Bursts of high frequency random noise stimulation (tRNS) increase arousal in a discriminative reaction time task.

Arousal reflects a state of generalized physiological activation, and different neuronal nuclei are responsible for its regulation through widespread projections to the cortex. Reaction times (RTs) have been used as a behavioral measure of arousal, and in a condition of increased arousal it has been showed a reduction of RTs, denoting a performance improvement. Recent modeling studies on transcranial electrical stimulation (tES) suggest a wide diffusion of the current across different regions of the cortex. On this basis it has been suggested that, sometimes, the effect of the tES might not be specific for the stimulated site but it might induces a general arousal. In the light of these results, the aim of the present study was to increase arousal using tES during a discriminative RTs task, in order to improve the performance. 16 healthy volunteers (8 F, mean age 25) participated to the experiment. The task was a continuative quasi-random visual presentation, for 100 ms, of digits from 1 to 9, with a variable inter-trial interval of 800-2000 ms. Subjects had to press response buttons for target digits (8-9) which were presented after a warning digit (1). We evaluated RTs using a paradigm that has been showed to be sensitive to the modulation of the arousal. Skin conductance response was recorded during the experiment to have also a physiological index of arousal. The experiment was divided in 6 blocks. During blocks 2, 3 and 4, bursts of high frequency random noise stimulation (tRNS) were administered to the subjects concurrently to the presentation of the warning digit, with the aim to increase the endogenous activation related to the preparation of the response. Each burst (2 mA) lasted for 900 ms. We used a pair of circular electrodes (22.8 cm2) placed with a fronto–occipital montage (FPz–Oz). Real and sham stimulation was balanced in a within subjects design. Results showed a significant reduction of RTs during the second block of stimulation (interaction between condition and block, p = <.001) denoting a performance improvement. Concerning skin conductance, analyses showed a main effect of condition (p = <.001) with a higher phasic activation during the real stimulation session, which was maintained over time. During the sham session, instead, the phasic activation decreased in a faster way, according to a physiological habituation to the repetitive stimuli. These results support the effectiveness of bursts of tRNS to increase arousal over time, measured both with a physiological (skin conductance) and a behavioral (RTs) index. Modulations over the behavioral performance, however, are weaker in comparison to the physiological ones, probably due to the less sensitivity of the behavioral measure used in the present research. The study of the tES effects on other behavioral tasks through the use of additional physiological measures of arousal (e.g., pupil dilation, EEG) are the future steps needed to support this promising research line.