Electroencephalography (EEG) is an electrophysiological monitoring method to record electrical activity of the brain. It is typically noninvasive, with the electrodes placed along the scalp, although invasive electrodes are sometimes used in specific applications. EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain
One of the big challenges in psychology is understanding what is happening at a neural circuit level in humans during cognitive processes. EEG, by quantifying activity in the brain, gives us an understanding of the regions of the brain involved in various cognitive processes, what sorts of processes might be active, and also provides different endpoints for evaluating the effectiveness of cognitive modulating compounds.
The most commonly employed strategies to monitor brain activity in humans are brain imaging by magnetic resonance imaging (MRI) and the measurement of electrical activity by electroencephalography (EEG). EEG is one of the most widely used measures of brain activity. EEG is conducted by attaching electrodes onto the scalp, which can record changes in electrical activity over time.
Figure 1. EEG cartoon and an example of the electrical waves that are read by each electrode
The advantage of EEG is that it is the least invasive measure of brain activity we have available, and provides lots of quantitative information during relevant cognitive processes. For example, some nootropic studies investigating the effects of compounds on attention will use EEG correlates of attention as an end-point, in addition to quantitative testing. EEG endpoints have the benefit of allowing the effects of compounds on brain function to be detected, even if changes in performance-based tests is not significantly improved. Thus, individuals or scientists can change dosages or ratios of nootropics to see if desired behavioral effects can be achieved.1In the medical realm, EEG has been used to identify early signs of Alzheimer's disease, and in fact, one 2015 study found that EEG waves associated with attention were more closely correlated with age-related cognitive decline, than EEG measures of discrimination, working memory processes, and perception.2
Recently, many start-up ventures marketing modular EEG headsets for consumers have arisen. The goal is to provide people with the tools to quantify their biology, so how do we connect EEG signals to cognitive processes and what are the relevant markers?
Each electrode on an EEG device measures electrical activity, that resolve as waves (think sine and cosine waves from trigonometry). Using mathematical transformation (Fourier), these raw signals can be resolved into distinct waves with different frequencies. The four most common waves are Beta, Alpha, Theta, and Delta.3
Figure 2. The four primary EEG waves and their associated frequencies
The brain is a modular organ, meaning particular regions play specific roles in cognitive processes. For that reason, the identification of these waves in distinct regions of the brain can give different information about which cognitive processes may be active at the time.
Alpha waves are the most widely investigated in the psychology literature. These rhythms can primarily be found in the occipital and posterior regions of the brain. Alpha waves can be induced by closing one's eyes and relaxing, and they are removed by any more intense cognitive process (e.g. thinking, mathematical calculations).3Thus alpha waves are often associated with a "relaxed" state, and there have been reports that L-theanine alone, or in combination with caffeine, can increase tonic alpha wave activity. These data indicate that L-theanine may promote a more relaxed state.1
Beta waves are associated with an awake and attentive state. Low-amplitude beta waves are associated with active concentration, or with a busy or anxious state of mind.4Over the motor cortex, beta waves are associated with various motor decisions (suppression of movement and sensory feedback of motion).5
Theta waves are found in many brain structures, with the best studied being the hippocampal theta rhythm. In humans this rhythm is associated with memory formation and navigation.7,8,9However, the measurement of theta waves in the hippocampus from scalp electrodes is technically difficult or impossible, because the hippocampus is a deep brain structure. So even though theta waves can be measured in many brain regions, the functional relevance of superficial theta rhythms (from brain structures closer to the scalp) are lacking.
Event-related potentials (ERP) or evoked potentials are voltage fluctuations that are associated with a distinct stimulus. Rather than a gross measurement of brain voltage over a large swath, ERPs are calculated by using many electrodes in combination to precisely define a particular electrical event in time.
Kelly, S. P., Gomez-Ramirez, M., Montesi, J. L., & Foxe, J. J. (2008). L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance. J Nutr, 138(8), 1572S-1577S.
Deiber, M. P., Meziane, H. B., Hasler, R., Rodriguez, C., Toma, S., Ackermann, M., . . . Giannakopoulos, P. (2015). Attention and Working Memory-Related EEG Markers of Subtle Cognitive Deterioration in Healthy Elderly Individuals. J Alzheimers Dis, 47(2), 335-349. doi:10.3233/jad-150111
Baumeister, J., Barthel, T., Geiss, K. R., & Weiss, M. (2008). Influence of phosphatidylserine on cognitive performance and cortical activity after induced stress. Nutr Neurosci, 11(3), 103-110. doi:10.1179/147683008x301478
Zhang, Y., Chen, Y., Bressler, S. L., & Ding, M. (2008). Response preparation and inhibition: the role of the cortical sensorimotor beta rhythm. Neuroscience, 156(1), 238-246. doi:10.1016/j.neuroscience.2008.06.061
Bland, B. H., & Oddie, S. D. (2001). Theta band oscillation and synchrony in the hippocampal formation and associated structures: the case for its role in sensorimotor integration. Behav Brain Res, 127(1-2), 119-136.
Brankack, J., Stewart, M., & Fox, S. E. (1993). Current source density analysis of the hippocampal theta rhythm: associated sustained potentials and candidate synaptic generators. Brain Res, 615(2), 310-327.
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