What is EEG (Electroencephalography) and How Does it Work?
Your brain runs the show. Think of the last time you tried to solve a crossword puzzle or started to learn a new language. Recall the last time you woke up in the middle of a weird dream or needed to find your way in a city you have never been before.
As you think, dream, see, and sense, your brain is constantly active, absorbing all information, compacting and re-connecting existing data, and integrating everything into a consistent experience. For you, that experience constitutes your reality.
Your brain is alive. Your brain shapes how you see your environment, filters or highlights objects and information most relevant to you. It creates its own stories based on your thoughts, emotions, desires and experiences, ultimately driving your behavior.
Electrical activity of the brain
The brain consists of billions of cells, half of which are neurons, half of which help and facilitate the activity of neurons. These neurons are densely interconnected via synapses, which act as gateways of inhibitory or excitatory activity.
Any synaptic activity generates a subtle electrical impulse referred to as a postsynaptic potential. Of course, the burst of a single neuron is difficult to reliably detect without direct contact with it. However, whenever thousands of neurons fire in sync, they generate an electrical field which is strong enough to spread through tissue, bone, and skull. Eventually, it can be measured on the head surface.
Think of this as a constant rumble of subtle earthquakes. Taken by itself, each burst might be too small to notice, but if several of them occur at the same time, in the same location, and in the same rhythm, they all add up to a mega-quake that will be noticeable even hundreds of miles away.
What is EEG and how does it work?
Electroencephalography , or EEG, is the physiological method of choice to record the electrical activity generated by the brain via electrodes placed on the scalp surface. For faster application, electrodes are mounted in elastic caps similar to bathing caps, ensuring that the data can be collected from identical scalp positions across all respondents.
Despite its somewhat daunting name (and pronunciation), grasping the essentials of electroencephalography is surprisingly simple:
- measures electrical activity generated by the synchronized activity of thousands of neurons (in volts)
- provides excellent time resolution, allowing you to detect activity within cortical areas -even at sub-second timescales
As the voltage fluctuations measured at the electrodes are very small, the recorded data is digitized and sent to an amplifier. The amplified data can then be displayed as a sequence of voltage values.
Price differences in EEG systems are typically due to the number of electrodes, the quality of the digitization, the quality of the amplifier, and the number of snapshots the device can take per second (this is the sampling rate in Hz).
EEG is one of the fastest imaging techniques available as it often has a high sampling rate. One hundred years ago the time course of an EEG was plotted on paper. Current systems digitally display the data as a continuous flow of voltages on a screen.
How can EEG data be interpreted?
As EEG monitors the time course of electrical activity generated by the brain, you can interpret which areas of the cortex are responsible for processing information at a given time:
- Occipital cortex
This part of the brain is primarily responsible for processing visual information. EEG experiments with visual stimuli (videos, images) often focus on effects in occipital regions.
- Parietal cortex
Parietal cortex is primarily responsible for motor functions and is active during self-referential tasks – when we are encountering objects or information that is important to us, for example.
- Temporal cortex
Temporal cortex has lateral aspects which are responsible for language processing and speech production. Medial (inner) regions are more active during spatial navigation.
- Frontal cortex
The frontal part of the human brain is enlarged compared to most other mammals. Basically, the frontal cortex is all about executive function: it helps us maintain control, plan for the future, and monitor our behavior. Apart from the regional characteristics of where certain electrical activity originates, you can also analyze which frequencies primarily drive the ongoing activity.
Whenever your brain is in a certain state, the frequency patterns change, giving insight into cognitive processes.
- Delta (1 – 4 Hz) – in sleep labs, delta waves are examined to assess the depth of sleep. The stronger the delta rhythm, the deeper the sleep. Increased delta power (an increased quantity of delta wave recordings) has also been found to be associated with increased concentration on internal working memory tasks .
- Theta (4 – 7 Hz) – theta is associated with a wide range of cognitive processing such as memory encoding and retrieval as well as cognitive workload . Whenever we’re confronted with difficult tasks (counting backwards from 100 in steps of 7, or when recalling the way home from work, for example), theta waves become more prominent. Theta is also associated with increased fatigue levels .
- Alpha (7 – 12 Hz) – whenever we close our eyes and bring ourselves into a calm state, alpha waves take over. Alpha levels are increased when in a state of relaxed wakefulness. Biofeedback training often uses alpha waves to monitor relaxation. They are also linked to inhibition and attention .
- Beta (12 – 30 Hz)- over motor regions, beta frequencies become stronger as we plan or execute movements of any body part . Interestingly, this increase in beta is also noticeable as we observe bodily movements of other people . Our brain seemingly mimics their limb movements, indicating that there is an intricate “mirror neuron system” in our brain which is potentially coordinated by beta frequencies.
- Gamma (>30 Hz, typically 40 Hz) – Some researchers argue that gamma reflects attentive focusing and serves as carrier frequency to facilitate data exchange between brain regions . Others associate gamma with rapid eye movements, so-called micro-saccades, which are considered integral parts for sensory processing and information uptake .
Analyzing EEG data can get quite challenging. Signal processing, artifact detection and attenuation, feature extraction, and computation of mental metrics such as workload, engagement, drowsiness, or alertness all require a certain level of expertise and experience to properly identify and extract valuable information from the collected data.
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