Participants
36 healthy participants (18 males, aged 18–31) were recruited through advertising. Following the local ethics committee guidelines (NRES committee South central – Oxford REC B 10/H0605/71), written and oral consent was obtained. Participants had a body mass index of 19–30, were physically fit as assessed by a medical doctor, and had normal laboratory values of thyroid and renal function. Additionally, females tested negative on a pregnancy test and were taking two forms of contraception. Participants were excluded if they took any psychotropic medication, had any past or current Axis 1 psychiatric disorder on DSM-IV, had any medical contra-indication, had current or past history of drug or alcohol dependency, smoked more than 5 cigarettes a day, had dyslexia, had any contra-indication to magnetic resonance imaging (MRI) scanning or were left-handed. A total of 3 participants were further excluded from the experiment and/or analysis due to treatment non-compliance (1x), unexpected adverse effect (1x) and incomplete MRI session (1x), leaving a total of 33 participants for analysis.
Experimental Design And Procedure
The present study followed a double-blind randomised design (as previously reported in Volman et al. 2021). An independent qualified researcher performed the randomisation on a 1:1 ratio for treatment (placebo/lithium) and gender (male/female). Lithium (“Priadel” prolonged release tablet) or placebo were administered orally at night for 11 days (± 1 day) on identical capsules. Lithium dosage was increased on a gradual fashion (day 1:400 mg, day 2:600 mg; days 3–11:800 mg) following previous procedures (Kohno et al. 2007; Monkul et al. 2007).
As previously described in Volman and colleagues (2021), participants visited the lab on three occasions. The initial assessment comprised a medical and psychiatric screening, as well as a blood drawing for testing thyroid stimulating hormone and creatine. Eligible participants were then asked to return for a baseline assessment in which the Beck Depression Inventory (BDI) (Beck et al. 1961), State-Trait Anxiety Inventory (STAI; (Spielberger 1983), Mood Disorder Questionnaire (MDQ; Hirschfeld et al. 2000), National Adult Reading Test (NART) IQ Scale (Nelson and Willison 1982), Eysenck Personality Questionnaire (EPQ; Eysenck and Eysenck 1984) and Emotion Regulation Questionnaire (ERQ; Gross and John 2003) were completed. Females were additionally required to take a pregnancy test at this stage. Upon completion, participants received the full treatment (lithium/placebo), the Befindlichskeit scale (BFS; Pichot & Olivier-Martin, 1974), the Positive and Negative Affect Scale (PANAS; Tran, 2013), the Bond and Lader Visual Analogue Scales (Bond and Lader 1974) and side-effects questionnaires, which had to be filled each day during treatment. Participants were contacted on days three and five to ensure no treatment side-effects and check appropriate compliance of the dosage regimen. Following the last treatment day, participants returned for testing. Blood was drawn to monitor lithium levels and participants underwent a behavioural and MRI session that lasted approximately 120 minutes. In the behavioural session, the BDI, STAI-state and MDQ as well as a battery of tasks (reported elsewhere) were completed. In the MRI session, functional and structural data were acquired while participants completed the ER task, the monetary incentive delay task (Volman et al. 2021), the checkerboard control task (Volman et al., 2021) and an MR spectroscopy scan (not included here).
Tasks
An adaptation of the original ER paradigm (Phan et al. 2005), previously used by Reinecke and colleagues (2015) was employed. In the task, participants were presented with 8 blocks of 5 negatively valanced images in each block (mean valence rating of 2.8 ± 1.7, mean arousal ratings of 6.0 ± 2.2 on 9-point Likert scales from 1 = unpleasant/low arousal to 9 = pleasant/high arousal) to which they were instructed to alternatively maintain (naturally experience the emotional state evoked), or reappraise (downregulate the provoked negative affect) through cognitive reappraisal. Employment of cognitive reappraisal was trained prior to the scan session. Following each block, participants had to rate on a 4-point rating scale (1 = neutral; 4 = negative) the intensity of the negative affect experienced. A more detailed description of the task as well as an illustrative figure can be found in supplementary material (Figure S1 supplementary material). Valence, arousal ratings and scene content matched between conditions and the order of the pictures within each condition remaining constant across all participants. The total duration of the task was of approximately 10 minutes.
A checkerboard control task (CCT: see Volman et al., 2021) was used to control for treatment-related possible cofounders on brain activation.
Mri Acquisition
Bold-oxygenation-level-dependent (BOLD) functional MRI (fMRI) data was acquired on a 3-Tesla MRI scanner (Magnetom, Siemens Medical systems) with a 32-channel head coil. Functional images during the ER task consisted of 45 T2-weighted echoplanar imaging (EPI) slices (TR = 3000ms, TE = 30ms, flip angle = 90°, field of view = 192 mm, voxel size = 3*3*3mm, 200 volumes, acquisition time (TA) = 10 minutes, 6 seconds). Functional images during the CCT consisted of 45 T2-weighted EPI slices (TR = 3000 ms, TE = 30 ms, flip angle = 87°, field of view = 192 mm, voxel size = 3*3*3mm, 120 volumes, TA = 6 minutes, 6 seconds). Additionally, fieldmaps volumes (magnitude and phase difference images) were acquired (echos at 5.19 and 7.65 ms, TR = 488, flip angle = 60°) to capture the inhomogeneities in the magnitude field. Structural scans were acquired via T1-weighted MR images (TR = 2040 ms, TE = 4.7 ms, flip angle = 8°, field of view = 192 mm, voxel size = 1*1*1 isotropic, TA = 5 minutes, 56 seconds).
Analysis Of Fmri Data
Data were analysed using FSL (FMRIB Software Library v6.05; www.fmrib.ox.ac.uk/fsl). Structural anatomical scans were brain extracted using FSL’s Brain Extraction Tool BET (Smith 2002). Fieldmap magnitude images were brain extracted by first registering these to their high-resolution structural images, inversing the created matrix, applying such matrix to the brain extracted structural mask with FMRIB’s Linear Registration Tool (FLIRT) (Jenkinson and Smith 2001; Jenkinson et al. 2002), and applying this mask to the whole brain magnitude image. These were then used to create a fieldmap rads image with the fieldmap phase difference image using the fsl_fieldmap_prepare tool.
Pre-processing of each participant’s functional data was done with FEAT (FMRI Expert Analysis Tool), part of FSL. This included motion correction using MCFLIRT (Jenkinson et al. 2002); spatial smoothing using Gaussian kernel of full width at half maximum (FWHM) 5mm; grand-mean intensity normalisation of the entire 4D dataset by a single multiplicative factor; high-pass temporal filtering of the functional timeseries at 90s; fieldmap distortion correction (Jenkinson 2003, 2004); registration of the functional images to their high-resolution structural images with Boundary-Based Registration (BBR) using FLIRT (Jenkinson and Smith 2001); and registration of the structural images to Montreal Neurological Institute (MNI)-152 standard space using linear registration with 12 degrees of freedom (DOFs), further refined using FNIRT non-linear registration with 10mm resolution (Andersson et al. 2007a, b).
Regarding the ER task, lower-level analysis was carried out to observe within-subject differences in brain activity across the two conditions by including the following contrasts: (1) Maintain vs. Baseline (M > B) to identify brain regions active when asked to naturally experience the emotion elicited; (2) Reappraise vs. Baseline (R > B) to identify brain regions active during voluntary suppression of negative affect using reappraisal techniques; (3) Reappraise vs. Maintain (R > M) to identify brain regions with greater activation when reappraising as compared to maintain; (4) Maintain vs. Reappraise (M > R) to identify brain regions with greater activation while maintaining as compared to reappraise; and (3) overall Picture Blocks vs. Baseline (M + R > B), to identify brain regions activated in response to negative images.
A custom 3-column format convolved with a gamma haemodynamic response function, and its temporal derivative were used to model the data. Time-series statistical analysis was carried out using FILM with local autocorrelation correction (Woolrich et al. 2001). Motion traces detected by MCFLIRT were included in the model as nuisance regressors to account for motion. Differences between groups in absolute and relative motion were tested for using Mann-Whitney non-parametric analysis. Groups did not differ in absolute nor relative motion (all U’s > 94, all p’s > 0.130).
In the higher-level analysis for the ER task, the contrast of parameter estimates (COPEs), their variance (VARCOPEs) and DOFs from the lower-level analysis were introduced into the analysis. Using a mixed-effects analysis with FLAME1 + 2 across the whole-brain, the following contrasts were analysed: (1) Placebo > Lithium (1, -1); (2) Lithium > Placebo (-1, 1); and (3) mean activation and deactivation across both treatment groups (1, 1; -1, -1). Due to lithium’s potential to promote GM changes, GM images of each participant were extracted using FMRIB’s Automated Segmentation tool (FAST; Zhang et al. 2001). These were then registered to standard space, smoothed to match the intrinsic smoothness of the fMRI data (2.63mm), voxel-wise demeaned across all subjects and added to the general linear model (GLM) of the ER task to remove any potential structural differences explaining the BOLD contrast differences. Significant activations were identified using cluster-based thresholding of statistical images with a height threshold of Z > 3.1 and a family wise error (FWE)-corrected cluster significance threshold of p < 0.05. Clusters thesholded at Z > 2.3 p < 0.05 were also reported for completeness and to compare the present results with previous studies using this less stringent statistical threshold (Worsley 2001). See Volman and colleagues (2021) for details on CCT analysis.
Small volume correction (SVC) analysis was performed using several regions of interests (ROIs), namely the bilateral amygdala, due to its implication in emotional processing; and bilateral vmPFC and middle temporal gyrus (MTG), key areas hypothesized to exert ER through cognitive reappraisal. Bilateral amygdala’s anatomical masks were created using the probabilistic map thresholded at 50 provided by the Harvard-Oxford Structural Atlas in FSL. Spherical masks of 10mm radius were created for the right and left vmPFC and MTG using coordinates previously reported (Buhle et al., 2014; right vmPFC: x = 6, y = 40, z=-20; left vmPFC; x=-6, Y = 40, z=-20; right MTG: x = 56, y=-32, z = 0; left MTG: x=-56, y=-32, z = 0). Significant activations were identified using cluster-based thresholding of statistical images with a height threshold of Z > 3.1 and a FWE-corrected cluster significance threshold of p < 0.05.
Connectivity analysis was performed to investigate a seed ROIs’ relationship with other brain areas throughout the task using psychophysiological interactions analysis in FSL (O’Reilly et al. 2012). Previously created masks for the bilateral amygdala were used as above. Bilateral vmPFC and MTG masks were obtained using the Talairach atlas (Talairach et al., 1988). Standard masks were transformed into individual’s standard space, thresholded and binarized. Time-series of each mask were extracted and entered in the lower-level analysis as a regressor to identify voxels where a significant effect is explained by such regressor. Task regressors were added (maintain, reappraise and instructions), in addition to the interaction between the masks’ time-series and the task conditions (maintain x mask time-series and reappraise x mask time-series). Contrast images were introduced in the higher-level analysis to identify brain differences in connectivity across treatment groups. Significant activations were identified using cluster-based thresholding of statistical images with a height threshold of Z > 3.1 and Z > 2.3 and a FWE-corrected cluster significance threshold of p < 0.05. BOLD parameter estimates of significant whole-brain or ROIs interactions were further explored plotted for visual inspection.
Statistical Analysis
Statistical analysis was performed with IBM SPSS 22 Software, with significant levels set at p < 0.05. Between group-differences on the sample’s tendency for ER (ERQ questionnaire) were analysed with an independent samples T-test after testing for normality. Treatment-derived differences on self-reported ratings of negative affect experienced throughout the task were further analysed with a mixed-model ANOVA, within-subject factor condition (maintain vs. reappraise) and between-subject factor treatment (lithium vs. placebo). Post-hoc analysis of the ANOVAs was carried out with pairwise comparisons.