[FieldTrip] Opposite DICS Beamforming results on source and sensor level on resting state data

[Tom Marshall]

Hey Haiteng,

Following up on Arjen's point; I've noticed that when people in the MEG 
close their eyes for a couple of minutes, their heads sometimes drop a 
little (ie nose moves toward chest). If your clinical group were feeling 
more drowsy during the recording and thus dropped their heads more, this 
would lead to exactly the kind of systematic SNR difference that Arjen 
is describing, and maybe most acutely in posterior sensors.

Best,
Tom

On 1/5/2015 3:39 PM, Stolk, A. (Arjen) wrote:
> Hey Haiteng,
>
> Is your contrast based on absolute signal frequency power? If so, did 
> you check for any systematic differences in headposition (and 
> especially in terms of distance to the sensors - the z-dimension) 
> across the groups? I presume such a systematic difference could yield 
> different results at the sensor- and source-level, but there are 
> probably also other possibilities out there.
>
> Yours,
> Arjen
>
> --
> Donders Institute for Brain, Cognition and Behaviour
> Centre for Cognitive Neuroimaging
> Radboud University Nijmegen
>
> Email: a.stolk at donders.ru.nl <mailto:a.stolk at donders.ru.nl>
> Phone:  +31(0)243 68294
> Web: www.arjenstolk.nl <http://www.arjenstolk.nl>
> ------------------------------------------------------------------------
> *From:* fieldtrip-bounces at science.ru.nl 
> [fieldtrip-bounces at science.ru.nl] on behalf of Haiteng Jiang 
> [haiteng.jiang at gmail.com]
> *Sent:* Monday, January 05, 2015 3:23 PM
> *To:* fieldtrip at science.ru.nl
> *Subject:* [FieldTrip] Opposite DICS Beamforming results on source and 
> sensor level on resting state data
>
> Hi  all,
>
>    I performed DICS beamforming on resting-state data ( eyes closed) 
> of a clinical population and controls. According to the sensor data, 
>  the control groups have more alpha-band (8-14
> Hz) activity over occipital  areas  after cluster statistic (attached 
> figure  upper plot) . Curiously, after beamforming ,  group 
> comparisons showed the reversed patters in visual cortex (attached 
> figure  bottom plot) .Hence, the source-level results are opposite to 
> the sensor-level results. This is *not* a problem of the design 
> matrix, or confusing the groups.  I check  the individual neural 
>  activity index on the single subject level .   They  make sense in 
> general .  I also tune the parameter a lot (tapper, central frequency 
> smooth frequency , regularization  parameter , et al ), the  opposite 
> pattern remains. I  understand that Beamformer images DO NOT DIRECTLY 
> CORRESPOND TO ANY sensor data.   However, the opposite pattern is 
> really weird.  I noticed that  Tobias Navarro Schröder had the similar 
> issue 4 years ago 
> (http://mailman.science.ru.nl/pipermail/fieldtrip/2011-May/003875.html). 
> Thus,  I am not the only one who encountered this problem.
>    Any tips and suggestions will be greatly appreciated.  Thanks in 
> advance!
>
>
>
>         Best,
>         Hatieng
>
>
>
> -- 
> Haiteng Jiang
> PhD candidate
> Donders Institute for Brain, Cognition and Behaviour
> Neuronal Oscillations Group
> Computational Cognitive Neuroscience Lab
> https://sites.google.com/site/haitengjiang/
>
>
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

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