Vladimir and Urs, thanks for your replies.
My reason for baseline correction is to minimise differences between
data epochs belonging to different conditions which are not dependent
on the experimental manipulation - e.g. low frequency drifts or
residual artefacts - as these differences could spuriously bias my
data analysis.
Since these spurious differences increase if the baseline is taken
further away from the relevant response, as far as I know, in the ERP
literature it is common to remove the baseline just before the
relevant stimulus, even in cases where the baseline might contain
previously evoked responses - see priming studies focused on the N400.
However, I seem to understand that this reasoning is not always valid
when looking at differences in source space, at least in the case in
which I first source-invert each condition separately, and in a second
step I test the difference between the two conditions in source space.
In this case, if I perform source inversion on EEG data
baseline-corrected on a temporal window containing evoked potentials,
for sure the single-condition source reconstruction will be distorted.
Now, if source inversion were perfectly linear, this would not harm if
I am only interested in comparing two conditions, but I am afraid
source inversion is not completely linear right?
Vladimir, I guess what you are suggesting is to perform source
reconstruction on the data epoched without any baseline correction
right? In this case, drifts would be only attenuated by the high-pass
filter I have previously applied to continuous data, but source
inversion should be more reliable.
Thanks for telling me if this reasoning looks wrong to you.
Best,
Marco
On 5 December 2012 22:50, Vladimir Litvak <[log in to unmask]> wrote:
> Dear Marco,
>
> I don't quite understand at what point you need that baseline and why you don't compare the activation images to each other without any baseline.
>
> Best,
> Vladimir
>
>
>
> On 5 Dec 2012, at 16:41, Marco Buiatti <[log in to unmask]> wrote:
>
>> Dear SPM Masters,
>>
>> Here's a more general and synthetic version of my previous question.
>>
>> My doubt: what is the optimal baseline for EEG source reconstruction,
>> and specifically:
>> 1) Is it problematic to set the baseline in a period that contains
>> (sensory) evoked responses? Is there a risk of suppressing the sources
>> of those evoked responses?
>> 2) Otherwise, is it problematic to set a baseline much before the
>> relevant stimulus (in my case, 800 ms before)? How do slow drifts in
>> the data impact on source reconstruction?
>>
>> Thanks,
>>
>> Marco
>>
>>
>> On 3 December 2012 11:59, Marco Buiatti <[log in to unmask]> wrote:
>>> Dear SPM Masters,
>>>
>>> I have yet another doubt on EEG source reconstruction which might be
>>> of general interest. I have a priming design in which the target
>>> stimulus is preceded by a prime stimulus, presented either 800 ms
>>> before (if it's auditory) or 360 ms before (if it's visual). In my
>>> simplest case, I want to test the difference between two different
>>> conditions, both comprising both visual and auditory primes. My
>>> problem is to choose the most appropriate baseline for EEG source
>>> reconstruction.
>>>
>>> At the sensor level, since I am testing a difference that should not
>>> depend on the evoked response of the prime, it is common use to take
>>> the baseline in the 100-200 ms window before the target presentation.
>>>
>>> For source reconstruction, is the fact that the pre-target baseline
>>> contains the evoked responses of the prime problematic? Could it
>>> suppress the same kind of responses and/or generate "phantom
>>> responses"?
>>> Should I choose a pre-prime baseline (e.g. 800 ms before the target),
>>> at the risk of having more variability due to low frequency drifts?
>>> Data have been high-pass filtered at 0.3 Hz before epoching, in this
>>> case I might increase the high-pass filter cut-off of the continuous
>>> data.
>>>
>>> Thanks a lot for your feedback,
>>>
>>> Best,
>>>
>>> Marco
>>>
>>> --
>>> Marco Buiatti, PhD
>>>
>>> CEA/DSV/I2BM / NeuroSpin
>>> INSERM U992 - Cognitive Neuroimaging Unit
>>> Bāt 145 - Point Courrier 156
>>> Gif sur Yvette F-91191 FRANCE
>>> Ph: +33(0)169.08.65.21
>>> Fax: +33(0)169.08.79.73
>>> E-mail: [log in to unmask]
>>> http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
>>>
>>> ***********************************************
>>
>>
>>
>> --
>> Marco Buiatti, PhD
>>
>> CEA/DSV/I2BM / NeuroSpin
>> INSERM U992 - Cognitive Neuroimaging Unit
>> Bāt 145 - Point Courrier 156
>> Gif sur Yvette F-91191 FRANCE
>> Ph: +33(0)169.08.65.21
>> Fax: +33(0)169.08.79.73
>> E-mail: [log in to unmask]
>> http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
>>
>> ***********************************************
--
Marco Buiatti, PhD
CEA/DSV/I2BM / NeuroSpin
INSERM U992 - Cognitive Neuroimaging Unit
Bāt 145 - Point Courrier 156
Gif sur Yvette F-91191 FRANCE
Ph: +33(0)169.08.65.21
Fax: +33(0)169.08.79.73
E-mail: [log in to unmask]
http://www.unicog.org/pm/pmwiki.php/Main/MarcoBuiatti
***********************************************
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