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