# redefinetrial_offset option

Natalia Grion grion at SISSA.IT
Fri Apr 16 14:42:01 CEST 2010

```Hi Michael,
When I wrote possible solution i meant: -(abs(trl(:,3))+(-51)) = -5958,
which is conceptually different from -6060.
In "definetrial" function when declaring trl(:,3)= - 6009 means that
1)trial start before trigger event (negative sign) and 2)the offset of
trigger is 6009 samples away with respect to the "begining" of the trial:
in sum: that the trial starts 6009 steps before trigger. If the sub-event
to which i want to realign happens 51 steps before trigger, then number of
steps with respect to t=0 are "-" 51, and this is applied when defining
data.time in the code of redefinetrial. But when redefining trl(:,3),
trigger  should get "closer" to beginning of trial, as beginning of trial
is still the old one. If what i' saying is correct, then also trl(:,1) and
(:,2) has to be modified relative to the new "sub-event".
Any reply will be great.
Natalia

> Hi Natalie,
>
> may be I am missing something but the shift by 51 samples implied by
>
> (-)6009 +(-51)
>
> is always to the "left" as you call it - irrespective of the sign of
"6009":
>
> 6009 +(-51) = 5958 < 6009 (left shift)
>
> -6009 +(-51) = -6060 < -6009 (left shift)
>
>
> Michael
>
> -----UrsprÃ¼ngliche Nachricht-----
> Von: Natalia Grion <grion at SISSA.IT>
> Gesendet: Apr 16, 2010 1:09:28 PM
> An: FIELDTRIP at NIC.SURFNET.NL
> Betreff: [FIELDTRIP] redefinetrial_offset option
>
>>Hi all,
>>         When inspecting in detail "redefinetrial" function I found a
>>striking point, I think there is an error on the code that could be
easily solved but maybe i'm missing something:
>> My trials are of fixed length: they go from -3sec to +3sec with 1 trigger
>>event as t=0. I want to realign the trials to a new (sub)event, so I
defined offset as: Nxsamples relative to t=0. For each trial, offset has
different signs as the event happened either before trigger event
(-#samples) or after it (+#samples).
>>In the code: when having for example -51 (samples relative to trigger)
data.time is shifted to the left, and this would be correct. But when
correcting "trial definition" this offset is summed to trl(:,3); the
>> point
>>is: my trl(:,3) is negative since is indicating  that the trial begins
before the trigger, (-)6009 +(-51) results in shifting the offset of
trigger to the right which is not the case: (possible solution:
>>abs(trl(:,3))+(-51).). Then: trl(:,1),trl(:,2) correction is omitted,
>> why?
>>as "event" changed, shouldn't beg and ensample follow this change?
sticking to +/-3sec defined as star/end of trial? In fact, data.time was
shift. In the rest of the code i don't see any line related to this
change.
>> Any help will be welcome!
>>Natalia
>>.............................
>>elseif ~isempty(cfg.offset)
>>  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>% shift the time axis from each trial
>>  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>offset = cfg.offset(:);
>>  if length(cfg.offset)==1
>>    offset = repmat(offset, Ntrial, 1);
>>  end
>>  for i=1:Ntrial
>>    data.time{i} = data.time{i} + offset(i)/data.fsample;
>>  end
>>  % also correct the trial definition
>>  if ~isempty(trl)
>>    trl(:,3) = trl(:,3) + offset;
>>  end
>>........................................
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>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users of
the
> FieldTrip  toolbox, to share experiences and to discuss new ideas for
MEG
> http://listserv.surfnet.nl/archives/fieldtrip.html and
> http://www.ru.nl/neuroimaging/fieldtrip.

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