# [FieldTrip] interactions between two factors

Arnaud Delorme arno at cerco.ups-tlse.fr
Thu Jul 26 18:35:49 CEST 2012

```Dear Marco,

I believe the first answer is the correct one. The problem is somewhat trivial but not implemented yet.
The LIMO toolbox will let you perform 2-way statistics with cluster correction for multiple comparisons.

Best,

Arno

On Jul 26, 2012, at 3:36 AM, Marco Buiatti wrote:

> Dear FieldTrippers,
>
> some time ago I have posted the message below concerning how to
> compute the statistical interaction between two factors in an EEG
> study with the FieldTrip cluster-based statistical analysis. Since I
> believe it is a problem of general interest, I was confident I would
> and I am trying to guess why:
>
> - the problem is trivial, I should go back to my statistics books and
> solve it myself;
> - the problem is ill-posed, I should go back to my statistics books
> and reformulate it correctly;
> - the problem is tabou, no one dares commits to a solution because it
> could be a wrong one.
> - the problem is solved: I should read message number #.
>
> Thanks a lot for your feedback,
>
> Best,
>
> Marco
>
> On 25 May 2012 15:58, Marco Buiatti <marco.buiatti at gmail.com> wrote:
>> Dear FieldTrippers,
>>
>> I am analysing an EEG study with 2x4 factors: one varies between 4
>> parametrically varying levels (1 to 4), the second between two levels.
>>
>> I have three questions concerning the use of Fieldtrip cluster-based
>> non parametric statistical analysis in this case:
>>
>> 1) How to compute the interaction between the two factors. Let's start
>> from the simplest case of a 2x2 design, factors varying between values
>> A1 and A2 for the first factor, B1 and B2 for the second. Please tell
>> me if it is correct to compute the interaction by:
>> - computing the difference diffA=ERP(A1)-ERP(A2) separately in
>> condition B1 and B2, for every subject
>> - performing a within-subjects statistical analysis between diffA in
>> condition B1 and diffA in condition B2 (function
>> statfun_depsamplesT.m).
>>
>> 2) Now consider that factor A varies parametrically between values 1
>> to 4. For the main effect of this factor, I have used the Fieldtrip
>> function statfun_depsamplesregrT.m and I'm satisfied with it. Is it
>> correct to compute the interaction by
>> - computing the regression
>> regrA=regression(ERP(A1),ERP(A2),ERP(A3),ERP(A4)) (computed as inside
>> function statfun_depsamplesregrT.m) separately in condition B1 and B2,
>> for every subject
>> - performing a within-subjects statistical analysis between regrA in
>> condition B1 and regrA in condition B2 (function
>> statfun_depsamplesT.m)?
>>
>> 3) Since BEFORE looking at the data (this is to prevent Eric's
>> contestation...) I expect a dipolar topography for the regression
>> (data are in average reference), I would like to combine into a joint
>> cluster negative and positive clusters. I have tried by changing
>> statfun_depsamplesregrT.m by just taking the absolute value of the
>> regression, but I get weird results (a huge, non significant cluster).
>> Is it possible that since values are now all positive, I should use a
>> different statistical test at the single bin level? Any other
>> suggestions?
>>
>>
>> Marco
>>
>>
>>
>> --
>> Marco Buiatti, PhD
>>
>> CEA/DSV/I2BM / NeuroSpin
>> INSERM U992 - Cognitive Neuroimaging Unit
>> Bât 145 - Point Courrier 156
>> Gif sur Yvette F-91191  FRANCE
>> Ph:  +33(0)169.08.65.21
>> Fax: +33(0)169.08.79.73
>> E-mail: marco.buiatti at gmail.com
>> http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
>>
>> ***********************************************
>
>
>
> --
> Marco Buiatti, PhD
>
> CEA/DSV/I2BM / NeuroSpin
> INSERM U992 - Cognitive Neuroimaging Unit
> Bât 145 - Point Courrier 156
> Gif sur Yvette F-91191  FRANCE
> Ph:  +33(0)169.08.65.21
> Fax: +33(0)169.08.79.73
> E-mail: marco.buiatti at gmail.com
> http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
>
> ***********************************************
>
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> fieldtrip at donders.ru.nl
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--
Arnaud Delorme, PhD
Centre de Recherche Cerveau et Cognition - UMR 5549
Pavillon Baudot, Hopital Purpan, BP 25202
31052 Toulouse Cedex, France

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