Running a Baby EEG experiment

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Introduction

WARNING; Still a work in progress!

This protocol is written for researchers, research assistants and students who are planning to run EEG experiments with infants in the ILS Babylab. A PDF version is also available, which also includes a comprehensive step-by-step guide to analysing your EEG data. In addition, a less-detailed checklist version can be found here.

Should you have any questions or suggestions, please contact the lab taff.

A preliminary remark on how to treat electrodes with care

Before you start your first testing day, make sure you have read and understood this section carefully. Should you have any questions, please do not hesitate to ask!

Electrodes are very fragile and an entire electrode bundle may become completely useless once only one of the electrodes dies. The lifespan of electrodes can be shortened drastically when handled without care. Therefore, it is important to keep a few things in mind when working with electrodes.

• NEVER let the electrodes touch metal. Always remove all of your jewellery before preparing the cap and cleaning the electrodes. Also, make sure your participant isn’t wearing any metal hairclips.
• When preparing the cap, let the bundles hang around your neck to prevent the ends from touching the floor.
• When cleaning the electrodes, make sure that the water is lukewarm. The gel will dissolve more easily when the water isn’t too cold, but too hot water will cause the electrodes to break faster.
• Never let the electrodes lay in the water for too long; when you put them into the water, clean and (gently) dry them immediately.
• NEVER let the other end of the bundle (which connects to the amplifier) touch water.
• NEVER let the electrodes sit under running water.
• Always clean the electrodes as soon as possible, before the gel gets the chance to dry.
• Be sure that no gel is left behind on the electrodes.
• Always clean the electrodes gently, using a soft brush. Don’t push or rub too hard.

The standard testing procedure

This section offers you a detailed description of how to run an EEG experiment with infants. In the lab, we make use of BioSemi equipment, and all experiments should be written in Presentation. Before you can start testing, make sure you have:

• Approval of the ethics committee
• Designed and piloted your experiment. NOTE: Make sure that all triggers are sent at the precisely right time from Presentation to Actiview!
• Scheduled your participants
• Reserved the EEG lab. There are two EEG labs at Janskerkhof 13: room K.13 and room K.02.

Note that a brief checklist of the entire procedure can be found here.

Preperation

The Babylab waiting area

At the beginning of your testing day, make sure you open up the waiting area (room K.01) and turn on the light. Check if there is still water in the watering can, the kettle and the Nespresso machine, and if the plant needs watering. Also make sure that the room looks clean and tidy.
General instructions for correct use of the waiting area can be found here. NOTE: this document is password protected, please contact Desiree Capel for the login details.

The lab(s)

The ILS Labs has two dedicated EEG labs; the Baby-EEG lab in K.02 and the regular EEG Lab in K.13. Both labs can be used for EEG experiments with infants.

It’s recommended you use the same lab for the entire duration of your experiment, as the change in scenery might influence your results.

General preparation

From the kitchen block in the control room (K.02) or the plastic cabinet next to the table (K.13), get out the following items:

• Electrode stickers (‘1’ in image 3 .7)
• Cotton pads (‘2’)
• Alcohol (‘3’)
• Electrode gel (‘4’)
• A syringe (‘5’)

Place all items on top of the cabinet/table in the test room.

image 3.7
Next, consider whether you want the child to sit in the baby chair, or whether you want it to be on the parent’s lap, for instance if it’s too young to sit down independently. Place the chairs according to your decision. Make sure that the child is seated right in front of the screen, in the middle of the two speakers. When testing in K.02, turn on the two speakers and the display(s) that you’re going to use by pushing the on/off-buttons. Keep the other displays turned off. When testing in K.13, check if the table is covered with the towel.

Once everything is prepared, start the experiment on the Presentation computer and check the sound level. There is a decibel meter on the shelf in the test room in K.13. Hold the tip of the decibel meter (circled in yellow in image 3 .8) at the place where the child’s ear will be during the experiment, and turn on the device using the switch circled in red in image 3 .8. You will see a number starting around 110 on the screen of the device, which will drop slowly until it keeps on circling around a certain number. This is the current sound level. The desirable sound level is approximately 65 decibel. If it is too high or too low, change the sound level in your experiment. For more information on how to use the decibel meter, please see here.

image 3.8

Changing the sound level in Presentation

If, after checking the sound level with the decibel meter, you find that the sound level is too high or too low, there are two ways to change the volume when using Presentation. Depending on your settings in Presentation, you could do it in either one the following manners.

One way to change the sound level, is to change the volume of the Presentation computer in the lower right corner of your screen (as you usually change the volume). This will work, as long as you haven’t installed a fixed sound level in your Presentation experiment. If you notice that the volume automatically jumps back to the original sound level after restarting the experiment, you need to adjust the volume in Presentation itself.

To do this, open up your experiment and go to the tab ‘Settings’ (circled in green in image 3 .9). On the left hand side, go to ‘Audio’. You will see the audio settings appear. Underneath ‘Volume’, you will probably see that the checkbox in front of ‘Set Device and Application Volume Device Volume’ is checked (circled in blue in image 3 .9). Adjust the sound level by typing in a higher or lower number between 0.0 and 1.0 in the square circled in red and click ‘Apply now’. Restart the experiment and check the sound level again. You should now see that the volume has changed.

image 3.9

Air circulation

When testing in K.13, you’ll find an air humidifier next to the kitchen block (image 3 .10). This device secures a pleasant air humidity inside the test room, to make sure that participants don’t blink their eyes too much.
Before you start testing, remove the entire grey part from the white part by lifting it up. Fill up the white box with water until the water level reaches ‘max’ (you can check the water level by looking at the indicator circled in red in image 3 .10). Put the grey part back on again, plug the humidifier in and press ‘power’ (circled in green).

image 3.10

The cabin in K.02 is connected to the buildings air circulation system, no setup is needed. The control room can be ventilated by using the fan in the window. The controls for the fan are located on the left wall next to the window, and consist of a on/off switch and a speed control. (Note: you might also need to pull the cord on the fan itself if someone used that by mistake).

Receiving the participant and preparing the cap

After you have prepared the control and test room, it is time to receive the parent and the baby in the waiting area. In the waiting area, shortly explain what is going to happen and let them sign the informed consent form. Take them to the EEG lab. Once they are seated, you can start the preparation of the cap.

Apply gel on the baby’s wrist

Before doing anything at all, apply a little bit of gel on the baby’s wrist using the syringe. Tell the parent that you’re doing this to make sure there won’t be an allergic reaction to the gel. You can show them that the syringe won’t hurt the child, by tapping the tip of the syringe onto your own palm. Tell the parent that the chances of getting an allergic reaction are extremely small, due to the fact that the gel is basically only a combination of water and salt.

Explain the procedure

While giving the baby’s skin a chance to react to the gel, explain the procedure to the parent. Mention the following things:

• What the experiment is about
• A brief explanation of the EEG procedure
• The duration of the actual experiment
• What the child is and isn’t allowed to do (e.g. play with the toys)
• Tell the parent not to speak during the experiment
• Also tell the parent to try to prevent the baby from pulling the cap or the electrodes as good as they can.
• Ask the parent to switch their phone to airplane mode, otherwise its signals will interfere with the EEG.
• Let them know that they are being watched through the webcam, and whether there will be a video recording of the experiment.
• If they want to stop the experiment, for whatever reason, they can wave to the camera.
• If the baby becomes too restless, you can stop at any time.
• They can wash their baby’s head at the end of the experiment.

After the explanation, check the baby’s wrist once more. If the skin hasn’t reacted to the gel, proceed with the preparation. If there seems to be an allergic reaction, quit the experiment.

Applying the mastoid electrodes

Before placing the cap on the child’s head, you have to apply the ‘mastoid electrodes’. These electrodes are also indicated by the names ‘EXG1’ and ‘EXG2’ (or EX1 and EX2; see image 3 .12). These are two separate electrodes, and are not part of a larger bundle.

The mastoid electrodes have to be placed behind the ears of the infant, on the ‘mastoid’. This is the most bony area behind the ear, where presumably no (or few) signals from the brain are received. In an EEG experiment, all electrodes on the left hand side are usually indicated by an odd number, and all electrodes on the right hand side are usually indicated by an even number. This means that EXG1 has to be placed behind the left ear, and EXG2 has to be placed behind the right ear of the child.

image 3.12

image 3.13

image 3.14

When applying the mastoid electrodes:

• First, put a little bit of alcohol onto a cotton pad (numbered ‘2’ and ‘3’ in image 3 .7).
• Degrease the area behind the child’s ears by rubbing it gently with the cotton pad with alcohol.
• Take one of the electrode stickers (numbered ‘1’ in image 3 .7).
• Stick it onto the electrode (as in image 3 .13).
• Remove the white protection layer (as in image 3 .14).
• Add a little drop of gel in the circle in the middle (as in image 3 .15). Do not forget to do this! It will hurt the child if you forget to add gel. However, also make sure you don’t add too much gel.
• Try to feel with your fingers where the mastoid is located and stick the electrode onto the mastoid (image 3 .16).
• Insert the other end of the electrode into the BioSemi amplifier (circled in blue in image 3 .20
• Watch out that the child doesn’t pull off the electrode right away!

image 3.15

image 3.16

NOTE: You can also prepare the cap first, and try to squeeze the mastoid electrodes underneath the cap afterwards to make sure the child cannot pull them off right away. A disadvantage of this method is that it’s harder to find the right location of the mastoids when the cap is already on the head. The importance of the mastoid electrodes also depends on whether you will use the linked left and right mastoid as reference for offline processing. Average reference may work fine. If you use the average as a reference, you don’t necessarily need the electrodes on the left and right mastoid (for more information about referencing your data, read this section).

Preparing the cap

Checking the signal

CMS and DRL

After all the electrodes are adjusted properly, it is time to check the signal. The first check is the blue light on the BioSemi amplifier (circled in yellow in image 3 .20). If this light is flickering, it indicates that the electrodes CMS and DRL are badly connected. These electrodes do not give you data, but form a feedback loop which acts as a reference and reduces the noise. Without a good CMS/DRL connection, your EEG measurements will be useless. Therefore, always make sure the blue light on the BioSemi is stable.

ActiView

After making sure that the CMS and DRL are working properly, it is time to start ActiView. Open up ActiView on the EEG1 (K.13)/Recording (K.02) computer. Before you start recording, make sure you have loaded the correct configuration file. A pop-up will open automatically in which you can select the correct configuration file. Go to ‘Desktop’, click ‘Configuration file’ and select ‘Infant_32channels.cfg’.

On your monopolar display, press ‘start’ (circled in yellow in image 3 .25). You will find that for each selected electrode, the signal is displayed on the screen (circled in red). Set the reference to Cz, by selecting ‘Free choice’ under ‘Reference’ (circled in green in image 3 .25) and selecting Cz in the list underneath. Check the signal for each electrode. The flatter the signal, the better. If you notice that one or more signals are very noisy, try adding more gel to that particular electrode, but not too much as you don’t want two electrodes to be connected through the gel. If you find that all signals are extremely noisy, add more gel to CMS and DRL or press the cap a little firmer onto the head with your hands. If the overall signal is better when you don’t select Cz as your reference, consider adding more gel to Cz. If you’ve tried to add gel to an electrode for a maximum of three times without seeing a significant improvement, remove the electrode from your display. You can do this by selecting ‘Free Choice’ under ‘Channels’ (circled in orange in image 3 .25, enlarged in image 3 .26), and deselect the particular electrode by pressing CTRL and selecting the channel from the list underneath. The data for this channel will still be saved, even though it is removed from the display.

image 3.25

image 3.26
While checking the signal, also look at the impedance by clicking ‘Electrode offset’ in the toolbar. Set the displayed range to 50 mV (circled in yellow in image 3 .27). You should see that the bars turn to blue. All bars should be within the range of +25 mV and -25 mV. The closer to zero, the better the signal. If one of the bars is above +25 mV or below -25 mV (for instance the bars circled in red in image 3 .27), consider adding more gel to the corresponding electrode. If you see that one of the bars is very unstable, also consider adding more gel to that particular electrode or remove it from the display.

NOTE: it may be difficult to get all electrodes with good signal when testing infants. Researchers are usually only interested in some electrodes rather than all. For instance, MMN is mostly visible at frontal-central electrodes. Hence, if the electrodes at the parietal and occipital locations are noisy, it is not worth it to spend a lot of time adjusting these electrodes, as children very easily get annoyed by people working on their head. Researchers need to be aware of the trade-off between a very good signal and crying babies. In addition, if you use left and right mastoids as reference, a few noisy electrodes can be tolerated, but if you want to use average reference in offline processing, decent signal from all electrodes is needed, otherwise your reference will not be accurate.

image 3.27

Testing

Start recording (Recording computer)

After you have checked the signal and everything is looking as good as possible, it is time to start testing. Close the door between the test room and the control room. NOTE: The cabin door in K.02 doesn’t close easily, so you might need to apply a bit more force than usual.

Press ‘Start File’ in the bottom right corner of ActiView (circled in blue in image 3 .25). A dialog pops up in which you have to type the participant number (image 3 .28). Type your participant number in the box under both ‘Enter Local Subject Identification’ and ‘Enter Local Recording Identification’. Press ‘OK’. After this, you will be able to select a folder in which you want to save the recording, type the same participant number again as the file name. Click “OK”.

NOTE: After you’ve pressed OK, you are not recording yet! Click ‘Pause Save’ at the right bottom corner of the screen when you want to start your recording. The buttons should look like the ones in image 3 .29, otherwise you are not recording!

Start experiment (Presentation computer)

Once you’ve made sure that you are actually recording, start your experiment in Presentation on the EEG2/Presentation computer. Click ‘run’, type in the participant number and press ‘enter’. Keep on pressing ‘enter’ until the sound starts playing.

Return to the ActiView screen and check whether the triggers are sent. You can see this in the window circled in red in image 3 .30. A colour or number should appear (depending on your settings) every time a stimulus is played. You can adjust the settings by selecting something else in the window circled in green in image 3 .30. This will not affect your data, but will only change your current view. If you do not see any triggers appear, stop the experiment and call lab support. Without triggers being sent, no offline processing can be performed, so you are recording for nothing!
You can always stop the experiment in Presentation halfway by pressing the Esc button. This will stop the entire block you were running. If a block has ended, press ‘enter’ a few times again to proceed on to the next block. It won’t be possible to pause a block and continue later. However, you can restart a block by starting the experiment again and skip the previous blocks (press ‘enter’ and ‘esc’ right after each other when starting a new block). Keep in mind that the log files from your experiment will overwrite themselves when restarting the experiment. If you want to keep those, make sure to copy them before you restart your experiment.

image 3.30
Once the recording has finished, press “pause save” at the right bottom of the screen, and then press “stop” at the up left of the screen.

Cleaning

After you completed the testing phase, remove the cap from the infant’s head and take off the mastoid electrodes. Guide the parent and child to the washing room and show them the towels and shampoo, or give them a Zwitsal paper cloth if they don’t want to wash the hairs immediately (this is usually enough cleaning for the youngest children without many hairs). Before you say goodbye to the parent and child, take them back to the waiting area and give them a gift (usually a book that’s appropriate for the child’s age) and reimburse travel expenses (if applicable). For more information about this, read the waiting area protocol. Also, give the parent the opportunity to ask any questions they may have.

Afterwards, fill the two plastic washtubs with lukewarm water (image 3 .31). Remove the electrodes from the cap and gently place them into the water. Do not let the electrodes sit under running water! Use the softest toothbrush to gently clean the electrodes. Pat dry them with a piece of paper towel. Hang the bundle back to it’s place and put the end with the electrodes back in the bag to prevent them getting (sun)light; image 3 .32 image 3 .32).
Place the cap in the other washtub. Use the other brush to remove the gel from the holes in the cap. Hang the cap up to dry on one of the hooks above the sink.
Fill the syringe a few times with lukewarm water and press it empty to remove the gel in the tip.

image 3.31 image 3.32

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