EEG Experiment
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Today, I would like to show you the procedure of EEG experiment. I will mostly focus on the EEG process such as how to appropriately place EEG electrodes to the head of the subjects, what needs to be notified to the subjects before implementing an experiment. (I will explain my work later.)
Check and prepare requirements
- Connect all the cables to proper connectors and make sure that an amplifier is turned on. If connectors are loosely connected or linked to a different channel, it cannot receive the brain signals properly and you might guess the machine has been broken or the subject’s brain is not working. As you can see in Figure 1, there are a number of channels and buttons that you have to care about so just double check that cables are properly linked together and buttons are pressed correctly before you go to the next step.
Figure 1. Each socket requires specific number of electrodes.
- Wash and dry electrodes and EEG cap. It is very important to remove all residuals such as hair and gel before reuse tools. If gel or water still remained in EEG cap this might cause the brain signals to be captured from one electrode and its adjacent electrodes at the same time. Each electrode should record the brain signals from one region but the neighboring brain signals should not be recorded. Therefore, EEG cap or electrodes should be clean and dry in order not to mix up the brain signals from multiple regions.
Figure 2. My team member Jaekoo kang participated in my experiment. A cap was clean and dry for successful experiment.
- Fill the conductive gel to a syringe and put toilet paper and antiseptic wipes near you. When electrodes are placed on the scalp, the conductive gel (which is very expensive) filled the gap between the scalp and the electrodes, reduces skin impedance in order to record the brain signals. Thus, we need to prepare a couple of syringes filled with the conductive gel. Also we need antiseptic wipes in case to clean up the skin for decreasing the skin impedance. In figure 3, an electrode is placed on the nose and this electrode has another purpose instead of recording the brain signal. Anyway, we will use antiseptic wipes in that case.
Figure 3. Inject the conductive gel to reduce the skin impedance.
EEG Setting Up
Now, we are ready to put an EEG cap to the subject. However, placing electrodes on the scalp and finding a spot where the brain signals are picked up is very time-consuming. This laborious procedure would take almost 40 minutes to 2 hours depending on the condition of the subject’s scalp. If your subject has a lot of dandruff or he or her sebum secretion is excessive then it will prevent the electrodes from capturing the brain signals adequately. (the funny thing is that we can predict the subject’s skin condition by the amount of time it took for setting up)
- Put a cap to the subject and adjust it until the holes are placed on the right place. There are many holes in a cap and we have to place those holes (by stretching and moving a cap) to the right brain regions. (See many holes in a cap shown in figure 4 and 5. Each electrode will be attached to their designated brain regions)
Figure 4 and 5. The left shows the connected electrodes on the subject head, and the right indicates the spots of the brain regions.
- Plug in electrodes to their designated brain regions. This will take a lot of time so the researchers need to prepare for an agony. Find an electrode that matches with the brain region in the cap, and put a bit of gel on the tip of the electrode and plug it in to the hole. By tweaking the electrode, we need to make the electrode to receive the brain signals. On the screen, you can check how well the electrodes are receiving the brain signals by color. (Figure 6 shows the color in each brain regions) Until all the regions are colored green (green means all electrodes are recording the brain signals stably.) tweaking and re-injecting the gel should be done repeatedly. As time goes by, all the electrodes would be stablized and then the experiment is good to go.
Figure 6. You will see such a figure on the screen. This head picture simplified the number of electrodes to 5. Real application will have more electrodes such as 32. Left shows no electrodes are receiving signals. Probably the electrodes are not yet plugged in or they are not receiving the brain signals because of high skin impedance or lack of the conductive gel. Middel picture tells that only one electrode in the right is able to capture the signals while others are still unstable. Yellow circle electrodes are detecting the signals but faintly. Right picture shows the stablized electrodes. In this stage, every electrodes are able to receive the brain signals properly so the researchers can implement an experiment.
Instruction to the subjects
Giving a proper instruction before experiment to the subjects is crucial. Therefore, the researchers need to elaborate a few instructions to their subjects.
- Make sure them not to shake their head during experiment. Your subject may not be familiar with the machine so he or she might feel uncomfortable with the EEG cap. I saw a few subjects move their head right and left during experiment and it not only added up a great noise to our data but also loosened the electrodes attached to the cap. Tell them they are only allowed to stretch their head or body during a break time but they should restrict their head movement when the experiment is in progress. Plus, ask them to raise their hands during experiment if they feel uncomfortable. In this case, the researchers should give a plenty of time for them to be relaxed.
- Elaborate the task as clear as possible. The researchers have to ask and check whether the subject understand the task procedure. Even though the clear instruction was delivered to the subjects, they don’t know whether they performed the task correctly or not. If the researchers have a hunch that the subject was not performed correctly, the researchers can demonstrate the task procedure and make sure them grasp it. This is very important because the subject who does not understand what he or she has to do in the experiment cannot give a best performance on the task and therefore, the researchers cannot expect a credible result from that subject.
Figure 7. Giving an instruction to the subject. To make an experiment successful, communication between the subject and the researcher is very important.
During Experiments
You might want to give a break for a moment after the experiment is just begun. However, you have to keep checking numerous things such as whether the subject is raising his or her hands for any reason, whether the brain signals are recorded properly (you can check if the subject is drowsy and moving their body through the signals), and checking the condition of the subject and the electrodes during a break time. Sounds boring? It might be but looking at the brain signals at each task event and the signals when the subject is sleeping is quite interesting.
Figure 8. The brain signal examination. During the experiment, the researchers keep checking the tendency of the signals and find any abnormal signals to correct the given problems.
After Experiments
If the experiment is finished, you have to take off the cap and the electrodes from the subject and wash them before the conductive gel becomes solid. The tip of the electrodes are especially fragile so the researchers should be careful when they handle the electrodes. (this one is also expensive!) Disconnect the cables from the amplifier and the control box and turn off the power. Finally, clean up around a laboratory, then the experiment is done.
Let me show you the EEG experiment that I participated in.
How was the story? I did my best not to use many jargons as possible and deliberate the EEG procedure with many pictures and personal experiences. Honestly, I skipped many details because it might prolongate the story. Furthermore, I cut the information that needed background knowledge. Anyway I want you to enjoy this story and please feel free to ask me if you have any questions.
Hyungwon Yang
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