The Combined Effects of Prefrontal tDCS and Slow-Paced Breathing on Stress and Emotions

Difficulties in stress and emotion regulation can undermine well-being. Yet, they could be tackled by intervention acting on the nervous system. Two promising interventions could achieve this via complementary pathways: slow-paced breathing (SPB), acting via peripheral mechanisms, and prefrontal transcranial Direct Current Stimulation (tDCS), targeting brain regions involved in autonomic regulation. Combining those techniques could therefore enhance their effects. This placebo-controlled study examined the effects of SPB and tDCS, alone and in combination, on stress and emotions. Participants were assigned to four groups combining SPB (5.5 bpm) or control breathing (15 bpm) with active (2 mA) or sham tDCS (ramp up/ramp down). They completed a baseline assessment, the intervention, a psychosocial stress task, a recovery phase and an emotional task with negative versus neutral images. During the different phases, we measured psychophysiological indices of stress and emotion regulation (heart rate variability [RMSSD], skin conductance [level and responses], pupil dilation during the emotional task and blood pressure [systolic and diastolic], appraisal of threat [Primary Appraisal Secondary Appraisal Scale (PASA)], affect [Affect Scale], and the use of emotion regulation strategies [State Emotion Regulation Inventory (SERI)]). tDCS reduced pupil dilation, prevented the decline in positive activating affect after stress recovery and the emotional task and of distraction during the emotional task. SPB decreased pupil dilation, reduced skin conductance responses, increased acceptance and distraction use and prevented the decline in positive activating affect during stress recovery. Combining active tDCS with SPB reduced the effects on pupil dilation, positive affect and acceptance. For the other variables, no effect of the intervention was observed. Rather than enhancing outcomes, combining them may impede each intervention. These findings highlight the independent efficacy of SPB and tDCS, caution against their concurrent use and highlight the need for further investigation.