This led to the development of a method that quantified respiratory sinus arrhythmia as an accurate index of cardiac vagal tone. Respiratory sinus arrhythmia is the functional manifestation of vagal influences on heart rate as described by Hering. The breathing-related changes in vagal influence on the heart are manifested as rhythmic increases and decreases in beat-to-beat heart rate, with greater vagal influences producing greater differences in the rhythmic increases and decreases. Respiratory sinus arrhythmia is a functional index of a neural feedback loop that dynamically adjusts the inhibitory influence of the vagus on the heart’s pacemaker. The feedback system has inputs from the lungs and heart going up to the brainstem and also projections from higher brain areas down to the brainstem. The output parameters of the feedback system provide measures of amplitude and frequency. The amplitude is a manifestation of vagal influence, and the periodicity reflects respiration rate.
our cognitive evaluations of risk in the environment, including identifying potentially dangerous relationships, play a secondary role to our visceral reactions to people and places
Within Polyvagal Theory, the neural process that evaluates risk in the environment without awareness is called neuroception (Porges, 2003, 2004