The neurophysiological activity of cortical neurons may be recorded using either surface electrodes or magnetom-eters (a magnetic recording device). The selection of one method of recording over another depends, at least in F I G U R E 7 – 1 . Examples of electroencephalography tracings illustrating activity in each of the four major frequency domains (1 second per block, sensitivity = 7 µV/mm).
T A B L E 7 – 1 . Major electroencephalography (EEG) bands, their respective frequencies, probable neural generators, and most characteristic location in a normal surface EEG recording
Band
Frequency
range (Hz) Principal neural generators Characteristic surface electrode location β (beta) >12.5 Corticocortical and thalamocortical networks
involved in information processing
Maximal over frontal and central regions
α (alpha) 8.0–12.5 Thalamic pacemaker neurons Occipital and perhaps central when eyes are closed θ (theta) 3.5–8.0 Thalamic pacemaker neurons under the influence
of inhibitory input from the reticular nucleus of the thalamus
If present in the waking record at all, amplitude is low and content is small; may be most obvious in central regions; becomes more obvious with drowsiness and sleep
δ (delta) <3.5 Oscillatory neurons in the deep cortical layers and within the thalamus
Not typically seen in the awake record of healthy adults; diffusely present in deeper sleep stages;
may be focally located over cortical lesions; may become prominent in frontal/central regions due to disruption of corticothalamocortical circuits
part, on the areas of cortex to be recorded. Because the cerebral cortex contains both gyral and sulcal surfaces, the columnar organization of cortex results in the production of both radially and tangentially oriented electrical dipoles (Figure 7–2). Radially oriented dipoles are gener-ated by gyral cortex; the dipole at the gyral surface would, if extended, form a radial line from the center of the head to the surface of the scalp (Figure 7–2, left). Tangentially oriented currents are generated by sulcal cortex, the ori-entation of which is tangential to the scalp surface that overlies them (Figure 7–2, right). Although both radially and tangentially oriented dipoles contribute to the elec-trical fields on the scalp, radially oriented currents are the predominant contributor to scalp surface electrical fields.
Tangentially oriented electrical fields generated by sulcal cortex are not as readily amenable to recording by a scalp electrode because they do not generate as substantial an electrical potential difference at the scalp surface as radi-ally oriented dipoles. However, tangentiradi-ally oriented electrical dipoles produce a magnetic field that is radially oriented with respect to the scalp that is detectable through magnetoencephalographic recordings using an appropriately positioned magnetometer (Figure 7–3).
Basic Methods of
Electroencephalographic Recording
Electroencephalographic methods are standardized to facilitate improved reliability of both recording and inter-pretation, particularly with respect to the detection and approximate localization of abnormal electrical activity.
In most clinical settings, electrodes are placed on the patient’s scalp according to the 10–20 International Sys-tem of Electrode Placement (Figure 7–4); higher density electrode arrays are sometimes used, particularly in neu-ropsychiatric research. Once electrodes are placed, they are connected to one another to create recording chan-nels. Multiple electroencephalographic channels are arranged in a variety of ways to create electroencephalo-graphic montages (see Figure 7–5 for a few examples).
Through these different arrangements, several different views of cortical electrical activity can be established that facilitate both identification and approximate localization of abnormal cortical activity (e.g., seizure focus, contu-sion, infarction, subdural hematoma).
Once recorded, the electroencephalographic record is visually inspected for normal and abnormal findings. Al-though this remains the most common and generally ac-cepted method of electroencephalographic interpreta-F I G U R E 7 – 2 . Illustration of the cortical mantle in
the coronal plane.
The radial orientation of electrical dipoles generated by gyral cor-tical columns is illustrated on the left, including their projection to the scalp surface. On the right, the tangential orientation of electri-cal dipoles generated by sulelectri-cal cortielectri-cal columns is illustrated. Note that the tangentially oriented dipoles do not project to the scalp surface directly overlying them. Instead, the electrical fields associ-ated with tangentially oriented dipoles eventually project to more distant (far-field) scalp areas. As a result of the longer distance and greater amounts of tissue traversed before emerging at the scalp, the electrical fields of tangentially oriented dipoles are relatively more attenuated and diffused before emerging at the scalp surface than are those of radially oriented electrical dipoles.
F I G U R E 7 – 3 . Illustration of the magnetic field generated by a tangentially oriented electrical dipole.
At the top of the diagram is the scalp surface. Below, a coronal cross section through two gyri is depicted. On the sulcal surface of the gyrus on the right, a single neuron in a cortical column is illustrated. When this neuron produces an electrical current, the magnetic field it generates is oriented perpendicular to that cur-rent. Many adjacent and simultaneously active tangentially ori-ented cortical neuronal columns produce magnetic fields whose flux lines are radially oriented with respect to the scalp and may be recorded by a magnetic recording device overlying this area.
tion, digitization and computer-assisted methods permit quantitative electroencephalographic analyses that are not possible through visual inspection alone (Hughes and John 1999). These methods include quantified analysis of the frequency composition of the EEG over a given pe-riod (spectral analysis), analysis of absolute and relative amplitude (µV/cycle/second) and power (µV2 /cycle/sec-ond) within a frequency range or at each channel, coher-ence (correlation between activity in two channels), phase (relationships in the timing of activity between two chan-nels), or symmetry between homologous pairs of elec-trodes (Hughes and John 1999; Neylan et al. 1997; Nu-wer 1990; Thatcher 1999). Values derived from quantitative electroencephalographic analyses can be mapped onto a representation of the entire scalp surface, a procedure known as brain electrical activity mapping (BEAM). Statistical probability mapping of BEAM data can be used to construct topographic maps of the results
of such analyses (Duffy et al. 1981), which offers a visual and potentially more intuitive method of inspecting these complex data sets (Figure 7–6).
There are reasonable concerns about the potential for misinterpretation and distortion of data subjected to quan-titative electroencephalographic analyses without concur-rent visual inspection by a qualified electroencephalogra-pher (Jerrett and Corsak 1988; Nuwer 1997). For example, spike detection using presently available QEEG software packages is poor, thereby limiting the application of quan-titative electroencephalographic procedures in the inspec-tion of records for epileptiform activity. Although these is-sues remain the subject of ongoing debate in the literature (Hughes and John 1999; Neylan et al. 1997; Nuwer 1997;
Thatcher 1999), quantitative electroencephalographic in-terpretation and analysis continue to hold promise for the investigation of neuropsychiatric disorders in general and the neuropsychiatric consequences of TBI in particular.
F I G U R E 7 – 4 . The 10-20 International System of Electrode Placement.
Electrodes are labeled according to their approximate locations over the hemispheres (F = frontal, T = temporal, C = central, P = parietal, and O = occipital; z designates midline); left is indicated by odd numbers and right by even numbers. A parasagittal line running between the nasion and inion and a coronal line between the preauricular points is measured. Electrode placements occur along these lines at distances of 10% and 20% of their lengths, as illustrated. In most clinical laboratories, the Fpz and Oz electrodes are not placed, but are instead used only as reference points. Fp1 is placed posterior to Fpz at a distance equal to 10% of the length of the line between Fpz-T3-Oz; F7 is placed behind Fp1 by 20% of the length of that line. O1 is placed anterior to Oz at a distance equal to 10% of the length of the line between Oz-T3-Fpz; T5 is placed anterior to O1 by 20% of the length of that line. F3 is placed halfway between Fp1 and C3 along the line created between Fp1-C3-O1; P3 is placed halfway between O1 and C3 along that same line. Right hemisphere electrodes are placed in similar fashion. Reference electrodes, in this case placed on the ears, are labeled A1 and A2.
Regardless of the method of electroencephalographic data analysis, the limitations of electroencephalographic recordings are important to acknowledge. Cerebrospinal fluid, meningeal tissue, bone, connective tissue, muscle, and skin attenuate the amplitude of high-frequency sig-nals, leaving at least part of the frequency spectrum (beta and higher) less than optimally represented on scalp sur-face recordings. These tissues, as well as sweat and skin oils, diffuse the electrical signal (now an electrical field) across the scalp surface. Hence, deeper sources of electri-cal signals within the brain are subject to greater attenua-tion and diffusion before arrival at the scalp surface. Con-sequently, surface electrodes tend to be relatively insensitive to signals of low strength or those generated by deep (e.g., subcortical, orbitofrontal, medial temporal, inferotemporal, and inferior occipitotemporal) struc-tures. Signal diffusion across the scalp presents serious
challenges to precise signal source localization using elec-trophysiological recording techniques, particularly with respect to localizing relatively deep signal sources. Place-ment of special (e.g., nasopharyngeal and sphenoidal) electrodes may modestly improve signal detection from the cortex to which they are most proximate, but in gen-eral these areas are relatively inaccessible to conventional EEG recording.
Basic Methods of
Magnetoencephalographic Recording
Magnetoencephalographic systems use superconduct-ing quantum interference devices (SQUIDs) to record cortically generated magnetic fields. Because fluctuat-ing magnetic fields (such as are produced by the cortex) induce electrical currents in conducting wires oriented F I G U R E 7 – 5 . Illustration of three common electroencephalographic montages, including referential (A), parasagittal bipolar (B; sometimes referred to as the double-banana montage), and transverse bipolar (C).
perpendicular to the direction of flow of the magnetic field, current is induced in the wire coil when it is placed over an area of active cortex (Reite et al. 1999). The wire detector is itself inductively coupled to the SQUID and its electronics, which together comprise a sensitive magnetic field measuring device. Because the magnetic fields produced by cortical activity are closer to the magnetic field detector than are most environ-mental sources, this device is reasonably sensitive to the fluctuating gradients produced by cortical activity and less affected by the more stable field gradients of distant environmental magnetic sources (Rojas et al. 1999). A variety of MEG detection coils are available, each dif-fering in their signal sensitivity and capacity for noise reduction. Modern magnetoencephalographic systems may have as many as 300 individual magnetic detectors (which are analogous to electroencephalographic elec-trodes). Pairing magnetic field detectors creates chan-nels for signal recording; these chanchan-nels can be arranged to create recording montages. Arrays of mul-tiple magnetoencephalographic channels may also be used for these purposes or arranged in a variety of ways to create magnetoencephalographic counterparts to electroencephalographic montages. Smaller arrays offer more limited and/or focused areas of signal detec-tion, as might be used in magnetoencephalographic evoked field or MSI recordings.
Magnetic field strength is not significantly attenuated by the tissue interposed between the source of the signal and the magnetometer positioned to detect it (Cuffin
1993). As such, MEG may be better able to detect both very high-frequency (up to 400–700 Hz) and ultra-low frequency (<1 Hz) signals that are not amenable to elec-troencephalographic recording (Lewine et al. 1999; Reite et al. 1999). However, there remain substantial technical challenges to recording cortically generated magnetic fields that offset this theoretical advantage (see Rojas et al.
1999 for a review). Although many of these technological challenges are manageable by presently available record-ing devices, the equipment, the magnetically shielded en-vironment in which it must be operated, and the routine operation of such recording systems are cost, expertise, and labor intensive. These challenges may be reasons for the limited availability and application of MEG in TBI research to date.