[FieldTrip] induced activity

Matt Craddock matt.craddock at uni-leipzig.de
Thu Jan 10 22:39:45 CET 2013

On 10/01/2013 19:56, Nenad Polomac wrote:
> Hi,
> I would like to calculate induced time frequency activity for my MEG
> data. I plan to do it as follows. First I calculate total time-frequency
> for every single trial. Second, I calculate evoked activity with
> ft_freqanalysis for the averaged ERF. Then I subtract evoked activity
> from every trial total activity. And finally I average these
> differences. I believe this is correct way...
> My problem is how to subtract evoked.powspctrm(3-D matrix) from
> total.powspctrm(4-D matrix).
> Thank you in advance!
> Nenad

Hi Nenad,

Another common approach is to first compute the ERP/ERF and then 
subtract it from each trial before performing the time-frequency 
transformations. This seems to be the more common approach I see in the 
literature, at least in a lot of EEG gamma band studies. Personally, 
I've found that doing the subtraction in the frequency domain (i.e. 
subtracting the power spectrum of the evoked activity from each trial) 
can give some odd looking results. There tends not to be a whole lot of 
difference between total and induced (ERP-subtracted) in my experience, 
so I stick with total rather than worry about subtracting something 
*into* the data.

Dr. Matt Craddock

Post-doctoral researcher,
Institute of Psychology,
University of Leipzig,
Seeburgstr. 14-20,
04103 Leipzig, Germany
Phone: +49 341 973 95 44

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