Source localization of brain activity using helium-free interferometer Available to Purchase

D.

 

Cohen

,  , ().

D.

 

Cohen

,  , ().

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Peplow

,  , ().

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Witchalls

,  , (), available at http://www.newscientist.com/article/mg20727735.700-nobel-prizewinner-we-are-running-out-of-helium.html.

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Sander

,

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Preusser

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Knappe

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, and

M.

 

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Nenonen

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, and

T.

 

Katila

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N.

 

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K.

 

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Y.

 

Suzuki

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A.

 

Tsukamoto

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D.

 

Suzuki

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Kandori

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, and

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Tsukada

,  , ().

Y.

 

Zhang

,

N.

 

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Liao

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S. C.

 

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C. C.

 

Lin

,

H. E.

 

Horng

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J. C.

 

Chen

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M. J.

 

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C. H.

 

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, and

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Yang

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Y.

 

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,

H.

 

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, and

A. I.

 

Braginski

,  , ().

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,

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, and

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Elbert

,  , ().

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Heim

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Öisjöen

,

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Schneiderman

,

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, and

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Winkler

,  , ().

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Faley

,

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Poppe

,

R. E.

 

Dunin-Borkowski

,

M.

 

Schiek

,

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,

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Dammers

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,

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,

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,

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, and

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Koshelets

,  , ().

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R. D.

 

Borkowski

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M.

 

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H.

 

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N.

 

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A.

 

Ermakov

,

V.

 

Slobodchikov

,

Y.

 

Maslennikov

, and

V.

 

Koshelets

,  , ().

In order to calibrate the high-T_c SQUID signal output, we performed noise measurements of about 2 min duration using both low- and high-T_c SQUID systems, simultaneously. After downsampling to 60 Hz, correlation analysis was applied to signal recordings from the high- and low-T_c SQUID system. The low-T_c SQUID sensor (A5 at the top of the helmet), which was orientated similarly to the high-T_c SQUID sensor, revealed a correlation coefficient of about 0.99. From these measurements, the signal output of the high-T_c SQUID system translates to ≈1 pT/mV.

Each high-T_c SQUID measurement location was defined by the location of the electrode position of the electroencephalography (EEG) cap from EASYCAP (EASYCAP GmbH, Berlin, Germany), which the subject was wearing without EEG electrodes. Before the measurement, the subject's head, including the electrode locations of the EEG cap, was digitized using a 3D digitizer (Polhemus, 3Space/Fastrak, USA) to define a head coordinate system. For a comparison of the source analysis between the two systems, coordinates from the high-T_c SQUID measurements were aligned with one of the low-T_c SQUID sensors. This was done by minimizing the distance between a line normal to the low-T_c SQUID sensor and the electrode location from the EEG cap where the high-T_c SQUID sensor was placed. To correct for the high-T_c SQUID sensor-to-head distance, each sensor coordinate was shifted in a normal direction by 20 mm (which was the approximate head to high-T_c SQUID sensor distance), assuming the electrode locations are distributed on a spherical surface. With this procedure, we derived a virtual sensor configuration comprising all high-T_c SQUID channel positions.

TFA was applied using the Python derivate of the MNE toolbox (http://martinos.org/mne). Each period of the task “eyes open” and “eyes closed” was limited to 7 s; to avoid interference due to the instruction of the task change, 1 s at the beginning and one at the end of each block was skipped. For each condition (eyes open vs. eyes closed), a total of 9 epochs with 7 s duration were used to calculate an average spectrogram. For comparisons between the two systems, the MEG signals were normalized to unit variance prior to analysis.

Artifacts in the signal of the high-T_c and low-T_c SQUID recordings were identified by using a peak-to-peak measure applied to the full data set using a sliding window of 200 ms length. With respect to the signals recorded with the whole head system, we used a peak-to-peak threshold of ≥8 pT, whereas a threshold of about $15 pT(≜15 mV)$ was used for the high-T_c SQUID recordings. The reason for the lower threshold used for the low-T_c SQUID system is that data recorded by that system include an online noise compensation. After artifact removal, MEG data from the auditory experiment were bandpass filtered from 3 to 30 Hz including a notch filter at the power line frequency of 50 Hz. Trials around stimulus onset were defined with 1 s length and a pre-stimulus time of 400 ms. On average, 293 and 294 trials were left after discarding trials with artifacts or unusually large amplitudes for the low- and high-T_c SQUID recordings, respectively.

T.

 

Lypchuk

, (, , ).

$GoF=1−⟨(mmeas−mfit)2⟩/⟨(mmeas)2⟩$⁠.

D.

 

Sheng

,

S.

 

Li

,

N.

 

Dural

, and

M. V.

 

Romalis

,  , ().