Changes in vagal tone in response to a startle : A new protocol

Abstract

Background:The brain's first response to stress is mediated through the vagus nerve of the parasympathetic nervous system (PSNS). In the presence of a stressor the vagal brake is either removed to facilitate peripheral engagement or bolstered to enhance cognitive flexibility. In contrast to other stress response systems, no standardized test exists to determine the integrity of the system in an individual. We attempted to develop such a protocol to reliably and robustly elicit a response from the PSNS.Methods:Participants viewed a series of three neutral and three startle videos wherein an auditory and visual stimulus rapidly appeared on the screen. Videos were onscreen for 15 s with an inter-video length of 150 s. Total viewing duration was 17 min and 30 s. Data were analyzed in MatLab. To test for statistical significance, we performed a repeated measures ANOVA followed by post-hoc, pairwise t-tests.Results:It was found that the video consistently caused a rapid decrease in vagal tone in response to the startle stimulus (Cohen's d = 0.922, p < 0.05). Following the decrease, vagal tone displayed a long term increase to levels higher than baseline perhaps reflecting heightened vigilance after exposure to startle (Cohen's d = 0.583, p < 0.001).Conclusions:This protocol reliably induced changes in vagal PSNS levels. Future studies need to explore inter-individual differences in reactivity patterns, as well as possible correlations with the response of the hypothalamic-pituitary-adrenal axis.