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.
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. 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.
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:
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 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 Headset – The International Federation of Societies for Electroencephalography and Clinical Neurophysiology, introduced the conventional electrode placement, also known as the 10-20 system. 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.
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). 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.
 Demos, J.N., 2019. Getting Started with EEG Neurofeedback. WW Norton & Company
 Burger, C., 2014. A novel method of improving EEG signals for BCI classification (Doctoral dissertation, Stellenbosch: Stellenbosch University).