clustering after sensor picking?

[Eric Maris]

Hi Walther,


> I am facing a question regarding the MCP in time-frequency-analysis. In
> order to boost the effect size, I picked several ROIs on the MEG scalp
> by running a Task-versus-Basline Comparison (values averaged over all
> subjects, all conditions, timepoints and frequencies) and selecting the
> highest raw effect patches. Since FDR-correction seems to be too
> conservative and yields no results in following analyses, I tried the
> cluster-method which works pretty well. Is it (statistically) correct to
> apply clustering after sensor picking? Since its test statistic refers
> to the clustering of /adjacent/ sensor-pairs of the whole spatiotemporal
> grid (which does not exist any more after the selection of ROIs), I
> wonder whether it is still appropriate. Any advice would be helpful.


If you have an independent localizer that you can thrust, then you should
use it, and you should use it for as many dimensions as possible.
Task-versus-baseline (with "task" being the average over all experimental
conditions) is an independent localizer the result of which you can
subsequently use for comparing the different experimental conditions. If you
want to use the localizer for all three dimensions (space, frequency, and
time), then you are done, because your three-dimensional array has been
reduced to a scalar. If you want to use the localizer for only a subset of
the dimensions, then you could use a cluster-based permutation test with
clustering over the remaining dimensions.

If you don't thrust your localizer, then you can do cluster-based
permutation test with clustering over all three dimensions.


Best,

Eric Maris


dr. Eric Maris
Donders Institute for Brain, Cognition and Behavior

Center for Cognition and F.C. Donders Center for Cognitive Neuroimaging

Radboud University
P.O. Box 9104
6500 HE Nijmegen
The Netherlands
T:+31 24 3612651
Mobile: 06 39584581

F:+31 24 3616066
E: e.maris at donders.ru.nl



MSc Cognitive Neuroscience: www.ru.nl/master/cns/





>
> Merry xmas in advance!
>
>
> Best
>
> Alex
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users of the
FieldTrip
> toolbox, to share experiences and to discuss new ideas for MEG and EEG
analysis.
> See also http://listserv.surfnet.nl/archives/fieldtrip.html and
> http://www.ru.nl/neuroimaging/fieldtrip.

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.