Chapter 1
Arrays and Montages

1.1 Arrays

As the field of electroencephalography developed, the need for a standardized array of electrodes became increasingly apparent, resulting, in 1958, in the adoption of the International 10-20 System. Subsequently, the need for more closely spaced electrodes to allow for more precise localization led to an expansion of this system into what has come to be known as the Modified Combinatorial 10-10 System.

One, very important, failing of the International 10-20 System has been its inability to capture epileptiform discharges arising from seizure foci in the medial and anterior aspects of the temporal lobe. Of the multiple attempts to remedy this failing, two have found their way into clinical practice: first, utilizing a portion of the 10-10 System, namely, the inferior temporal chain; and, second, using one of the supplemental noninvasive electrodes, namely, the "true" anterior temporal electrode. Both of these are discussed at the end of this chapter.

1.1.1 International 10-20 System

The International 10-20 System was first described by Jasper in 1958 (Jasper 1958) and has become a worldwide standard. In developing this system, Jasper decided that "[p]ositions of electrodes should be determined by measurement from standard landmarks on the skull." and that these "[m]easurements should be proportional to skull size and shape, insofar as possible." The standard landmarks Jasper chose were the nasion, the inion, and the preauricular points, and to maintain proportionality to the individual skull size, Jasper, rather than using predetermined distances in centimeters, decided on using percentages of the distances between these points on the individual patient's head.

The first line to be drawn lies in the midline and extends from the nasion to the inion. The first point on this line, according to Jasper (1958), is the Fp midline point, and this point "is 10 per cent of the nasion-inion distance above the nasion; the second point (F) is 20 per cent of this distance back from the point Fp, and so on in 20% steps back for the Central, Parietal, and Occipital points (hence the name 10-20 system)."

The next line to be drawn is "based upon the Central coronal plane. The distance is first measured from the left to right preauricular points ." and Jasper (1958) cautions that we "[b]e sure the tape is passing through the predetermined Central point at the vertex when making this measurement. Ten per cent of this distance is then taken for the Temporal point up from the pre-auricular point on either side. The Central points are then marked 20 per cent of the distance above the temporal points."

The next line to be drawn is an "A-P line of electrodes over the temporal lobe, frontal to occipital, (and this) is determined by measuring the distance between the Fp mid line point . through the T position of the Central line, and back to the mid-occipital point. The Fp electrode position is then marked 10 per cent of this distance from the mid-line in front, and the Occipital electrode position 10 per cent of the distance from the mid-line in back. The inferior Frontal and posterior Temporal positions then fall 20 per cent of the distance from the Fp and O electrodes, respectively along this line" (Jasper 1958).

"The remaining mid-Frontal (F3 and F4) and mid-Parietal (P3 and P4) electrodes are then placed along the Frontal and Parietal coronal lines respectively, equidistant between the mid-line and temporal line of electrodes on either side" (Jasper 1958).

In explaining the rationale for the names of the various electrodes, Jasper (1958) commented that "[t]raditional anatomical terms have been employed to designate electrode positions over the various lobes of the brain, with the exception of the Central region which is, strictly speaking, partly frontal and partly parietal."

He goes on to note that "[i]n order to differentiate between homologous portions over left and right hemispheres it was decided to use even numbers as subscripts for the right hemisphere, and odd numbers for the left hemisphere . Electrodes at the mid-line in Frontal, Central and Parietal regions were originally designated F0, C0 and P0 but this led to some confusion since P0, for example, might be interpreted as parieto-occipital. Consequently the midline positions have been changed to Fz, Cz and Pz (z for zero!)." The complete system of placements is shown in Figure 1.1 and schematically in Figure 1.2.

Figure 1.1 The complete International 10-20 System.

SOURCE: Jasper (1958)/with permission of Elsevier.

The full names of each of Jasper's 21 alphanumeric designations are given in Table 1.1. Subsequently, however, two alternative names have gained currency. The first alternative is trivial: "auricle" has been replaced by "ear." The second, however, is far more significant. In the same year that Jasper published his study, Abraham and Ajmone Marsan (1958) referred to F7 and F8 not as "inferior frontal," but as "anterior temporal," and this designation has stuck, despite the fact, as discussed below, that F7 and F8 do not overly the temporal lobe at all.

1.1.2 Anatomical Position of Electrodes of the International 10-20 System

Jasper (1958) stated that "[a]natomical studies should be carried out to determine the cortical areas most likely to be found beneath each of the standard electrode positions," and in his paper he briefly described two such studies.

"Two methods were employed: (1) metal clips placed along the Central and Sylvian fissures at operation were then used to identify these fissures in X-ray studies of the skull after the EEG electrodes had been applied, and (2) electrode positions were carefully marked on the heads of cadavers, drill holes placed through the skull, and the cortex marked with India ink in each position before removing the brain for examination. Brains with gross lesions or focal atrophy were excluded."

Figure 1.2 The complete International 10-20 System in schematic form.

SOURCE: Jasper (1958)/with permission of Elsevier.

Table 1.1 Electrode names in the International 10-20 System

Fp1 left frontopolar Fp2 right frontopolar F7 Left inferior frontala F3 Left frontal Fz Frontal midline F4 Right frontal F8 Right inferior frontala A1 Left auricle T3 Left mid-temporal C3 Left central Cz Central midline C4 Right central T4 Right mid-temporal A2 Right auricle T5 Left posterior temporal P3 Left parietal Pz Parietal midline P4 Right parietal T6 Right posterior temporal O1 Left occipital O2 Right occipital

a F7 and F8 are also known as "anterior temporal" electrodes.

"Although some variability was found, and is to be expected, the position of the two principle (sic) fissures should be within plus or minus about 1?cm. of that indicated on the drawings, provided the head measurements are carefully made and the brain is free of gross distortion due to expanding or contracting lesions. Due to the obliquity of the Central Fissure the upper central electrodes will usually lie pre-central while the lower ones will be post-central in most cases."

Jasper's original "anatomical studies" were followed up by Homan et al. (1987), who capitalized on the availability of computed tomography (CT) scanning to provide a more detailed correlation, and the results are summarized in Table 1.2.

The findings of Homan et al.'s study have significant implications for cases of suspected temporal lobe epilepsy. To begin with, although as noted earlier, F7 and F8 are often referred to as "anterior temporal," they do not, in fact, overly the temporal lobe at all. Consequently, if one wishes to adequately sample the anterior portion of the temporal lobe, it will be necessary to utilize a supplemental electrode, namely, as discussed further below, a "true" anterior temporal electrode, T1 or T2. Next, since none of the temporal electrodes (T3 and T4, T5 and T6) overly the inferior temporal gyrus, it will be necessary, if one wishes to sample this area, to place an "inferior temporal" chain, which is a fragment of the Modified Combinatorial 10-10 System, as discussed immediately below.

Table 1.2 Electrode positions as determined by CT scanning

Electrode position Cortical location F7,8 "Inferior frontal gyrus rostral portion of pars...