Phase confusion: how inconsistent cardiac labelling obscures interoception research

Research on the interaction between interoception and external sensory processing has grown rapidly, with a key focus on how different phases of the cardiac cycle (systole and diastole) affect stimulus processing. This typically involves synchronizing stimulus presentation with specific cardiac phases. However, the definition of these phases varies significantly across studies. As in biomedical disciplines, some studies define systole as the period from the R-peak to the end of the T-wave in the electrocardiogram (ECG), with diastole encompassing the rest of the cycle. Others use a time-based heuristic approach, defining systole based on the estimated time for afferent signals from the heart to reach the brain. These studies assume that baroreceptor activity near the heart plays a key role in cognition, though conclusive evidence for this assumption is lacking. This variation in definitions has led to a confusing characterization of cardiac phases, where systole in one study may correspond to diastole in another (and vice versa). To address this, we review current research on how cardiac phases affect external stimulus processing and highlight methodological inconsistencies in their operationalization. We debate the use of time-based heuristic approaches and discuss additional sources of variability, such as differences in recording equipment and measures of interoceptive abilities. Rather than focusing on specific study findings, we emphasize the broader challenges posed by these inconsistencies, including issues with replicability and interpretation. Finally, we provide recommendations for standardizing methodologies to improve the reliability and comparability of future research.