Page 68 - Like me, or else... - Michelle Achterberg

Chapter 3
reactions on a 7-point scale, with 1 representing very little and 7 representing very much.
MRI data acquisition
MRI scans were acquired with a standard whole-head coil on a Philips 3.0 Tesla scanner (Philips Achieva TX). The SNAT was projected on a screen that was viewed through a mirror on the head coil. Functional scans were collected during two runs T2*-weighted echo planar images (EPI). The first two volumes were discarded to allow for equilibration of T1 saturation effect. Volumes covered the whole brain with a field of view (FOV)= 220 (ap) x 220 (rl) x 114.68 (fh) mm; repetition time (TR) of 2.2 seconds; echo time (TE) = 30 ms; sequential acquisition, 38 slices; and voxel size= 2.75 x 2.75 x 2.75 mm. Subsequently, a high-resolution 3D T1scan was obtained as anatomical reference (FOV= 224 (ap) x 177 (rl) x 168 (fh); TR=9.76 ms; TE=4.95 ms; 140 slices; voxel size 0.875 x 0.875 x 0.875 mm).
MRI data analyses
Preprocessing
MRI data were analyzed with SPM8 (Wellcome Trust Centre for Neuroimaging, London). Images were corrected for slice timing acquisition and rigid body motion. Functional scans were spatially normalized to T1 templates. Due to T1 misregistration, one participant was normalized to an EPI template. 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 never exceeded 1 voxel (<3 mm) in any direction for any participant or scan (movement range: 0.001-1.22 mm, M=0.055, SD=0.036).
First-level analyses
Statistical analyses were performed on individual subjects’ data using a general linear model. 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 for the length of the noise blast duration (i.e., length of button press) and with separate regressors for noise blast after positive, negative, and neutral feedback. Trials on which the participants failed to respond in time were marked as invalid. Note that his happened rarely, on average 3.78% of the trials were invalid. 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.
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