Vagal tone is an internal biological process referring to the activity of the vagus nerve, the tenth cranial nerve located in the medulla oblongata of the brainstem. The vagus nerve serves as the key component of the parasympathetic branch of the autonomic nervous system, regulating the homeostasis (or “resting state”) of the majority of the body’s internal organ systems that operate on a largely subconscious level, such as the heart, lungs, eyes, glands and digestive tract. Due to the regulatory nature of the parasympathetic nervous system (PSNS), vagal activity is continuous, chronic, and passive (“tone” in this usage is analogous to “tension”).
In the context of psychophysiological research, vagal tone (and specifically its influence on heart rate) represents an index for the functional state of the entire parasympathetic nervous system. Heart rate is normally controlled by multiple centers in the brainstem; one of these centers, the nucleus ambiguus, increases parasympathetic nervous system input to the heart via the vagus nerve. Vagal tone decreases heart rate by inhibiting the firing rate of the sinoatrial node (S-A node, the "pace-maker" tissue of the heart). The absolute level of cardiac vagal activity or vagal tone appears to result from the excitatory drive from peripheral baroreceptors. In animals, cardiac vagal activity disappears at very low pressures or if the afferent nerves are cut. Thus, a major determinant of resting heart rate is the beat to beat activation of the arterial baroreflex with each cardiac cycle. Cardiac vagal tone has been treated as a physiologic substrate of regulation of emotion and arousal.
Relation to respiratory sinus arrhythmia
Respiratory sinus arrhythmia (RSA) is a naturally occurring variation in heart rate that occurs during a breathing cycle. RSA is also a measure of parasympathetic nervous system activity - which denotes "rest and digest" behaviors.
Vagal tone cannot be directly measured. Instead, other biological processes are measured that represent the functionality of vagal tone. An increase in vagal tone both slows the heart and makes heart rate more variable (i.e. there is more beat-to-beat change between heart beats). During the process of RSA inhalation temporarily suppresses vagal activity, causing an immediate increase in heart rate. Exhalation then decreases heart rate as it causes vagal activity to resume. Thus, while vagal tone is not explicitly measured, the resultant changes in heart rate are. This is done by measuring periodic changes in the heart rate during a resting state of cardiovascular activity, a process known as heart rate variability (HRV). There are about 40 published methods of quantifying HRV, but the vast majority of studies use only a few of the available measurements. Most common is high-frequency HRV, a measurement of the amount of heart rate variability there is between typical breathing cycles (approximately 2.5 to 6.7 seconds).
On an electrocardiogram ECG, RSA is seen as subtle changes in the R-R interval (time between two of the distinctive, large, upward "R" spikes on an electrocardiogram) synchronized with respiration. The R-R interval on an ECG is shortened during inhalation and prolonged during expiration. Meditation and relaxed breathing techniques can temporarily alter RSA.
RSA is very pronounced in children, but without sufficient cardiovascular exercise it gradually disappears as a person approaches their teenage years. Typically, expression of RSA decreases with age; however, adults in excellent cardiovascular health, such as endurance runners, swimmers, and cyclists, are likely to have a more pronounced RSA. Professional athletes typically maintain very high vagal tone and RSA levels. RSA also becomes less prominent in individuals with diabetes and cardiovascular disease.
In general, young children with a high vagal tone level present more positive psychophysiological, behavioral, and social performance, as well as predictive outcomes in mental, motor, and social skills. Heart rate variability has been linked to a wide array of variables, including: behavioral inhibition, sympathy, instrumental coping, attention, and temperament. While a higher vagal tone is typically indicative of more adaptive functioning, newborn infants with high baseline levels of vagal tone were found to be highly reactive and irritable. By the age of five to six months, vagal tone was found to be positively related to interest and positive expressiveness, while negatively related to internalizing stress. This buffering effect continues well into adolescence; studies have found that high vagal tone in eight to twelve years olds buffers against intense verbal marital conflict. It is a very strong effect which has been shown to protect the child against some of the negative effects of parental alcoholism. With the exception of the neonatal, high baseline vagal tone levels in children lead to a variety of positive effects.
Use in research
Heart rate variability is measured using an electrocardiogram (ECG), which measures the amount of variability of time between heartbeats. Ten electrodes are attached to the skin and their voltages are recorded by an external device. Electrodes are applied to each wrists, and the lateral calf muscle of each leg. Six more electrodes are placed in a horizontal line between the right of the sternum and the left of the left pectoral muscle. The results of the ECG are recorded graphically. The x-axis denotes the time between heartbeats in seconds while the y-axis represents the voltage of vagal activity in millivolts. Each square denotes 40 milliseconds in time and .1 millivolt in voltage. Measurements of RSA in psychology research have primarily been examined in studies of adolescence, where it is used as a predictor of an individual’s emotion regulation.
Strengths as a research tool
Measurements of HRV are widely used in psychological, biomedical and physiological research. There are several reasons for this: 1) They do not suffer as many kinds of experimental bias that are often found in verbal responses, questionnaires, or structured interviews. 2) HR is a very easy measure to take and record, compared to other physiological techniques, especially EEG or fMRI data. Measuring HR also is a relatively quick process in regards to the time between the participant entering and the recording beginning. There is a variety of free software available for the calculation of HRV indices. 3) Changes in the heart occur both immediately (within a few seconds) in the presence of interesting, shocking or attentionally-capturing stimuli, but also over long periods of time in different mood states (over several minutes) and over 24-hour sleep/wake cycles. This is more prompt than neuroendocrine or inflammatory changes. 4) In experimental work in social psychology, heart rate can be taken while social situations occur without destroying their face validity or interfering with the task. It is almost impossible to act 'naturally' in an fMRI machine, as they are extremely noisy and remove the possibility of face-to-face interaction.
However, there are some significant professional and technical challenges to HRV data collection and usage: 1) Data collection and analysis is frequently poorly handled. While having a broader access to research method is a very good thing, it also inevitably means that non-experts use the method. 2) HRV provides information about the autonomic nervous system, and frequently psychological researchers have a poor understanding of the interface between social/emotional/personality characteristics and the body. 3) The diverse ways of calculating HRV means researchers may attempt to compare behavioral data to several of these different analyses and then pick suitable measures.
Studies have shown that the efficiency of pulmonary gas exchange is improved by RSA, suggesting that RSA may play an active physiological role. The matched timing of alveolar ventilation and its perfusion with RSA within each respiratory cycle could save energy expenditure by suppressing unnecessary heartbeats during expiration and ineffective ventilation during the ebb of perfusion (delivery of blood from arteries to capillaries for oxygenation and nutrition).
RSA or heart rate variability in synchrony with respiration is a biological phenomenon, which may have a positive influence on gas exchange at the level of the lung via efficient ventilation/perfusion matching.
Yasuma, F. & Hayano, J. (2004). Respiratory sinus arrhythmia: why does the heart beat synchronize with respiratory rhythm? Chest, 125(2), 683-90. doi:10.1378/chest.125.2.683
Due to age differences in expression of RSA, the majority of vagal tone research as it relates to social behavior and human psychology has focused on children. Typically, researchers are concerned with baseline vagal tone, treating it either as a potential predictor of behavior or examining its relationship with mental health (particularly emotion regulation, MDD, anxiety, and internalizing and externalizing disorders). However, it is worth noting that individual suppression of vagal tone during a particularly challenging task (“vagal reactivity”) may be influenced by regulation of attention; some studies have found that vagal tone can predict a variety of different, sometimes even opposite, effects in youth.
Studies have shown that children with high levels of vagal tone tend to exhibit greater psychophysiological health, higher mental and motor functioning, and more adaptive behavioral and social performance than those with lower vagal activity; these results hold true over a wide range of experimental contexts.[unreliable medical source?] There are implications that parental relationships have a significant effect on later expression of vagal tone, even surpassing the childhood years. Research indicates that children with more secure attachments with their mothers exhibited greater empathetic responsiveness, less social inhibition, and higher vagal tone, again highlighting the vagus nerve’s regulatory effect on emotional and social function.
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