Dear Jing,
Low-pass filtering is basically smoothing so it may cause activity to
appear earlier in time than it actually is, even before the stimulus.
If timing is very important for you you need to think carefully about
your filter settings. There is a possibility to apply one directional
filter (it's accessible from batch and script). This kind of filter
can only shift activity to later in time, but this is also distortion
just a different kind. Any filtering distorts the original signal, you
just need to know what features are important for you.
Best,
Vladimir
On Mon, Jun 20, 2011 at 4:16 PM, <[log in to unmask]> wrote:
> Dear Vladimir,
>
> Thanks for your email!
>
> I did the operation you asked me to do, I did "epoch-> artifact ->
> averaging -> baseline correction" without filtering, and the data shows
> the response at the correct position(at 2 ms) without shifting-error( at
> -7 ms).
>
> May I ask what kind of problem it would be? How can I avoid this error
> happen?
>
> Many thanks!
>
> Jing
>
>
>
>
>> Dear Jing,
>>
>> Do you still get this shift without any filtering (just with baseline
>> correction)? If you do I might need to look at an example of you raw
>> data file.
>>
>> Best,
>>
>> Vladimir
>>
>> On Mon, Jun 20, 2011 at 2:16 PM, Jing Kan <[log in to unmask]> wrote:
>>> 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
>
>
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