Abstract A subtle change in heart rate variability might indicate ongoing cognitive processing.

Perhaps the most stunning example of brain-heart connection is the phenomenon of “broken heart” syndrome, when heartbreaking news leads to sudden worsening of cardiac function and acute heart failure. The brain-heart connection, mediated by the autonomic nervous system, has been an intense focus of research. Raimondo et al. exploit this connection to study conscious processing. By using a simple series of auditory beats and simultaneous analysis of electroencephalographic (EEG) activity and heart rate variability, the authors could differentiate between patients in a vegetative state and those with minimal consciousness with some ability to improve.

The study examined 70 vegetative or unresponsive wakefulness state (VS/UWS) patients and 57 patients in minimally conscious state (MCS). Patients in VS/UWS are characterized by wakefulness without conscious response to sensory stimulation. In contrast, patients in MCS—although with profound alterations of consciousness—have some remnant of nonreflexive movements to stimuli. The experimental paradigm used EEG and electrocardiogram (EKG) recordings while an auditory stimulus was played; a cycle of five equal tones was played followed by a cycle composed of the same four tones and a slightly different fifth tone. This slight alteration in the last tone promotes cognitive processing. The study found that MCS patients had a shift in their cardiac cycle associated with the tone change, which was not present in patients in VS/UWS. When EKG data in response to the stimuli were added to EEG measures, prediction of the clinical state was improved. In summary, an almost trivial unexpected auditory change triggers an autonomic response to change the heart rate variability that can be used to help differentiate between levels of consciousness. More in-depth studies to address the clinical/cognitive meaning of a more intact brain-heart interaction may be an avenue for further research into studies of consciousness.