Eric Maris maris at NICI.RU.NL
Thu Nov 30 21:44:03 CET 2006

Hi Marco,

> 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?

If you had decided to run Cluster Randomizaton Analysis on the absolute
value of the ERP's WITHOUT HAVING LOOKED AT YOUR DATA, then you would have
controlled your Type 1 error (false alarm) rate at 0.05.

This does not imply that, for your data, the null hypothesis holds.

I would advise you to run Cluster Randomization Analysis on the data of each
of the subjects separately. If the same significant spatiotemporal pattern
shows up in the majority of the subjects, then I would be convinced that
something interesting is going on here. However, in this way, you are not
testing a random effects null hypothesis. Maybe you can live with this. I
can live with it, and so do all monkey neuroscientists with their N=2



> Thanks,
> Marco
> --
> Marco Buiatti - Post Doc
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