about cluster randomization analysis

Marco Buiatti marco.buiatti at UNIV-PARIS5.FR
Fri Oct 28 13:40:26 CEST 2005

Dear FieldTrippers,

I'm using fieldtrip cluster randomization functions to investigate
statistically significant differences between the ERPs of 8 subjects
relative to two different experimental conditions. In order to prove the
test performance, I'm using an experiment that shows an evident
significant difference, easily detectable with a simple t-test. I have
some difficulties though in interpreting the results, because they look to
me very dependent on the parameters I use. My main question is therefore:
is there any general rule of thumb you can propose in choosing the

The procedure I am following now is a sort of two-steps method: in the
first place, I choose a wide time interval and a low minimum number of
channels. I end up with many clusters that are far from being
significative. I then shorten the time interval to include just one
cluster (starting from the most significant one), and increase the minimum
number of channels, and run the analysis again. In this case, I eventually
got a significative cluster where I was expecting it from a simple
observation of the t-test. Do you think this procedure is right or am I
doing something wrong? Is it correct to temporally focus on a cluster to
check its significance?

Another couple of questions:
1) Minnbchan. I understood it is the minimum number of significative
neighbor (channel,time) points for a  (channel,time) point to enter a
cluster, no matter if adjacency is more in channel space or time
direction. Am I right? Since time and channel space are quite different
dimension, would it be better to set a minimum channel number separately
for the two?
2) Maybe because my data are average-referenced, I often end up with a
positive and negative cluster emerging almost at the same time. Have you
thought about any way to include the search of dipole-like configurations?

Thank you and have a good day.

Marco Buiatti - Post Doc

Cognitive Neuroimaging Unit  - INSERM U562
Service Hospitalier Frederic Joliot, CEA/DRM/DSV
4 Place du general Leclerc, 91401 Orsay cedex, France
Telephone: +33 1 69 86 77 65    Fax: +33 1 69 86 78 16
E-mail: marco.buiatti at gmail.com    Web: www.unicog.org

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