Introduction

What is EEG? What is EEG? what do you know?
"Newcomer Technician's Electroencephalogram Measurement Notes", where newcomer engineers who started in such a situation post what they felt when they actually touched the EEG.

I'm going to tell you a raw voice that challenges without prior knowledge.

Eye movement 1: Blink

For electroencephalography (EEG), which captures brain activity as electrical signals, it is necessary to distinguish whether the observed signals are brain activity or external signals.
For an electroencephalograph, external signals come from the human body (signals from sensory organs, muscle movements, etc.) and from external sources (power supply noise, wireless signals, etc.).
In this chapter, I would like to actually observe and consider how eyeball and eyelid movements affect the output of an electroencephalograph (EEG).

Since there are various movements of eyeballs and eyelids, I will explain "blinking" this time.

Blinking is the movement of the eyelids to open and close, and one blink is said to take an average of 100-150msec.
Since it is difficult to conveniently observe voluntary blinking (blinking that we usually do unconsciously), this time we performed 4 sets of voluntary blinking (blinking that simulates the opening and closing of the eyelids) twice. I observed* how the value of the electroencephalograph changed during the period.
Spontaneous blinking and voluntary blinking observe similar signals, but please understand in advance that in spontaneous blinking, there is a slight time difference between the appearance times of positive and negative signals, so it is not a perfect reproduction.

Now let's look at the experimental results.

* Here, the position of each sensor is described using the electrode name of the 10-20 method. For details, please refer to the electrode placement method.

The first set of eyelid opening and closing movement is quick without leaving a gap, and the second and subsequent sets are deliberately opened and closed more slowly than the first set.

As you can see from graphs 1 to 6, in the forehead (F3/F4), which is closest to the eyes, signals much larger (100 times or more) than EEG signals are output in conjunction with eyelid opening and closing movements. I understand this.
In addition, the central (C3/C4) and parietal (P3/P4) areas were also affected, though not as much as the frontal area (F3/F4), and the greater the distance, the smaller the impact. I understand.
Why are such signals observed?
It comes from the structure of the eye.

1. Structure of the eyeball
A structure in which the retina has a negative charge due to its resting potential, and the cornea consequently has a positive charge

2. Eye movement during blinking
The structure of the eyeball that faces upward when the eye is closed and faces forward when the eye is opened

Due to the above structural characteristics, when you blink, electricity flows through your eyelids and propagates through your skin to your head.
Originally, an electroencephalograph that is intended to observe only brain waves observes electrical signals, so artifacts due to blinking, which is the same electrical signal, are observed.
The effect is large, and the voltage is more than 100 times that of the electroencephalogram signal.

Graph 7 is the superimposed EEG outputs of F3 and F4.
In general, when blinking, both eyes are opened and closed at the same time, so signals are generated at the same timing on the left and right sides.
In addition, as mentioned above, the signal due to blinking tends to decrease as the distance increases, and it is possible to distinguish whether this signal is due to blinking by looking at all the signals.

Countermeasures/Requirement of countermeasures/Difficulty of countermeasures

【measures】
Since the magnitude and frequency of the signal change from time to time, try to remove it with a general-purpose filter, etc. If it is still difficult to remove it, the data for that period is discarded.
As a preliminary experiment, we asked the subjects to intentionally blink, and used it for the above measures.

[Necessity of measures]
Required

[Countermeasure difficulty]
During ~

Related information

New Technician's Electroencephalogram Measurement Record ~Artifact 2 Eye movement 2 Eyes closed and opened one by one~

How to choose an electroencephalograph (EEG)

New Technician's Electroencephalogram Measurement Notes Extra Edition (Supplement)

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