EEG Instrumentation, Montage, Polarity, and Localization

Fig. 2.1

Pyramidal neuron

Fig. 2.2

Parallel arrangement of the pyramidal neurons allows for summation of the individual potentials

Cortical neuronal alignment effectively creates an electrical dipole. Whether a positive or negative potential is recorded on the scalp electrode depends on the location of the recording electrode with respect to these dipoles (Fig. 2.3). Epileptiform discharges (spikes or sharp waves) are commonly surface negative. Simultaneous intracranial and scalp recordings confirmed that at least 6 cm² of synchronous cortical activation is indeed necessary to detect an individual epileptic spike on scalp electrodes [1].

Fig. 2.3

Orientation of the sulci and therefore the dipoles determine what potential is recorded from the scalp

Recording the EEG

Commonly used electrodes for scalp EEG have a contact surface made of non-depolarizing chloride-treated silver. International standards specify that electrode resistance should be between 100 and 5000 Ω. Properly applied electrodes show a resistance of a few hundred ohms.

A minimum of 21 electrodes are recommended for scalp EEG. The international 10–20 system is commonly used for the placement of these electrodes (Fig. 2.4). With this system, inter-electrode distances average from 4 to 6 cm, as the “10” and “20” mean that the distances between adjacent electrodes are either 10% or 20% of the total nasion-inion or right ear–left ear distance of the skull. In addition, only the superior lateral temporal region is covered. The 10–10 system is more extensive and includes subtemporal electrodes.

Fig. 2.4

International 10–20 system for electrode placement

The EEG potentials are displayed in channels; each channel represents the difference in potential between two electrodes. By convention, if the difference between two electrodes is negative, then it is represented by an upward deflection, while a downward deflection represents a positive difference.

Montages

The 10–20 system employs 21 electrodes. Differences in potentials between these electrodes constitute channels. Combinations of different channels are called montages. The two main montage types are the bipolar and the referential.

In a bipolar montage, channels are arranged in chains that follow an anterior-to-posterior or a transverse arrangement (Figs. 2.5 and 2.6). The chains imply that the second lead in the first channel is the first lead in the second channel, and so forth until the end of the chain. In a referential montage, each channel represents the difference of the potential of any given electrode with a single chosen electrode (Fig. 2.7).

Fig. 2.5

Bipolar montage with anterior-to-posterior chains (longitudinal bipolar or double-banana montage)

Fig. 2.6

Bipolar montage with transverse chains

Fig. 2.7

Referential montage using Cz as the common reference

Each configuration has its advantages and disadvantages. In a bipolar montage, external noise can easily be canceled out as it measures the difference in potential between contiguous electrodes, hence amplifying local potentials. Visual detection of differences in local potentials is easier on a bipolar montage particularly when “phase reversal” is seen, signifying a negative event taking place in the region of the electrode that is common to the two channels where polarity changes (Fig. 2.8).

Fig. 2.8

To the left is a representation of a negative potential and its field as recorded from the scalp. Right upper is a representation of that potential as represented on a referential montage. Note that the amplitude of the spike corresponds to the proximity of the recording electrode to the negative filed maximum. Right lower is the same potential as recorded from the same electrodes but arranged in a longitudinal bipolar montage. Each channel represents the difference in potential between two adjacent electrodes in the same chain. The highest negative potential is recorded from contact C; this would lead to B–C to have a positive value (down-going tracing on EEG), while C–D will have a negative value (up-going). This would result in the so-called phase reversal on a bipolar montage, where the common electrode is closest to the maximum negative potential as recorded from the scalp and within that chain

A referential montage on the other hand would be highly susceptible to external noise but it would be able to detect both local (near field) and distant (far field) potentials. The amplitude of the deflection on a referential montage would be a closer representation of the absolute potential at an electrode.

Acquiring, Filtering, and Displaying the EEG Signal

Electrocerebral potentials are in the microvolt range and contaminated by significant ambient electrical noise. In order to record, isolate, and represent an interpretable tracing, certain processing of the signals is required.

Differential amplifiers and common mode rejection: Each electrode records potentials generated by both the brain and the environment. Filtering out the surrounding noise is done with a differential amplifier, which excludes the signals recorded by both electrodes in a channel and amplifies the differences in between. This function is also known as common mode rejection.

Filtering the EEG signal: Conventional EEG interpretation requires the exclusion of very low frequencies using a high-pass (or low frequency) filter, very high frequencies using a low-pass (or high-frequency filter) filter, or a specific band of frequencies using a high-pass filter.

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