Did you know the neurons in your brain generate roughly 25 Watts of electricity, which is enough to power a light bulb? The brain is not only a fascinating physiological phenomena, but it can also be thought of as an amazing piece of electrical machinery. Although individual neurons only generate limited amounts of electricity, altogether your brain’s neurons have amazing electrical capabilities and can produce some fascinating brain waves. While we have been talking about electroencephalography (EEG) and other neuroimaging techniques at meetings, what types of brain waves might we measure for in an EEG? What is the difference between these brain waves, and when are they generated? How does pathology affect these brain waves in different people? Let's dive right into it.*
*First, for those of you who want more background information, I will be doing a short recap on the mechanisms of action potentials. Otherwise, feel free to skip ahead to the actual content!
Recap: How does a neuron generate electricity?
Neurons generate electricity via an impulse called an action potential. When a neuron is not receiving or sending signals, it is in its resting state, meaning it has a negative charge within the neuron versus the positive charge in its environment. This state is maintained by sodium-potassium pumps, or transport proteins, along the axon that push many sodium ions outside the cell and move some potassium ions inside the cell. However, an action potential is generated when an incoming signal causes a sodium pump to open up and allow an incoming rush of sodium ions inside the cell. This flips the intracellular and extracellular charges so that the outside environment is now negatively charged and the intracellular environment is now positively charged. This causes a potassium pump a little farther down the axon to open and push out potassium ions in attempt to repolarize the charge distribution and return it to normal. This alteration of the charge distribution occurs all the way down the axon until it reaches the end and initiates the release of neurotransmitters across the synaptic gap to stimulate the next neuron and thus propagate neuronal communication.
What is an EEG?
Electroencephalography (EEG) uses non-invasive electrodes placed on the scalp to measure the electrical activity of the brain. Specifically, the electrodes measure fluctuations in the voltage produced by neurons in the occipital cortex (visual processing), parietal cortex (motor functioning), temporal cortex (speech production and language processing), and frontal cortex (higher thinking). EEG has an extremely high temporal resolution, measuring electrical signals on a millisecond scale. However, it does not have a very high spatial resolution due to its nature as a non-invasive measurement technique and thus the impedance of electrical signals due to the thick skull. There are also many artifacts, or unintentional errors that can mess up data, that may affect EEG results, such as patient movement and sweating. The electroencephalogram, or resultant display of measured EEG data, amplifies the small electrical charges measured from your neurons and produces a readable graph. Because it displays the basic waveform of your brain activity and can detect sudden bursts in activity or patient response to stimuli, EEG's are often used to diagnose disorders involving abnormal brain activity. For example, epilepsy, or seizure, displays on the electroencephalogram as many rapid and spiking waves. But what other diseases can be recognized via EEG?
*Here is a great module on that gets more technical about EEG acquisition:
Diagnosis via EEG:
EEG can often be used to detect the presence of “abnormal” brain waves, like in diseases such as brain lesions (which result in slower brain waves) or Alzheimer's disease or some psychoses (ie- sleep disorder, narcolepsy). There are several specific examples of EEG being used in research to detect abnormal brain waves that can serve as biomarkers for certain diseases. For example, the Jeste lab has shown that children with autism have been shown to produce lower frequency alpha waves ("Peak alpha frequency is a neural marker of cognitive function across the autism spectrum"). Additionally, gamma waves are diminished in Alzhiemer's patients ("An Observational Trial of Alzheimer's Disease Treatment With Combination of 40Hz Light and Cognitive Therapy"). Furthermore, one study showed using EEG that narcoleptic patients have a disturbance of delta and theta waves in their NREM sleep ("Spectral Analysis of Polysomnography in Narcolepsy"). So what do all these fancy wave names mean, and why are they so important?
What are the different brain waves? When are they generated?
Your brain produces waves of different frequencies, or the "speed" of a wave (the number of cycles/periods per second), depending on the specific cognitive process being performed. Therefore, if we can see what brain waves appear to be abnormal, then we can understand more about the neurobiological mechanisms behind certain cognitive abnormalities and thus use that electrical abnormality as a biomarker for those specific diseases. Let's take a closer look at what these waves are and what cognitive processes they give us insight into via EEG recordings.
*This video is referenced later in this section during the discussion of gamma waves.
A Note From the Blogger (aka - me!):
I hope you all enjoyed this article and learned something about EEG acquisition and its uses as a diagnostic tool based off of abnormal brain activity. I know this article was a bit more neuroscience heavy than neurotech focused, but I think it is critical to understand neurological processes before we can understand and appreciate the many uses and powers of neurotechnology. I hope you all have an amazing week 6, and keep producing those gamma waves as you conquer your midterms!