The difficulties in defining interoception are reflected in

The difficulties in defining interoception are reflected in its characterisation and measurement. For example, it has been suggested that individual differences in interoceptive ability should be considered on three dimensions rather than one: objective interoceptive sensitivity (the degree to which an individual can accurately perceive the state of their body); subjective interoceptive sensibility (an individual’s beliefs about their interoceptive accuracy/sensitivity); and interoceptive awareness (a metacognitive measure which reflects the degree to which an individual’s sensibility accurately reflects their sensitivity; Garfinkel et al., 2015a; see Table 1: Glossary); with interoceptive sensitivity serving as the core construct. It is worth noting that these dimensions generally refer to explicit interoception (conscious perception of, or beliefs about, one’s internal state), but that interoception can also be implicit, for example during homeostasis, when subconscious perception of internal states allows regulation of the bodily state, or when subconscious perception of internal states alters behavioural, neural or bodily responses in the absence of conscious awareness. It is clear then that individual differences in ‘interoceptive ability’ may be a product of, 1) the interoceptive signal itself (e.g. there may be individual differences in the extent to which individuals become aroused, meaning that for some individuals there is a weaker interoceptive signal to be perceived), 2) differences in the transduction of the interoceptive signal or its transmission to the central gap junctions (and there may be developmental influences on this process; Feng et al., 2013), 3) the degree to which unconscious perception of interoceptive states impacts on bodily states, neural activity, and ongoing cognition (Azevedo et al., 2016a; Martins et al., 2014; Suzuki et al., 2013; Garfinkel et al., 2013, 2014; Fiacconi et al., 2016; Gray et al., 2009, 2010, 2012; which we refer to as ‘implicit interoception’ within Expression vector paper), or 4) the degree to which individuals can consciously perceive, and recognise/differentiate, interoceptive signals (which we refer to as ‘explicit interoception’; see Table 1 for a Glossary). Measurement of explicit interoceptive sensitivity has relied almost exclusively on tasks assessing heartbeat perception, typically heartbeat tracking and heartbeat discrimination tasks (e.g., Schandry, 1981; Katkin et al., 1983; Whitehead et al., 1977). In the former, participants are required to count their heartbeats over a specified interval and their count is compared to the actual number of heartbeats in that period. In the latter, participants hear two auditory stimuli, one in-phase with their heartbeat and one slightly delayed, and are required to indicate which signal is in-phase. While the reliability of these tests has been well-established (e.g., Brener and Kluvitse, 1988; Jones, 1994; Wildman and Jones, 1982), several factors affect their suitability for research. First, the tests are extremely insensitive at lower ability levels; approximately 30% of typical, healthy individuals have no conscious awareness of their heartbeat at all (Khalsa et al., 2009a). This insensitivity makes them ill-suited to index interoceptive ability in populations that may be characterised by reduced interoceptive ability caused by ill health or developmental stage. Second, heartbeat may be perceived via (exteroceptive) touch receptors due to the vibration of the chest wall (Khalsa et al., 2009a,b). The degree to which the heartbeat may be perceived via this route depends on factors such as the percentage of body fat (Rouse et al., 1988), systolic blood pressure (O\'Brien et al., 1998) and resting heartrate and heartrate variability (Knapp-Kline and Kline, 2005). Again, all of these factors may change as a function of developmental stage (Umetani et al., 1998; St-Onge, 2005; Franklin et al., 1997; Yashin et al., 2006).