Bursts of transcranial electrical stimulation increase arousal in a continuous performance test

Arousal reflects a state of generalised physiological activation, and its key role in cognition and behaviour has been extensively described. The regulation of arousal is controlled by specific nuclei located in the brainstem that contain widely distributed projections to the cortex and form the arousal systems. In humans, arousal has been commonly studied and modulated through behavioural paradigms, whereas in animals, direct electrical stimulation has been used to confirm the important role of these widely distributed structures. Recent evidence suggests that it might be possible to use transcranial electrical stimulation (tES) to non-invasively induce currents in the brainstem regions of the brain. Therefore, we hypothesise that, using a specific electrode arrangement, it might be possible to employ tES to stimulate subcortical-cortical neuromodulatory networks, inducing modulation of general arousal. The aim of the present study was to determine if it is possible to increase arousal during a discriminative reaction times (RTs) task, through the application of tES, to improve the subjects' performance. We developed 3 experiments: Experiment 1 validated the behavioural task, which was an adapted version of the continuous performance test. Experiment 2 aimed to show the task sensitivity to the level of activation. The results confirmed that the task was sensitive enough to reveal modulations of arousal. In Experiment 3, we applied bursts of tES concurrent with the onset of the relevant stimuli of the task to increase the physiological phasic activation of arousal. The skin conductance response was recorded during the experiment in addition to the RTs. The results showed a reduction of RTs and a concurrent increase in skin conductance during the real stimulation condition, which is consistent with a general increase in arousal. In all, these data support the effectiveness of bursts of tES in modulating arousal.