Respiratory phases modulate heartbeat-evoked cortical responses

Heartbeat Evoked Responses (HERs) are EEG event-related potentials time-locked to the heartbeat that reflect the cortical processing of cardiac activity. HER amplitude has been related to various behavioural measures of interoception, but the reliability of such findings is controversial, given possible confounding effects driven by the electric cardiac field, as well as by other somatosensory or interoceptive factors. One of such interoceptive factors may be the respiratory phase. In fact, despite recent studies demonstrating optimized neural processing of incoming exteroceptive (sensory) information during inhalation, virtually nothing is known about mechanisms through which respiratory activity influences incoming interoceptive (cardiac) information. We simultaneously recorded 64-channel EEG, cardiac and respiratory activity in 20 healthy volunteers during an eyes-open resting state. We extracted HERs detected during inspiratory (when respiratory activity is at maximum) and post-expiratory (when respiratory activity is at a minimum) phases. In order to reduce cardiac field artefacts, HERs were time-locked to the T-peak and pruned with ICA-based procedures. HERs were compared performing a repeated-measures, two-tailed cluster mass permutation test (10 000 permutations), including all time points between 80 and 350 ms post T-peak. We found that respiratory phases modulate HER amplitude, which was significantly higher during post-expiratory phases as compared to inspiratory phases, in a time window ranging from 176ms to 254ms post T-peak. This “respiratory HER effect” was detected in central and parietal areas known to be involved in various experimentally-induced HER modulations. Present finding indicates an often-unnoticed influence of respiration on cardiac interoception, suggesting increased neural processing of the heartbeat during post-expiratory phases, when respiratory interoceptive afference is at a minimum, as compared to inspiratory phases. Finally, starting from recent studies that independently showed that cardiac interoceptive accuracy increased during breath-holding and is predicted by HER amplitude, we conducted another study investigating if: i) the performance on the heartbeat tapping task changes depending on the specific respiratory phase, and ii) these changes correlate with the “respiratory HER effect”.