Magnetenzephalogramm, MEG
MEG
EEG vs. MEG
Differences between EEG and MEG
• MEG is only sensitive to the tangential component of the dipoles, but insensitive to the radial component. EEG measures both. This implies that MEG recordings are mainly based on activity in the sulci, but not in the gyri
(1/3 of the cortex).
• In contrast to EEG, MEG is insensitive to the inhomogeneities of skull and scalp which result in field spreading. As a consequence, the ERF is often more
focal than the ERP.
Mismatch negativity (MMN)entdeckt durch Näätänen 1978
Winkler et al. 1999EEG Mismatch Negativity
Erwerb eines finnischen Vokalkontrastes durch Ungarn
Phillips et al. 2000 J Cog Neuroscience
Phillips et al. 2000 J Cog Neuroscience
Phillips et al. 2000 J Cog Neuroscience
The inverse problem
Aim: Finding the source distribution underlying a given scalp potential map.
Problem: The inverse problem in EEG and MEG has no unique solution. For any given potential (or magnetic field) distribution over the scalp surface, a variety of possible neural source distributions exists that can
produce the same surface map.
The number of possible current source distributions that matches a given set of surface data may be large.
Dipole analysis• A head model is assumed, e.g. a three-shell model (brain, skull, scalp), or a more realistic head model.
• This model allows the calculation of the scalp electrical potential generated at a particular location on the scalp by
an intracerebral source with a particular location, orientation and strength.
• In a number of iterative steps the source parameters can be changed until the difference between the modelled and
the recorded waveforms is minimized.
Constraining the inverse problem
Actual solutions often involve information about neurophysiology and anatomy to reduce the solution space.
• Sources (dipoles) may change strength, but not location or orientation during a specified time interval (spatial-
temporal constraint).
• Sources are all located at the same depth (e.g. in the neocortex). This approach is referred to as “spatial
deconvolution”, “de-blurring”, or “cortical imaging”. It is based on the unique relation between surface potentials
and sources at a fixed depth.
Dipole analysisA source dipole is defined by its location,
orientation, and strength
Sources for the Bereitschaftspotential: fit with 1 and 2 stationary dipoles
(extension of the left middle finger)
Die Kombination von hämodynamischen und
elektromagnetischen Daten kann Informationen
über räumliche UND zeitliche Eigenschaften von
Hirnaktivierungen bieten.
Geschätzte Zeitfenster der Wortproduktionsprozesse
Konzeptuelle Vorbereitungvon der Bildpräsentation bis zum lexikalischen Konzept 175 ms(Thorpe et al.,1996; Schmitt et al., 2000)
Lemmazugriff 115 ms(Levelt et al., 1992; Roelofs, 1992; Schmitt et al., 2001)
Formenkodierung* Wortformzugriff 40 ms(van Turennout et al., 1998)* Syllabifizierung 125 ms(van Turennout et al., 1997; Wheeldon & Levelt, 1995) * Phonetische Enkodierung bis Beginn der Aussprache 145 ms
Gesamt 600 ms
(Jescheniak & Levelt, 1994; Levelt et al., 1998; Damian et al., 2002)
Salmelin, Hari, Lounasmaa, & Sams (1994), Fig. 1
Picture naming: MEG
Gemessene (links) und erwartete (rechts) Zeitfenster beiBildbenennung
MEG data from:
Salmelin et al., 1994; Levelt et al., 1998; Maess et al., 2002
Indefrey, P. and Levelt, W.J.M. (2004) Cognition