Epilepsy Awareness Program - EEG Resources

EEG Resources :


Neuroscience | EEG Background | EEG History | Diagnostic Application of EEG | Design of EEG Equipments| | Components of EEG Equipments | EEG Montages | 10 20 electrode placement system | Reference Books | EEG Settings | Commercial EEG Equipment Manufacturers |



*- Clinical Neurophysiology on the Internet

*- Neurosciences on the Internet

*- Electroneurodiagnostic Studies

*- eMedicine: Neurology

*- Neuromonitoring

*- The Whole Brain Atlas

*- New Scientist magazine

*- Cerebral Lobes, Cerebral Cortex, and Brodmann's Areas

*- The Discovery of Bioelectricity


EEG Background:

An electroencephalogram (EEG) machine is a device used to create a picture of the electrical activity of the brain. It has been used for both medical diagnosis and neurobiological research. The essential components of an EEG machine include electrodes, amplifiers, a computer control module, and a display device. Manufacturing typically involves separate production of the various components, assembly, and final packaging. First developed during the early twentieth century, the EEG machine continues to be improved. It is thought that this machine will lead to a wide range of important discoveries both in basic brain function and cures for various neurological diseases. The function of an EEG machine depends on the fact that the nerve cells in the brain are constantly producing tiny electrical signals. Nerve cells, or neurons, transmit information throughout the body electrically. They create electrical impulses by the diffusion of calcium, sodium, and potassium ions across the cell membranes. When a person is thinking, reading, or watching television different parts of the brain are stimulated. This creates different electrical signals that can be monitored by an EEG. The electrodes on the EEG machine are affixed to the scalp so they can pick up the small electrical brainwaves produced by the nerves. As the signals travel through the machine, they run through amplifiers that make them big enough to be displayed. The amplifiers work just as amplifiers in a home stereo system. One pair of electrodes makes up a channel. EEG machines have anywhere from eight to 64 channels. Depending on the design, the EEG machine then either prints out the wave activity on paper or stores it on a computer hard drive for display on a monitor. It has long been known that different mind states lead to different EEG displays. Four mind states—alertness, rest, sleep, and dreaming—have associated brain waves named alpha, beta, theta, and delta. Each of these brain wave patterns have different frequencies and amplitudes of waves. EEG machines are used for a variety of purposes. In medicine, they are used to diagnose such things as seizure disorders, head injuries, and brain tumors. A trained technician in a specially designed room performs an EEG test. The patient lies on his or her back and 16-25 electrodes are applied on the scalp. The output from the electrodes are recorded on a computer screen or drawn on a moving piece of graph paper. The patient is sometimes asked to do certain tasks such as breathing deeply or looking at a bright flickering light. The data collected from this machine can be interpreted by a computer and provides a geometrical picture of the brain's activity. This can show doctors exactly where brain activity problems are.

More EEG Background Resources: Wikipedia | WebMD | MedLine Plus | Epilepsy Action | epilepsy.com | emedicine |


EEG History:

The EEG machine was first introduced to the world by Hans Berger in 1929. Berger, who was a neuropsychiatrist from the University of Jena in Germany, used the German term elektrenkephalogramm to describe the graphical representation of the electric currents generated in the brain. He suggested that brain currents changed based on the functional status of the brain such as sleep, anesthesia, and epilepsy. These were revolutionary ideas that helped create a new branch of medical science called neurophysiology.
For the most part, the scientific community of Berger's time did not believe his conclusions. It took another five years until his conclusions could be verified through experimentation by Edgar Douglas Adrian and B. C. H. Matthews. After these experiments, other scientists began studying the field. In 1936, W. Gray Walter demonstrated that this technology could be used to pinpoint a brain tumor. Walter used a large number of small electrodes that he pasted to the scalp and found that brain tumors caused areas of abnormal electrical activity.
Over the years the EEG electrodes, amplifiers, and output devices were improved. Scientists learned the best places to put the electrodes and how to diagnose conditions. They also discovered how to create electrical maps of the brain. In 1957, Walter developed a device called the toposcope. This machine used EEG activity to produce a map of the brain's surface. It had 22 cathode ray tubes that were connected to a pair of electrodes on the skull. The electrodes were arranged such that each tube could show the intensity of activity in different brain sections. By using this machine Walter demonstrated that the resting state brain waves were different than brain waves generated during a mental task that required concentration. While this device was useful, it never achieved commercial success because it was complex and expensive. Today, EEG machines have multiple channels, computer storage memories, and specialized software that can create an electrical map of the brain.

More EEG History Resources: Faculty.washington.edu | Mapping the Brain |

Diagnostic Application of EEG

The main diagnostic application of EEG is in the case of epilepsy, as epileptic activity can create clear abnormalities on a standard EEG study. A secondary clinical use of EEG is in the diagnosis of coma, encephalopathies, and brain death. EEGs can also help to identify causes of other problems such as sleep disorders and changes in behavior as well it can be used to evaluate brain activity after a severe head injury or before heart or liver transplantation.
EEG used to be a first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.

More EEG Usage Resources:Wikipedia | epilepsy.com | Epilepsy Foundation | WebMD | emedicinehealth |                                      MedicineNet | Epilepsy Ontario | Epilepsy Action | NCBI | emidine.medscape |

Design of EEG Equipments


The basic systems of an EEG machine include data collection, storage, and display. The components of these systems include electrodes, connecting wires, amplifiers, a computer control module, and a display device.

The electrodes, or leads, used in an EEG machine can be divided into two types including surface and needle electrodes. In general, needle electrodes provide greater signal clarity because they are injected directly into the body. This eliminates signal muffling caused by the skin. For surface electrodes, there are disposable models such as the tab, ring, and bar electrodes. There are also reusable disc and finger electrodes. The electrodes may also be combined into an electrode cap that is placed directly on the head.

The EEG amplifiers convert the weak signals from the brain into a more discernable signal for the output device. They are differential amplifiers that are useful when measuring relatively low-level signals. In some designs, the amplifiers are set up as follows. A pair of electrodes detects the electrical signal from the body. Wires connected to the electrodes transfer the signal to the first section of the amplifier, the buffer amplifier. Here the signal is electronically stabilized and amplified by a factor of five to 10. A differential pre-amplifier is next in line that filters and amplifies the signal by a factor of 10-100. After going through these amplifiers, the signals are multiplied by hundreds or thousands of times.
This section of the amplifiers, which receive direct signals from the patient, use optical isolators to separate the main power circuitry from the patient. The separation prevents the possibility of accidental electric shock. The primary amplifier is found in the main power circuitry. In this powered amplifier the analog signal is converted to a digital signal, which is more suitable for output.
Since the brain produces different signals at different points on the skull, multiple electrodes are used. The number of channels that an EEG machine has is related to the number of electrodes used. The more channels, the more detailed the brainwave picture. For each amplifier on the EEG machine two electrodes are attached. The amplifier is able to translate the different incoming signals and cancels ones that are identical. This means that the output from the machine is actually the difference in electrical activity picked up by the two electrodes. Therefore, the placement for each electrode is critical because the closer they are to each other, the less differences in the brainwaves that will be recorded.

The software provided with some EEG machines can be used to create a map of the brain.

Various other accessories are used with an EEG machine. These include electrolytic pastes or gels, mounting clips, various sensors, and thermal papers. EEG machines used in sleep studies are equipped with snoring and respiration sensors. Other uses require sensory stimulation devices such as headphones and LED goggles. Still other EEG machines are equipped with electrical stimulators.

Source of article: Made How

Components of EEG Equipments

EEG machine consist of different parts and components; including electrodes, amplifier, Photic flash light, storage, output devices and advanced medical software used for analysis and display of the obtained brain signals.

A brief description of these parts can be explained as below:


A)- Electrodes:


One of the keys to recording good EEG signals is the type of electrodes used. Electrodes that
make the best contact with a subject's scalp and contain materials that most readily conduct
EEG signals provide the best EEG recordings.

Electrodes are metal discs placed on the scalp in special positions. These positions are identified by the recordist who measures the head using the International 10/20 System. This relies on taking measurements between certain fixed points on the head. The electrodes are then placed at points that are 10% and 20% of these distances.
Each electrode site is labelled with a letter and a number. The letter refers to the area of brain underlying the electrode e.g. F - Frontal lobe and T - Temporal lobe. Even numbers denote the right side of the head and odd numbers the left side of the head.
There is a great variety of electrodes that can be used. The majority are small discs of stainless steel, tin, gold or silver covered with a silver chloride coating. These normally have a lead attached.

Some of the types of electrodes available include:Reusable disks, EEG Caps with disks, Adhesive Gel Electrodes, Subdermal Needles.




Electrodes are usually used for EEG recording which minimise the contact resistance between electrode and skin surface and guarantee a stable, reliable contact. So-called silver / silver chloride electrodes meet these demands best. By means of a special conductive gel, which is applied in the hair between electrodes and scalp, the electrical resistance can be minimised and the derivation of the EEG can be optimised. Since an EEG usually requires some dozens of electrodes, all electrodes have to be applied to the represented cap carefully one by one and tested. For particularly good contacts, the surface of the scalp has to be roughened mechanically or abraded. The time expenditure and the required know how are considerable, so the EEG derivation is generally left for specialists and the derivation of brain waves for controlling machines so far appeared to be too complex.

B)- Amplifier:

A minimum of 25 electrode inputs (21 on the scalp, the system reference, ground, and 2
extra) are recommended and 32 electrode inputs (9 extra electrodes) are preferred when
recording. Ideally, the system should provide two reference inputs in order to prevent the
loss of data should one reference electrode become dislodged during recording.
The input impedance should be greater than 10 mega-ohms. The common mode rejection
ratio should be at least 100 dB for each input.

C)- Photic Flash:


The flashing light device consists of a Photic light or also can be called flash light where intensity, frequency and duration of the emitted light are operator controlled. The Flash light is positioned about a meter distance from the patient head and usually flash light is not given continuously when frequency is changed as a pause for a certain time separates different flash light frequency values.
The Photic stimulation is usually used as a part of routine EEG test and can provoke seizure in certain percentage of patients.

Response to Photic stimulation

*- Asymmetrical: unilateral destructive occipital lesion

*- Photomyoclonic response at f=12-18 Hz; associated with brainstem lesion or psychiatric disorders but not epilepsy

*- Photoparoxysmal response most easily elicited at f= 15-20 Hz; not time locked to flash stimulation

Photic driving response

Rhythmic occipital dominant waveform
Occurs at stimulus frequency 5-30 Hz, especially at 8-13 Hz
Associated with lamda and POST

D)- Computer and Software:




Normally medical grade computers are preferred to be used which are tested and confirmed to be used in medical equipments. There are no extra components required to be used on the computer except video card of certain capabilities required in most EEG equipments for video monitoring.
The vast majority of Neurophysiology equipment manufacturers operate their PC's using Microsoft Windows operating system environments.
Specialized medical software is used to analyze, display and provide special functions and options which can be helpful in diagnosis of epilepsy.


Montage means the placement of the electrodes. The EEG can be monitored with either a bipolar montage or a referential one. Bipolar means that you have two electrodes per one channel, so you have a reference electrode for each channel. The referential montage means that you have a common reference electrode for all the channels.

Basic Terminology

Montage: patterns of connection between electrodes; usually 16 or more electrodes

Referential: background rhythm interpretation

Bipolar: adjacent electrodes are linked along longitudinal (parasaggital) or transverse (coronal) lines; useful for localization.

Additional electrodes are sometimes required e.g. T1/T2, sphenoidal, nasopharyngeal


Commonly used montages

Polarity convention: Upward = negative field

Input 2 is the nearest neighbouring electrode and changes from channel to channel (bipolar derivation)

Input 2 is a distant electrode common to all channels (common reference)

Input 2 is computed (A1+A2 linked ear reference or laplacian reference varies from channel to channel)

Montage in details:

Bipolar montage: Each channel (waveform) represents the difference between two adjacent electrodes. The entire montage consists of a series of these channels. For example, the channel "Fp1-F3" represents the difference in voltage between the Fp1 electrode and the F3 electrode. The next channel in the montage, "F3-C3," represents the voltage difference between F3 and C3, and so on through the entire array of electrodes.

Referential montage: Each channel represents the difference between a certain electrode and a designated reference electrode. There is no standard position for this reference; it is, however, at a different position than the "recording" electrodes. Midline positions are often used because they do not amplify the signal in one hemisphere vs. the other. Another popular reference is "linked ears," which is a physical or mathematical average of electrodes attached to both earlobes or mastoids.

Average reference montage: The outputs of all of the amplifiers are summed and averaged, and this averaged signal is used as the common reference for each channel.

Laplacian montage: Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes.

10 20 electrode placement system

The International 10-20 system is an internationally recognized method to describe and apply the location of scalp electrodes in the context of an EEG test or experiment. This method was developed to ensure standardized reproducibility so that a subject's studies could be compared over time and subjects could be compared to each other. This system is based on the relationship between the location of an electrode and the underlying area of cerebral cortex. The "10" and "20" refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front-back or right-left distance of the skull.

Each site has a letter to identify the lobe and a number to identify the hemisphere location. The letters F, T, C, P and O stand for Frontal, Temporal, Central, Parietal, and Occipital, respectively. Note that there exists no central lobe, the "C" letter is only used for identification purposes only. A "z" (zero) refers to an electrode placed on the midline. Even numbers (2,4,6,8) refer to electrode positions on the right hemisphere, whereas odd numbers (1,3,5,7) refer to those on the left hemisphere.

Two anatomical landmarks are used for the essential positioning of the EEG electrodes: first, the nasion which is the point between the forehead and the nose; second, the inion which is the lowest point of the skull from the back of the head and is normally indicated by a prominent bump.

When recording a more detailed EEG with more electrodes, extra electrodes are added utilizing the spaces in-between the existing 10-20 system. This new electrode-naming-system is more complicated giving rise to the Modified Combinatorial Nomenclature (MCN). This MCN system uses 1, 3, 5, 7, 9 for the left hemisphere which represents 10%, 20%, 30%, 40%, 50% of the inion-to-nasion distance respectively. The introduction of extra letters allows the naming of extra electrode sites. Note that these new letters do not necessarily refer to an area on the underlying cerebral cortex.




Video designed and published by Biomedresearches - Emad El Alem

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Fisch, Bruce J. Fisch and Spehlmann's EEG Primer. Elsevier Science, 1999.

Othmer, Kirk. Encyclopedia of Chemical Technology. Vol. 22, 1992.

Neocortical Dynamics and Human EEG Rhythms: Paul L. Nunez

Analysis of the Electrical Activity of the Brain: Franco Angeleri, Stuart Butler

Webster, J. G. Medical Instrumentation Application and Design. 2nd ed. 1992.

Wong, Peter K. H. Digital EEG in Clinical Practice. Lippincott Williams & Wilkins, 1995.

Electroencephalography: Basic Principles, Clinical Applications, and Related Fields: Ernst Niedermeyer, F. H. Lopes

Atlas of Electroencephalography: R.R. Clancy, H.L. Chung, J.P. Temple

Basic Mechanisms of the EEG: St. Zschocke, E.J. Speckmann

Current Practice of Clinical Electroencephalography: David D. Daly, Timothy A. Pedley

Digital EEG in Clinical Practice: Peter K. H. Wong, MD

EEG in Clinical Practice: John R. Hughes

EEG Interpretation: Problems of Overreading and Underreading: Eli S. Goldensohn, Steven Wolf, Sam Koszer

The Electroencephalogram: Its Patterns and Origins: John S. Barlow

Basic Mechanisms of the EEG (Brain Dynamics): St. Zschocke, E.J. Speckmann

Functional Neuroscience: C. Barber

EEG: Clinical Neurophysiology by Levin & Luders

EEG Settings


The standard filtering settings for routine EEG are:
Low frequency filter: 1 Hz
High frequency filter: 50-70 Hz
Sensitivity: 7 µv/mm
Impedance: A measure of the impediment to the flow of alternating current, measured in ohms at a given frequency. Larger numbers mean higher resistance to current flow. The higher the impedance of the electrode, the smaller the amplitude of the EEG signal. In EEG studies, should be at lest 100 ohms or less and no more than 5 kohm

Commercial EEG Equipment Manufacturers

Natus Medical (Nicolet, Biologic, Xltek, Dantec, NeuroCom, Schwarzer, Grass, Stellate- http://www.natus.com/

eb Neuro - http://www.ebneuro.biz/en/

Braintronics - http://www.braintronics.nl/

Compumedics - http://www.compumedics.com/

Dantec Medical - http://www.dantecmedical.co.uk/

Axon Systems - http://www.axonsystems.com/

Mitsar - http://www.mitsar-medical.com/

NeuroConn - http://www.neuroconn.de/

Nihon Kohden - http://www.nihonkohden.com/

NR Sign - http://www.nrsign.ca/

Cadwell - http://www.cadwell.com/

WR Medical Electronics - http://www.wrmed.com/

Neuroscan - http://www.neuroscan.com/landing.cfm

Moberg Research - http://www.mobergresearch.com/

Lifelines Neurodiagnostic Systems - http://www.lifelinesneuro.com/

Biola - http://www.biola.ru/index_e.html

Micromed - http://www.micromed.eu/

Advanced Brain Monitoring - http://www.b-alert.com/index.html


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See Also: Epilepsy Health Corner

See Also: Neurophysiology Health Corner

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