracy compared to healthy individuals; however, once
lidocaine Lidocaine, also known as lignocaine and sold under the brand name Xylocaine among others, is a local anesthetic of the amino amide type. It is also used to treat ventricular tachycardia. When used for local anaesthesia or in nerve blocks, lid ...
was applied to the patient's chest over the region of maximum cardiac sensation and the test was run again, the patient did not sense any change in heartbeat whatsoever. This suggested that somatosensory information from afferents innervating the skin outside of the heart may provide information to the brain about the heart's pounding through the somatosensory cortex.


Interoception and emotion

The relationship between interoception and emotional experience is an intimate one. In the late 19th century,
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
noted and discussed the involvement of sensations from the viscera by describing similarities between humans and animals reactions to fear in his book, ''The Expression of Emotions in Man and Animals''. Later,
William James William James (January 11, 1842 – August 26, 1910) was an American philosopher, historian, and psychologist, and the first educator to offer a psychology course in the United States. James is considered to be a leading thinker of the lat ...
and Carl Lange developed the James-Lange theory of emotion, which states that bodily sensations provide the critical basis for emotional experience. The somatic marker hypothesis, proposed by
Antonio Damasio Antonio Damasio ( pt, António Damásio) is a Portuguese-American neuroscientist. He is currently the David Dornsife Chair in Neuroscience, as well as Professor of Psychology, Philosophy, and Neurology, at the University of Southern California ...
, expands upon the James-Lange theory and posits that decisions and the ensuing behaviors are optimally guided by physiological patterns of interoceptive and emotional information. Ensuing models focusing on the neurobiology of feelings states emphasized that the brain's mapping of different physiological body states are the critical ingredients for emotional experience and
consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
. In another model, Bud Craig argues that the intertwining of interoceptive and homeostatic processes is responsible for initiating and maintaining motivational states and engendering human self-awareness.


Interoception and mental illness

Disturbances of interoception occur prominently and frequently in psychiatric disorders. These symptom fluctuations are often observed during the most severe expression of dysfunction, and they figure prominently in diagnostic classification of several psychiatric disorders. A few typical examples are reviewed next.


Panic disorder

Palpitations Palpitations are perceived abnormalities of the heartbeat characterized by awareness of cardiac muscle contractions in the chest, which is further characterized by the hard, fast and/or irregular beatings of the heart. Symptoms include a rap ...
and dyspnea are hallmarks of panic attacks. Studies have shown that panic disorder patients report a heightened experience of interoceptive sensations, but these studies have failed to clarify whether this is simply due to their systematic bias toward describing such feelings. However, other studies have shown that panic disorder patients feel heartbeat sensations more intensely when the state of the body is perturbed by pharmacological agents, suggesting they exhibit heightened sensitivity to experiencing interoceptive sensations.


Generalized anxiety disorder

Patients with generalized anxiety disorder (GAD) frequently report being bothered by interoceptive feelings of muscle tension, headaches, fatigue, gastrointestinal complaints, and pain.


Posttraumatic stress disorder

Functional neuroimaging studies have shown that
posttraumatic stress disorder Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threat ...
(PTSD) patients exhibit a decreased activation in the right anterior insula, a region of the brain that is largely responsible for identifying the mismatch between
cognitive Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought ...
and interoceptive states. Further, because PTSD patients have shown decreased activation within many nodes of the lamina I homeostatic pathway—a pathway through which the thalamus sends interoceptive information to the
anterior insula The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian br ...
and
anterior cingulate In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involved i ...
—it has been suggested that PTSD patients experience reduced interoceptive awareness. Approaches such as
somatic experiencing Somatic experiencing (SE) is a form of alternative therapy aimed at treating trauma and stressor-related disorders like PTSD. The primary goal of SE is to modify the trauma-related stress response through bottom-up processing. The Clients’ ...
use an interoceptive approach to treat PTSD.


Anxiety disorders

The broad consensus of studies investigating the link between interoceptive awareness and anxiety disorders is that people with anxiety disorders experience heightened awareness of and accuracy in identifying interoceptive processes. Functional imaging studies provide evidence that people with anxiety disorders experience heightened interoceptive accuracy, suggested by hyperactivation in the
anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involve ...
—a region of the brain associated with interoception—in several different kinds of anxiety disorders. The insula has been suggested to be abnormal in a large scale study across anxiety disorders in general. Other studies have found that interoceptive accuracy is increased in these patients, as evidenced by their superior ability in heartbeat detection tasks in comparison to healthy controls.


Anorexia nervosa

Anorexia nervosa Anorexia nervosa, often referred to simply as anorexia, is an eating disorder characterized by low weight, food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thin. ''Anorexia'' is a term of Gre ...
(AN) has been associated with interoceptive disturbances. Patients with AN often develop insensitivity to interoceptive cues of hunger, and yet are highly anxious and report disturbed interoceptive experiences, both inside and out. While AN patients concentrate on distorted perceptions of their body exterior in fear of weight gain, they also report altered physical states ''within'' their bodies, such as indistinct feelings of fullness, or an inability to distinguish emotional states from bodily sensations in general (called
alexithymia Alexithymia is a personality trait characterized by the inability to identify and describe emotions experienced by oneself. The core characteristic of alexithymia is marked dysfunction in emotional awareness, social attachment, and interpersonal ...
).


Bulimia nervosa

Studies suggest that patients with and recovered from
bulimia nervosa Bulimia nervosa, also known as simply bulimia, is an eating disorder characterized by binge eating followed by purging or fasting, and excessive concern with body shape and weight. The aim of this activity is to expel the body of calories eaten ...
(BN) exhibit abnormal interoceptive sensory processing and reduced interoceptive awareness under resting physiological conditions. Specifically, patients with BN report reduced sensitivity to many other kinds of internal and external sensations, exhibiting increased thresholds to heat pain compared to healthy subjects and an increased gastric capacity. Neuroimaging literature suggests a pattern of abnormal interoceptive processing in patients with BN based on increased activity and volume in the insula and
anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involve ...
—regions associated with interoception and taste processing—when looking at food.


Major depressive disorder

Major depressive disorder Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Intro ...
(MDD) has been theoretically linked to interoceptive dysfunction. Studies have shown that women with MDD are less accurate on heartbeat counting tasks than are men with MDD and that, in general, patients with MDD are less accurate at counting heartbeat than are patients with panic or anxiety disorders. However, patients with MDD do not always exhibit reduced cardiac interoceptive accuracy; depressed patients experiencing high levels of anxiety will actually be more accurate on heartbeat detection tasks than depressed patients with lower levels of anxiety.


Somatic symptom disorders

Patients with somatic symptom disorders score lower on heartbeat detection tasks than healthy controls, suggesting that interoceptive accuracy is poor in psychosomatic disorders. It has also been found that patients with
psychosomatic disorders Psychosomatic medicine is an interdisciplinary medical field exploring the relationships among social, psychological, behavioral factors on bodily processes and quality of life in humans and animals. The academic forebear of the modern field o ...
who are anxious or stressed report physical symptom discomfort at lower heart rates during exercise treadmill tests, implying poorer interoceptive distress tolerance in somatic symptom disorders with comorbid psychiatric conditions.


Obsessive compulsive disorder

Results from a study investigating the relationship between obsessive compulsive disorder (OCD) and internal body signals found that patients with OCD were more accurate on a heartbeat perception task than healthy controls and anxiety patients heightened interoceptive awareness.


Autism spectrum disorder

Patients with
autism spectrum The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...
disorder (ASD) may have poorer interoceptive awareness than control subjects. It is hypothesized that this decrease in interoceptive accuracy is due to
alexithymia Alexithymia is a personality trait characterized by the inability to identify and describe emotions experienced by oneself. The core characteristic of alexithymia is marked dysfunction in emotional awareness, social attachment, and interpersonal ...
, which is often associated with ASD. However, it has also been found that children with ASD actually show greater interoceptive sensitivity than ontrol subjectswhen measured over a long period of time. Further investigation into the relationship between interoception and ASD is needed in order to fully understand the interoceptive aspect of the disorder.


Current theories of interoceptive processing


Embodied predictive interoception coding (EPIC)

The EPIC model proposes a method of understanding the brain's response to stimuli contrary to the classic "stimulus-response" model. The classical view of information processing is that when a peripheral stimulus provided information to the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...
, it was processed in the brain, and a response was elicited. The EPIC model deviates from this and proposes that the brain is involved in a process of
active inference The free energy principle is a mathematical principle in biophysics and cognitive science that provides a formal account of the representational capacities of physical systems: that is, why things that exist look as if they track properties of the ...
, that is, assiduously making predictions about situations based on previous experiences. These predictions, when coupled with incoming sensory signals, allow the brain to compute a prediction error. Interoceptive prediction errors signal the occurrence of discrepancies within the body, which the brain attempts to minimize. This can be done by modifying the predictions through brain-related pathways, altering the body position/location in order to better align incoming sensory signals with the prediction, or altering the brain's method of receiving incoming stimuli. Interoceptive prediction error signals are a key component of many theories of interoceptive dysfunction in physical and mental health.


Research and treatments

As attention on interoception increases among the scientific community, new research methods and treatment tactics are beginning to emerge. Because there currently is not a clear consensus as to what exactly interoception is, or the best ways in which to measure it, most research and questionnaires on the subject only measure individual facets of interoception. The convergence and interrelatedness of the questionnaire items between the constructs has been found to be low. Ongoing research in interoception has shown the importance of perturbing interoceptive systems. This allows researchers the ability to document the effects of non-baseline states, which occur during times of panic or anxiety. It also provides the participant the ability to gauge the intensity of sensations within the body. This can be done through pharmacological interventions, balloon distensions, or respiratory breathing loads depending on the interoceptive system of interest. Another research method used to study interoception is specialized flotation environments.Oaklander, Mandy. "Float Hopes". ''Time''. Time, n.d. Web. 29 June 2017. Floating removes external stimuli so that individuals can more easily focus on the interoceptive sensations within their bodies. One idea with floating is that over many float sessions, patients with different kinds of disorders may learn to become more attuned or tolerant of their interoceptive sensations not only in the float tank but also in their everyday lives. Whole body
hyperthermia Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extrem ...
may provide a new treatment technique for
major depressive disorder Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Intro ...
. It is thought that reducing one of the bodily symptoms of depression, which is increased
inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
, using whole body hyperthermia will also reduce depressive feelings represented in the brain. In theory, these techniques will help patients better attune themselves to their interoceptive sensations, allowing them a better understanding of what occurs in their bodies. Acupuncture is an alternative treatment type for many people with anxiety and depression. Typically, it is self-prescribed by patients; however, results are inconclusive on its ability to manage symptoms of depression. Recently, Massage therapy has been shown to have the ability to reduce symptoms of generalized anxiety disorder.
Meditation Meditation is a practice in which an individual uses a technique – such as mindfulness, or focusing the mind on a particular object, thought, or activity – to train attention and awareness, and achieve a mentally clear and emotionally calm ...
and
mindfulness Mindfulness is the practice of purposely bringing one's attention to the present-moment experience without evaluation, a skill one develops through meditation or other training. Mindfulness derives from ''sati'', a significant element of Hind ...
have been looked into as possible techniques to enhance interoceptive awareness based on their tendency to redirect focus within oneself. Studies show that meditation and mindfulness practices promote attention to interoceptive sensations. In reality, with all the benefits of mindfulness, many may be ascribed to an increase in interoception. However, interoceptive awareness and accuracy in specific domains such as breath or body monitoring, may be more independent from a broad improvement in interoceptive accuracy. Meditation and mindfulness practices have been shown to modify the insula, which is considered central to our interoceptive abilities. Attention is also being given to the
subtle body A subtle body is a "quasi material" aspect of the human body, being neither solely physical nor solely spiritual, according to various esoteric, occult, and mystical teachings. This contrasts with the mind–body dualism that has dominated ...
and its function as a CNS map in traditional Tibetan and Indian medicine.Loizzo JJ. "The subtle body: an interoceptive map of central nervous system function and meditative mind-brain-body integration". ''Ann N Y Acad Sci.'' 2016 Jun;1373(1):78-95. . Epub 2016 May 10. . Although a universal definition of interoception has not been reached, research on interoception and
psychiatric disorders A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitt ...
has shown a link between interoceptive processing and mental disorders. It has been proposed that exposure therapy, a commonly employed treatment for
anxiety disorder Anxiety disorders are a cluster of mental disorders characterized by significant and uncontrollable feelings of anxiety and fear such that a person's social, occupational, and personal function are significantly impaired. Anxiety may cause phy ...
s, may provide a basis for a model of
interoceptive exposure Interoceptive exposure is a cognitive behavioral therapy technique used in the treatment of panic disorder. It refers to carrying out exercises that bring about the physical sensations of a panic attack, such as hyperventilation and high muscle ten ...
therapy that could be incorporated into treatment plans of different psychiatric disorders. One proposal states that multiple interoceptive challenges assessing different physiological systems could provide diagnosticians with the ability to create an "interoceptive profile" for a specific individual, creating a patient-specific treatment plan.


See also

*
Inflammatory reflex The inflammatory reflex is a neural circuit that regulates the immune response to injury and invasion. All reflexes have an afferent and efferent arc. The Inflammatory reflex has a sensory afferent arc, which is activated by cytokines, and a ...


References

{{reflist


External links


Interoception Library
at Center for Open Science
Laureate Institute for Brain Research Interoception Summit 2016
on YouTube Human body Human physiology Human anatomy