How to mount an EEG net? Flashcards
How is the electrolyte made?
With 1 liter of warm water and 1 spoon of shampoo and 1 spoon of potassium.
For what do we use the electrolyte?
The electrode nets conduct electrical potentials from the scalp through little sponges that need to be soaked in a salty solution; the electrolyte.
How long does the EEG net need to sit in the electroltyte solution?
10 minutes
What needs to be measured before putting on the EEG net?
The subject’s head circumference.
How is the location of the central electrode (Cz) determined?
50% of the distance nasion-inion, 50% of the distance ear-ear. This spot can be marked with a red marker.
How does the EEG-net need to be applied?
From the back of the head to the front, while standing in front of the subject.
We need to make sure Cz electrode is on top of the marked spot. What other electrodes are important to check whether they are on the correct place?
The Fz and Oz electrodes, they need to be aligned in the saggital plate. These are number 11 and 75 on the 129-electrode net).
Why should you always move several electrodes together?
It reduces the risk of breaking the electrodes.
What is the impedance?
The definition of an impedance is any obstruction, or the measure of the opposition of an electric current to the energy flow when voltage is applied.
What is the next step after you have mounted the net?
Check how well the electrodes are conducting the electrical potentials from the scalp.
What does a high impedance mean?
The higher the impedance, the worse they conduct, the more noise the EEG will contain. The impedance, which is similar to resistance, should be as low as possible.
When you measure the impedance you can set a threshold for e.g. 70 kOhms. This means that above this value, electrodes are considered bad and will turn red. Here, the impedance measurement window will pop up on the computer screen. What do you need to try with the red electrodes?
Try to move aside the hair underneath bad electrodes and lightly scrub the scalp with the electrode. If this does not help, use the plastic pipet from the toolbox on the table to add a few drops of electrolyte solution to the sponge around the bad electrode.
No more than 10 electrodes should be red. What do you need to take into consideration when having 10 red electrodes left?
Red electrodes should not be located on any crucial location (10-20, reference) and should not be out of proportion with the other ones.
When you are done fixing the impedances, what needs to be done?
Provide the subject with earplugs and apply cling foil around the subject’s head to prevent the EEG-net from drying out. More or less tightly wrap the foil around the head twice. The impedances usually improve after doing this, because the cling foil will tighten the electrode net around the subject’s head, which can improve the contact with the scalp.
What is a notch filter?
Filters out the 50 or 60 Hz signal coming from the electricity grid.
