[FieldTrip] ft_definetrial

[Schoffelen, J.M. (Jan Mathijs)]

Dear Hannah,

From your error, it seems that you called ft_REdefinetrial, rather than ft_definetrial, I am not sure whether that was intended.
The way forward would be (in your case)

cfg = ft_definetrial(cfg);

% this yields a cfg.trl matrix that defines epochs  - from - marker1 - to - marker2

Then, you can use ft_preprocessing(cfg) to load in the epoched data, followed by a call to ft_redefinetrial to cut the epochs into 2 second snippets.

Alternatively, you can write a fancy ’trialfun’ that also incorporates the cutting into epochs of desired length (+desired overlap). Perhaps you can have a look at Appendix 2 of this tutorial: http://www.fieldtriptoolbox.org/tutorial/coherence/, which gives an example of such a trialfun.

Best wishes,
Jan-Mathijs




On 19 Jun 2020, at 05:31, Hannah Anderson <handerson at usc.edu.au<mailto:handerson at usc.edu.au>> wrote:

Dear FieldTrip community

I am attempting to define a trial based on a start and end marker, and then cut the trial into 2 second segments with 0.5% overlap.

    Marker Segmentation
    cfg                                      = [];
    cfg.dataset                       = d(data).name;
    cfg.continuous                 = 'yes';
    cfg.trialfun                        = 'trial_fun_startend'; %trial_function for marker segmentation
    cfg.trialdef.prestim          = 0.2; %in seconds
    cfg.trialdef.poststim        = 0; %in seconds
    cfg.trialdef.ntrials             = inf;

    cfg                                       = ft_definetrial(cfg);  % define trials

I created a trial fun which is successful in creating a trial from start marker to end marker but I get an error I get an error;
-Index in position 2 exceeds array bounds (must not exceed 2)
Error in ft_redefinetrial (line 267)
 offset    = newtrl(:,3);

when I try and then segment the trial into 2 second trials with overlap.

This is my trialfun:

function [trl, event] = trial_fun_startend(cfg);

hdr          = ft_read_header(cfg.dataset);
event        = ft_read_event(cfg.dataset);

detectflank  = 'up';

value        = [event(find(strcmp('STATUS', {event.type}))).value]';
sample       = [event(find(strcmp('STATUS', {event.type}))).sample]';

trl          = [];
sel          = find(strcmp({event.type}, 'STATUS'));
event        = event(sel)

begsample = event(1, 2);
endsample = event(1, 3);
offset    = newtrl(:,3);
trl      = [begsample endsample];

I am not sure what else I can try as the other define trial options segment the data into two trials (one for each trigger) and I need it to be one dataset starting at trigger 1 and ending at trigger 2. If anyone could offer some advice/assistance it would be greatly appreciated.

Kind regards,
Hannah

University of the Sunshine Coast  l  CRICOS 01595D
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