139 Electrophysiological Techniques FIGURE 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 FpzT3-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 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 period (spectral analysis), analysis of absolute and relative amplitude (µV/cycle/second) and power (µV2/cycle/second) within a frequency range or at each channel, coherence (correlation between activity in two channels), phase (relationships in the timing of activity between two channels), or symmetry between homologous pairs of electrodes (Hughes and John 1999; Neylan et al 1997; Nuwer 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 quantitative electroencephalographic analyses without concurrent visual inspection by a qualified electroencephalographer (Jerrett and Corsak 1988; Nuwer 1997) For example, spike detection using presently available QEEG software packages is poor, thereby limiting the application of quantitative electroencephalographic procedures in the inspection of records for epileptiform activity Although these issues remain the subject of ongoing debate in the literature (Hughes and John 1999; Neylan et al 1997; Nuwer 1997; Thatcher 1999), quantitative electroencephalographic interpretation and analysis continue to hold promise for the investigation of neuropsychiatric disorders in general and the neuropsychiatric consequences of TBI in particular 140 TEXTBOOK OF TRAUMATIC BRAIN INJURY FIGURE 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) 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 signals, leaving at least part of the frequency spectrum (beta and higher) less than optimally represented on scalp surface 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 electrical signals within the brain are subject to greater attenuation and diffusion before arrival at the scalp surface Consequently, 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) structures Signal diffusion across the scalp presents serious challenges to precise signal source localization using electrophysiological recording techniques, particularly with respect to localizing relatively deep signal sources Placement of special (e.g., nasopharyngeal and sphenoidal) electrodes may modestly improve signal detection from the cortex to which they are most proximate, but in general these areas are relatively inaccessible to conventional EEG recording Basic Methods of Magnetoencephalographic Recording Magnetoencephalographic systems use superconducting quantum interference devices (SQUIDs) to record cortically generated magnetic fields Because fluctuating magnetic fields (such as are produced by the cortex) induce electrical currents in conducting wires oriented 141 Electrophysiological Techniques FIGURE 7–6 An example of spectral mapping This map describes relative power (percentage of total power) in the right hemisphere across several frequency ranges in a 25-yearold man with diffuse intermixed slowing on visual inspection of the electroencephalography record 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 environmental 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 differing 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 electrodes) Pairing magnetic field detectors creates channels for signal recording; these channels can be arranged to create recording montages Arrays of multiple 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 detection, 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 (