[FieldTrip] NAI noise normalization beamforming

[Merkel, Nina]

Dear Fieldtrip Community,

I am using NAI normalization for my resting state MEG data. It does something to my data that I don't quite understand😳.

Before normalization I have the average power of groupA > groupB (also true for the sensor data). After normalization the pattern turns, while the variance of groupA decreases and the variance in groupB increases. I am not exactly sure what that means. Is the change in power completely explainable by the higher variance of groupA? Are there any limitations of using NAI? Is there a situation where I should not use it?

I am especially wondering about the variance since during my whole analysis it was quite clear that this a a major difference between the groups.

I am posting this question since I was not able to figure out how exactly the NAI is estimated during source reconstruction.

Does anyone have more information about the computation of NAI?

Thanks so much

Nina
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