Page 128 - Like me, or else... - Michelle Achterberg

Chapter 5
Preprocessing
MRI data were analyzed with SPM8 (Wellcome Trust Centre for Neuroimaging, London). The exact same preprocessing steps were used in preprocessing MRI data from W1 and W2. Images were corrected for slice timing acquisition and rigid body motion. Functional scans were spatially normalized to T1 templates. Some participants did not finish the T1 scan and were normalized to an EPI template (W1: n=5 at W1; n=10 at W2). Volumes of all participants were resampled to 3x3x3 mm voxels. Data were spatially smoothed with a 6 mm full width at half maximum (FWHM) isotropic Gaussian kernel. Translational movement parameters were calculated for all participants. Participants that had at least two out of three runs of fMRI data with <3 mm (1 voxel) motion in all directions were included in subject-specific analyses (W1: n=385; W2: n=358).
Subject-specific analyses
Statistical analyses were performed on individual subjects’ data using a general linear model, previously described in Achterberg et al. (2018b). The fMRI time series were modeled as a series of two events convolved with the hemodynamic response function (HRF). The onset of social feedback was modeled as the first event, with a zero duration and with separate regressors for the positive, negative, and neutral peer feedback. The start of the noise blast was modeled as the second event, with the HRF modeled for the length of the noise blast and with separate regressors for noise blast after positive, negative, and neutral judgments. Trials on which the participants failed to respond in time were modeled separately as covariate of no interest and were excluded from further analyses. Additionally, six motion regressors (corresponding to the three translational and rotational directions) were included as covariates of no interest. The least squares parameter estimates of height of the best-fitting canonical HRF for each condition were used in pairwise contrasts. The pairwise comparisons resulted in subject-specific contrast images.
Confirmatory ROI analyses
ROI selection
Regions of interest were based on higher-level group analyses of W2 in an independent reference group (the non-randomized dummy control group, n=41, Table S1). The advantage of this approach is that the participants were in exactly the same study protocol, but were not included in the subsequent analyses, leading to an independent selection of ROIs (Poldrack, 2007). Using comparable sample sizes, we previously reported replicable results of main effects of the social network aggression task (Achterberg et al., 2017). We first investigated social feedback (positive, neutral, negative) versus fixation (see supplementary materials, Figure S2a and Table S1). SPM8’s MarsBaR toolbox (Brett et al., 2002)
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