[Eeglablist] Triggers issues

[Andreas Widmann]

Hi Alex,

I had a look at the data. Phillip's diagnosis is correct. You indeed have to reset the trigger channel to zero (or one; depending on TTL logic) after a certain delay. The delay is system specific. For BioSemi amplifiers one to two samples should be fine. You actually do reset the trigger channel but only some time before the next trial (~750ms; therefore the stimulus triggers are unaffected). You should do that directly after stimulus and response trigger, respectively.

The problem is complicated due to either your stimulation system being faster for rising TTL edges than for falling or the BioSemi amplifier detects rising edges TTL edges faster than falling. Therefore you sometimes get overlapping stimulus and response triggers (actually also resetting the non-active trigger bits to zero) by a single sample introducing the spurious 22, 42, etc. triggers.

For new recordings you should correct your experiment logic. For existing recordings you may use my BioSemi importer you find here:
https://github.com/widmann/erptools/blob/master/pop_bdfread.m
This gives you the same results as with BrainVision Analyzer (no missing response triggers as with biosig but also including the spurious 22, 42 triggers; your description was misleading with respect to the latter). If response trigger latency is irrelevant you may just ignore the spurious events. If response trigger latency matters you have to additionally correct all response trigger latencies *if they were directly preceded by a spurious event* by minus one sample.

Hope this helps! Best,
Andreas

> Am 17.10.2018 um 11:31 schrieb Phillip Alday <phillip.alday at mpi.nl>:
> 
> What software are you using to send the triggers? I've seen this type of
> thing happen when the trigger channels aren't reset to zero after being
> sent. In the VMRK file, this should also be somewhat apparent because
> the duration field will be a non-trivial number.
> 
> Phillip
> 
> On 10/16/2018 05:38 PM, Andreas Widmann wrote:
>> Hi Alex,
>> 
>> the raw data are in BrainVision file format? Which function/plugin do you use for import? In case bva-io I will have a look into the issue if you provide (Dropbox etc.) an example file (raw data) showing the problem.
>> 
>> Best,
>> Andreas
>> 
>>> Am 16.10.2018 um 14:39 schrieb Alex Lepauvre <alex.lepauvre at gmail.com>:
>>> 
>>> Dear All,
>>> 
>>> We are currently having an issue with the response triggers of our experiment. It is design as followed: the participants are presented to a stimuli, to which they have to answer, during which we recorded brain activity using 64EEG cap system. On answer, a triggers is sent (S1 for right, S2 for wrong). We therefore theoretically have 2 triggers per trial: a stimulus onset trigger (S10, S20, S30 depending on condition) and an answer trigger. The time between stimulus onset and answer varies, as participant can answer within five seconds after stimulus onset. The issue is that when importing the data in EEGlab, the response triggers get added to the onset triggers. We therefore don't have any S1 or S2 triggers, but only S12, S22... Moreover, some of the answer triggers disappear. We should indeed have an answer trigger following each onset trigger, which is not systematically the case. However, we processed the data using brain vision analyzer, but the problem did not appear with this software (we still have the S1 and S2 triggers, nor any missing triggers inbetween), despite using exactly the same raw data. The problem seems to be due to the importation of the data in EEGlab, as the problem is already present following importation.
>>> We could not figure out where the problem lies, and we are hoping you could help us figure it out, as this could potentially impair our statistical analysis.
>>> 
>>> Thanks in advance for your help,
>>> 
>>> Best
>>> 
>>> Alex Lepauvre
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