[FieldTrip] Odd behavior in DICS

[Nakhnikian, Alexander]

Hello All,


I'm getting strange results from DICS applied to EEG data. I'm wondering if anyone else has run into these issues and/or can suggest a solution.


I don't have individual sMRI so I'm using the standard BEM and MRI that come with FT. I constructed the lead fields as follows. First, I constrain the dipoles of interest to the cortical mantle using the AAL atlas. I then compute the lead fields using ft_prepare_leadfields; I don't have a baseline for these data so the lead fields are normalized. I feed the precomputed lead fields and data into ft_sourceanalysis to find the source-level power for each subject.


1) DICS, but not eLORETA, returns extremely large results at certain voxels. These extrema dominate the grand averages. In each subject, these voxels are always at the edge of the cortex, near the skull. The voxel locations are not exactly the same across subjects, but for any given subject they tend to appear in one of a few locations (i.e. over the right occipital cortex). The lead fields have the same magnitude (following normalization) across all voxels. I did notice, however, that voxels at which extreme values appear tend to have a larger condition number (~12) than voxels at which abnormal values do not appear in any subject (~2). I do not know whether this is relevant but I thought it might have some effect on adaptive but not non-adaptive filters. Since I'm constructing the forward model using FT templates, I thought someone else might have encountered this problem before.


2) As I mentioned above, eLORETA does not return such extreme results; furthermore, when we contrast controls with schizophrenia patients using an independent samples t-test with cluster control for multiple comparisons we find significant differences between groups using eLORETA but not DICS. The raw differences between the grand averages returned by DICS and eLORETA are similar, but when we run stats using DICS most of the correct p values are equal to 1 with one or two in the 0.8-0.9 range. I'm looking for converging evidence between different source analysis methods and it's not clear to me why DICS and eLORETA return such different results.


3) I thought that voxels are which unusually large values occur in controls and patients could be throwing off the stats; however, DICS does not return significant contrasts between control and schizophrenia even when I mask out the extreme voxels.


Thanks in advance for any advice.


Best,


Alexander
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