EEG electrodes
Types of electrode placement
- EEG electrodes placed separately on scalp
- EEG electrodes mounted as special band on head
Types of EEG electrodes
- Disk electrodes
- Ear clip electrode
- Intracortical electrodes
Type of electrode connections
- Between each member of a pair (bipolar)
- Between one monopolar lead and a distant reference
- Between one monopolar lead and the average of all
International 10-20 system
It’s a method to describe and apply the location of scalp electrodes. This system is based on the relationship between the electrode and underlying cerebral cortex.
The 10 and 20 refer the actual distance between the adjacent electrodes .
In general 25 electrodes are used in general EEG test but number of electrodes may vary as per the EEG requirement and area of investigation.
In 25 electrode EEG system:
- 23 electrode are active electrodes
- 1 is ground electrode
- 1 is ref electrode
- A1-left ear
- A2-right ear
- Fp-frontal pole leads
- F-frontal leads
- P-parietal leads
- C-central leads
- T-temporal leads
- O-occipital leads
Montages
Sequential montage
- Each channel represents the difference between two adjacent electrodes
Referential montage
- Each channel represents the difference between a certain electrode and a designated reference electrode
Average reference montage
- The outputs of all of the amplifiers are summed and averaged
Laplacian montage
- Each channel represents the difference between an electrode and a weighted average of the surrounding electrodes
EEG Artifacts
Mains Interference
- Mains voltage of 110/230 volts, exceeds the EEG's 50 to 100 microvolts by 126dB
- Amplifier notch filters are designed to suppress a certain amount of mains interference
Biological Artifacts
- Eye-induced artifacts - eye blinks, eye movements
- ECG and EMG induced artifacts
- Glossokinetic artifacts
Environmental Artifacts
- Movement by the patient, or even settling of the electrodes
- Presence of an IV drip that can cause rhythmic, fast, low-voltage bursts, which may be confused for spikes