[FieldTrip] Fwd: ASSR and midline activity

[Schoffelen, J.M. (Jan Mathijs)]

Begin forwarded message:

From: Olga Sysoeva <olga.v.sysoeva at gmail.com<mailto:olga.v.sysoeva at gmail.com>>
Subject: ASSR and midline activity
Date: 17 December 2016 at 23:47:38 GMT+1
To: <jan.schoffelen at donders.ru.nl<mailto:jan.schoffelen at donders.ru.nl>>

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:

       1. preprocessing/artefact detection;

  1.  bandpass filtered form 35 to 45 Hz using Infinite Impulse Response (IIR) Butterworth two-pass filter of 4th order (forward and reverse), which results in a zero-phase shift of ERP signal;

  2.  Preprocessing of individual MRI/head model and so on construction;

  3.  Than the data were averaged for each ear separately, and covariance matrix was calculated based on the whole epoch;

  4.  The output of this stage analysis was used for calculation the beamformer filter transformation from sensor to the source level. In particular I used Linearly Constrained Minimum Variance (LCMV).

  5.  ITPC (as well as Evoked and total power) was calculate at each vxl.

a) 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.

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.

  6.  Statistical analysis with permutation test : activity vs  baseline (midline cluster is also highly significant).

I am also attaching my script for details.

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 “ Vectorview” , Neuromag System.

Best Regards,

Olga.

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