Many organs produce electrical signals, which can also be measured in various examinations. A well-known example is an electrocardiography (ECG), which visualizes the electrical currents in the heart. The nerve cells of the brain also produce characteristic currents. Similar to the ECG, they can be “derived” from the body surface with electrodes. This measurement is called the electroencephalogram (EEG). With the EEG, the wires are attached at certain points of the head and connected via cables to an EEG device. The electrodes measure the activity of the brain, which is then displayed as a curve on a monitor.
When will an EEG be made?
An EEG is recorded, for example, in suspected nervous diseases such as epilepsy or brain damage. Sometimes anesthesia is monitored during surgery. In intensive care units or a sleep laboratory it can give information about the brain function. An EEG can also detect the brain death of a human.
Earlier, the EEG also played an essential role in the diagnosis of strokes or brain tumors. Today, more imaging studies such as CT or MRI are used.
How is the examination performed?
Usually, 21 electrodes are used for an EEG. To facilitate attachment, they are generally attached with cables in a hood for placement on the head. Before the examination, the electrodes are coated with contact gel.
A shave is not necessary. The hair should be washed and without residues of products such as stiffener, gel or hair spray.
It is measured in a relaxed, quiet position when lying down or sitting. During the examination, a medical professional gives instructions, for example, to open the eyes or to inhale and exhale violently. Sometimes certain stimuli such as flickering light are used to stimulate brain activity. The examination takes about 20 to 30 minutes.
What does an EEG image show?
Unlike a cardiac curve on an ECG, an electroencephalogram does not show jagged patterns but multiple waves. The course of the waves mainly depends on how active the brain is – whether you are awake or asleep, concentrated or tired. Each stream provides information about the activity of nerve cells in a specific brain region.
Differences are mainly:
- Alpha waves (frequency 8 to 13 per second (Hertz, Hz)): They describe the brain activity at rest with closed eyes, but the awake state, the so-called “basic rhythm.”
- Beta waves (14 to 30 Hz): Higher, irregular frequencies occur with open eyes, sensory stimuli, and mental activity.
- Gamma waves (above 30 Hz): They can occur with increased attention and learning processes.
- Theta waves (4 to 7 Hz): Reduced frequencies occur, for example, when falling asleep or very tired.
- Delta waves (0.5 to 3.5 Hz): The slowest, mostly synchronous waves signal deep sleep.
Each person shows a typical standard EEG picture for him. This means that “normal” EEGs can look very different. In children, the range is even more pronounced. Also, the wave pattern runs much slower and more irregular than in adults.
When an EEG is used for diagnosis, it is taken into account how often the waves (frequency) and how high they are (amplitude). Deviations from typical wave patterns may indicate a disease or disorder. For example, epileptic convulsions occur due to unusually high and steep waves (so-called spike waves). But many drugs that affect the brain change the EEG image.