EEG Lead System
International Federation of EEG society has suggested 10 – 20 electrode placement system for EEG recording. Silver / silver chloride electrodes are used as surface electrodes in this setup. On the scalp, distances between two electrodes are given as 10% and 20% of the distance between specified points.
Nasion and Inion are the two reference points near the ear lobes of the human. Over the head, the distance between nasion and inion are divided into 5 points. The nasion – inion distance via the vertex is measured and three electrodes are placed as 1 in frontal, 1 in central and 1 in parietal. They are placed at a distance of 10%, 20%, 20%, 20%, 20% and 10% of this length. Now, similarly nasion – inion distance is measured along the temporal lobes and five electrode are placed as two in frontal, 2 in temporal and 1 in occipital lobes respectively. They are also placed at a distance of 10%, 20%, 20%, 20%, 20% and 10% of this length on either side. Lastly, on the peripheries of the circle remaining six electrodes are fixed as 2 in frontal, 2 in central and 2 in parietal. So, in the setup totally 19 electrode are placed on the brain scalp and one electrode which acts as reference is placed at the ear lobe. This is popularly known as 10 – 20 EEG system.
EEG Recording Setup
Here 21-electrode system is used. Electrodes are connected to 8-channel selector. Output from 8-channel connector goes to the differential amplifier bank. Differential amplifier is made of preamplifiers that are used to reduce noise. 50 Hz noise is created due to A.C interference that can be reduced using differential amplifier. The output obtained from differential amplifier is connected with signal processing unit. After further processing, the display unit displays the data.
This system helps to record the potentials generated from the sensory parts of the brain. To achieve this, output unit is connected with audio stimulus, visual stimulus and touch stimulus. It can also measure the time delay between stimulus and response from brain. In addition, we have a filter bank consisting of low pass, high pass and band pass filters. They help to remove noise from the brain waves. For the output recording, we can use either pen recorder or CRO. Three modes namely Unipolar, Average mode and bipolar recording are used to measure EEG.
Applications of EEG
- Epilepsy diagnosis.
- Anesthetic level.
- Brain injury.
- Monitoring during surgery.