Combined MEG and EEG methodology for non-invasive recording of infraslow activity in the human cortex
Introduction
Slowly changing depolarisations of the cerebral gray matter are observed in border zones of ischemic foci. These slow potential shifts are referred to as periinfarct depolarisations (PID) and cortical spreading depressions (CSD). PID and CSD caused infarct expansion in animal models of focal cerebral ischemia (Hossmann, 1994). Experimental neuroprotective treatments for stroke decreased the incidence of PID and CSD (Williams et al., 2003). Advances in non-invasive measurements of infraslow potential shifts, as surrogates of PID and CSD, are likely to support the design of novel diagnostic and therapeutic strategies in the treatment of stroke. To date, CSD measurements are restricted to invasive techniques, such as recordings with subdural electrocorticographic strip electrodes (Strong et al., 2002, Fabricius et al., 2006, Dreier et al., 2006). However, these invasive techniques are only possible in very few human individuals after craniotomy.
Cerebral infraslow electrical activity can be measured non-invasively with two different techniques: Direct Current-Magnetoencephalography (DC-MEG) and Direct Current-Electroencephalography (DC-EEG). In this study, the term “infraslow” activity is used to describe very slow current changes or field changes in the range below 0.1 Hz (Vanhatalo et al., 2005). In the past, non-invasive recordings of very slowly changing cortical depolarisations were compromised by large baseline drift artefacts, which were much higher than the biological signals of interest (Barkley et al., 1991). First experimental non-invasive infraslow magnetic field recordings in humans were reported by employing a modulation-based MEG technique (Cohen, 1969). Over the last decade, MEG techniques have been refined. Now, these techniques enable the measurement of pathological infraslow magnetic field changes (Bowyer et al., 2005), as well as physiological infraslow magnetic field changes (Mackert et al., 1999, Mackert et al., 2001). The detection of slow electrical potential shifts requires a direct coupled (DC) EEG with a bandwidth starting from 0 Hz (Vanhatalo et al., 2003). Recent advances in amplifier technique and electrode design now allow the detection of infraslow potential shifts in humans non-invasively (Vanhatalo et al., 2003, Tallgren et al., 2005).
MEG and EEG are sensitive to different orientations of source generators (Barkley et al., 1991). The human cortex is folded forming fissures with dipole layers orthogonal to the cortex surface. MEG is insensitive to radial dipoles, but sensitive to dipoles that are tangential to the scalp, such as currents in the wall of fissures. By contrast, EEG is most sensitive to dipoles that are oriented radially to the scalp, e.g., sources generated by dipoles in the crown and floor of fissures.
In the present study, we aimed to detect, localize and quantitatively monitor cortical infraslow activity in the human cerebral cortex using simultaneous MEG and EEG measurements. This specific combination allows to benefit from the physical advantages of each method. To induce changes in focal cortical infraslow activity, a physiological motor activation paradigm was used, in which subjects performed self-paced repetitive slow and very slow finger movements.
Section snippets
Methods
Healthy right-handed subjects (n = 7, three females; aged 24–28 years) performed self-paced very slow (around 0.5 Hz) and slow (around 1.5 Hz) finger movements (according to motor task performance rates definition of Sadato et al., 1996) of the right hand. Movements were trained prior to the recording session. Very slow finger movements were performed at 0.5–0.7 Hz and slow finger movements at 1.5–2.0 Hz as determined from the light switch signal. Handedness of the subjects was tested using the
Results
In all seven subjects, very slow and slow right finger movements revealed motor-related infraslow magnetic field changes over the left motor cortex that were clearly above noise level. These infraslow magnetic field changes were characterized by a direct and steep increase of field strength at the beginning of activation, sustained field amplitude during the 30 s activation, and a slower decrease after the end of the finger movement (Fig. 1, right side). The EEG data set of one subject was
Discussion
This study demonstrates the feasibility of combined measurements of electrical potentials and magnetic fields in the infraslow frequency range to non-invasively localize and quantitatively monitor physiological, low amplitude, infraslow cortical activity in humans. Simultaneously recorded EEG-derived infraslow potential changes and MEG-derived magnetic field changes could be correlated directly.
MEG recordings demonstrated long lasting motor task related infraslow field changes over the
Acknowledgements
This work was supported by BMBF/Berlin NeuroImaging Center Grant GF GO 01184601, 01 GO 0208, 01 GO 0518 and DFG Cu 36/1–3, 5, 6. Helpful discussions with M. Carbon are gratefully acknowledged.
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