[FieldTrip] second-level stats with TFR

[jan-mathijs schoffelen]

Cher Seb,

In order to be able to compute a single subject T-statistic on the TFR you indeed need to specify cfg.keeptrials = 'yes', prior to calling ft_freqanalysis. Are you absolutely sure that you did this? If so, we need to look into this. Could you create yourself an account on our bugzilla file-server (bugzilla.fcdonders.nl) and file this as a bug. Just copy and paste your e-mail message and also upload some script+data facilitating the reproduction of the problem?

Cheers,

Jan-Mathijs

On Dec 7, 2011, at 7:52 AM, miellet at psy.gla.ac.uk wrote:

> Hello,
> I've got problems doing second-level stats with TFR because of the data structure.
> I previously did it with ERF and didn't experience any difficulty. I'm just comparing 2 conditions so the design is very simple. The dimord for ERF was rpt_chan_time so I had access to the different trials.
> For ERF, I computed individual t-values between my conditions and across trials; then averaged the result across participants and compared the grand-average to zero. Do you think this strategy makes sense?
> For TFR, the dimord is chan_freq_time (with cfg.keeptrials='yes', before or after ft_freqbaseline). Obviously I don't want to compute the individual t-values across channels, freq bands or time points but across trials.
> Do you have any idea where I could find this information please? (I look in previous.....previous but couldn't find anything useful. I also specify keeptrials at each step of the process)
> Thank you very much for your help,
> Sebastien
> 
> ----------------------------------------------
> Dr. Sébastien Miellet, Lecturer
> 
> Department of Psychology
> University of Fribourg
> Faucigny 2
> 1700 Fribourg
> Switzerland
> 
> tel: +41 26 300 7666
> ----------------------------------------------
> 
> 
> 
> Quoting Sara Bögels <s.bogels at psy.gla.ac.uk>:
> 
>> Hi all,
>> 
>> I have been trying to do second-level statistical inference (as described in one of the FAQs) on ERFs, but I am not sure whether I am doing everything correctly.
>> 
>> In the first step I calculate the T-values for the difference between two conditions (twice), which are between items, with ft_timelockstatistics. I put the output of all participants in a cell (called 'stat1a' and 'stat1b'). (I tried to use ft_timelockgrandaverage to combine the subjects together but it needs a field avg).
>> 
>> Then I use ft_timelockstatistics again but  subject level. I first want to look at the difference between the two conditions. This difference is reflected in the T-values of the first step so I create a dummy which is the same as 'stat1' but I replace all the values in the field 'stat' per participant with zeros. Then I call (with appropriate cfg parameters):
>> 
>> stat2a = ft_timelockstatistics(cfg,stat1a{:},dummy{:});
>> stat2b = ft_timelockstatistics(cfg,stat1b{:},dummy{:});
>> 
>> To compare the two differences (stat1a and stat1b) and thereby look at an interaction, I call:
>> 
>> stat2a-b = ft_timelockstatistics(cfg,stat1a{:},stat1b{:});
>> 
>> I am uncertain whether the dummy works (or is there a way to compare the t-values to zero directly?) and whether the stat1a{:} trick works with ft_timelockstatistics.
>> 
>> Thanks in advance for your answer.
>> 
>> Sara
>> 
>> 
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> 
> 
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Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

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