Commonly used methods for assessing cardiac interoceptive accuracy have been criticised for

assuming that all individuals perceive their heartbeat at the same delay following contraction of

the heart, despite evidence for notable variability across individuals. However, it remains unclear

whether some individuals perceive their heartbeat at a particular phase of their cardiac cycle –

that is, at a relative point in the cycle that may vary in absolute timing depending on heart rate -

rather than at a specific delay. Identification of all heartbeat perceivers is critical for accurate

measurement of cardiac interoceptive accuracy, individual differences in which are theorised to

play a role in several aspects of higher-order cognition as well as health and wellbeing. In the

current study, data from 526 participants who completed the Phase Adjustment Task (PAT) as a

measure of cardiac interoceptive accuracy were examined. In this task, participants are asked to

adjust a virtual dial until tones appear synchronous with their heartbeats. Data were analysed

using a novel framework that allows differentiation between delay-based and phase-based

response patterns. Of 76 interoceptive individuals identified, 21% (N = 16) demonstrated

response patterns consistent only with phase-based responding. These novel findings challenge

current assumptions regarding individual differences in the perception of heartbeats, and suggest

that many commonly used measures may underestimate the true proportion of heartbeat

perceivers.