Removal of magnetoencephalographic artifacts with temporal signal-space separation: demonstration with single-trial auditory-evoked responses - PubMed (original) (raw)
Removal of magnetoencephalographic artifacts with temporal signal-space separation: demonstration with single-trial auditory-evoked responses
Samu Taulu et al. Hum Brain Mapp. 2009 May.
Abstract
Magnetic interference signals often hamper analysis of magnetoencephalographic (MEG) measurements. Artifact sources in the proximity of the sensors cause strong and spatially complex signals that are particularly challenging for the existing interference-suppression methods. Here we demonstrate the performance of the temporally extended signal space separation method (tSSS) in removing strong interference caused by external and nearby sources on auditory-evoked magnetic fields-the sources of which are well established. The MEG signals were contaminated by normal environmental interference, by artificially produced additional external interference, and by nearby artifacts produced by a piece of magnetized wire in the subject's lip. After tSSS processing, even the single-trial auditory responses had a good-enough signal-to-noise ratio for detailed waveform and source analysis. Waveforms and source locations of the tSSS-reconstructed data were in good agreement with the responses from the control condition without extra interference. Our results demonstrate that tSSS is a robust and efficient method for removing a wide range of different types of interference signals in neuromagnetic multichannel measurements.
(c) 2008 Wiley-Liss, Inc.
Figures
Figure 1
Geometry of a MEG measurement. The green (internal) and red (external) volumes contain the brain and interference sources, respectively. The white region in between is free of magnetic sources, except when artifacts arise from magnetic impurities, braces, stimulators, or other nearby disturbance sources.
Figure 2
Original signals from one left‐temporal‐lobe gradiometer channel during the Control condition (top), External interference condition (middle), and Nearby interference condition. Stimulus times are denoted by dashed vertical lines. Arrows point to visible auditory responses.
Figure 3
Original and processed signals from one magnetometer during the three conditions. From top to bottom: Original data, SSS reconstruction for signal arising from the outside of the sensor array (SSS external), SSS reconstruction for signal arising from the inside the sensor array (SSS internal), and tSSS reconstruction for the internal signal.
Figure 4
Top: The mean angle between two randomly chosen vectors as a function of their dimension, n. The vectors were composed as linear combinations of orthonormal sine and cosine components, with uniformly distributed random amplitudes ranging from −1 to 1. Bottom: The effect of window length T on the tSSS‐reconstructed auditory response. The values of T were 1 s (black dashed), 2 s (red), 4 s (blue), and 8 s (black solid; under the blue curve during the peak). The data were low‐pass filtered at 70 Hz.
Figure 5
Top: Averaged auditory responses from the 306 channels during the Control condition. The display shows all channels as viewed from above the head, with the subject's nose pointing up in the plane of the paper. The sensor array contains sensor triplets, with two orthogonal planar gradiometers and one magnetometer in each triplet. Bottom: Original (left) and tSSS‐processed (right) averaged signals from a subset of two gradiometers and one magnetometer indicated by the rectangle in the upper figure.
Figure 6
Field patterns based on the original signals (left column) and the tSSS‐reconstructed signals (right column) for the Control, External interference, and Nearby interference conditions. The contour step is 100 fT in all field patterns. The red color corresponds to magnetic flux coming out of the sensor surface, and the blue color indicates field in the opposite direction.
Figure 7
Spatial distribution of a single auditory response before (top left) and after (top right) tSSS processing. No further temporal filtering was applied. The channel of the close‐up is indicated by a rectangle. Bottom: The corresponding field distribution of a single auditory response before (left) and after (right) tSSS processing. The contour step is 100 fT.
Figure 8
Single‐trial plot of 340 consecutive individual auditory responses (108, 102, and 130, respectively, for the three different conditions) after three different processing procedures: Original (top panels), data processed by the standard SSP operator corresponding to usual environmental interference (middle panels), tSSS‐reconstructed data (bottom). In all boxes, the different conditions are displayed in the order Control (i), External interference (ii), and Nearby interference (iii); these conditions are separated in each panel by thin white horizontal lines.
Figure 9
Thirty consecutive tSSS‐reconstructed auditory responses from the Control condition. The upper curves contain the whole frequency band 0.1–200 Hz, whereas the lower signals have been low‐pass filtered at 40 Hz.
Figure 10
Locations of current dipoles of single auditory responses superimposed on the subject's own magnetic resonance image slices. The goodness‐of‐fit values were for all plotted dipoles better than 85%.
Figure 11
Histograms of x‐, y‐, and _z‐_coordinates for all current dipoles of Figure 10. N refers to the number of observations, and solid and dotted lines correspond to the Control (i) and External interference (ii) conditions. The _x_‐axis runs from left to right preauricular point, the y axis points toward the nasion, and the _z‐_axis is orthogonal to _xy‐_plane and is directed to the top of the head.
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