- EEG Lead System Definition: The EEG lead system is defined as the placement of electrodes on the scalp using the 10-20 system, recommended by the International Federation of EEG society.
- Electrode Placement: Electrodes are positioned at specific percentages along the nasion-inion line and temporal lobes to capture brain activity accurately.
- EEG Recording Setup: An EEG recording setup involves a 21-electrode system connected to an 8-channel selector and a differential amplifier, leading to a signal processing unit.
- Noise Reduction: Differential amplifiers are essential in reducing 50 Hz noise from AC interference in EEG recordings.
- Recording Modes and Filters: The setup includes low pass, high pass, and band pass filters, and uses Unipolar, Average mode, and bipolar recording methods for EEG recording.
EEG Lead System
EEG measurement uses a lead system on the scalp. The International Federation of Clinical Neurophysiology (the EEG societies’ successor) recommends the 10-20 electrode placement system for EEG recording. This system uses silver/silver chloride electrodes as surface electrodes. The distances between electrodes on the scalp are set at 10% and 20% of the distance between specific skull landmarks.
Nasion (bridge of the nose) and inion (occipital bump) are two midline skull landmarks. The left and right preauricular points sit near the ears. The nasion-inion distance over the head is divided into five segments. Electrodes are placed at the frontal, central and parietal points along this line.
Those electrodes sit at 10%, 20%, 20%, 20%, 20% and 10% of this length. The same percentages are marked along each temporal line from nasion to inion, with five electrodes: two frontal, two temporal and one occipital. The same 10%, 20%, 20%, 20%, 20% and 10% spacing is used on either side.
Finally, six more electrodes are placed around the periphery: two in the frontal, two in the central and two in the parietal regions. The common 10-20 set uses 19 electrodes on the scalp plus ear or mastoid references, often 21 contacts in total.
EEG Recording Setup
The block diagram shows a teaching example with a 21-electrode system connected to an 8-channel selector. Clinical EEG now often uses 16 channels or more. The selector output goes to a differential amplifier bank, which includes preamplifiers to reduce noise. 50 Hz (or 60 Hz) mains interference is reduced by the common-mode rejection of the differential amplifier. The processed output is then sent to a signal processing unit and shown on the display unit.
The same hardware can also record evoked potentials from sensory parts of the brain. For that test the output unit is connected with audio stimulus, visual stimulus and touch stimulus. It can also measure the time delay between stimulus and response from the brain.
The system also includes a filter bank with low pass filters, high pass filters and band pass filters to remove noise from brain waves. The output can be recorded using a pen recorder, a digital display or, in a teaching lab, a CRO. Three recording montages are unipolar (referential), average mode and bipolar recording.
Applications of EEG
- Epilepsy diagnosis.
- Anesthetic level.
- Brain injury.
- Monitoring during surgery.
