Although EEG is discussed in more detail by Epstein elsewhere in this issue, we review some crucial concepts of EEG that are important to understand when reviewing VEEG PSG. Scalp EEG reflects fluctuating electrical voltage fields that result from continuous changing or oscillating extracellular current flow in the underlying cerebral cortex. Most of the EEG activity recorded represents a constantly changing summation and integration of excitatory and inhibitory postsynaptic potentials developed by the cell bodies and large dendrites from thousands of neighboring groups of neurons cross-talking in the underlying cerebral cortex. A single action potential does not contribute to the EEG because it is too brief, small, and distant from the recording electrode(s).

The pyramidal neurons in layers III and V of the cerebral cortex are believed to contribute most of the scalp EEG signal because they are aligned perpendicular to the surface of the cerebral cortex and tend to fire together. When a group of radially oriented pyramidal cortical neurons depolarize, a tangential net negativity is seen on the cortical surface, with a relative positivity in the deeper neocortical layers forming a vertical dipole for current flow.

One of the most striking features of EEG recorded from intracranial electrodes in animals and humans is the difference in electrical activity from electrode to electrode, even when the electrodes are only 1 to 2 mm apart.² The similarity of electrical activity from 2 scalp electrodes separated by more than a few millimeters is probably because the neurons in the vicinity of the electrode are driven by a common source. Neurons of at least 6 to 10 cm² of cortical surface area must fire synchronously for an IED to be detected on scalp EEG.^3,4 Neuronal generators that are deep-seated or dipoles that are horizontal to the cortical surface may not produce recognizable scalp EEG potentials.

EEG electrodes are applied to the scalp for routine EEG recordings using the International 10–20 System of electrode placement (Fig. 1).^5,6 In 1991, the American Clinical Neurophysiology Society (ACNS) recommended modifying the naming of the midtemporal and posterior temporal electrode placements: the left and right midtemporal electrodes (T3 and T4) were to be called T7 and T8, respectively, and the posterior temporal electrodes (T5 and T6), P7 and P8 (Fig. 2).⁷ This recommendation was made to be in concordance with the International 10-10 System of electrode placement, which provides names and locations for additional electrodes by further dividing the distances between standard 10-20 placements. The additional electrodes of the 10-10 system are most often used for epilepsy surgery evaluations in patients with medically resistant epilepsy, detailed topographic mapping of EEG, and research. They are particularly useful when recording EEG in patients with suspected temporal lobe epilepsy (TLE) because the T7 and T8 electrodes also record activity from the lower part of the frontal lobe, and the location of the maximal electronegativity of a discharge can be helpful in differentiating mesial from neocortical TLE.

Fig. 1 The International 10-20 System seen from (A) left and (B) above the head. A, ear lobe; C, central; F, frontal; FP, frontal polar; O, occipital; P, parietal; Pg, nasopharyngeal.

(Adapted from Sharbrough F, Chatrian GE, Lesser RP, et al. American Electroencephalographic Society guidelines for standard electrode position nomenclature. J Clin Neurophysiol 1991;8:200–2; with permission.)

Fig. 2 The International 10-20 System with modified combinatorial nomenclature for the names and locations of electrodes (A). Note how the right and left midtemporal and posterior-temporal electrodes are named T7, T8, P7, and P8. This modification was first recommended by the ACNS to be in concordance with the names and locations of electrodes in the expansion of the 10-20 System, the International 10-10 System shown in (B). The modified combinatorial nomenclature is used to detail topographic mapping of EEG activity.

([B] Adapted from Sharbrough F, Chatrian GE, Lesser RP, et al. American Electroencephalographic Society guidelines for standard electrode position nomenclature. J Clin Neurophysiol 1991;8:201; with permission.)

The International 10-20 System ensures that the EEG electrode placement is standardized across laboratories. Properly placed electrodes ensure the electrode placements are symmetric and evenly spaced over the correct anatomic location. When reading an EEG, electrical activity over 1 area is compared with that on the same (homologous) area on the contralateral side. Meaningful asymmetries or absence of electrical activity expected to be seen over a region (eg, sleep spindles or the alpha rhythm) can be confirmed only knowing that electrode placement is the same over time. Unequal interelectrode distances for homologous pairs of electrodes cause a false amplitude asymmetry: the shorter the interelectrode distance, the lower the amplitude. Proper electrode placement becomes even more critical in routine PSG given the limited number of EEG derivations recommended by the American Academy of Sleep Medicine (AASM) scoring and recording guidelines.⁸

Montages are systematic and logical combinations of multiple pairs of electrodes that allow for simultaneous recording of EEG activity over the scalp.⁹ Most digital EEG systems have 18 to 21 amplifiers that permit simultaneous recording of 18 to 21 channels of EEG. Each electrode is connected to an EEG amplifier. The digital EEG or PSG system connects different pairs of electrodes in any number of configurations to best show EEG activity based on the clinical question.

Montages are designed to compare EEG activity from homologous electrodes between 2 hemispheres. There are 3 basic montage designs: common reference, average reference, and bipolar. In a common reference (referential) montage, multiple scalp electrodes (lead or grid 1) are connected to a common reference (lead or grid 2). Each amplifier records the difference in electrical activity between a scalp electrode and the reference electrode. Electrodes most frequently chosen as the reference electrode(s) are the left and right mastoid (M1, M2) or the left and right auricular (A1, A2). All of the electrodes on the left scalp are typically referenced to the ipsilateral left auricular or mastoid; those on the right to the right auricular or mastoid. Alternatively, each electrode can be referenced to the linked auricular or mastoid (ie, A1 + A2, M1 + M2) placements.

A common reference montage displays electric potential differences between each active recording electrode and the relatively biologically indifferent or neutral common reference electrode(s). If the common reference is biologically inactive, then each active scalp electrode can display the true amplitude, frequency, and phase of EEG activity over it.⁹ In reality, common reference electrodes are rarely bioelectrically silent. Common reference montages are often used to confirm hemispheric asymmetries suspected on a bipolar montage and assist in the localization of epileptic abnormalities.

In routine EEG recording, the mastoid (M1 or M2) or auricular (A1 or A2) electrode placements are the most often selected common reference(s). However, a common reference should be chosen that is least likely to be involved in the electrical field of the scalp region of interest. For example, the use of the left mastoid (M1) as a reference is not advised when mapping the distribution of a left temporal spike discharge because M1 is likely to be within the field of the waveform of interest and contaminate the electrodes connected to M1. If the left temporal spike is unilateral, the contralateral mastoid (M2) could be chosen as a reference, although an electrode placement outside the temporal region is preferred.

Selecting the midline central (CZ, vertex) or parietal (PZ) as the common reference can display the wake EEG well. Reformatting and reviewing the wake EEG using a referential montage can help identify the dominant posterior rhythm, its posterior-anterior gradient, and voltage asymmetries and confirm and localize artifacts, malfunctioning electrodes, and obvious asymmetries. Fig. 3A shows the dominant alpha rhythm during wakefulness maximal over the parietal and occipital regions and eye movements over the frontopolar regions on a referential montage using CZ as the common reference. As shown in Fig. 3B, CZ is an active reference, and therefore a poor choice for common references during sleep because sleep spindles, vertex waves, K-complexes, and saw tooth waves are particularly prominent over the vertex and appear falsely distributed across all the EEG channels. The AASM Scoring Manual recommended EEG montage is an example of a common reference montage, linking right frontal, central, and occipital electrode placements to the left mastoid electrode (F4-M1, C4-M1, O2-M1), but recording the contralateral homologous EEG derivations (F3-M2, C3-M2, O1-M2) as backup.

Fig. 3 CZ can be a good choice of a common reference during wakefulness (A) but a poor choice during nonrapid eye movement (NREM) 2 sleep, when sleep spindles are falsely projected in all the EEG channels (B). Note the distribution of the alpha rhythm in this 14-year-old girl, maximal in the occipital leads (arrow, A) and the widespread distribution of the K-complex (arrow, B) surface-negative at CZ.

An average reference montage may also be used to confirm the localization of EEG activity. An average reference is created by measuring, summing, and averaging electrical activity from all (or most) of the active scalp electrodes before being passed through a high-value resistor. The resulting signal in lead 2 is then used as the average reference electrode and connected to input 1 of each amplifier. Contemporary digital EEG systems perform these calculations seamlessly, providing instantaneous moment-to-moment averaged values of EEG activity from all recording electrodes. The average reference compares EEG activity over each scalp electrode with the average value of all the electrodes in use. The average reference is particularly useful in determining the maximal negativity of a focal discharge and whether bilateral discharges can be lateralized to a particular hemisphere and region.

A bipolar montage consists of serial pairs of electrodes connected together in straight lines from the front to the back of the head (longitudinal anterior to posterior), transversely (left to right) or circumferentially. Each pair of electrodes enters lead 1 and lead 2, with the lead 2 sharing lead 1 of the next channel. When an electrode is common to 2 channels (eg, F4-C4, C4-P4), it is connected to input lead 2 of the first and to input lead 1 of the next. Each EEG electrode (and the amplifier connected to it) measures and displays fluctuating voltage differences between 2 adjacent biologically active scalp electrodes.

Bipolar EEG montages are particularly useful for localizing IEDs and focal background abnormalities by identification of a phase reversal.⁹ An inward or negative phase reversal is one in which the deflections point toward each other on a bipolar montage. A surface-negative phase reversal identifies which electrode is the site of maximum electronegativity (the estimated source of the IED). Most IEDs on scalp EEG are surface-negative. Fig. 4 shows frequent spike-wave discharges that show maximal electronegativity over the left frontal (F3) region confirmed by the surface-positive downward deflection in F3-C3 and surface-negative upward deflection in F3-C3. Positive phase reversals are less common, seen in patients with skull defects, head trauma, malformations of cortical development, neonatal, and invasive EEG recordings. A positive phase reversal occurs when the summed EEG activity generates a horizontally oriented dipole of current flow. The alternative EEG montage of the AASM Scoring Manual is primarily a bipolar montage. For example, the 2-channel bipolar montage, FZ-CZ, CZ-OZ, is well suited to confirm that sleep spindles are typically maximal over CZ. When K-complexes are of equal amplitude over FZ and CZ, the FZ-CZ linkage may result in cancellation effects.¹⁰

Fig. 4 Note the frequent spike-wave discharges that phase reverse over the left frontal (F3) area (arrow). The phase reversal is confirmed by the surface-positive downward deflection in F3-C3, and the simultaneous surface-negative upward deflection in F3-C3. A surface-negative phase reversal identifies which electrode is the site of maximum electronegativity (the estimated source of the IED). Most IEDs on scalp EEG are surface-negative.

The AASM Scoring Manual requires that manufacturers of digital PSG systems permit reformatting of EEG into different montages. However, many sleep specialists and technologists are unaware of this requirement, or rarely take advantage of it. Reformatting even limited EEG montages recorded in routine PSG can be useful. When IEDs are observed in an expanded EEG tracing, it is best to make sure that the reference does not involve the electrical field of the discharge.

Review of an EEG commonly begins with a longitudinal (anterior-posterior direction) bipolar montage that connects electrodes from anterior to posterior, creating a temporal and a parasagittal chain of electrodes over each cerebral hemisphere (Fig. 5). This montage is often referred to as a double-banana montage. This electrode configuration allows comparison of the left and right parasagittal and temporal chains for symmetry.

Fig. 5 The longitudinal anterior-posterior bipolar montage is the most common montage used in routine EEG.

The EEG can be reformatted into a longitudinal transverse (left-right direction) bipolar montage to evaluate (1) the symmetry of sleep spindles and EEG activity over the midline regions; (2) whether EEG activity is dominant over the temporal or parasagittal region in a particular hemisphere; and (3) IEDs or seizures that arise from midline or deep interhemispheric regions. Fig. 6 shows how PSG signatures of nonrapid eye movement (NREM) sleep are displayed on a transverse bipolar montage: vertex waves typically show phase reversals over the midline central (CZ, vertex) and K-complexes over the midline frontal (FZ).

Fig. 6 Note how the polysomnographic signatures of NREM sleep (vertex activity, sleep spindles, and K-complexes) are well displayed using a transverse bipolar EEG montage.

A third commonly selected bipolar montage is a circumferential bipolar montage, also called a coronal or hatband montage. Adjacent electrodes are connected in a coronal fashion from left to right, highlighting the midline and anterior to posterior differences. This electrode configuration is particularly helpful for identifying whether IEDs or focal slowing in the posterior regions lateralize to 1 side or are maximal over the posterior temporal or occipital region. A reverse hatband montage can be used to evaluate frontopolar or anterior temporal activity. Box 1 details the common bipolar EEG montages recommended by the ACNS when recording routine EEGs.¹¹

Asymmetries suspected on a bipolar montage are best confirmed using a referential montage. Reformatting and reviewing the wake EEG using a referential montage can help identify the dominant posterior rhythm, its posterior-anterior gradient, and voltage asymmetries and confirm and localize artifacts, malfunctioning electrodes, and obvious asymmetries (Fig. 7).

Fig. 7 Note how a common reference montage using the ipsilateral auricular electrode as reference shows the symmetry of the dominant posterior rhythm seen in the parietal and occipital regions, and eye movements noted over the frontopolar regions.

VEEG PSG has several advantages over routine PSG, including the ability to analyze behavior, correlate behavior with EEG, and more accurately detect seizure activity caused by additional recording electrodes. Aldrich and Jahnke¹² reviewed their experience with 122 patients with suspected parasomnias who underwent VPSG with 12 to 16 channels of EEG. VEEG PSG provided a definite diagnosis of epilepsy or a sleep disorder in 35% of cases, supportive evidence of either in another 30%, but was inconclusive in the rest. These investigators further found that VEEG PSG confirmed the diagnosis in 78% of 36 patients with known epilepsy, 69% of 41 patients whose paroxysmal motor nocturnal behaviors were prominent, but only 41% of 11 patients who were referred for minor motor activity in sleep.

Oldani and colleagues¹³ evaluated the reliability of routine VEEG, daytime VEEG after sleep deprivation, and nocturnal VPSG to diagnose nocturnal frontal lobe epilepsy (NFLE) in 23 patients with normal awake VEEG recordings. Nocturnal VPSG confirmed the diagnosis in 87% of patients, daytime VEEG with sleep deprivation in 52%. A study of 100 consecutive adults with a history of sleep-related injury found VEEG PSG was diagnostic in 65% and helpful in another 26%, but often more than 1 night of recording was needed to confirm the diagnosis.¹⁴