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, our brain is constantly active, absorbing all information, compacting and re-connecting existing data, and integrating all 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
Under the hood, the brain consists of hundreds of thousands of cells, so-called neurons, densely interconnected via synapses, which act as gateways of inhibitory or excitatory activity. In other words: Synapses propagate information across neurons (excitatory) or prevent the passage of information from one neuron to the next (inhibitory).
Any synaptic activity generates a subtle electrical impulse referred to as post-synaptic potential (post = behind). Of course, the burst of a single neuron is too tiny to be noticed. However, whenever a smaller group of neurons (about 1000 or more) fires 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 thousands of miles away.
What is EEG and how does it work?
Electroencephalography (encephalon = brain), or EEG, is the physiological method of choice to record all of the electrical activity generated by the brain from 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 biophysics behind electroencephalography is actually surprisingly simple:
- EEG measures electrical activity generated by the synchronized activity of thousands of neurons. The electric activity is measured in voltages.
- EEG provides excellent time resolution, allowing you to analyze which brain areas are active at a certain time – even at sub-second timescales.
Since 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 can take thousands of snapshots per second (256 Hz or higher). 100 years ago the EEG time course was plot on paper. Current systems display the data as 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 brain areas 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) focus on effects in occipital regions.
- Parietal cortex. Parietal cortex is primarily responsible for fusing various bodily reference frames (eye-centered, head-centered, hand-centered, body-centered). Also, parietal cortex 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. You might have heard about hippocampus: This is the brain region in temporal cortex where we form spatial and autobiographical memories from early childhood days, for example.
- Frontal cortex. The frontal part of the human brain is enlarged compared to other mammals. Basically, frontal cortex is all about cognitive control: It keeps us from running after flashing lights, makes us pursue Graduate studies and PhD careers, and binds various memories and experiences into a consistent conglomerate.
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 – your brain kicks into gears.
- 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. Interestingly, delta waves are only present in non-REM phases – when we’re not dreaming, for example.
- Theta (4 – 8 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.
- Alpha (8 – 12 Hz). Whenever we close our eyes and bring ourselves into a relaxed, wakeful state, alpha waves take over. Alpha is reduced with open eyes and drowsiness. Therefore, alpha coordinates multi-sensory processing, attention, and concentration. Biofeedback training often uses alpha waves to monitor relaxation.
- Beta (12 – 25 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 coordinated by beta frequencies.
- Gamma (< 25 Hz, typically 40 Hz). At the moment, gamma frequencies are the black holes of EEG research. 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.
When it comes to analyzing EEG data, it admittedly can get quite challenging. Signal pre-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.
We’re here to help – reach out to our team of experts at iMotions to learn more about EEG data collection and analysis. We’re happy to give you a hand and point you to the right resources.