Hi Panagiotis,
Thanks for your reply!
I know that the downsampling of SPM will truly generate some error shift
of the signal in some cases as you said. However, I avoid to use the
downsampling data here but directly using the raw data, this error-shift
problem still exist.
My data is from Elekta 306 system. I also did the preprocessing with the
Elekta software. It shows the response occur at 2.7 ms after stimulus
which sounds much reasonable than occur at -7 ms by SPM. I have no idea
what else I can do to solve this problem.
I will be really appreciate of any of the clues.
Many thanks,
Jing
> Hi Jing,
>
> Just a thought...
>
> Did you downsample a continuous file before epoching? I have noticed
> that if I downsample a continuous (CTF-MEG) file after filtering and
> before epoching, the triggers shift in a weird way (up to +/-10ms) even
> if he downsampling rate is very close to the original one... (I mean,
> you expect a reasonable jitter in the triggers if you go from 1000Hz to
> 200Hz, but not +/-10ms if you go from 1000Hz to 950Hz for example). I
> wanted to investigate this issue further and provide a proper report but
> I did not have the time to do it yet... and maybe I got something else
> wrong.
>
> Anywayz,
> Good Luck :)
>
>
>
> On 6/20/2011 11:45 AM, Jing Kan wrote:
>> Dear SPMers,
>>
>> Thanks for your concern!
>>
>> I am dealing with the MEG data pre-processing. After the steps of
>> filter,
>> epoch, artifact detect, averaging and baseline checking, I realized the
>> response shifted a few seconds, i.e. the strong response occurred after
>> the stimulus shifted into 7 ms before the stimulus.
>>
>> May I ask is anyone know the reason about this?
>>
>> Many thanks in advance!
>>
>> Jing
>
>
> --
> Panagiotis S. Tsiatsis
> Max Planck Institute for Biogical Cybernetics
> Cognitive NeuroImaging Group
> Tuebingen, Germany
>
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