[FieldTrip] redefine trials using different trigger as zero point

[Anne Urai]

Dear Katrin,

if you use a custom trialfun (http://fieldtrip.fcdonders.nl/example/making_your_own_trialfun_for_conditional_trial_definition) you can code the time between stimulus onset and response in an additional column. That way, you can shift the time axis of the data by this amount on each trial. For example:

        % redefine trials
        offset      = (data.trialinfo(:,RTcol));
        prestim     = 1;
        poststim    = 2;
        
        cfg           = [];
        cfg.begsample = round(offset - prestim*data.fsample);
        cfg.endsample = round(offset + poststim*data.fsample);
        data = ft_redefinetrial(cfg, data);
        
        % then shift the time axis
        cfg = [];
        cfg.offset = -offset;
        data = ft_redefinetrial(cfg, data);

        % timelock
        cfg = [];
        cfg.keeptrials = 'yes';
        data = ft_timelockanalysis(cfg, data);

That way, your new data structure will now be response locked. Note that the timelockanalysis is optional, and only works when you have trials that are all the same length. 

If you do not want to use a custom trialfun, you could also rerun the same analysis as you have for the stimulus-locked data, but then taking the response code as your eventvalue.

Good luck,

---
Anne E. Urai, MSc
PhD student | Institut für Neurophysiologie und Pathophysiologie | Universitätsklinikum Hamburg-Eppendorf
Martinistrasse 52, 20246 Hamburg, Germany | http://anneurai.wordpress.com





On 28 Sep 2014, at 11:54, KatrinH Heimann <katrinheimann at gmail.com> wrote:

> Dear all,
> I know I asked this already twice, but I did not get the right answer yet and just cannot figure it out myself.
> So, I did cut my data in quite large trials using ft_define trial and logging it to the beginning of a slide. 
> 
> cfg                     = [];
> cfg.dataset             = name;
> cfg.trialfun            = 'ft_trialfun_general'; % this is the default
> cfg.trialdef.eventtype  = 'trigger';
> cfg.trialdef.eventvalue = 'ceck'; % the value of the stimulus trigger 
> cfg.trialdef.prestim    = 0.216; % in seconds
> cfg.trialdef.poststim   = 7.284; % in seconds (max time of check slide)
> cfg                     = ft_definetrial(cfg);
>  
> % cancel out training trials
> cfg.trl([1:4],:) = [];
>  
> %change timeline according to constant offset of 16 ms = 8 samples (because recorded with 500 hz)in
> %structure trial - plus delay reported by EGE = 18 ms = 9 samples
> cfg.trl(:,3)=cfg.trl(:,3)-17
>  
>  
> 
>  Then I preprocessed these trials 
> 
> %% preprocess data
> cfg.channel    = 'all';
> cfg.preproc.detrend       = 'yes';
> cfg.preproc.demean = 'yes';
> cfg.preproc.baselinewindow = [-0.1 0]
> % 
> mov_data           = ft_preprocessing(cfg);
> % 
> save (strcat(sb,'mov_data') , 'mov_data')
>  
> 
> Now I wanna cut out smaller pieces that are centered around another trigger - the response of the subject. I did not use this trigger at the beginning as then defining a baselinewindow is impossible (as the response is always happening at a different time).
> 
> I tried to just use ft_definetrial again, as I don't see a possibility to use ft_redefine trial:
> 
> cfg = []
> cfg.dataset= strcat(sb,'mov_data.mat')
> cfg.trialdef.eventtype  = 'trigger';
> cfg.trialdef.eventvalue = 'resp'
> cfg.trialdef.prestim    = 0.75
> cfg.trialdef.poststim   = 0.75
>  
> mov_data_small = ft_definetrial(cfg)
> 
> but I get the error message:
> 
> Error using ft_read_header (line 1833)
> unsupported header format (matlab)
> 
> Error in ft_trialfun_general (line 71)
> hdr = ft_read_header(cfg.headerfile, 'headerformat', cfg.headerformat);
> 
> Error in ft_definetrial (line 169)
>     [trl, event] = feval(cfg.trialfun, cfg);
>  
> 
>  I wonder if that is as the file produced by fieldtrip during the preprocessing is not one that is specified for ft_read_header - but how do I deal with this?
> 
> Thanks a lot for your help
> 
> Katrin
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

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