[FieldTrip] ft_sourcestatistics not working as expected

[Akul Satish]

Dear Fieldtrippers,

I am trying to run a paired-samples t-test at source level using the output
structure of the ft_sourceanalysis function. Although the script does run
without any errors, I am getting a p-value of 0.002 and "stat" = Infinity
at all 3294 voxels within the brain. Note that I have attempted to run
these tests with 1) no correction for multiple comparisons, 2) cluster
correction, and 3) varying participant numbers. I am getting the same
result regardless of these 3 changes.

However, when I plot the grand-averaged raw 'pow' differences, the plots
make sense - there is localised activity from some brain regions that could
be involved in the task, but no activity in other brain regions.

I am not sure what I am doing wrong and any help would be appreciated!

I have run the analysis in the following way:

1) LCMV beamrforming to obtain time-series data at each voxel/virtual
sensor/dipole (using the 3D volumetric grid)
2) time-frequency decomposition to determine baseline-normalised power at
each point in the voxel x frequency x time data matrix
3) averaged data over a certain freq x time cluster region, at each voxel
4) For each participant and condition of interest, created a dummy output
variable from ft_sourceanalysis and replaced the averaged power from step
(3) into the 'avg.pow' variable at each voxel
5) Run ft_sourcestatistics using the following parameters:

cfg                     = [];
cfg.dim              = SourceTF_forstat.overlap{1}.dim;
cfg.method        = 'montecarlo';
cfg.statistic        = 'depsamplesT';
cfg.parameter        = 'pow';
cfg.correctm         = 'no';
%cfg.alpha           = 0.05;
%cfg.numrandomization = 1000;
%cfg.tail             = 0;
%cfg.correcttail      = 'prob';

% Specify Design Matrix

subj = numel(SourceTF_forstat.overlap);

design = [1:subj, 1:subj];
design(2,1:subj) = 1;
design(2,subj+1:2*subj) = 2;

cfg.design = design;
cfg.uvar = 1;
cfg.ivar = 2;

cfg.spmversion = 'spm12';

SourceClusterResult = ft_sourcestatistics(SourceTF_forstat.overlap{:},
SourceTF_forstat.nonoverlap{:})

I used the following documentation for help with ft_sourcestatistics:
https://www.fieldtriptoolbox.org/example/source_statistics/

Thanks a lot,
Akul

*___________________________________________Dr Akul Satish | Post-D**octoral
Research Associate*
B218, Department of Psychology
University of York
York YO10 5DD
akul.satish at york.ac.uk
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