interoception interoception interoception is the collection of senses understanding 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, thalamus, insula, somatosensory, and anterior cingulate cortex—allowing for a nuanced representation of the physiological state of the body. this is important for maintaining homeostatic conditions in the body and, potentially, facilitating self-awareness. interoception is involved in many different physiological systems like the cardiorespiratory system, gastrointestinal system, nociceptive system, endocrine and immune systems. 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 this graph shows the number of articles that reference the term "interoception" specifically from 1905 to 2015. a clear increase in the number of publications per year on the topic is seen in the 21st century. 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. 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 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 and self-awareness. for example, interoception is the fundament of the modern view on allostasis and allostatic load. 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". 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 this table defines many of the different facets or components 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 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" 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.this graph shows the number of articles published on interoception vs. articles published on interoceptive facets without directly referencing the term from 1905 to 2015. the timeline starts one year before the publication of charles sherrington's book, the integrative action of the nervous system, which introduced "interoception". interoceptive physiology cardiovascular system this table illustrates the most common tasks that are used to assess cardiovascular interoception, broken down by the corresponding facets that they test. the exemplars at the bottom refer to specific studies or researchers who developed the task. 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, 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 disorders that are characterized by heightened sympathetic nervous system 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 co2 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.