Just to comment on Gareth's answer, both Gareth and I are right. If
you filter the continuous data, you get better results, but you are
more limited at later stages. So you can use wide-band data and
filtering during the inversion to find more or less the optimal band,
but then go back and filter the continuous data using these settings.
Vladimir
On Wed, Apr 21, 2010 at 1:06 PM, Gareth Barnes
<[log in to unmask]> wrote:
> Hi Anette
>
> Here are some answers to the filtering issues.
>
> Best wishes
>
> Gareth
>
>
>
> Filtering:
>
>
>
> First, I would like to understand the difference between filtering during
> preprocessing (spm_eeg_filter) and filtering during inversion (inverse.lpf &
> inverse.hpf). Does it make any difference at all whether I filter during
> preprocessing or during inversion?
>
>
>
> It shouldn’t matter. But the more you filter initially the fewer options you
> will have later on.
>
>
>
> Second, what is the difference between filtering and windowing my data?
> Similarly, what would you suggest for steady state responses: Filter
> frequencies of interest prior to inversion or window them after inversion?
>
>
>
> Windowing is a preprocessing step to ensure an accurate estimate of the
> frequency content of the data- it basically attenuates the edges of the
> timeseries. Typically I would say you have a better chance of seeing the
> response if you filter prior to inversion (NB this could be done at the
> inversion stage)
>
>
>
> 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, that sounds like what is happening. Remember also that the filter has a
> finite fall off slope (i.e. 45Hz cut off means that the power will be approx
> halved at this frequency but invevitably there will be 40Hz attenuation
> also), so best to get your band of interest somewhere in the middle of the
> bandpass.
>
>
>
>
>
> Source Priors:
>
>
>
> I would like to insert auditory and visual source priors for localization.
> However, in different conditions I expect either the auditory, the visual
> cortex or both cortices to be active. So far I invert all conditions
> simultaneously (as suggested in the manual) and defined a general ROI (which
> comprises visual and auditory cortex). However, I was wondering if it makes
> more sense to invert each condition by itself and to define specific ROIs
> for each condition (e.g. auditory cortex for auditory conditions, visual
> cortices for visual conditions and auditory and visual cortices for the
> multimodal conditions).
>
>
>
>
>
> Inversion:
>
>
>
> Is there a way to determine phase locking of the steady state responses
> after inversion?
>
> Lastly, do you have any idea why I get reasonable results with GS, while ARD
> does not work at all?
>
>
>
>
>
> Looking forward to your answers!
>
>
>
> Thanks a lot for your help,
>
> Anette
|