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  • Updated 07.31.2022
  • Released 01.11.2018
  • Expires For CME 07.31.2025

Technical aspects of EEG



The electroencephalogram (EEG) remains, as it has been since Hans Berger made his first recording in the 1920s, a pivotal diagnostic 2-dimensional clinical tool for assessing the brain’s 3-dimensional electrophysiological activity. EEG is a graphic display of a voltage differential between 2 sites of the brain function recorded over time. Because surface electrodes are routinely used to record EEG, the scalp, skull, and meninges serve as barriers to influence the EEG; therefore, features of the cortical waveforms may be altered. To ensure the accuracy of the recording where very low amplitude electrophysiological potentials are encountered, the technical aspects of recording are important to understand and maintain. Interpreting EEG necessitates a baseline understanding of electrophysiology, biological aspects of recording, and physical principles in order to achieve accurate pattern recognition. Adherence to the 10 to 20 international system of electrode placement, principals underlying instrumentation and electrical safety, standardized digital recording, and applying crucial recording concepts throughout the performance of the EEG are a few important technical requirements. The advances in technology have been many. However, the technical challenge of recording interpretable EEG with little or no artifact has remained the same. It is essential that the EEG be run by an experienced and knowledgeable technologist capable of supervising the technical aspects of the recording from application of electrodes to storing and archiving the data. In this article, the authors discuss important technical aspects of EEG that may influence the quality of the recording and, as a result, impact the clinician application of scalp EEG.

Key points

• The EEG begins with the accurate measurement and placement of the electrodes using the international 10-20 system. EEG instrumentation includes all of the components that connect the patient to the EEG machine. With each component of a recording system, there is potential to encounter interference or noise from outside sources.

• The electrodes are not part of the EEG machine itself and reflect a separate connection subject to technical limitations. Silver-silver chloride and gold electrodes applied after debridement of the skin surface with an adhesive have shown the best electrical conduit (22). Interelectrode impedances up to 10 kilo-ohms (kohms) are acceptable but optimal recording still requires impedances that are balanced (12). New electrode technologies including dry electrode cap models that don’t require gel are being tested to reduce the delay for obtaining an EEG in acute neurologic settings and eliminate the need for trained EEG technologists to secure individual electrodes (16).

• The electrode pins are connected to the jack box with site-specific locations reflecting the anatomical location of electrode placement on the head. The jack box is connected via an insulated input cable that allows each electrode to be selected for montage arrangements either in pairs or in single configuration that is then amplified. The electrodes are then typically arranged in a left-over-right array and displayed by montages in a final arrangement for visual analysis (01).

• Amplifiers are designed to increase sensitivity of detecting waveforms recorded from the brain, as well as filter the frequencies recorded. The EEG machine has a low frequency filter, a high frequency filter as well as a notch (60 Hz) filter. Standard settings of 7 uV/mm sensitivity, 70 Hz high frequency filter, and 1 Hz low frequency filter should be the initial settings of the EEG.

• Sampling rates are based on the Nyquist-Shannon theorem and the minimum sampling rate should be at least 256 samples per second; however, 512 hertz (Hz) is preferred to prevent aliasing on high resolution screens (04).

• The risks associated with performing an EEG are very low but the potential for adverse electrical safety exists if the ground is faulty and electricity is able to be passed through the patient. This can be prevented through having patients connected to multiple grounds. In addition, fire hazards represent a concern when high levels of oxygen are in use during function of the electrical machinery.

Historical note and terminology

The first known tracing in animals of fluctuating cortical potentials that constitute the EEG was performed by Richard Caton, a scientist from Liverpool, England, in 1875 using a galvanometer (25). In 1924, Hans Berger, a German psychiatrist, performed the first EEG recording in humans using Siemens double-coil galvanometer and published his first report of a human 3-minunte EEG recording in 1929 recognizing the alpha rhythm. Berger was not able to observe the recording live; rather, he had to develop the paper form of EEG, similar to a photograph in order to visualize the recording (05). Berger continued to make advances in his attempts to record EEGs as technology improved. His colleagues at the Institute of Brain Research in Berlin were able to make significant improvements in the quality of the EEG with the help of JF Tonnies who was an engineer at the Institute. From the initial 1-channel EEG, Tonnies is credited with developing the first ink writing oscillograph, which he called the neurograph. He went on to develop the first differential amplifier as well as the concepts of impedance and volume conduction (25). In the late 1920s, electrical engineer, Harry Nyquist determined that the number of independent pulses that could be put through a telegraph transmitting the number of channels over a unit of time was limited when it was less than twice the bandwidth of the channel (04). In the 1940s, Claude Shannon expanded on the Nyquist theorem and developed the Nyquist-Shannon sampling rate. Current sampling rates in digital equipment are based on the Nyquist-Shannon sampling theorem to prevent “aliasing” of the EEG data and false representation of the brain signal caused by a lower sampling frequency than the actual waveform. Most proprietary systems now offer 512 Hz sampling rates as a result to ensure adequate representation of a wide range of frequencies in the EEG.

By the 1950s, EEG technology started to become invasive, and the use of invasive electrodes and the exploration of deep intracerebral regions began. In the 1980s, developments in data collection and analysis allowed the EEG to be digitalized and recorded on videotape. The first-generation commercial digital EEG systems were introduced in the 1990s (25). Over the years, computed networking enabled remote EEG reading and simultaneous video recording of the patients, making continuous EEG (cEEG) a reality (17; 15). As manual review and interpretation of cEEG became increasingly labor-intensive, methods were developed to assist in rapid and accurate EEG interpretation. In the last 2 decades, complex algorithms enabling quantitative EEG analyses, such as wavelet analysis and Fourier analysis, were developed to improve display of the EEG signal (08). Automated spectral analysis was introduced to study spectral arrays using the Fast Forier Transform to generate a spectrogram, a color plot providing the temporal dispersion of the EEG frequency spectrum separated by power contained in independent components for analysis (15). These methods have improved the ease and diagnostic power of EEG by displaying quantitative frequency representation in trends, especially in intensive care units (21).

Terminologies. Understanding the technical aspects of recording EEG requires a foundation of terminology that encompasses its instrumentation. The following is a glossary of terms that are used to facilitate the concepts involved in recording EEG.

Aliasing. Aliasing is a signal processing term. Aliasing occurs when a system is measured at an insufficient sampling rate and creates a frequency misrepresentation of the recorded activity.

Amplifier. Amplifier is an electronic device that can increase the power of a signal. An amplifier functions by using electric power from a power source to increase the amplitude of the voltage or current signal. The amplifier gain is the ratio of the output signal to the input signal.

Analog to digital converter (AD converter). Analog to digital converter is a system that converts an analog signal into a digital signal.

Artifacts. Artifacts are noncerebral signals that often contaminate the recordings in both temporal and spectral domains within a wide frequency band. Internal source of artifacts may be due to physiological activities of the subject (eg, ECG, EMG or muscle artifacts, EOG) and its movement. External sources of artifacts are environmental interferences, recording equipment, electrode pop, and cable movement. In addition, some artifacts appear focal whereas others appear diffusely.

Bandwidth. Bandwidth refers to a frequency range.

Bipolar recordings. Bipolar recordings are carried out where active electrode pairs are compared with each other to record the difference between each pair.

Calibration. Calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. A mechanical calibration is performed at the beginning and end of each recording to test the accuracy of the amplifiers.

Capacitance. Capacitance is the ability to store an electrical charge.

Channel. Channel refers to the output of an amplifier that displays electrical information.

Common-mode rejection. Common-mode rejection refers to the characteristic of differential amplification where a signal that is the same in the 2 amplifier inputs is “rejected,” or not recorded (there is no potential difference). Common-mode rejection ratio relates to the ratio of signal to noise.

Chart drive. Chart drive refers to the motion component of the EEG.

Derivation. Derivation refers to recording from an electrode pair with the output displayed in 1 channel of the recording.

Electrode. Electrode is a solid electric conductor through which an electric current enters or exits an electrolytic cell or other medium. The electrode is considered to be the first component in a series of instruments involved in recording EEG.

Electrode test. Electrode test is the test applied to electrodes to assess impedance.

Epileptiform discharges. Epileptiform discharges are the EEG patterns of spikes and sharp waves associated with an increased risk for developing seizures and epilepsy.

Filter. Filter refers to particular circuits within the amplifiers that attenuate frequencies or frequency bands.

Gain. Gain is the ratio of the output signal to the input signal or the amount of magnification being used to amplify or increase the voltage of a signal.

Galvanometer. Galvanometer is an electromechanical instrument for detecting and measuring electric current.

High frequency filter. High frequency filters reduce the sensitivity of the EEG to high frequencies. It can be adjusted by a stepped control available on all EEG machines that perform digital recording.

Impedance. Impedance is the measurement of resistance to an alternating electrical current. In order to obtain a record with minimal electrical noise, impedance of the scalp electrodes should be under 10 kOhms.

Input I. Input refers to the first of 2 inputs measured by the EEG differential amplifier.

Input II. Input II refers to the second of 2 inputs measured by the EEG differential amplifier.

Interictal. Interictal refers to the period between seizures, or convulsions, that are characteristic of an epilepsy disorder.

Low frequency filter. Low frequency filter reduces the sensitivity of the EEG to record frequencies that are lower than the cut-off filter setting.

Jack box. Jack box is the electrode board where each individual pin of the electrodes is plugged to pre-amplify and convert the analogue signal to one that is digital. Each site is labeled with the anatomical name of the electrode and is also configured to represent a diagram of a head to alleviate confusion as to where to plug in the electrodes.

Nyquist-Shannon theorem. Nyquist-Shannon theorem states that the sampling rate must be at least twice the highest analog frequency component.

Ground. Ground is the reference point in an electrical circuit from which voltages are measured.

Montage. Montage is a standardized arrangement of selected pairs of electrode channels that are displayed in a “map” of the brain’s electrical activity chains for review. The most common montage is the A-P (anterior-posterior) longitudinal bipolar montage (aka the “double banana”) because the electrode configuration appears like 2 bananas laid front to back over each of the brain hemispheres. In a bipolar montage, neighboring electrodes may be paired in a chain to form an array either anterior to posterior (longitudinal bipolar) or side to side (transverse bipolar). The ACNS recommends that a standard EEG recording should contain at a minimum: 1 longitudinal bipolar montage, 1 transverse bipolar montage, and 1 referential montage.

Notch filter. Notch filter is a circuit that filters out narrow band frequencies, for example a 60 Hz (or 50 Hz) signal. This is particularly important when recording in intensive care unit settings where a variety of electrical equipment is in use.

Oscillograph. Oscillograph is a device used for recording the waveforms of changing currents, voltages, or any other quantity that can be translated into electric energy, as sound waves.

Phase reversal. Phase reversal refers to the principle means of localization in bipolar recording. The phase reversal reflects the maximal amplitude electrophysiological phenomenon of interest (eg, spikes or sharp waves) where waveforms “point” towards each other in adjacent channels.

Polarity. Polarity refers to negative, positive, or neutral values and relates to the polarity convention, which dictates that an upward deflection is surface-negative, and a downward deflection is surface-positive.

Reactivity. Reactivity refers to alteration of EEG activity by external sensory stimuli.

Reference electrode. Reference electrode refers to an electrode that is relatively inactive compared to the active electrode site and remains consistent for all electrodes in the montage. Examples of a reference electrode include ipsilateral ear/mastoid or vertex location. In a referential recording, the activity from the active electrode is compared to the reference to produce a potential difference.

Resistance. Resistance is opposition to direct electrical current (DC) flow.

Sensitivity. Sensitivity refers to the ratio of input voltage to output recorded in a channel of the EEG recording.

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