Interoception refers to the process by which the nervous system senses and integrates signals originating from within the body, providing a momentary mapping of the body’s internal landscape and its relationship to the outside world. Active inference is based on the premise that afferent sensory input to the brain is constantly shaped and modified by prior expectations.
In this review, the authors propose that interoceptive psychopathology results from two primary interoceptive dysfunctions: First, individuals have abnormally strong expectations of the situations that elicit bodily change (i.e., hyperprecise priors), and second, they have great difficulty adjusting these expectations when the environment changes (i.e., context rigidity).
These dysfunctions potentially manifest in mental illness. Interventions aimed at altering interoceptive processing can help the brain create a more realistic model of its internal state.
It’s not the strongest species that survive, nor the most intelligent, but the most responsive to change.
— Leon C.Megginson on Sir Charles Darwin’s On the Origin of Species
Traditionally, interoception has been considered to be a one-way street in which bodily signals traveling to the brain cause sensation and elicit regulatory responses when bodily homeostasis is disrupted. More recently, interoception has been adopted into the conceptual framework
of active inference, which is based on the premise that afferent sensory input to the brain is constantly shaped and modified by the individual’s expectations. Thus, interoception can be conceptualized as a bidirectional process between the brain and the body, with feedback and feedforward loops, and as a rich internal model aimed at predicting future states of the body.
Homeostasis and allostasis are important constructs in the context of interoception.
Homeostasis is a dynamic process by which a state of balance is maintained according to a predetermined set point. The body is constantly engaged in maintaining a multitude of homeostatic interoceptive set points, the majority of which never reach conscious awareness. The active processes responsible for maintaining this balance are not readily apparent, giving rise to a misleading sense of quiescence under resting physiological conditions.
Allostasis is a proactive process aimed at minimizing energy costs by adaptively anticipating future needs of the body. An allostatic load occurs when there is a persistent deviation that continuously engages this adaptive process. It can sometimes lead to allostatic stress, whereby the body’s recovery mechanisms fail to adequately compensate for the increased burden, leading to a new and potentially harmful set point.
Based on the notion of active inference, the conceptual models to better understand and measure interoception suggest that interoceptive perceptions are dynamically constructed by the brain, meaning that interoception is the result of an iterative process of comparing the brain’s expectation of sensation with the incoming sensation.
Active inference starts with the premise that the perceptual process is an interaction between the brain’s model of what is to be expected and its comparison to the actual sensory evidence. The goal of this process is to generate the most accurate model of the world to help guide the most adaptive behavior, despite the inherent uncertainties of nature. Perception is not what we sense but a computational compromise between our expectation of what we believe we should be sensing and the actual sensation experienced. Thus, perception emerges from processing the external or internal world within the context of a prior model.
At any point in time an individual has a number of models in mind, and for each of these models the individual holds a belief as to how likely this model is to be correct, that is, to be a true representation of the world. Interoceptive afferences provide the brain with sensory evidence for what is happening inside the body and are evaluated relative to a given expectation (model). Perception changes future expectations, meaning that as a consequence of processing these afferences, the individual’s beliefs about the probabilities of the different models will change.
The manner in which these changes occur is best described in
probabilistic terms according to Bayes’ rule:
P(that a certain model is correct given what has been sensed) =
P(what has been sensed given a certain model) ∗ P(of a certain model) /
P(what has been sensed given all available models)
An important aspect of Bayes’ rule is that model and evidence contribute to the perception as a function of their precision, which can be defined as the certainty with which a model is believed to be true and the certainty of a particular afferent given an expectation.
For example, a person at home experiencing a tap on their shoulder by their partner might have a strongly positive perception, whereas the same experience in a dark alley might be perceived as frightening. However, a tap on their shoulder in a crowded space when waiting for their partner might elicit a much more complex perception based on the certainty with which the person believes that their partner should be present.
Thus, the perceptual process updates the expectations for future perceptions.
In that sense, perception is the iterative process of updating models with evidence from the inside or outside world. If the sum of the evidence provides information that is different from the model, the brain generates a prediction error. For example, a tap on the shoulder by a burglar at home would generate a strong prediction error. In the context of interoceptive afferences, we have previously called this a somatic error or a body prediction error that serves as a signal to the brain to adjust the underlying model.
It is important to recognize that the degree of adjustment is related to the precision of the model and the precision of the evidence. Specifically, if the model is highly precise (i.e., the individual holds a strong expectation about the state of the inside or outside world and does not believe that there are alternative models of the world that can properly account for the afferent signal), then even in the presence of somatic error the model will be adjusted only slightly. In comparison, if the evidence is not very precise (i.e., it is not clear what the afferences are signaling), the model will not be strongly adjusted even if it differs significantly from the evidence.
It has been hypothesized that the central nervous system implements active inference perceptual processing hierarchically and bidirectionally. This means that models exist at different levels, and models at a lower level of the hierarchy serve as evidence for models at a higher level. In return, higher-level models modify the expectations of the lower-level ones. For example, an accelerated heartbeat may serve to increase the expectation of arousal, which in turn could provide evidence for fear or excitement, depending on the context. Each of these models is thought to consume energy, a finite resource, leading to competition within the organism for an optimal trade-off between model complexity, accuracy, and fitness—that is, a weighing of the model’s accuracy in representing the world against the energy spent to entertain it. Minimization of this energetic cost, based on the free energy principle proposed by Friston, aims to provide the computational explanation for how the brain optimizes (selects) perceptions in the presence of multiple expectations and models.
Probability (that a certain model is correct given what has been sensed)
Bayes’ rule
=
Probability (what has been sensed given a certain model)
∗ Probability (of a certain model)
__________________________________________________
Probability (what has been sensed given all available models)
A consequence of active inference models is that certain psychiatric disorders, especially those characterized by chronic and unrelenting anxiety, are preferentially susceptible to top-down constructed dysfunctions; that is, they are the consequences of a persistent mismatch between predicted body states and afferent signals from the body.
In an adaptive individual, corrective action in the presence of somatic error can be achieved by adjusting expectations (priors) to match the current physiological state or by engaging in regulatory actions that change the afferent signal, leading the current physiological state to conform more closely with expectations.
In a nonadaptive person, persistent somatic errors are hypothesized to be important for generating feeling states aimed at motivating actions toward homeostasis. For example, a persistently hyperprecise prior of expecting a bear instead of a rabbit (Figure 1) may lead the individual to experience anxiety that motivates withdrawal or avoidance—that is, the person might withdraw from the environment associated with these overly strong beliefs. Second, context rigidity (Figure 2), or the lack of the ability to adjust expectation as a function of context, may contribute to the persistent experience of somatic error because the individual does not adjust prior beliefs about different models in a new environment, a hypothesis that can be tested with a hierarchical Gaussian filter approach using separate parameters for precision, prediction errors, and mean expectations on different temporal hierarchies.
This interoceptively focused model of psychopathology is predicated on the assumption that a primary function of the nervous system is to develop accurate models of the world in an effort to establish conditions ideal for optimizing bodily functioning. Corrective action failure results in homeostatic dysregulation, which can manifest as sudden changes in symptom intensity, mislabeling of symptoms, a predominant focus on bodily signals in daily life, extreme hypervigilance toward disturbed body states, aberrant self-related thinking patterns, and attempts to reduce these aversive experiences via avoidance or escape behaviors.
Interoceptive attention:
the process of observing internal bodily sensations that is either goal directed (top down) or driven by bodily signals (bottom up)
Interoception is essential for maintaining a homeostatic balance between the brain and the body in a dynamically changing world.
If Darwin’s thesis is true, and the survival of the species depends on the ability to adaptively respond to change, then the active inference framework described here highlights two potential failures of adaptation: hyperprecise priors and context rigidity.
Both create an insensitivity to change, leading to persistent somatic error, unsustainable allostatic load, maladaptive corrective actions, and eventually psychopathology.
Surtil : 