Induced activity

[Adrian Bartlett]

Hi Bobby,

This sounds like you may have a case of line noise contamination, which is
usually seen at 50/60Hz (depends on which continent you are on). While I am
not familiar with it's implementation in FIELDTRIP, there are notch
filtering solutions to eliminate this contamination. Using multitaper FFT
will usually result in the 0 power (post-notch-filtering) at ~60 Hz being
smoothed over by the narrowband frequency bias.

- A

On Thu, Mar 25, 2010 at 1:33 PM, Bobby Stojanoski <
stojanoski at utsc.utoronto.ca> wrote:

> Dear Fieldtrippers,
>
>
>
> I am a relatively new user of fieldtrip and am very impressed!
>
>
> I am interested in comparing differences at certain frequencies – induced
> 40-100 Hz – between 2 experimental conditions.  My understanding was that
> I can calculate induced activity in the gamma range by calculating the power
> for each trial (subject average -- freqanalysis) and then averaging across
> subjects (grand average -- freqdescriptives/freqgrandaverage).
>
>
> To my dismay, when I plotted the results of my grandaverage I found a band
> of power at 55 - 65 Hz for the entire duration of my epoch. I should add
> this was not the case when I plotted power across trials for each
> participant.
>
>
> Earlier discussions mention computing induced+evoked (using freqanalysis
> and freqgrandaverage) and subtracting that from evoked (using
> timelockanalysis+freqanalysis) to get extract induced only activity.
> However, later posts suggest that this is not a valid approach.
>
>
> 1. Where have I made my mistake?
>
>
> 2. If ((induced+evoked)-evoked)) is not valid, what is the correct approach
> to calculating induced activity at 40 - 100 Hz?
>
>
> Any help would be greatly appreciated!
>
>
> Thank you
>
> Bobby Stojanoski
>
>
>
> ----------------------------------
>
> The aim of this list is to facilitate the discussion between users of the
> FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and
> EEG analysis.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/
>

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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