<div>Abbreviation: ECG: Electrocardiogram; E-Field: Electric Field; HRV:
Heart Rate Variability; MRI: Magnetic resonance Imaging; tDCS:
transcranial direct current stimulation.</div><div>Heart rate variability</div><div>Cardiac activity was measured continuously and then separated in four
time-points: 5-min of baseline (baseline), initial 5 minutes of the tDCS
session (5-min tDCS), 10-min during the 0-back task (during 0-back) and
the last 5-min of the tDCS session (last 5-min tDCS). Cardiac activity
was acquired using a Biopac ECG100C amplifier (Biopac Systems Inc., USA)
and Biopac Acqknowledge software 4.3. For the amplifier, the gain was
set at 5000, with a high pass filter of 0.05 Hz and a low pass filter of
35 Hz. The sampling rate was set at 1000 Hz. The electrocardiogram was
set-up using the lead I configuration
(Francis, 2016), with two
Ag/Agcl electrodes attached below the right and left clavicle and a
third reference electrode placed under the left ribs. The
electrocardiogram data were analyzed with PhysioData Toolbox (version
0.6.3) which allows for automated R-peak detection and
inter-beat-interval (IBI) extraction. Misidentified R-peaks were
manually corrected after visual inspection of the data.</div><div>HRV (in milliseconds) was assessed by calculating the Root Mean Squared
of Successive Differences (RMSSD) of the detrended IBI data
(Tarvainen et al., 2002 )and
High-Frequency-HRV (HF-HRV, 0.15 to 0.40 Hz), using the absolute power
of the HF band, as calculated using the Lomb-Scargle method.
Importantly, although there are several ways to investigate HRV, only
RMSSD and HF-HRV were analyzed here because they present robust effects
when using NIBS intervention over the PFC and they are markers of the
parasympathetic system
(Makovac et al., 2017).</div><div>RMSSD and HF-HRV were calculated for time epochs of 5 min, as
recommended elsewhere (Malik
et al., 1996).Therefore, for the time-point in which we recorded the
HRV during the 0-back task (10min), the average of two epochs of 5 min
was obtained.</div><div>tDCS</div><div>TDCS was performed using a NeuroConn device (DC-Stimulator Plus,
NeuroConn, Germany). The session consisted of a 20-minute direct
current, delivered using two rubber electrodes of 25cm² (5x5cm) applied
to the scalp with a conductive gel. The anode and cathode were placed
over the left and right neuronavigated DLPFC, respectively ( left: x =
-38, y = +44, y = +26; right: x = 38, y = +44, y = +26)
(Blumberger et al., 2018),
pointing towards Cz. Stimulation at these coordinates has been found to
achieve optimal anticorrelation with the ACC in functional brain studies
(Fox et al.,
2012)(Razza et al., 2022).
The active tDCS sessions were performed using currents of 1.5mA and 3mA.
The sham protocol was identical but consisted of a brief active period
of 30 seconds fade-in and 30 seconds fade-out at the beginning and end
of the session to a current intensity of 3mA with the device set to 0mA
for the remainder. The ‘study mode’ of the tDCS device was used to
deliver active or sham current based on a randomized imputed code,
allowing for double-blinding of participants and study personnel. The
codes were randomized via<i>https://www.randomizer.org</i>and were managed by an independent party not directly involved in data
acquisition.</div><div>Self-reported measures</div><div>The State-Trait-Anxiety Inventory (STAI) state inventory was
administered at the beginning of each session
(Spielberger et al., 1970).
The STAI state measures how participants are feeling at that moment
using a 4-point Likert scale (1 = not at all, 2 = a little, 3 =
moderately, 4 = very much). This measurement was collected to identify
the baseline psychological state of each participant before the tDCS
session. Moreover, the Visual Analogue Scale (VAS) was collected two
times (at baseline and after the tDCS session). In VAS participants are
asked to indicate from ‘zero’ to ‘one-hundred’ (“I do not experience
this at all”, “I experience this very much”) how much they were
feeling the following moods: ‘angry’, ‘tense’, ‘sad’ ‘happy’, ‘stressed’
and ‘anxious’. Higher scores indicate higher levels of perceived stress
or negative affect.</div>