Do not duplicate

I ran into the following error when trying to use a script to make Marsbar extract betas

Error in pr_stat_compute at 34

Indices too large for contrast structure

This problem occurred when I was trying to extract the betas for an unusual participant who had an empty bin for one condition, and for whom I had therefore had to manually alter the set of contrasts.  In doing this, it turns out that I had inadvertently duplicated one contrast vector.  Although the names were different, the number of contrasts had been amended to reflect the number of unique contrast vectors in SPM.xCon but not in Marsbar’s D, meaning that pr_stat_compute’s ‘xCon = SPM.xCon’ (line 23), did not return the same value as my own script’s ‘xCon = get_contrasts(D)’.  This was causing the two xCons to differ in their length and the resultant error in pr_stat_compute.

The solution lay in removing the duplicate contrasts from the contrast specification for that participant.

Below (indented) is a straightforward MATLAB/SPM/Marsbar script for generating separate Marsbar ROI and Nifti ROI spheres (user-defined radius) from a set of user-defined MNI coordinates stored in a text file (‘spherelist.txt’ saved in the same directory as the script).  ROI files are saved to mat and img directories that the script creates.
I use this script to generate *.mat files as seeds for resting connectivity analyses.  Once a *.mat ROI is generated, Marsbar can be used to extract raw timecourses from this region to feed into connectivity analysis as the regressor of interest.  Because MRIcron doesn’t read *.mat Marsbar ROI files, I render the equivalent *.img seed regions on a canonical brain when I need to present them graphically.

% This script loads MNI coordinates specified in a user-created file,
% spherelist.txt, and generates .mat and .img ROI files for use with
% Marsbar, MRIcron etc.  spherelist.txt should list the centres of
% desired spheres, one-per-row, with coordinates in the format:
% X1 Y1 Z1
% X2 Y2 Z2 etc
% .mat sphere ROIs will be saved in the script-created mat directory.
% .img sphere ROIs will be saved in the script-created img directory.
% SPM Toolbox Marsbar should be installed and started before running script.

% specify radius of spheres to build in mm
radiusmm = 4;

% Specify Output Folders for two sets of images (.img format and .mat format)
roi_dir_img = ‘img’;
roi_dir_mat = ‘mat’;
% Make an img and an mat directory to save resulting ROIs
% Go through each set of coordinates from the specified file (line 2)
spherelistrows = length(spherelist(:,1));
for spherenumbers = 1:spherelistrows
% maximum is specified as the centre of the sphere in mm in MNI space
maximum = spherelist(spherenumbers,1:3);
sphere_centre = maximum;
sphere_radius = radiusmm;
sphere_roi = maroi_sphere(struct(‘centre’, sphere_centre, …
‘radius’, sphere_radius));

% Define sphere name using coordinates
coordsx = num2str(maximum(1));
coordsy = num2str(maximum(2));
coordsz = num2str(maximum(3));
spherelabel = sprintf(‘%s_%s_%s’, coordsx, coordsy, coordsz);
sphere_roi = label(sphere_roi, spherelabel);

% save ROI as MarsBaR ROI file
saveroi(sphere_roi, fullfile(roi_dir_mat, sprintf(‘%dmmsphere_%s_roi.mat’,…
radiusmm, spherelabel)));
% Save as image
save_as_image(sphere_roi, fullfile(roi_dir_img, sprintf(‘%dmmsphere_%s_roi.img’,…
radiusmm, spherelabel)));

UPDATE: WordPress messed with the characters in the above script, so here is a link to the script file and an example spherelist.txt file.