Interoceptive inference: homeostasis and decision-making.

Body In a recent article in TICS [1], Seth makes the compelling claim that interoceptive inference (i.e., the approximate Bayesian inference about internal bodily states) underlies body ownership and selfhood. In this letter, we argue that the significance of interoceptive inference extends beyond this. In particular, we emphasise the role of interoceptive inference in both homeostasis and allostasis (i.e., the process of achieving homeostasis) [2], and how this role grounds decision-making and motivated behaviour, when contextualised in the setting of hierarchical active inference. To persist over time, organisms must restrict themselves to a small subset of possible biophysical states [3]. Under the free energy principle [3], this homeostasis arises because organisms minimise surprise, conditioned on their model of the world. In other words, they seek out the states they expect to occupy, where these ‘familiar’ states are innately valuable [3]. There are two ways to achieve this: first, through homoeostatic control of the internal milieu of the agent via autonomic responses [1,4,5]; and, second, through allostatic actions on the external world [2,3] (Figure 1). Consider an agent whose blood sugar level falls below a certain threshold to an undesirable or surprising level. In this situation, homeostasis can be maintained through the metabolism of bodily fat stores (when food is not available), or consummatory behaviour (when food is available). In real-life situations, both autonomic reflexes and somatic reflexes conspire to maintain homeostasis. Crucially, both processes can also be explained as surprise or prediction-error minimisation, albeit acting through different mechanisms. We argue that interoceptive inference is not only involved in the first (homeostatic) process, but also participates in the latter (allostatic) process by informing value-based choices about the internal state of the body. We believe this notion can explain the key role of the insular cortex in decision-making [6]. A popular taxonomy considers the representation of internal and external states as a prelude to decision-making, followed by valuation, action selection, outcome evaluation, and, finally, learning and information updating [7]. The insular cortex