Phase confusion: how inconsistent cardiac labelling obscures interoception research

Research on the interaction between interoception and external sensory processing has expanded rapidly, particularly in how stimulus processing is influenced by concurrent cardiac phases (systole and diastole). However, definitions of these phases vary widely. Some studies follow biomedical conventions, defining systole as the period from the R-peak to the end of the T-wave in the electrocardiogram (ECG), with diastole encompassing the remainder of the cycle. Others adopt a time-based heuristic approach, defining systole based on the estimated arrival of afferent cardiac signals in the brain. This approach assumes that baroreceptor activity plays a key role in cognition, though conclusive evidence for this is lacking. This variability leads to inconsistencies, where systole in one study may correspond partly or entirely to diastole in another. To address this, we first review studies examining how cardiac phases modulate stimulus perception. Second, we highlight methodological inconsistencies in defining cardiac phases, particularly in time-based heuristic approaches. Third, we discuss additional sources of variability, such as differences in recording equipment and measures of interoceptive ability. Rather than focusing on specific findings, we highlight broader challenges in replicability and interpretation. Additionally, we introduce the HEARTS framework, a set of guidelines for standardising methodologies in interoception research. This consists of six recommendations: Harmonizing physiological terms across fields, Establishing clear nomenclature, Avoiding selective reporting, Refining mechanistic understanding, Tailoring methods for precision, and Standardizing study comparability. Implementing these recommendations can improve methodological consistency, cross-study reliability, and interdisciplinary collaboration, ensuring a more accurate understanding of how cardiac interoceptive signals shape perception and cognition.