<div dir="ltr"><div><div><div><div><div>Hi Olga<br><br></div>I haven't looked at your code in detail. Just wondering whether you have considered the possibility to be picking up genuine ASSR source activity in frontal areas?<br></div>Additional ASSR generators outside auditory cortices have been described previously using both MEG (<a href="https://www.ncbi.nlm.nih.gov/pubmed/15925187">https://www.ncbi.nlm.nih.gov/pubmed/15925187</a>) and PET (<a href="https://www.ncbi.nlm.nih.gov/pubmed/15276678">https://www.ncbi.nlm.nih.gov/pubmed/15276678</a>).<br></div>It wouldn't be surprising that beamformer is showing sources not apparent at sensor-level.<br><br></div><div>On the other hand, beamformer results might show a bias towards the center of the brain unless normalized leadfields or pseudo-t/z are used (but just by looking at your plots this doesn't seem to be the case, sources seem to fall nicely over auditory cortices).<br>Beamformer output can be also affected by highly correlated sources, but since you've used monoaural stimulation that shouldn't be a great deal here.<br></div><div>It may be worth trying a different source estimation method and compare the results.<br></div><div><br></div>Best wishes<br></div>Marc<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Sun, Dec 18, 2016 at 11:55 AM, Schoffelen, J.M. (Jan Mathijs) <span dir="ltr"><<a href="mailto:jan.schoffelen@donders.ru.nl" target="_blank">jan.schoffelen@donders.ru.nl</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<span><b>From: </b></span><span>Olga Sysoeva <<a href="mailto:olga.v.sysoeva@gmail.com" target="_blank">olga.v.sysoeva@gmail.com</a>><br>
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<span><b>Subject: </b></span><span><b>ASSR and midline activity</b><br>
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<span><b>Date: </b></span><span>17 December 2016 at 23:47:38 GMT+1<br>
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<span><b>To: </b></span><span><<a href="mailto:jan.schoffelen@donders.ru.nl" target="_blank">jan.schoffelen@donders.ru.nl</a>><br>
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<p style="margin-bottom:0in">Recently, I've got some unexpected results from localization of Auditory Steady-State Response (ASSR) with Fieldtrip. The stimulation was monaural 40 Hz clicks train (500 ms). In addition to the clear two clusters of significant
activity in left and right auditory cortex, I see big and pronounced increase in ITPC (as well as total and evoked response) along the midline structures (see Figure 1 as example of source reconstruction of grandaverage of 40Hz ITPC values (N = 30). The monaural
presentation of 40Hz clicks into left ear). Can it be some artifact of the method I used for localization? That was:</p>
<p style="margin-bottom:0in"><span style="text-decoration:none"><span style="font-weight:normal"> 1. preprocessing/artefact detection;</span></span></p>
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<p style="margin-bottom:0in"><span style="text-decoration:none"><span style="font-weight:normal">bandpass filtered form 35 to 45 Hz using Infinite Impulse Response (IIR) Butterworth
</span></span>two-pass filter of 4th order (forward and reverse), which results in a zero-phase shift of ERP signal;</p>
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<p style="margin-bottom:0in">Preprocessing of individual MRI/head model and so on construction;</p>
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<p style="margin-bottom:0in"><span style="text-decoration:none"><span style="font-weight:normal">Than the data were averaged for each ear separately, and covariance matrix was calculated based on the whole epoch;</span></span></p>
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<p style="margin-bottom:0in"><b><span style="text-decoration:none"><span style="font-weight:normal">The output of this stage analysis was used for calculation the beamformer filter transformation from sensor to the source
level. In particular I used </span></span>Linearly Constrained Minimum Variance (LCMV).</b></p>
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<p style="margin-bottom:0in">ITPC (as well as Evoked and total power) was calculate at each vxl.
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<p style="margin-bottom:0in">a) <span style="text-decoration:none">
<span style="font-weight:normal">we performed frequency decomposition using wavelet method based on Morlet wavelets for the frequency of 40 Hz for 200-500 ms post click onset. As the output we used complex Fourier spectra to be able to extract the
ITPC information. </span></span></p>
<p style="margin-bottom:0in"><span style="text-decoration:none"><span style="font-weight:normal">b) For each voxel the total power was calculated as mean of absolute value, and ITPC as sum of absolute value divided by the averaging,
which takes into account the phase difference.</span></span></p>
</li><li><span style="text-decoration:none"><span style="font-weight:normal">Statistical analysis with permutation test : activity vs baseline (midline cluster is also highly significant).<br>
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<p style="margin-bottom:0in">I am also attaching my script for details.</p>
<p style="margin-bottom:0in">My doubts about the "reality" of this midline cluster came from observation of the sensor level individual response (see picture from Neuromag). By the way, here in Moscow MEG Center we use Elekta 306-channel
<font face="serif"><font size="2"><b>“</b></font></font><font face="serif"><font size="2"> Vectorview</font></font><font face="serif"><font size="2"><b>”</b></font></font><font face="serif"><font size="2">
, Neuromag System.</font></font></p>
<p style="margin-bottom:0in">Best Regards,</p>
<p style="margin-bottom:0in">Olga.</p>
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