The body intervenes: How active inference explains depression's clinical presentation.

The low mood that characterises depression is accompanied by changes in bodily processes, manifested in symptoms such as insomnia, reduced appetite and fatigue. The active inference framework provides an explanation as to how mood-related symptoms are linked. It suggests that affective experiences arise from predictions about interoceptive states and their corresponding prediction errors, with the relative influence of each modified by precision weighting. Moods reflect long-term predictions about the state of the body, incorporating parameters related to sleep, appetite and energy levels. Depression emerges from the interplay between reduced confidence in long-term prospects and heightened expectation of shorter-term negative affect, which sees a re-weighting of the precision of interoceptive prediction errors. The ensuing bodily changes contribute to the emergence of depressed mood; and underpin disturbances in shorter-term interoceptive predictions and the experience of emotions such as anxiety and irritability. This framework details how interoceptive processes shape the phenomenological and symptomatic experience of depression, helping us to understand the disorder's multifaceted and often idiosyncratic clinical presentation, and with implications for the way we understand and treat depression and its co-morbidities.