190 Chapter 2 Diomagnetism 2.4 Neuromagnetism 2.4 N euromagnetism b. THOMAS ELBERT ~ ~ ~ ~ The Brain is just the weight of God ­ ~ : For - Heft them - Pound for Pound ­ : : ~. : ~ And they will differ - if they do - ~ ~ ~ As syllable from Sound - : EMILY DICKINSON, 1862 Fig. 2.58: Example of simultaneous EEG (left) 2.4.1 Introduction to Magnetencephalography are attached on places as indicated (F: frontal, C: central, P: parietal, T: According to Emily Dickinson, in 1862, brain function cannot be measured by physical voltage measured between these posi scales. Nevertheless, as we near the end of the 20th century our attempts to measure, (c) At the sametinie interval, the IT "pound for pound", pieces of brain structure and function have not been abated. We a function of time, horn 37 sensor I, carry on, despite an ongoing struggle between the frustration of our inability to un­ left temporal region as indicated. A derstand the functioning of the brain and the illusion that we may, at some point, becomes also visible in t,he EEG. SI be able to fit the pieces of our knowledge into the grand picture. Indeed, with every (Data courtesy to Dr. C. Wienbruch observation of normal and abnormal brain processes, the chances of determining the etiology of pathological conditions increases and the prospects of developing better diagnoses and treatment procedures improves. Measuring brain function as precisely originate from the volume currents, like Elec' as possible in its fluctuation in time and space on different scales constitutes the en­ evoked or event-related potentials (EP, ERP) trance for the understanding of brain functioning on a holistic, systemic level. When decades ago, and furthermore, have become activational brain patterns have simple spatial configurations, magndoencephalogra­ cognitive and behavioral neuroscience. T-,:eE phy allows the macroscopic description of active neural sources with high spatiaf and electrodes attached to the surface of the sca nearly arbitrary temporary resolution. No other method currently available provides measures the voltage fluctuationsori the sur comparable informationI. trades. For our purposes it is significant t The present chapter provides an introduction to Magnetoencephalography (MEG) body not only evokes an electric potential di and presents selected examples of studies employing MEG and the MEG-based meth­ also elicits a measurable magnetic field. AI ods used to locate active regions in the brain (magnetic source imaging -MSI). Reviews magnetic counterparts are found in measure of basic work on perceptual processing and studies of movement-related activity in­ Fig. 2.58), and evoked or event-related magi clude the ones by Hari (1990); Hari & Ilmoniemi (1986); Hoke (1988); Williamson & Biomagnetic fields have a very low amp Kaufman (1987). Clinical applications are summarized by Lewine & Orrison (1995); true for the magnetic field which appears a Makela et al. (1998), and Naatanen et al. (1994) outline the potential of MEG for the activity, the amplitude of the MEG lies ar studies of human cognition. almost 5-6 a-rders of magnitude less than th; activity generated by power lines, cars or I fields of a sensory stimulation lie somewher 2.4.1.1 Overview Tesla), one order of magnitude smaller. The sour.ces of biomagnetic signals result mostly from processes of neuronal or muscu­ Measuring the extremely weak biomagr lar excitation. The signals originate from an intracellular current flow with a relatively free of contact with the subject or patient, high current density. The excited portion of the nerve endings or muscle tissue repre­ for technical equipment. Biomagnetic me1 sents a local source of current. At different locations the current penetrates through the scientific and clinical importance that the cell membrane such that the circuit can be closed over the volume conductor Le., the clear advantages that they hold over t by current pathways through extracellular body tissue. The bioelectric potentials that alone. A major advantage lies in the fa( are vertical to the body's surface essentia 1 ]0 the future, optical methods may promise similar or even greater poteritial but non-invasive whereas the electric potential distribution optical imaging of human brain function remains to be developed. / 2.4 Neuromagnetism 191 : : :: ~ ~ : ~ ~ ~, : : ! ~ :: : Fig. 2.58: Example of simultaneous EEG (left) and MEG (right) recordings. (a) Electrodes are attached on places as indicated by the black dots and also at the earlobes (F: frontal, C: central, P: parietal, T: temporil.l sites). (b) The EEG refers to the voltage measured between these positions and the eadobes as a function of time. (c) At.the same time interval, theto:agnetiCfield{MEG) was meaSured, also as a functio~ -ofiime"-from 37simsor locations. (d) Sensors were 'locatedover the left temporal region as indicated. A clear spike can be detected in the MEG and becomes also visible in the EEG.-Spikes are comlllon in patients with epilepsy. (Data courtesy to Dr. C. Wienbruch) originate from the volume currents, like Electroencephalogram (EEG - Fig. 2.58) and evoked or event-related potentials (EP, ERP), were integrated into clinical diagnostics decades ago, and furthermore, have become a fundamental parameter of research in cognitive and behavioral nenroscience. TheEEG refers tothevoltage derived from two electrodes attached to the surface of thescalp,-w1i.ifetn-e Electrocorticogram (ECoG) measures t.he voltage fluctua.t.ions on the surface of the brain, using intracranial elec­ trodes. For our purposes it is significant that the current which runs through t.he body not only evokes an electric potential dist.ribution on the surface of the body, but also elicits a measurable magnetic field. Analogous to the electric potentials, these magnetic counterparts are found in measures of the Magnetoencephalogram (MEG ­ Fig. 2.58), and evoked or event-related magnetic fields (EF, ERF). Biomagnetic fields have a very low amplitude (see Fig. 2.1). This is part,icularly true for the magnetic field which appears as a result of neuronal act.ivity. For brain activity, the amplitude of the MEG lies around 1 pT (1 picoTesla = 10-12 Tesla), almost 5-6 orders of magnitlldeless than that of urban noise (resulting from magnetic activity generated by power lines, cars or elevators). The evoked cortical magnetic fields of a sensory stimulation lie somewhere around 100 IT (100 femtoTesla "" 10-13 Tesla), one order of magnitude smaller. Measuring the extremely weak biornagnetic fields is completely non-invasive and free of contact with the subject or patient, but it requires a great initial investment for technical equipment. Biornagnetic methods, however, would never have gained .the scientific and clinical importance that they increasingly have if it were not for the clear advantages that they hold over the measurements of bioelectric potentials alone. A maj\Jr advantage lies in the fact that magnetic field components which are vertical to the body's surface essentially result from intracellular current flow, whereas the electric potential distribution is brought to the surface by the volume " / 2.4 Neuromagnetism 192 Chapter 2 Biomagnetism --~----"--------1 current and is therefore still measurable at considerable distances from the SOurce. Log size T~e IIleasurem~nt (One example of this is the measurement of EKG from extremities: I m - is ·successful even at a great distance from the heart, such as from the distal section Brain of the extremities.) Consequently, the volume currents are considerably distorted, as 0.1 m - I·.cm body tissue varies greatly in its degree of conductivity, and may be even markedly Map - anisotropic-like, for instance, in muscle tissue. As muscles also cover areas of the mm - scalp, and as conductivities vary greatly for scalp, skull, cerebro-spinal fluid and brain Column 0.1 mm - with their complex geometries, neuronal sources can be modeled only to a very limited extent when information is based on EEG alone. Under most conditions, biomagnetic Neuron 10 I'm - measurements allow for the determination of the source of biological activity with a Dendrile I I'm - better spatial resolution (up to only a few mm) than is possible with the measurement Synapse 0.1 I'm - i of electric potentials. This is particularly true for the source of magnetic fields which ms are evoked by various sensory modalities within the primary representational zones of cerebral cortex. In many instances, this acitivity can be modeled as a single current dipole. The accuracy of the localization of the "equivalent current dipoles" (ECD) Fig. 2.59: Temporal and spatial re lies somewhere below one half of a centimeter (Liitkenhi:iner et al., 1990, 1996) and is function: EEG Electroen< thus not only considerably better than EEG-based source analysis but also superior bination of functional ma to the localizations of brain-imaging methods which are based on blood flow measures functional Magnetic ResOI such as PET (Positron Emission Tomography) or SPECT (Single Photon Emission phy (see Sections 3.2.2 an, Computed Tomography). The accuracy of source localization is not identical with the accuracy of separating different, simultaneously active sources. While the localization The advantages of MEG-ba; accuracy, as seen above, lies somewhere around a few millimeters (particularly the source imaging is accompanied relative localization accuracy), the ability to separate many different sources is about a series of requirements in order one order lower. measure the extremely weak magnl Currents flowing perpendicularly to the surface of the head emit magnetic fields fields. Since the discovery of with a relatively small signal strength outside the body. They are, so to speak, mag­ Josephson Effect and the developm netically silent, but do create a pronounced electrical potential which can be measured of the SQUID, which resulted fr on the surface. Because magnetic fields and electric potentials contain complementary this discovery, these requirements h, information with respect to their source(s), the ability to measure both types of signals been fulfilled.