Arousal levels can be used to predict the ongoing effects of neuromodulation
In this study, we aimed to verify whether the behavioural effects induced by a common prefrontal tDCS montage were dependent on the participants’ arousal levels. Pupillary dynamics were recorded during an auditory oddball task while applying either a sham or real tDCS. The tDCS effects on reaction times and pupil dilation were evaluated as a function of subjective (STAI-Y State scores) and physiological (pre-target pupil size) arousal predictors. We showed that prefrontal tDCS hindered task learning effects on response speed such that performance improvement occurred during sham, but not real stimulation. Moreover, both predictors significantly explained performance during real tDCS, with interaction effects showing performance improvement only with moderate arousal levels; likewise, pupil dilation was affected according to the ongoing levels of arousal. These findings highlight the potential role of arousal in shaping the neuromodulatory outcome, and thus encourage a more careful interpretation of null or negative results.
Steps to reproduce
A single-blind within-subject design was implemented. The testing sessions were organized in two days separated by at least 48h in order to exclude any tDCS carryover effects. In each session, participants completed the task twice: at baseline (T1) without any electrodes mounted on their scalp, and subsequently either during sham or real stimulation (T2) . We collected behavioral and pupil data for the whole task duration (~18 min). Participants were randomly assigned and counterbalanced across two session-orders of tDCS protocol, and they were kept blind to the ongoing experimental condition (i.e., sham or real). The same participant was tested at around the same hour of the day. Participants seated in a soundproof dark room at the distance of about 55 cm from a 17-in LCD monitor and with the only source of light provided by a grey fixation cross. The auditory oddball task was presented using E-Prime presentation software by means of two constant-loudness speakers. In every task condition there was a fixed total number of trials (420) of which 20% included targets (84) and 80% standards stimuli (336). The interstimulus interval was set to a range of 2.1-2.9 s and both stimuli lasted for 70 ms. In so doing, we ensured enough time (~8 s) for any pupil dilation to return to baseline before overlapping to the next target trial. At the end of each experimental session participants were given a questionnaire to rate the perceived sensations or discomforts that influenced their performance. A battery-driven current stimulator (Brain- STIM, EMS, Bologna, Italy) was used to deliver 1 mA (0.028 mA/cm2) direct current stimulation via two rubber electrodes (35 cm2) which were inserted inside two saline-soaked sponges. Conductive electro-gel was also applied. The electrodes montage consisted in placing the anode over the area F3 of the EEG 10-20 system and the return (cathode) electrode over the right supra-orbital area. The duration of the stimulation consisted of about 17 min (1040 s) with 15 s of currents fade-in and fade-out. Configuration of the sham condition included 15 s of fade-in, 10 s of actual current delivery and 15 s of fade-out given at the beginning of the experiment only. Pupil diameter was recorded with an EyeLink 1000 Plus system and at 500 Hz sampling rate with left-monocular and pupil-CR tracking mode. Pupil signal was processed offline. A shape-preserving piecewise cubic interpolation method was chosen to interpolate values ranging from 70 ms before blink onset to 300 after blink offset. Epoch segmentation and baseline correction were then carried out (respectively for -1 s to +2.5 s and -800 to +200 ms relative to target onset). We extracted two variables of interest: (i) pupil dilation (PD), as the peak value of the maximum dilation after targets presentation and (ii) Pre-stimulus pupil diameter (PrePD) as the mean of 1 s data prior to tone presentation.