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 40 channels. Depending on the design, the EEG machine then either prints out the wave activity on paper (by a galvanometer) 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.
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.
Numerous raw materials are used in the construction of an EEG machine. The internal printed circuit boards are flat, resin-coated sheets. Connected to them are electronic components such as resistors, capacitors, and integrated circuits made from various types of metals, plastic, and silicon.
The electrodes are generally constructed from German silver. German silver is an alloy made up of copper, nickel, and zinc. It is particularly useful because it is soft enough to grind and polish easily. Stainless steel (which has a higher concentration of nickel) can also be used. It tends to be more corrosion resistant but is harder to drill and machine.
An adhesive tape is used to attach surface electrodes to the patient. Since the electric signals are weakly transmitted through the skin to the electrodes, an electrolyte paste or gel is typically needed. This material is applied directly to the skin. It may be composed of a cosmetic ingredient like lanolin and chloride ions that help form a conductive bridge between the skin and the electrode allowing better signal transmission. Polytetrafluoroethylene (Teflon) is used as a coating for the wires and various kinds of electrodes.
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. In the United States, the FDA (Food and Drug Administration) has proposed production suggestions for manufacturers of EEG machines.
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.
A variety of output printers and monitors are available for EEG machines. One common device is a galvanometer or paper-strip recorder. This device prints a hard copy of the EEG signals over time. Other types of devices are also used including computer printers, optical discs, recordable compact discs (CDs), and magnetic tape units. Since the data collected is analog, it must be converted to a digital signal so electronic output devices can be used. Therefore, the primary circuitry of the EEG typically has a built-in analog to digital converter section. 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.
The different parts of an EEG machine are produced separately and then assembled by the primary manufacturer prior to packaging. These components, including the electrodes, the amplifier, and the storage and output devices, can be supplied by outside manufacturers or made in-house.
At each step in the manufacturing process, visual and electrical inspections occur to ensure the quality of each EEG device being produced. Since circuit fabrication is sensitive to contamination, assembly work is done by line operators in air-flow controlled, clean rooms. Operators must also wear lint-free clothing to reduce the chance of contamination. The functional performance of each completed EEG device is also tested to make sure it works. This is done by powering up the device, turning it on, and running a series of standard tests. To simulate real-life use, these tests are done under different levels of heat and humidity.
In general, manufacturers set their own quality specifications for their EEG machines. However, in the United States the Food & Drug Administration (FDA) provides production recommendations that are usually adapted by the industry. Various other medical and governmental organizations also propose standards and performance suggestions. Some factors considered important are standardized input signal ranges, accuracy of calibration signal, frequency responses, and recording duration.
In the future, EEG machines will be improved in their manufacture and their applications. From a manufacturing standpoint, the components that makeup the internal electronics of the device will likely get smaller. This will allow for smaller, more portable machines. It will also make the devices less expensive. This will be important because some experts suggest that future applications will make it desirable for individual consumers to have EEG machines.
While manufacturing improvements will come from research done in the general field of electronic manufacturing, specific research on EEG machines has focused on new uses and applications. For example, a device has recently been introduced that may make it possible to screen for Alzheimer's disease. This machine contains a cap that is fitted with electrodes. When worn it provides an electronic picture of a patient's brain activity. This picture is compared to the brain activity of healthy people and differences are noted.
A similar machine has been developed which can use information received from EEG electrodes to control computers. With this device the user wears an electrode-containing cap and looks at a computer screen. After a training session with the computer, users have been able to control the movement of a cursor on the screen just by using their thoughts. If fully developed, this technology could be a revolutionary development for paraplegics. Individual consumers may also benefit using such a device to control household lights, computers, and appliances just by thinking.
Fisch, Bruce J. Fisch and Spehlmann's EEG Primer. Elsevier Science, 1999.
Othmer, Kirk. Encyclopedia of Chemical Technology. Vol. 22, 1992.
Webster, J. G. Medical Instrumentation Application and Design. 2nd ed. 1992.
Wong, Peter K. H. Digital EEG in Clinical Practice. Lippincott Williams & Wilkins, 1995.
Sabbatini, Renato M.E. "Mapping the Brain." Brain & Mind 15 November 2001. < http://www.epub.org.br/cm/n03/tecnologia/eeg.htm#topography >.