EEG Data Collection

When the Participant Arrives

1. Welcome the participant to the lab!
2. Verbally walk them through consent form and other relevant questionnaires
3. Measure the participant's head to determine cap size (ask for consent, let the participant know you will be touching them!)
4. Move on to preparing the cap while the participant fills out forms

Measuring Head Circumference

The current cap sizes our lab carries are 54, 56, 58, and 60. Our tape measurer is usually kept on the shelf right outside of the EEG testing room.

1. Start measuring at the spot on the forehead between the eyes (the nasion) and include the bump at the back of the skull (the inion)
2. This circumference will usually be between 54-60 cm. If it is odd, round down to an even number

Prepping the Cap, Ground, and Gel

Plugging the Electrodes into the Cap

• Each splitter box has 32 electrodes. The numbers on the electrode wires correspond to the numbers on the cap.
• Electrode Set #1 = Green
• Electrode Set #2 = Yellow

While the participant is filling out forms, plug the electrodes into the correct size cap to match the following electrode layout:

• Make sure that each electrode is completely pushed into the cap. Also make sure all the wires run toward the back of the cap.

Plugging the Ground Electrode into the Cap

• Plug the ground electrode's wire into the front of the amplifier in the socket marked “GND." (The notch has to be facing the top of the socket.)
• Plug the ground electrode into the black space in the front and center of the cap. (This can also be done once the cap is on.)

Putting on the EEG Cap

• Get the participant settled in the EEG room (315B).
  1. If the participant has glasses, ask them to remove them temporarily.
  2. Put the cap on by centering the front of the cap on the participant's forehead and then pulling the rest of the cap backwards over their head. This is easier with two people.

Aligning the Cap

• Cz (green 24) should be on the midline (i.e. center) of the head, halfway between the nasion and inion. Use the measuring tape to double-check.
• If the cap is out of place by <1cm, you can wiggle it into place. If it is farther off, take the cap off and try again.
• The tab on the back of the cap should be sticking out and lying flat (i.e. not preventing gel depositing by blocking an electrode socket).

Securing the Cap and Electrode Bundles

Connecting Electrodes to the Amplifier

1. IMPORTANT: Unplug the actiPOWER battery (the box directly below the amplifier).
2. Roll the cart with the amplifier and battery on it closer to the participant's chair, and warn them not to push the chair back or stand up suddenly.

Attaching the EOG Electrodes

1. Put a double-sided sticker on each EOG electrode (small, round electrodes with colorful wires).
2. Fill the well of the EOG electrodes with gel from the syringes.
3. Peel off the backing on the stickers.

Gelling the Scalp Electrodes

1. Double-click the blue PyCorder Desktop icon on the computer just outside of room 315B. Hit "Accept" on the popup message.
2. Switch to “Impedance Mode” in PyCorder. The LEDS on all 65 scalp electrodes should light up red. The electrodes will light up red on the screen as well.
3. Depending on the particular task: You may want to start up the task and walk the participant through the instructions/practice while you complete setup.

To apply gel to each electrode:

• Carefully push the blunt syringe tip through the plastic groove.
• Twirl the syringe to move the hair away from the scalp. Circular and crescent motions work best. (See picture below.)
• Squeeze a small amount of gel onto the scalp under the electrode until the LED turns yellow or green.
• Let the gel settle for a few minutes as you move onto depositing gel to a different electrode. Circle back and repeat gelling if the electrode stays yellow.

Checking the Signal Quality in PyCorder

1. When all (or most) of the electrodes are green, switch to "Shielding Mode" in PyCorder.
2. Check the "baseline correction" box on the right-hand side.
3. Adjust the display however you like using the drop-down menus under "Display" on the right-hand side. (See below.)

Adjusting the PyCorder display

Timebase

• This controls the scale of the x-axis, which plots time.
• We typically leave this at 10 s (the default).

Scale

• This controls the scale of the y-axis, which plots voltage.
• For the scalp electrodes, we typically set this to 200 μVolts.
• For the EOG electrodes, we typically set this to 20 μVolts.

Channels

• This controls which electrodes are displayed on the screen.
• "EEG 1-32" displays the green set of electrodes (splitter box 1).
• "EEG 33-64" displays the yellow set of electrodes (splitter box 2).
• "Aux" displays the EOG electrodes.

For more info, please see the attached PyCorder manual.

4. Wait a minute or so for the EEG signals to flatten out in PyCorder. Apply a bit more gel to to particular electrodes as needed to try to improve the signal quality.

5. Make note on the experiment's running sheet of any electrodes that are still very noisy.

6. Switch to the “Aux” view, and ask the participant to blink a few times and look from side to side. Check the EOG signals in PyCorder for obvious deflections corresponding to blinks and saccades.

Starting the Task

1. Once you're satisfied with the signal quality, click "Start Recording" in PyCorder. Enter a file name before hitting "OK".
2. If you haven't already, start up the task, and walk the participant through the instructions as needed.
3. Instruct the participant to try to minimize their muscle movements as much as possible, particularly facial movements and humming or speaking. (They can blink and move their eyes normally, and they can re-adjust/fidget during breaks in the task.)
4. Before leaving the testing room, show the participant the call button in case they need to alert the experimenter during/after the task.

After the Task

1. Click "Stop Recording" in PyCorder; hit "OK" to confirm. Close PyCorder.
2. Detach the EOG electrodes from the participant's face and ear.
3. Unhook the chin strap (or ask the participant to do it), and remove the cap.
4. Unplug the splitter box cables, the ground electrode, and the EOG electrodes from the amp/pre-amps.
5. IMPORTANT: Re-plug in the actiPOWER battery.
6. Offer the participant a towel and shampoo, and direct them to the showers on 4th floor of Metcalf.
7. Pay the participant or grant them SONA subject pool credit. Paid participants must sign the compensation sheet.
8. Thank the participant for coming in!

PROBLEM: Impedances are not going down during impedance testing

SOLUTION: This is most likely a problem with the ground electrode: The impedance must be low on the ground before any other electrodes will work properly. Add a bit more gel, and give it time to settle.

• Alternatively, it could be a problem with the reference electrode (we use CPz). The reference must also have a low impedance before other electrodes will work properly.
• If there is still no improvement, remove the ground electrode from the cap and gently abrade the surface with a piece of fine-grain sandpaper before replugging it.

PROBLEM: PyCorder computer displays error message that amplifier battery has low voltage

SOLUTION: Either the battery was not plugged in after it was last used (oh no!), or something has gone wrong with the collection computer. A simple reboot of the computer should fix the latter issue.

PROBLEM: Signals in PyCorder appear to flatline in the middle of recording

SOLUTION: The amplifier has saturated, most likely because of one or two electrodes with particularly large fluctuations in voltage. To continue data collection, stop recording, restart PyCorder, and resume recording as quickly as possible.

PROBLEM: Quick, large changes in impedances during impedance testing

SOLUTION: The actiPOWER battery is probably still plugged in. Unplug the battery and plug it back in as soon as data collection is finished.

Additional Resources