Atypically high interoception has been argued

Atypically high interoception has been argued to characterise both panic disorder and anxiety syndromes (Ehlers, 1993; Clark, 1999; Paulus, and Stein, 2006). Indeed, individuals with heightened rates of anxiety (and panic disorder) report greater awareness of bodily sensations (see Domschke et al., 2010 for a review) and show higher interoceptive sensitivity on heartbeat tracking measures than typical individuals (e.g., Eley et al., 2004; Ehlers and Breuer, 1992; Domschke et al., 2010). Whilst early theories suggested oversensitivity was a result of increased attention to bodily signals (Ehlers, 1993), more recent proposals suggest that carboxypeptidase anxiety symptoms arise from discrepancies between the individual’s actual and expected bodily state (i.e. greater prediction error; Paulus and Stein, 2006). Despite differing proposals regarding the process that drives oversensitivity to interoceptive information in anxiety disorders, the majority of theories suggest that oversensitivity, coupled with a bias to interpret bodily signals in a negative manner, contributes to both the cognitive (e.g., worry) and behavioural (e.g., avoidance) symptoms associated with these syndromes (Ehlers, 1993; Paulus, and Stein, 2006). The theoretical and empirical work described above demonstrates the clinical relevance of atypical interoception, and suggests that alexithymia is a marker of atypical interoception. Although further evidence is required, the relationship between alexithymia and atypical interoception may hold important implications for understanding psychopathology. Despite the current classification methods used to delineate psychiatric disorders into discrete categories, recent research indicates substantial overlap between disorder categories, so much so that one factor − the ‘p-factor’- appears to underlie a number of conditions that are presently assumed to be isolated entities (Caspi et al., 2014; Laceulle et al., 2015). The p-factor subsumes the previous two-factor structure of psychopathology which incorporated an internalising and externalising factor (e.g., Krueger et al., 1998) by demonstrating that both of these factors load onto a single factor. We have recently proposed that interoceptive ability constitutes the p-factor (for a more detailed discussion see Brewer et al., 2016a,b), based on evidence of interoceptive difficulties in a broad range of psychiatric conditions. For example, poor interoceptive sensitivity has been observed in individuals with depression (see Harshaw, 2015) and schizophrenia (Ardizzi et al., 2016). Likewise, high obsessive compulsive traits in non-clinical samples have been associated with reduced propensity to utilise internal bodily signals for gauging arousal (Lazarov et al., 2010). The contribution of atypical interoception to a number of other disorders including addiction (see Verdejo-Garcia et al., 2012; Naqvi and Bechara, 2010), eating disorders, somatic symptom disorders and obsessive compulsive disorder (see Khalsa and Lapidus, 2016; Brewer et al., 2015b; Brewer et al., 2016a,b; Stern, 2014) is also well-recognised. In contrast, atypically heightened interoceptive sensitivity is associated with anxiety and panic disorder (Paulus and Stein, 2006; Ehlers and Breuer, 1992). Alexithymia has been found to co-occur with a number of disorders − including all of the disorders loading onto the externalising factor described by Caspi et al. (2014), such as alcohol (Rybakowski et al., 1988) and Substance Abuse (Michael, 1990), and Conduct Disorder (Deborde et al., 2015), as well as conditions that load onto the internalising factor, such as Major Depressive Disorder (Honkalampi et al., 2000), Anxiety (Hendryx et al., 1991) Obsessive Compulsive Disorder (Grabe et al., 2006) and Schizophrenia (Van\'t Wout et al., 2007). Whilst a high pattern of co-occurrence is not unique to alexithymia, these findings, together with the association between alexithymia and poor interoception, are consistent with the idea that atypical interoception may be the ‘p-factor’ accounting for symptom commonalities between psychiatric disorders. Under this model atypical interoception would contribute towards the emotional difficulties (such as emotion regulation, expression, recognition, empathy, and emotional lability), learning and decision-making impairments, and sensory symptoms (including affective touch, pain, and somatic symptoms) seen across a number of diagnostic categories.