Effects of Transcranial Electrical Stimulation on Physiological Responses to Acute Stress: A Systematic Review

Physiological responses to acute stress generally proceed through two phases: first, a rapid and transient sympathetic and parasympathetic nervous system response, and second, a slow and relatively sustained hypothalamic-pituitary-adrenal (HPA) response. These responses are associated with changes in heart rate, heart rate variability, electrodermal responses, blood pressure, and salivary biomarkers such as alpha amylase and cortisol. Through the interactions of biochemical responses to stress and distributed brain regions including the amygdala, hippocampus, striatum, and prefrontal cortex, acute stress exposure can also adversely affect cognitive function. Transcranial electrical stimulation (tES), including transcranial direct, alternating, and random noise stimulation (tDCS, tACS, tRNS), has been examined for its potential ability to modulate physiological responses to stress and buffer negative impacts of stress on cognitive function. This systematic review surveyed research examining the effects of tES on physiological responses to acute stress in healthy neurotypical samples, examining cardiorespiratory (e.g., blood pressure, heart rate), peripheral (e.g., electrodermal), and biochemical (e.g., alpha amylase, salivary cortisol) outcomes. Sixteen reports were identified for inclusion. The articles showed low to high risk of bias, used a diverse set of acute stress induction techniques and tES methodologies, and examined a wide range of physiological outcomes. We found mixed evidence for the effectiveness of tES in reducing acute stress responses, and suggest that some tES methodologies may be more promising than others. Several directions for continuing research are proposed to help elucidate the mechanisms underlying tES effects on acute stress responses and clarify its potential for application to cognitive enhancement domains.