Dear Annette,
What I was trying to say was that if you don't see your auditory
activation when you include low frequencies in the inversion, you
shouldn't include those low frequencies i.e. you should filter the
data before the inversion. Time-frequency contrast computed after the
inversion can make some difference because it can specifically give
you the sources activated at your frequency of interest. But if those
sources were not localized properly because of interference from the
low frequencies then time-frequency contrast won't help.
Vladimir
On Mon, Apr 26, 2010 at 2:47 PM, Anette Giani
<[log in to unmask]> wrote:
> Dear Vladimir,
>
> Thanks very much for your fast and helpful reply! However, I am uncertain
> concerning one of your answers.
>
>
>
>> Interestingly, windowing my data around 40 Hz (which corresponds to
>> the auditory stimulation frequency) reveals auditory activation.
>> However, if I apply a more broadbent window of 3:45 Hz all auditory
>> activation seems to be gone. In this case, all activation seems to be
>> localized in visual cortex (Importantly, visual stimulation occurred
>> at a frequency of 6 Hz.) Is it possible that stronger visual activation
> outperforms auditory activation?
>>
>
> Yes, it's possible and the results you get empirically answer your question.
> The inversion aims at producing a solution which explains maximal variance
> in the data. Therefore if the signal at low frequencies is much stronger
> than at high frequencies the algorithm will focus at modelling the low
> frequencies. Windowing after inversion will not solve this because at this
> stage the solution is already computed. So if this is indeed a problem for
> you, you should filter your data to the range of interest before the
> inversion.
> If I understand you correctly
>
>
>
> If I understand your answer correctly, windowing has no effect on the
> solution of the source localization. However, depending on the frequencies I
> chose for windowing, auditory activation becomes "visible" (that is in the
> case of narrowband windows) or "invisible" (in the case of broadbent
> windows). Since the solution has been determined during inversion, why can
> the auditory activation only been "seen" during narrowband windowing?
>
> Thanks again and best,
> Anette
>
>
>
>
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