Stefan Debener s.debener at UKE.UNI-HAMBURG.DE
Thu Nov 30 23:22:02 CET 2006

Dear Marco,

I have noticed the same phenomenon with other channel-wise statistics
(like sample-wise ttests) before. A nose-tip recorded ERP P3, for
instance, may show a significant condition effect (e.g. target vs
non-target) at parietal electrodes. This condition effect often is
"mirrored" to, and "shared" with, frontal sites after re-referencing the
data to the common average. At these formerly "inactive" frontal
electrodes, the time course and morphology of the  parietal P3 now shows
up, with the absolute amplitude being decreased and the polarity
reversed (due to re-referencing). As a result, if you look at the
spatial pattern of p values before/after re-referencing, a  previously
significant effect at parietal sites may now turn out to be
non-significant. Probably a good argument for doing statistical analysis
on the source level (which may come with other prolems).


Marco Buiatti wrote:

> Dear FieldTrippers,
> I have a question on the use and interpretation of Cluster
> Randomization Analysis on ERPs. I analised the statistical difference
> between 2 conditions (9 subjects, within-subjects design), and I
> obtained two NON-significant clusters, one negative in the frontal
> lobe (p=0.24) and one positive in posterior regions (p=0.08). These
> clusters emerge and die at almost the same times, so they really look
> like reflecting the SAME process. I tested this hypothesis by
> performing the same analysis on the absolute value of the ERPs: now a
> significant positive cluster (p=0.02) emerges at the same latency of
> the previous two, and spatially overlapping to their topography.
> Do you think that this procedure is plausible, or alternatively that
> it is not correct and the effect is just too weak? Anyone faced a
> similar problem and has alternative procedures?
> Thanks,
> Marco

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