[FieldTrip] Biosemi eventtype problem

[Elmeri Syrjänen]

We have the same problem with reading the status correctly from Biosemi.
Our experiment software (presentation) sends a zero as first trigger so a
simple value(:) = value(:) - value(1); in the trial function will remove
the offset from the triggers.

/elmeri

On Thu, Jan 22, 2015 at 12:09 PM, Ricarda Braukmann <
r.braukmann at donders.ru.nl> wrote:

> Hi Vitoria,
>
> I had a problem with biosemi markers not being read in correctly by FT as
> well.
>
> First of all, if I remember correctly, using ft_read_event only worked for
> me with .bdf files (and not .edf files).
> Still even with the .bdf files, the numbers were not correct.
> This was caused by the fact that the biosemi system always sent out two
> markers to the EEG (one constant marker and one marker specific to stimulus
> presentation).
> FT for some reason did not recognize these markers as 2 (8bit) markers but
> created 1 16 bit marker from it.
>
> Once I knew this it was easily solved, I just recoded the markers.
> Not sure whether this is what is happening with your set-up as well (might
> be different with newer ft versions), but maybe it helps.
>
> In any case, I belief that the biosemi STATUS markers are indeed not
> strings.
>
> Best,
> Ricarda
>
>
> On Wednesday, January 21, 2015, Alexander Maye <a.maye at uke.de> wrote:
>
>> Hi Vitoria,
>>
>> with this minimal description it's hard to say what the problem is, but
>> these
>> are the things that come to my mind:
>> - Did you setup/modify a config file for the ftbuffer, and did you start
>> the
>> buffer with this config?
>> - Sometimes the higher bits of the parallel port are set, giving you event
>> values >60.000. Maybe you could mask out the bits that you are interested
>> in?
>> Another possibility is that ftbuffer's event values are in
>> two's-complement
>> format. In any case you could check the output of the ftbuffer program -
>> if
>> your events aren't there, your program will not see them either.
>> - Transition from some value to zero are not detected as events as it
>> seems.
>>
>> Hope this helps,
>>
>>                 ALEX.
>>
>>
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