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Subject:

Re: problems with trigger sampling

From:

Gustavo Sudre <[log in to unmask]>

Reply-To:

Gustavo Sudre <[log in to unmask]>

Date:

Wed, 17 Jun 2009 11:33:37 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (55 lines)

Olaf,

I've seen this problem as well, but I'm afraid I don't have any clever  
solutions to share. At least with my triggers, I imagine it is related  
to how fast the bits in the parallel port are changing, which take  
longer than the actual sampling rate of the system (and hence, why  
you'd see it more often with a higher sampling rate). For example,  
let's say that my transition was supposed to be from 5 to 0 (or 101 to  
000 in binary). Then, for one sample I would have something like a 4  
(100 in binary) in the middle, because not all the bits switched at  
the same time. But this is just an example, and if I remember  
correctly the values of the trigger would have to very somewhat high  
for it to happen. But I don't have much background to give any more in- 
depth explanation about it... this is just my naive explanation for it.

As far as fixing it, it depends on what you're using for data  
analysis. You could have MNE output all trigger line transitions and  
then manually (Excel?) delete all the entries that don't make much  
sense. You could write a simple matlab script to do it too. If you're  
going to use matlab, you might as well just extract the single  
channel, fix the trigger values to what they were supposed to be (if  
you look at the bits explanation above, you should be able to figure  
out in your case what constant was being added to the values... that's  
what I did when I saw that), and then write the clean channel back to  
the raw fif file. Another option would be to use a mask to the trigger  
channel in MNE when opening the file (if that's what you're using).

Best,

Gus

On Jun 17, 2009, at 8:09 AM, Olaf Hauk wrote:

> Hi,
>
> we have recently become aware of a problem with trigger sampling in  
> the Neuromag system. Externally delivered trigger values are not  
> always read correctly at the beginning or the end of a trigger pulse  
> (both in the digital trigger channels as well as the combined  
> channel STI101). The first and last sample point may show an  
> intermediate value between 0 and the actual trigger value (e.g. ...0  
> 0 240 255 255... instead of ...0 0 255 255 255...). We tested  
> sampling rates of 1000Hz and 5000Hz, and it appears that there is  
> always a delay of exactly one sampling point (but more erroneous  
> triggers occurred for the 5000Hz case).  We therefore conclude that  
> the problem must lie on the acquisition (rather than the  
> presentation) side.
> Has anyone else come across this problem before, and possibly found  
> a (hardware) solution? Not every analysis software automatically  
> handles this problem correctly, and it has already caused some  
> confusion in the Cambridge MEG community.
>
> Regards,
>
> Olaf Hauk

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