FieldTrip support

Thomas Sauvigny tsauvigny at GOOGLEMAIL.COM
Wed Aug 25 11:26:03 CEST 2010


Dear fieldtrip users!

 

 

May I ask for your support because of a problem concerning my within-subject
permutation test (EEG-Data)?

 

The goal is to compare  two grandaverages with two different conditions,
each grandaverage consisting of 4 datasets from 4 subjects, each dataset
about 40-50 single trials.

I did the preprocessing, timelock and grandaverage: 4 datasets (called x1, 

x4) for the first condition with the following command:

 




cfg = [];

cfg.keeptrials = 'yes';

timelock1 = timelockanalysis(cfg, x1);

timelock2 = timelockanalysis(cfg, x2);

timelock3 = timelockanalysis(cfg, x3);

timelock4 = timelockanalysis(cfg, x4);

 

 

cfg.channel = 'all'

cfg.latency = 'all'

cfg.keepindividual = 'yes'  

cfg.normalizevar = 'N-1'

 

da_test1= timelockgrandaverage (cfg, timelock1, timelock2, timelock3,
timelock4) 

save da_test1

 

Same way for the second condition: da_test2

Now I start the permutation test (file attached). As result there are no
significant clusters highlighted (although there are  quite large
differences in the ERP-plot). I think because there is a mistake concerning
the number of single-trials which fieldtrip takes as the basis for the
statistic test.

 

Because the output in the command window shows:

 




selected 30 channels

selected 2501 time bins

selected 1 frequency bins




total number of measurements     = 8

total number of variables        = 2

number of independent variables  = 1

number of unit variables         = 1

number of within-cell variables = 0

number of control variables      = 0

using a permutation resampling approach

repeated measurement in variable 1 over 4 levels

number of repeated measurements in each level is 2 2 2 2 

computing a parmetric threshold for clustering

computing statistic

estimated time per randomization is 0 seconds

computing statistic 1 from 100




found 7 positive clusters in observed data

found 11 negative clusters in observed data




stat = 

 

                   prob: [30x2501 double]

            posclusters: [1x7 struct]

    posclusterslabelmat: [30x2501 double]

        posdistribution: [1x100 double]

            negclusters: [1x11 struct]

    negclusterslabelmat: [30x2501 double]

        negdistribution: [1x100 double]

                   mask: [30x2501 logical]

                   stat: [30x2501 double]

                    ref: [30x2501 double]

                 dimord: 'chan_time'

                  label: {30x1 cell}

                   time: [1x2501 double]

                    cfg: [1x1 struct]

 

 

cfg = 

 

    xlim: [0 0.1000]

    zlim: 'maxmin'

 

the input is timelock data with 30 channels and 4000 timebins

applying preprocessing options

averaging trials

averaging trial 1 of 4

averaging trial 2 of 4

averaging trial 3 of 4

averaging trial 4 of 4

reading layout from file easycap32ch-avg.lay

 

So, could this be the mistake that fieldtrip guesses “4” to be the number of
all trials (perhaps because 4  datasets in each grandaverage??) And how can
I solve this problem? Is there a way to account for the single-trial number
when using the permutation test?

 

Thank you very much for your help!!

 

Kind regards 

 

Thomas Sauvigny

Tübingen University

 


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