1 THE AUTONOMIC PHYSIOLOGY OF TERROR MANAGEMENT ...

systems predominate over the older systems (Porges, 1998). Thus, withdrawal of the
mammalian vagal system may potentiate both the reptilian vagal freezing response to
threat, and sympathetic mobilization responses to threat. From this Polyvagal Theory, the
mammalian vagus seems in general to buffer against threat responses, whether mediated
by the reptilian vagus or the SNS.
Measurement of Parasympathetic Activity
Innovation in measuring PNS activity in humans has occurred relatively recently,
within the past 20 years, and is having a substantial influence on the field of
psychophysiology. Specifically, the level of parasympathetic influence at the heart,
termed vagal tone, can be estimated by obtaining an electrocardiogram (EKG) signal
generally of two minutes or more. However, vagal tone cannot be ascertained by HR
alone. More vagal outflow slows HR, but one cannot assume that a slowed heart always
corresponds to increased vagal input. Heart rate may also slow due to SNS withdrawal. If
the SNS and PNS always acted and responded reciprocally, we could accurately infer
vagal and SNS changes from HR changes. For example, a HR increase would always
signal vagal withdrawal and an increase in SNS activity. But as mentioned, the PNS and
SNS do not necessarily work in a reciprocal fashion (Bernson, Cacioppo, & Quigley,
1991; Bernson, Cacioppo, & Quigley 1993).
A method has been derived, however, to extract PNS influence from the EKG
signal. Specifically, respiratory sinus arrhythmia (RSA), extracted from EKG data,
provides this vagal tone index. Respiratory sinus arrhythmia is the variability in interbeat
intervals due to breathing. As Porges (1992) describes, the reason vagal influence can be
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