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Hey Haiteng,<br>
<br>
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.<br>
<br>
Best,<br>
Tom<br>
<br>
<div class="moz-cite-prefix">On 1/5/2015 3:39 PM, Stolk, A. (Arjen)
wrote:<br>
</div>
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<div style="direction: ltr;font-family: Tahoma;color:
#000000;font-size: 10pt;">Hey Haiteng,<br>
<br>
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.<br>
<br>
Yours,<br>
Arjen<br>
<div><br>
<div style="font-family:Tahoma; font-size:13px">
<div style="font-family:Tahoma; font-size:13px">--<br>
Donders Institute for Brain, Cognition and Behaviour<br>
Centre for Cognitive Neuroimaging<br>
Radboud University Nijmegen<br>
<br>
Email: <a moz-do-not-send="true"
href="mailto:a.stolk@donders.ru.nl" target="_blank">a.stolk@donders.ru.nl</a><br>
Phone: +31(0)243 68294<br>
Web: <a moz-do-not-send="true"
href="http://www.arjenstolk.nl" target="_blank">www.arjenstolk.nl</a></div>
</div>
</div>
<div style="font-family: Times New Roman; color: #000000;
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<hr tabindex="-1">
<div style="direction: ltr;" id="divRpF292681"><font
color="#000000" face="Tahoma" size="2"><b>From:</b>
<a class="moz-txt-link-abbreviated" href="mailto:fieldtrip-bounces@science.ru.nl">fieldtrip-bounces@science.ru.nl</a>
[<a class="moz-txt-link-abbreviated" href="mailto:fieldtrip-bounces@science.ru.nl">fieldtrip-bounces@science.ru.nl</a>] on behalf of Haiteng
Jiang [<a class="moz-txt-link-abbreviated" href="mailto:haiteng.jiang@gmail.com">haiteng.jiang@gmail.com</a>]<br>
<b>Sent:</b> Monday, January 05, 2015 3:23 PM<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:fieldtrip@science.ru.nl">fieldtrip@science.ru.nl</a><br>
<b>Subject:</b> [FieldTrip] Opposite DICS Beamforming
results on source and sensor level on resting state data<br>
</font><br>
</div>
<div>
<div dir="ltr">Hi all,
<div><br>
</div>
<div> 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
<div>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 (<a
moz-do-not-send="true"
href="http://mailman.science.ru.nl/pipermail/fieldtrip/2011-May/003875.html"
target="_blank">http://mailman.science.ru.nl/pipermail/fieldtrip/2011-May/003875.html</a>).
Thus, I am not the only one who encountered this
problem.</div>
<div> </div>
<div> Any tips and suggestions will be greatly
appreciated. Thanks in advance!</div>
<div> <img
src="cid:part4.03040009.05060203@fcdonders.ru.nl"
height="562" width="537"><br>
</div>
<div><br>
</div>
<div><br>
</div>
<div>
Best,</div>
<div>
Hatieng</div>
<div> </div>
<div>
</div>
<div><br>
</div>
<div> </div>
<div> <br clear="all">
<div><br>
</div>
-- <br>
<div class="gmail_signature">
<div dir="ltr">
<div>
<div>Haiteng Jiang </div>
<div>PhD candidate</div>
<div>Donders Institute for Brain, Cognition and
Behaviour</div>
<div>Neuronal Oscillations Group </div>
<div>Computational Cognitive Neuroscience Lab</div>
</div>
<div><font face="arial, helvetica, sans-serif"><a
moz-do-not-send="true"
href="https://sites.google.com/site/haitengjiang/"
target="_blank">https://sites.google.com/site/haitengjiang/</a></font></div>
</div>
</div>
</div>
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</div>
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<br>
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