[FieldTrip] Decreased coherence at 60Hz?

[Herbert J Gould (hgould)]

Using a steeply notched filter will distort your waveforms  this looks like you might have had a significant impedance mismatch on several of your electrodes.

Herbert jay Gould, Ph.D.
School of Communication Sciences and Disorders
The University of Memphis


From: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-bounces at science.ru.nl] On Behalf Of Pelt, S. van (Stan)
Sent: Tuesday, March 17, 2015 11:03 AM
To: FieldTrip discussion list
Subject: Re: [FieldTrip] Decreased coherence at 60Hz?

Hi Daria,

As far as I know, signal power influences coherence measures. So, in your case, it might be that you have relatively large power and/or SNR differences between your electrodes, that might underlie your results. You might consider using a notch filter to filter the 60 Hz signal out, or stratifying your trials to compensate for SNR differences.

Best,
Stan
--
Stan van Pelt, PhD
Donders Institute for Brain, Cognition and Behaviour
Radboud University
Montessorilaan 3, B.01.34
6525 HR Nijmegen, the Netherlands
tel: +31 24 3616288


From: fieldtrip-bounces at science.ru.nl<mailto:fieldtrip-bounces at science.ru.nl> [mailto:fieldtrip-bounces at science.ru.nl]<mailto:[mailto:fieldtrip-bounces at science.ru.nl]> On Behalf Of Daria Boratyn
Sent: dinsdag 17 maart 2015 16:42
To: fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>
Subject: [FieldTrip] Decreased coherence at 60Hz?

Hi all,

Using FieldTrip, I created bipolar montages on each hemisphere and then calculated the coherence between those. I am getting an odd result and seeing a decrease in coherence at 60Hz for some of my runs. My understanding is that 60 cycle noise should be picked up by every electrode and would therefore have high coherence values across the board. Would anyone have a suggestions as to what could be causing this? I've attached a screenshot of what I'm seeing.
Thank you,
Daria
[cid:image001.png at 01D060A8.18FB77D0]

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