Whither vagal tone.

Measures of respiratory sinus arrhythmia (RSA) have been widely applied in the physiological and psychophysiological literature as an index of vagal control of the heart. Despite an extensive literature, however, differences in interpretation remain within the field. A guiding conception for several contributions in this issue is the notion of separate brainstem centers involved in parasympathetic control, with distinct evolutionary origins and significance and with divergent influences on cardiac vagal tone as it relates to psychological and behavioral processes. We here consider the biological foundations for this view, discuss some methodological and interpretative caveats for RSA applications and offer suggestions for further development of the field.