Dear Antanas,
On Thu, Aug 27, 2009 at 2:52 PM, Spokas
Antanas<[log in to unmask]> wrote:
> Thank you for your answer, you're right that predicted and observed scalp
> maps match at that time point, it's just that in the region there is
> negative activation and source activation is shown high positive activation,
This is quite normal. The relation between source polarity and scalp
polarity depends on the orientation of the source. You are used to
sources oriented upwards but this source as you are saying is on the
inferior surface of the frontal cortex (but see below) so it is
oriented downwards and when it has positive activation what you see on
the top of the head is negative. I again refer you to the dipole
simulator from my previous e-mail.
> and it's quite a sharp peak lasting for very short time in that source time
> course and it is hard for me to interpret that activation, as it's at 100ms
> poststimulus and in the averaged time series and this activated source is in
> the inferior frontal cortex, when I would only expect occipital activation?
When I hear something like this I immediately think 'eyeblink'. It's
indeed unlikely that you will see true orbitofrontal source but it is
very likely that your data is contaminated with eye-blinks and since
your subjects were probably consistently blinking around the stimulus
presentation these eyeblinks are not averaged out. Since SPM does not
model sources in the eyes, this activation is explained by the closest
source on the mesh which is orbitofrontal cortex. There are several
things you can do ranging: just ignore this source with the hope that
SPM will still model the occipital brain sources correctly, go back to
your single trials and reject the ones with eyeblinks, high pass
filter your data hoping to get rid of at least some it etc. In general
the best solution is to instruct your subjects to fixate and not blink
during stimulus presentation and then reject all the trials with
blinks.
> Another thing is that in ERP literature it's quite usual to find direct
> reference from active electrodes to the brain area activated, e.g. occipital
> electrodes will be indicating occipital cortex involvment...? Thank you.
>
This is perhaps true for the occipital cortex where because of the
anatomy the brain sources project to the back of the head, but not
true in general.
Best,
Vladimir
> ________________________________
> From: Vladimir Litvak <[log in to unmask]>
> To: [log in to unmask]
> Sent: Thursday, 27 August, 2009 11:30:13
> Subject: Re: [SPM] imaging source reconstruction
>
> Dear Antanas,
>
> The relation between where the source is and where the electrodes
> record the maximal deflection is not a simple one (you can play with
> the tool from
> ftp://www.besa.de/be/besa.de/free_tools/DipoleSimulator-2009-03-27-Install.exe
> to get some intuition). Did you try to compare in the rendering tool
> the predicted and the observed scalp maps? If they match then perhaps
> SPM does a good job after all. If not then I can look at the example
> and tell you if there is something wrong. Also remember that SPM does
> not reconstruct each time frame separately (like some other packages)
> but looks at temporal and spatial modes computed over the whole time
> window.
>
> Best,
>
> Vladimir
>
> On Thu, Aug 27, 2009 at 10:21 AM, Spokas
> Antanas<[log in to unmask]> wrote:
>> Dear Vladimir,
>>
>> I've incounted troubles with imaging source reconstruction, I've just
>> noticed, that at some point in time series, it's estimated source with
>> highest activation over the whole brain in the region, where none of the
>> electrodes have recorded much of a potential and completely ignored much
>> higher potential at different sight?.. meaning that the whole estimation
>> flies out of the window? I've checked again all electrode positions in
>> cooregistration, and all looks good there. Could you please explain
>> Vladimir, thank you.
>>
>> Regards,
>> Antanas
>>
>>
>>
>
>
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