This study is based on the data of a previous trial that measured working memory performance of healthy subjects after a course of NIBS protocols. The original study used a factorial, double-blinded, within-subjects design, in which participants were allocated to receive four different NIBS protocols (tDCS, iTBS, combined tDCS + iTBS and placebo) on different days, in randomized order (Razza et al. 2022) (A study’s design figure can be found in the Sup. Material Figure S1). Here, we report results from the tDCS, iTBS, and placebo groups.
Participants
Right-handed subjects, aged 18 to 45 years, without neuropsychiatric disorders and/or clinical diseases were included. Participants were prescreened by e-mail and those who met inclusion criteria underwent on-site screening by a trained psychologist checking for previous or current psychiatric diagnoses based on the DSM-5 (American Psychiatric Association [APA], 2013), the Hamilton Depression Rating Scale (HDRS) (Hamilton 1960), the Beck Depression Inventory (BDI) (Beck et al. 1961), and the Positive and Negative Affect Schedule (PANAS) scales. Exclusion criteria were specific contraindications for NIBS interventions and MRI (e.g., metal implants), habitual smoking (> 10 cigarettes/day) or abuse/dependence on other drugs, pregnancy, and use of psychoactive drugs (including antidepressant drugs, benzodiazepines, and Z-drugs).
Procedure
First, an anatomical T1-weighted acquisition of the brain was performed using a 3T Magnetic Resonance Imaging (MRI) scanner, followed by a real-time MRI-guided neuronavigation system (Brainsight, Rogue Resolutions, Inc) using the T1-weighted to target the left and right DLPFCs (MNI152 stereotaxic coordinates, -38, + 44, +26 and + 38, +44, + 26, respectively) (Fox et al. 2012). The experimental session was composed of prior baseline measures (HDRS and BDI), respectively followed by the NIBS sessions. The working memory task was assessed right after the end of the simulation session.
2-Back task
The 2-back task was programmed in E-prime 2.0 software (Psychology Software, Tools Inc Pittsburgh, Pennsylvania, USA). The visual stimulus consisted of letters (A to Z) that appeared in a pseudo-randomized order on a computer screen of 15 inches. The applied protocol consisted of three blocks of 30 letters. Letters were displayed on the screen for 500ms, with an interstimulus interval of 3000 ms. Each block consisted of 10 ‘target’ letters, representing a total of 30 ‘targets’. Targets were letters identical to the ones presented two steps earlier in the trial sequence. Participants were instructed to press different buttons on the keyboard for target (key ‘2’) and non-target stimuli (key ‘0’). A brief practice containing 20 stimuli was conducted prior to the task. The 2-back was chosen because it was previously associated with working memory improvement after NIBS in healthy participants (Brunoni and Vanderhasselt 2014).
NIBS protocols
Based on previous studies investigating the cognitive performance after tDCS in healthy volunteers (Wischnewski et al. 2021), electrodes were positioned over the left (anode) and right (cathode) DLPFC located via neuronavigation. tDCS was applied with a current of 2mA through saline-soaked sponges of 25cm² and lasted 20 minutes (Neuroconn DC-Stimulator, Ilmenau, Germany). Sham tDCS used the same montage but delivered only an active current of 30 seconds of fade-in and fade-up on the beginning of the tDCS session.
iTBS protocol used a TMS coil applied with an angle of 45 degrees relative to the midline. The protocol consisted of 54 cycles of 10 triplet bursts with a train duration of 2s and an interval of 8s between trains (1620 pulses) at 110% of the resting motor threshold. The protocol lasted 8 minutes and 40 seconds. The coil B65 Active/Placebo Magventure was used for both active and sham protocols. The coil has two identical sides for delivering active or sham stimulation depending on the randomized codes imputed on the device.
MRI-acquisition
All brain structural MRIs were acquired in a 3-Tesla MR system (General Electric PET / MRI equipment). Volumetric images were based on T1-weighted sequences using a 3D fast-field echo pulse sequence with the following parameters: field of view (FOV) of 25.6, time of repetition (TR) of 7.7ms, time of echo (TE) of 3.1ms, and 202 slices.
Neuroimaging processing and cortical thickness quantification
The T1-weighted image of each participant was processed using the CAT12 toolbox (Dahnke et al. 2013) within SPM12 software (Penny et al. 2011) using MATLAB. We performed a voxel-based processing for voxel-based morphometry (VBM), followed by a surface-based processing for surface-based morphometry (SBM) and, finally, a region-based processing for region-based morphometry (RBM) (Destrieux et al. 2010). The VBM analysis incorporates tissue segmentation, spatial registration, adjustments for volume changes due to registration (modulation), as well as a convolution with a Gaussian kernel matrix (spatial smoothing with an 8 mm full-width to half-maximum (FWHM) filter). The latter steps are followed by the SBM, which also incorporates several different steps such as surface creation, surface registration and spatial smoothing (applied using a 15 mm FWHM filter). In the surface creation step, a projection-based thickness method estimates both initial cortical thickness and central surface considering partial volume information, sulcal blurring and asymmetries (Dahnke et al. 2013). The cortical thickness measurement captures the width of the gray matter band as the distance between its inner and outer boundary in thousands of points. Then, topological correction is performed with spherical harmonics, followed by a surface refinement, resulting in the final central, pial and white surface meshes (Yotter et al. 2011). The pial and white matter surfaces are used to refine the initial cortical thickness using the FreeSurfer thickness metric (Yotter et al. 2011). Finally, data was visually inspected.
In the RBM step, the Destrieux atlas (Destrieux et al. 2010) was used to fit individual surfaces using the spherical registration parameters determined during surface-based processing. Cortical thickness was then calculated for each ROI in native space.
According to our a priori hypothesis, the investigated ROIs were the DLPFC, mPFC and PCC of both hemispheres and subregions of the Destrieux atlas were used to parse each of these ROIs. The DLPFC was composed of inferior frontal gyrus, inferior frontal sulcus, middle frontal gyrus, middle frontal sulcus, and superior frontal sulcus. The mPFC consisted of the superior frontal gyrus and the frontal pole. Finally, the PCC was composed of its posterior ventral and posterior dorsal portions (Fig. 1). ROIs thickness was obtained as the average of the sum of all the subregions included in each ROI.
Insert Fig. 1 about here.
Statistical Analysis
Statistical analyses were performed using R Version 4.1.2 (Team and Others 2013). Reaction time (in milliseconds (ms)) and accuracy (binary outcome) of the target stimuli were considered the dependent variable while ROI thickness and protocol were the independent variables. Missed responses were considered errors and reaction times < 200ms and > 2500ms were considered not genuine responses and were excluded (Whelan 2008; Razza et al. 2022). Due to the non-normal distribution of reaction time responses (Sup. Material Figure S2), generalized linear mixed models (GLMM; using the ‘lme4’ package) with an inverse-gaussian error distribution and an inverse link function were used (Lo and Andrews 2015). For accuracy, GLMMs with binomial distributions were employed. All models used the interaction between ROI thickness and protocol and were controlled for the variable session, indicating in which order the different NIBS protocols were performed. The variable subject was included as a random intercept. Analyses were performed using ROIs of both hemispheres, resulting in a total of six models for each outcome.
These analyses were not corrected for multiple comparisons since they were hypothesis-driven. All models were controlled for the effects of age and gender. All results were considered significant at a p threshold of 0.05.