How the Heartbeat Influences Conscious Perception

Post by Flora Moujaes 

What's the science? 

The heart and the brain are two of the most important organs in the body. It is common knowledge that the brain influences the heart. For example, when we experience danger the heart beats faster. But does the heart influence the brain? A number of recent studies have shown that even though humans are mostly not aware of their heartbeats, the state of the heart (e.g. whether it is contracting or expanding) can influence their perception of external stimuli. This week in PNAS, Al et al. combine electroencephalography and electrocardiography to explore how and why the heartbeat influences conscious perception. 

How did they do it? 

To investigate how the heartbeat influences conscious perception, weak electrical stimuli were administered to either the left index or middle finger of 37 participants. After each stimulus, participants were asked if they could detect the electrical stimuli, and if so if they could identify which finger the stimuli had been administered to. Electrocardiography (ECG) was used to measure the electrical activity of the heart, and electroencephalography was used to measure a number of key brain responses including (1) The heartbeat-evoked potential (HEP) which indicates the brain’s response to the heartbeat and so is a marker of cardiac interoception; (2) The P300 response, which is related to conscious external somatosensory perception. 

What did they find? 

The brain’s attentiveness to the heartbeat influenced conscious perception. The researchers found that if the brain’s response to the heartbeat, measured using the HEP, was higher, people were less able to detect the stimulus. The researchers explained that this might be due to the fact that the brain divides its attention between external signals in the environment and internal signals within the body. If the brain is more focused on internal signals such as the heartbeat, it may then be less aware of external signals from the outside world. This is further evidenced by the fact that higher HEPs were followed by decreases in brain responses related to conscious external somatosensory perception (e.g., P300). 

The timing of the stimulus relative to the cardiac cycle influenced conscious perception. The researchers also found that if the stimulus was presented during systole when the heart was contracting, participants were less able to detect or localize it. This was also reflected by the fact that if stimuli were presented during systole there was a decrease in the P300 response: the brain response related to conscious external somatosensory perception. This indicates that during systole, external perception becomes less sensitive. The researchers understood this finding using a predictive coding framework, which posits that the brain is continuously producing and updating a model of sensory input. In the predictive coding framework, the heartbeat is treated as a predictable event and so is largely ignored by the brain. As the human finger contains a lot of blood vessels, weak neutral external stimuli at the finger may be mistaken as heartbeat-pulse associated noise and ignored by the brain. 

What's the impact? 

Overall this study shows that both the timing of the stimulus relative to the cardiac cycle and the brain’s attentiveness to the heartbeat influence the conscious perception of external stimuli. The researchers also offer an explanation for why the heartbeat may influence conscious perception: spontaneous shifts between interoception and exteroception may affect our conscious perception, while the predictive coding mechanism that allows the brain to ignore predictable events such as the heartbeat may result in similar stimuli also being ignored. These results are important as they further our understanding of how body-brain interactions can shape our cognition.

Al et al. Heart–brain interactions shape somatosensory perception and evoked potentials. PNAS (2020). Access the original scientific publication here.