Heartbeat-locked auditory deviations slow down cardiac activity
Our responses to environmental inputs depend on the variations of our own physiological activity. However, the mechanisms by which the integration of sensory information with interoceptive signals shape bodily responses to external events remain debated. In this pre-registered study, we hypothesized three possible mechanisms underlying such exteroceptive-interoceptive integration: cardiac surprise, active inference, and dynamic coupling. To test them, we implemented a closed-loop stimulation procedure to play auditory deviations from sequences either synchronized or not with heartbeats which varied in type (omissions or rare tones) and predictability (random or regular intervals). First, we replicated previous findings that cardiac activity slows down in response to sound omissions only when sounds are synchronized with heartbeats. Second, we showed that this effect extends to rare tones, excluding the dynamic coupling hypothesis. Third, we demonstrated that these responses do not depend on the predictability of auditory deviations, excluding both cardiac surprise and active inference hypotheses. In a control experiment, we further observed that behavioral responses depend on the type and predictability of auditory deviants: participants can discriminate subjectively which sounds were synchronized with their own heartbeats without evidence of a relationship to interoception nor cardiac responses. Overall, these results demonstrate that auditory deviations slow down cardiac responses when locked to heartbeats but independently from their type and regularity, calling for novel hypotheses to account for the interoceptive-exteroceptive integration of sensory signals into cardiac activity. Impact statementUsing a cardio-audio synchrony task, we show that cardiac responses slow down upon heartbeat-locked auditory deviations independently from their type or regularity, suggesting a simple, fundamental mechanism of integration of internal and external signals into bodily responses to the environment. The heart may provide a straightforward way to study basic self-related processes, without depending on behavior or self-report, which is especially valuable for individuals who are unable to respond or communicate.