Induced activity

[Bobby Stojanoski]

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

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