Subject Preparation: EOG
Electroculogram (EOG) Eye Lead Placement
Used to identify and monitor eye blinks (VEOG - vertical eye movement) and saccades (HEOG - horizontal eye movement). This is a one channel montage which is designed to pick up horizontal and vertical eye movement from both eyes. There are many ways to monitor eye movements- some montages use one channel and some use two channels. Selection is usually based upon the reason(s) for monitoring the eye movement.
- 1. Prepare the skin near the eyes and behind the ears where electrodes will be placed:
- Abrade the skin with Nuprep.
- Use an alcohol wipe to remove the gritty residue.
- 2. If using Ten/20 paste, carefully scoop into the electrode cup. Otherwise, press the electrodes onto the skin and apply electrolyte gel through the electrode opening with a syringe.
- 3. Using large EEG adhesive washers, place the electrodes firmly as follows:
- Left Infra Orbital (LIO) - on the orbital ridge centered directly under the eye.
- Left Outer Canthus (LOC) -at the lateral junction of the upper and lower eye lid.
- Ground electrode on forehead or the left mastoid bone behind the ear.
- 4. Check the impedances using the Grass Impedance Meter:
- Plug the left infra orbital electrode into jack# 1 and the left outer canthus electrode in jack# 2 and the ground into jack# 3.
- Push the red ON button.
- The Electrode Selector should be at 1 or 2.
- EOG impedance should be < 50k ohms.
- Refer to section entitled “Electrode Impedance Meter Check” for more detailed information on reducing impedance.
- 5. Secure all electrodes with Transpore (medical) tape.
- If you require further assistance, please request technical assistance from MEG staff.
Reducing Skin Impedances
Nuprep is a topical abrasive used for reducing skin impedance. It cleans the skin by removing any natural or applied oil, lotions and/or dead skin. Remove with water.
- Insert the wooden end of a cotton swab through the opening of the electrode to push hair aside (in one direction) until the skin is clearly visible.
- If the hair is long, lift up the electrode a little and let it down again while moving back the swab to grab a new bunch of hair (this will prevent the hair from slipping back).
- Dip the swab into the Nuprep and insert through the electrode opening.
- Twirl the swab against the skin using the thumb and index finger.
Ten/20 Conductive Cream is an opaque white, water soluble electrolyte substance used as a conductive and/or fixing/adhering medium to attach electrodes. Removed with warm water.
- *If using Ten/20 - scoop paste into a cup electrode (this paste is too thick to squirt from a syringe).
- Press firmly on the skin.
- *If using a liquid electrolyte like Electrogel, Elifix or Abralyt
- Fill a syringe with the solution.
- Squirt into the middle of each electrode opening.
- *Completely fill the electrode space (the space from the skin to the electrode) with cream / paste so that there are no air pockets and the electrolyte makes good contact with the skin.
If the electrodes are closely spaced, be careful to avoid electrical bridging. This happens when electrolyte from adjacent electrodes pools together. To avoid this, draw back the syringe concurrently to prevent electrolyte from swelling underneath the electrode rim and bridging to the adjacent electrodes. When using Ten/20 do not allow the paste from adjacent electrodes to touch
Electrode Impedance Meter Check
The Grass S88 Impedance Meter measures AC impedance (resistance and capacitance). An impedance check should be performed on all electrodes used for EEG or EMG. When two or more electrodes are plugged into the electrode switch positions, all electrodes except the one selected electrode being measured are connected together and provide the reference for the one being measured.
- 1. The Electrode Selector Switch should be at Jack position 1 or 2.
- 2. Plug the ground electrode (FPz) into electrode Jack 1 position.
- 3. Plug the other electrodes into the Jack (2, 3, 4, etc) positions in a systematic manner one at a time.
- 4. Press the Red ON Button to activate the impedance check. The power will automatically shutoff after approximately 2 minutes. (There is no OFF button.)
- 5. If the impedance is too high - take a cotton tip swab dipped in Nuprep (Abralyt or other skin prep) and twirl against the skin until impedance is at an appropriate level. Then re-insert Elifix electrolyte.
- Impedance Recommendations
EEG Electrodes ≤ 5Kohms EMG/EOG/ECG ≤ 50Kohms
- 6. After checking the impedance of each electrode plug the electrode into the appropriate Electrode Jackbox / Amplifier Position.
*If the electrode impedance exceeds 199.9K ohm, a “1” will appear in the left display window with all other digits blanked out indicating the impedance is too high. Take a cotton swab that has been dipped in Nuprep and insert it through the electrode opening and twirl against the skin. Re-apply electrolyte. Re-check the impedance.
MEG DSQ 3500 Amplifier Setup
Located inside the MSR on the table near the MEG machine. The DSQ 3500 Amplifier has inputs for:
- Fiducial Coils - Nasion, Left Ear, Right Ear, Inion(unused) and Vertex(unused);
- 21 Unipolar Jacks - there are 21 unipolar channels which measure the difference between an electrode and a common reference channel. The reference channel is common to all of the EEG unipolar channels;
- 8 Bipolar Channels - that measure the potential difference between two individual electrodes. These channels are labelled A - G on the jackbox, however, the channels are numbers 56 to 62 in the parameters;
- ZIF Connector/Jig Connector - for the EEG EasyCap System which can accommodate up to 56 electrode positions;
- Ground Jack - Z;
- Reference Jack - Ref.
Please note: There is a Static Dissipation Button which you must touch before touching anything else on the amplifier and before plugging any electrodes into the amplifier.
- If Using a One Channel Bipolar EOG:
- Plug the electrodes into a bipolar channel.
EOG ELECTRODE Location Bipolar Channel 56
(Channel 56 is the first Bipolar Channel on the Amplifier)
Lt OC Left Outer Canthus Channel-056 A+ (black) Lt IO Left Infra Orbital Channel-056 A- (blue)
Note: if recording only EOG (no EEG) you will still need to have a ground electrode.
If you are using disposable electrodes, discard the electrode and clean the electrode lead with alcohol.
If you are using reusable electrodes, they must be sent to Sterile Processing Services for sterilization.
1. Place soiled electrodes in the special two-part ziploc bags provided in the lab (on the left side of the supply cabinet. The electrodes should be on the interior pocket, with the leads tucked in the exterior pocket.
2. Fill out the sterilization request form, according to the following template. It is important to mark the following instructions on the form so that the electrodes are not damaged during handling:
- Distilled/Deionized water only
- Keep out of direct light when not in use
- If you notice that the electrodes are returned in poor condition, please let the lab manager know as soon as possible so SPS can be notified.
3. Place the electrode baggie inside the amber three-part transport container, with the request form on top. You can leave it on Anna's desk for collection.
4. Contact SPS to notify them that the MEG Core has electrodes to pick up. They will usually send someone within the next 24 hours.
- Sterile Processing Services: 301.402.2853
- B.B. Avila: 301.435.4096