too many trials/overlapping trials

Sameer Walawalkar sameer at ANDREW.CMU.EDU
Tue Feb 6 14:32:44 CET 2007


I am currently familiarizing myself with Fieldtrip by using  the
information gleaned from$g_started_with_reading_raw_eeg_or_meg_data
beginning with the most basic application which starts with defining the

I use the first three lines to set up cfg, and then issue the command
Note: the data is from Elekta Neuromag 306 channel MEG.

It returns some thousand odd events and finds STI 101 and STI 301 as Stim

So next I do the following:

>> cfg =  []    ;
>> cfg.dataset = 'JD_012507_Beeps_One.fif' ;
>> cfg.trialdef.eventtype  = 'STI101'  ;
>> cfg.trialdef.eventvalue = 7    ;
>> cfg.trialdef.prestim = 1.500;
>> cfg.trialdef.poststim = 2.501;
>> cfg =  definetrial(cfg);
evaluating trialfunction 'trialfun_general'
found 61361 events
created 60 trials

My first problem is, I should have 30 trials for this event. The file
contains a total of 60 trials, but the other 30 are for eventtype = 11
So it is clearly doing something wrong.

Next, I decided to see what is in cfg.trl and I get the Nx3 matrix that I
find the following problems with it

1> each trial should be 4000 milli seconds long. That happens here,
however, there seem to be overlap between trials which does not make
sense. This is what leads to the double counting. Thus I get 4000 msec
long trials bunched in group of two where the second trial starts about
1000 to 1300 msecs after the beginning of the first trial.

2> If the first trial is at 8502, all subsequent trials should be in
increments of integral multiples of 4000 ( plus or minus a few). But that
does not seem to happen (and I am looking at on the leading
trial of the pair and discounting the trials that follow 1000
msecs later.)

Thanks for your help.


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