[FieldTrip] LCMV time window

[Jim Cavanagh]

I get radically different outputs depending on the time window used for
LCMV (primarily MEG, but also a bit with EEG).

My data are sampled at 250 Hz, filtered .5 to 20 Hz, calculated as the
percent change (sourcediff = (source A - source B)  / source A).

With 3 samples (12 ms), LCMV outcomes are uninterpretable (i.e. values of
100,000 % change).  Increasing samples yield increasingly realistic values,
especially at about 25 samples (100 ms). I presume this has to do with the
covariance, especially since there doesn't appear to be much noise in the
data.  This is unfortunate since some of the ERF events that I'd like to
capture are short-lived.

1) Does anyone have an explanation for this issue so I can account for it?
2) Or, barring that, feedback on an optimal way to determine temporal
window size for LCMV?


-- 
Jim Cavanagh
Assistant Professor
University of New Mexico
https://sites.google.com/site/jcavanaghunm/home
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