[This text was adapted from here: https://imotions.com/blog/eeg/]
One of the most versatile brain imaging techniques is electroencephalography, called EEG for short. Literally, electro-encephalo-graphy means writing of the electric activity of the brain Why writing? Similar to a seismometer, EEG recordings were initially done on paper.
Electroencephalography records electrical activity and brain waves using electrodes placed on the scalp. Measuring electrical activity from the brain is useful because it reflects the combined activity of many neurons.
There are several reasons why EEG is an exceptional tool for studying the neurocognitive processes underlying human behavior (Cohen, 2011):
Cognitive, perceptual, linguistic, emotional and motor processes are fast. Most cognitive processes occur within tens to hundreds of milliseconds – much faster than the blink of an eye. In addition, the events triggering cognitive processes occur in time sequences that span hundreds of milliseconds to a few seconds. Similar to a high-speed camera, EEG has a high time resolution and can capture the physiological changes underlying the cognitive processes much better than other brain imaging techniques (such as MRI or PET scanners).
Your brain is constantly active, generating electrical activity which of course is very subtle (significantly less than a 9V battery) but detectable with the right device. EEG sensors are able to pick up these tiny signals from the scalp surface. International brain research has been obtaining consistent findings and established well-accepted theories on how the EEG signals relate to cognitive, affective or attentional processing. Techniques like MRI only measure neural activity indirectly and require a much deeper understanding of the relationship between what is measured and how it relates to cognitive processing.
Activity of a single neuron is too small to be detected from outside the skull. However, if postsynaptic potentials occur at the same time and in synchrony for hundreds of thousands of similarly oriented neurons, they sum up and generate an electric field, which is propagated at nearly the speed of light throughout brain tissue and skull. Eventually, it can be measured from the scalp.
Think of this as an audience applauding after an concert. At first everyone claps in their own rhythm, causing white noise without any observable pattern. After a short while, however, the audience gets in sync – all of a sudden everyone is clapping at the same time, in the same rhythm. This synchronized clapping is much louder than the white noise a few minutes ago. At a certain point in time, the synchronization will fade.
Irrespective of whether it’s neural activity, the clapping of a crowd or the rumbling of an earthquake, all of these phenomena occur because of a synchronization of oscillation patterns.
The billions of neurons in the human brain have highly complex firing patterns, mixing in a rather complicated fashion. The neural oscillations that can be measured with EEG are even visible in raw, unfiltered, unprocessed data. However, the signal is always a mixture of several underlying base frequencies, which are considered to reflect certain cognitive, affective or attentional states. Because these frequencies vary slightly dependent on individual factors, stimulus properties and internal states, research classifies these frequencies based on specific frequency ranges, or frequency bands: Delta band (1 – 4 Hz), theta band (4 – 8 Hz), alpha band (8 – 12 Hz), beta band (13 – 25 Hz) and gamma band (> 25 Hz).
Being the slowest and highest amplitude brainwaves, oscillations in the 1 – 4 Hz range are characterized as delta waves (Niedermeyer & da Silva, 2012). Delta waves are only present during deep non-REM sleep (stage 3), also known as slow-wave sleep (SWS). In sleep labs, delta band power is examined to assess the depth of sleep. The stronger the delta rhythm, the deeper the sleep. Delta frequencies are stronger in the right brain hemisphere, and the sources of delta are typically localized in the thalamus. Since sleep is associated with memory consolidation, delta frequencies play a core role in the formation and internal arrangement of biographic memory as well as acquired skills and learned information.
2. Theta band (4 – 8 Hz)
Brain oscillations within the 4 – 8 Hz frequency range are referred to as theta band (Niedermeyer & da Silva, 2012). Studies consistently report frontal theta activity to correlate with the difficulty of mental operations, for example during focused attention and information uptake, processing and learning or during memory recall. Theta frequencies become more prominent with increasing task difficulty. This is why theta is generally associated with brain processes underlying mental workload or working memory (Klimesch, 1996; O‘Keefe & Burgess, 1999; Schack, Klimesch, & Sauseng, 2005). Theta can be recorded from all over cortex, indicating that it is generated by a wide-ranging network involving medial prefrontal areas, central, parietal and medial temporal cortices. Apparently, theta serves as carrier frequency for cognitive processing across brain regions that are further apart (Mizuhara, Wang, Kobayashi, & Yamaguchi, 2004).
3. Alpha band (8 – 12 Hz)
First discovered by Hans Berger in 1929, alpha is defined as rhythmic oscillatory activity within the frequency range of 8 – 12 Hz (Niedermeyer & da Silva, 2012). Alpha is generated in posterior cortical sites, including occipital, parietal and posterior temporal brain regions. Alpha waves have several functional correlates reflecting sensory, motor and memory functions. You can see increased levels of alpha band power during mental and physical relaxation with eyes closed. By contrast, alpha power is reduced, or suppressed, during mental or bodily activity with eyes open. Alpha suppression constitutes a valid signature of states of mental activity and engagement, for example during focused attention towards any type of stimulus (Pfurtscheller & Aranibar, 1977). You could also say that alpha suppression indicates that your brain is gearing up to pick up information from various senses, coordinating attentional resources and focusing on what really matters in that particular moment.
4. Beta band (12- 25 Hz)
Oscillations within the 12 – 25 Hz range are commonly referred to as beta band activity (Niedermeyer & da Silva, 2012). This frequency is generated both in posterior and frontal regions. Active, busy or anxious thinking and active concentration are generally known to correlate with higher beta power. Over central cortex (along the motor strip), beta power becomes stronger as we plan or execute movements, particularly when reaching or grasping requires fine finger movements and focused attention. Interestingly, this increase in beta power is also noticeable as we observe others’ bodily movements. Our brain seemingly mimics the limb movements of others, indicating that there is an intricate “mirror neuron system” in our brain which is coordinated by beta frequencies (Zhang et al., 2008).
5. Gamma band (above 25 Hz)
At the moment, gamma frequencies are the black holes of EEG research as it is still unclear where exactly in the brain gamma frequencies are generated and what these oscillations reflect. Some researchers argue that gamma, similar to theta, serves as a carrier frequency for binding various sensory impressions of an object together to a coherent form, therefore reflecting an attentional process. Others argue that gamma frequency is a by-product of other neural processes such as eye-movements and micro-saccades, and therefore do not reflect cognitive processing at all. Future research will have to address the role of gamma in more detail.