The electrical activity of neurons falls primarily into 2 categories: postsynaptic potentials and action potentials. Postsynaptic potentials are slow fluctuations of the voltage across neuronal cell membranes, extending over many milliseconds. These fluctuations represent excitatory and inhibitory postsynaptic potentials, caused by the arrival of various neurotransmitters at synapses on the cell body and dendrites. Such potentials may also arise by direct electrical contact in so-called gap junctions.

Action potentials are brief, explosive depolarizations that only last a millisecond or less and are triggered by postsynaptic potentials near the origin of the axon at the cell body, and then conducted rapidly along axons for distances ranging from millimeters to meters. Action potentials are too small, brief, and unsynchronized for their effects to do anything but cancel out over the great distance between the brain and the scalp. Instead, EEG recorded at the scalp seems to represent the ever-changing sum of synchronous postsynaptic potentials arising from broad cerebral cortical areas.

For EEG signals to be detected through the skull and other intervening tissues, synchronous activity must extend over cortical areas of 10 cm² or more, which does not mean that all of the neurons in that area are behaving synchronously; perhaps only a small fraction of them are. Ten cm² is the cortical expanse possessed by a medium-sized rodent.¹ Because a rodent brain is capable of doing many different things simultaneously, this consideration may place significant limitations on the level of detail in the information that can be extracted from scalp EEG.

Fig. 2 shows a sketch of large pyramidal cells oriented vertically through the layers of the human cerebral cortex. Excitatory neurotransmitter receptors tend to be clustered distally in the dendrites and inhibitory receptors closer to the cell body. Excitation of the dendritic membrane generates a net flow of negative charge into the extracellular space of the upper cortical layers. Inhibition of the proximal cell membrane produces a net positivity in the extracellular space at deeper layers. The resulting extracellular dipoles tend to be distributed vertically through the depth of the cortex. Multiplied across millions of neurons, this stratification of electrical charge produces the scalp EEG.

Fig. 2 The larger neurons in the layers of the cerebral cortex, with long vertically oriented dendrites. The + and − signs indicate vertical separation of charge in the extracellular space during postsynaptic potentials. Synchrony of such potentials across many square centimeters of cortex produces a detectable potential difference at the overlying scalp.

The resting potential across the neuronal membrane represents an astonishing voltage in a microscopic space; scaled up to familiar sizes, it would throw sparks and produce serious shocks. By the time this activity reaches the scalp, their neuronal voltages are attenuated thousandfold, to the range of 20 to 50 μV. This magnitude is roughly the size of a weak radio signal and must be amplified enormously before it can be processed and displayed.

A 50 μV EEG signal at the scalp is far smaller than the many sources of noise that threaten to disrupt or obliterate it and is subject to distortion at many stages of the recording process. First, the signal must be amplified many thousands of times for proper analysis; the multiplication factor is called gain. But amplification of the EEG is straightforward; more bothersome is the large range of other electrical noise (from the body and the external environment) that can swamp the EEG entirely (Fig. 3). Certain electrical activity (from muscle contraction, cardiac conduction, and eye movement) represents noise when it appears in the EEG channels but it is a desired PSG signal when recorded in the appropriate PSG channels.

Fig. 3 The enormous range of electrical activity that can be recorded from the human body. Note that the vertical axis is a logarithmic scale in volts. EKG, electrocardiogram; EMG, electromyogram.

Many artifactual potentials that corrupt the EEG signal in PSG arise from the clutter of electrical wires and equipment that surrounds us in modern buildings, producing electrical noise that permeates our bodies whenever we are inside them. The largest source of such noise is the 60-cycle field from nearby electrical equipment and power lines, which can easily be hundreds of times larger than the EEG.

Many years ago, frustrated neurophysiologists attempted to avoid environmental noise by recording inside metal enclosures (Faraday cages). Such enclosures remain necessary for magnetic resonance imaging and for the infinitesimal signals of magnetoencephalography but in EEG and PSG they can usually be avoided with the use of modern amplifiers.

Because electrical noise can be orders of magnitude larger than EEG and other neurophysiological signals (see Fig. 3), it may seem surprising that we ever see the EEG at all. The key is that enormous as the interfering signals may be, many of them are almost identical on closely adjacent portions of the body. Thus, an exact subtraction of the activity from nearby electrodes can remove the noise those electrodes have in common (common-mode signal) and record only the activity that is different at those electrodes (differential-mode signal.)

This accurate subtraction of the common-mode signal requires a precisely balanced input stage called a differential amplifier, which classically involves a 3-electrode input: a hot positive terminal, a hot negative terminal, and a neutral or ground connection. The ground lead is explicit in electromyography (EMG) and electrocardiography (EKG). In EEG and PSG, a single ground electrode serves multiple channels and is most often placed on the forehead.

The cleanup factor for a differential amplifier is called the common-mode rejection ratio and is supposed to be 10,000 or greater. In practice, problems like unequal electrode properties, poor ground connections, or noise sources in direct contact with the body can degrade amplifier performance, allowing 60-cycle artifact and other noise to appear in the EEG.

It is easy to think that the 60-Hz noise can simply be removed with the notch filters but other types of common-mode signals that accompany it cannot. Thus, the notch filters should be left off whenever humanly possible, allowing 60-Hz noise to serve as the electrical equivalent of a canary in a mine shaft.

EEG, EMG, and EKG electrodes provide the essential interface between lead wires and human tissues. Placing dry metal electrodes directly on dry skin is usually ineffective at conducting electrical potentials; skin preparation is necessary to remove cutaneous oils and reduce the insulating effect of keratin. An ionic solution, or electrolyte, provides an essential conductive layer on the scalp.

However, the interface between the electrolyte and the metal electrode produces a chemical half-cell potential, which is, essentially, half of an electric battery and can be much larger even than the 60-Hz noise (see Fig. 3). Avoiding possible interference requires careful design of the amplifier, use of a low-frequency filter (LF), and matching electrodes to produce minimal size and variation in the half-cell potential.

For the latter reason, gold and silver-silver chloride electrodes have been widely preferred. Disposable stick-on electrodes with a layer of conductive gel and a small amount of silver-silver chloride tend to be more uniform and are easier to use on areas that are free of hair. However, all skin connections are imperfect and manifest a certain amount of impedance, which is the term for all of the factors that oppose current flow.

The effectiveness of the differential amplifier depends on equal impedances at the 2 hot input terminals. Large and unequal electrode impedances can allow unpredictable amounts of common-mode noise to appear at the output. Most often, the common-mode noise is a 60-Hz artifact. Occasionally it represents other activity, including EKG and pickup from the ground electrode on the forehead. Fig. 4 is a complex example of the latter. Poor electrode connections were obscured because the 60-Hz filter was turned on, but this artifact was detected because of its pattern across multiple derivations. With fewer channels it might have been extremely difficult to recognize. The best way to avoid unpredictable common-mode artifacts is not to apply the 60-Hz filters routinely but to use good technique and keep the electrode impedances low.

Fig. 4 Ground recording artifact in a segment of paper EEG using an ipsilateral ear reference montage. Note that only selected channels from the left and right hemisphere are shown, to give a more compact display. Outlined in red is frontal slow activity over the left hemisphere. Directly below it there seems to be a burst of posterior temporal, parietal, and occipital slowing on the right. Close inspection shows that the right-sided posterior activity is actually the inverse of the left-sided frontal activity. The latter has been picked up from the ground electrode on the forehead. All of the apparent slowing on the right is ground recording artifact. HF, high frequency filter.

In North America, ground has 3 different but overlapping meanings: (1) True earth ground (a physical connection to the whole planet through metal plumbing or other low-resistance conduits) is a wonderful zero-potential point—a near-infinite reservoir for storing or withdrawing electrons. Stray charges, like lightning, head for earth ground. (2) Ground in electrical circuits is a reference point that remains at a virtual zero voltage and may or may not be connected to earth. (3) In the classic 3-terminal differential amplifier, the recording ground is the neutral reference lead.

Be advised that confusing the 3 meanings of ground can be fatal, because one of the 2 wires in alternating current (AC) power lines is almost always connected to earth ground at the local power station. To operate electrical devices plugged into the AC wall jack, the current flows from the hot wire of the power line to the neutral wire at earth ground. Connecting the reference ground on patients to earth ground would, therefore, connect patients to part of the power system. If patients are connected to earth ground, any stray currents (especially those from the hot power line) will happily flow through patients on the way there.

All grounded metal surfaces and grounded electrodes are potential routes through which a stray current can electrocute patients. A third power wire, which represents a second and more direct earth ground connection, is required for all hospital equipment. This additional ground, commonly referred to as “the” ground, increases safety overall, but in rare situations also constitutes a risk if the connection fails or patients contact voltage from another source.

The core principal of modern neurophysiological safety standards is that the recording ground on patients cannot form a direct connection to the power line grounds or to other sources of earth ground. Patients should not be in contact with any other earth ground, although they may inevitably encounter it in the form of hospital plumbing fixtures. In most modern electronic equipment, every electrode jack is separated from patients and the rest of the recording system by an optical isolation system.

Traditionally, discussions of electrical safety have focused on ground loops, which may be formed when patients are connected to more than one recording ground through different pieces of medical equipment. With proper isolation of the electrodes, ground loops are no longer a safety issue but may still contribute substantially to electrical noise.