The human brain is by far the most complex physical object known to us in the entire cosmos.” -Owen Gingerich
In order to understand what EEG Devices are and how they work, we need to understand the basic anatomy and functional areas of the brain. This article will provide you a basic overview of that.
Our brain controls every task we do- both unconscious or conscious. The building blocks of our brain are nerve cells which are called neurons(shown in the figure below). These nerve cells transmit information throughout the body in the form of electrical impulses.
What are Neurons?
Neurons consist of three major sections: Dendrites, Cell body(Soma) and Axon, as seen in the figure below. Neurons are connected to the other neurons in a complex yet well defined circuitry which still is not completely understood by modern science[1]. The dendrites are connected to other neurons and act as a receiver, that is, they take the information to the cell body. Then the axons carry the electrical current to the terminals, where they transmit the information forward.
This network of neurons acts just like a complex network of computers forming the internet. A simplified example could be, if you wish to watch a YouTube video on your computer, the cable from your internet provider which comes to your house is the dendrite. Your router acts as a cell body which decides on where to route the video and finally the ethernet cable is the axon which carries the video to your computer.
Brain regions and Functions
The outermost layer of the nerve tissue present in a human brain is known as the cerebral cortex, and it is naturally separated in four lobes namely frontal lobe, parietal lobe, temporal lobe, and occipital lobe (see the image below). Each of these lobes has a discrete set of functions as well as connections to the different parts of the brain. Let us focus on each lobe now:
- Frontal lobe: It is responsible for immediate and sustained attention, social skills, emotions, empathy, time management, working memory, and character. The frontal lobe is generally known as an executive planner i.e. it helps in maintaining control, plan for the future, and monitoring the behavior.
- Parietal lobe: This works as a navigation system for our body. It integrates all the information from different senses and generates a coherent representation of the environment. For example, assume you are reading a book and having a cup of tea. Now, you don’t have to look where exactly your cup is every time. While focusing on the book, you can pick up the cup, have your tea and then put it back. This is possible because the parietal lobe stores and retrieves the shape, size and orientation of objects to be grasped in our mind,
- Temporal lobe : This houses the auditory cortex which processes the auditory signals. It also has involvement in the memory-making process, especially verbal memory. The Left temporal lobe is involved with language interpretation written and verbal whereas the right temporal lobe is more involved when listening to music, and understanding social cues.
- Occipital lobe: This is where all the visual processing takes place. Anything in our visual field seen through our eyes is routed to the occipital lobe where it is processed and hence it has strong connection to the entire brain networks be it the frontal lobes for problem definition, parietal lobe in locating objects or memory regions which are present under the cerebral cortex.
EEG Device Basics
As seen above, different regions of the brain are constantly communicating with each other. This communication is accomplished via neurons. When clusters of neurons fire in synchrony using electrical impulses, they produce an electric potential which is captured by the EEG sensors placed on the scalp.
Since these signals originate deep within the brain, voltage captured by the sensors is in μVs(1/1,000,000th of a Volt), usually between 10–50 μVs. This is extremely low and can be easily corrupted very easily with small eye movements, muscle movements, sweat etc. This particularly makes it very difficult processing EEG data and hence the biggest limitation of EEG devices is that the user has to be sitting calm and relaxed to capture any usable information. This limitation is sometimes a huge hindrance.
Capturing EEG
Capturing EEG signals which could be used for analysis later is a bit complex then just placing a sensor over the head. 3 sensors are used to capture 1 channel: two active and one ground. This is done using differential amplifiers. The use of the amplifiers is to return the difference between two active sensor values. Have a look at the image below, the red and the green sensors are the active sensors represented by V1 and V2. The output is the difference of voltages captured at those locations. Yellow sensor is referred as the ground sensor, which is usually placed on the ear lobe or mastoid bones to minimize the activity captured by these sensors.
EEG Sensor Placements in Headset
EEG Headset — The International Federation of Societies for Electroencephalography and Clinical Neurophysiology, introduced the conventional electrode placement, also known as the 10–20 system[2]. Like any other scientific system, it was introduced to maintain a standardized system to ensure that any study can be reproduced and effectively analyzed.
The international 10–20 system contains 19 sensor locations, and each sensor location is designated based on the lobe it is placed on. Odd numbers are on the left, even numbers on the right and Z on the centerline.
- Fp — Frontal poles
- F — Frontal lobe
- P — Parietal lobe
- T — Temporal lobe
- O — Occipital lobe
- C — Center line
- A — Reference electrodes
Brain Wave Patterns
In EEG signals, it is observed that they have certain characteristics. These characteristics change as a person ages as well as the state the person is in (sleeping or awake)[2]. Due to these characteristics, brain waves can be broken up into five categories. These 5 categories are called alpha (α), theta (θ), beta (β), delta (δ), and gamma (γ) and represent a band of frequencies. The image below shows the frequency range of each category as well as some of the mental functions associated with those categories.
The following image is a snapshot from an actual EEG Headset recording, where the brown lines are 1 second marks. If you observe closely, a combination of alpha(red), beta(yellow) and gamma(grey) bands can be observed in the data, which was expected because during the recording the user was sitting with eyes closed but in a wakeful state.
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