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Anatomical basis of an EEG

Here’s some more interesting information about the origin of the EEG waves. All this time, we’ve been explaining these concepts without direct relation to structures in the brain in our previous 2 articles - Basics of EEG and Basics of an EEG wave.

Let me introduce a few other players in this game- the thalamus and the RAS (reticular activating system). The basic connection is as follows:


The cortex is constantly receiving and sending impulses to the thalamus, which acts as kind of a relaying station. The RAS, if you’ll recall, is responsible for the state of being awake. It complies and conveys information from different places to and from the brain. Here, we’ll look at its primary function - keeping us awake. By now, you should have thought of two specific waves - the alpha and beta waves!

The alpha waves will fire only in the presence of this system. Without the cortico-thalamic system, there will be no alpha waves. Instead, you’ll have delta waves!

Let me explain.

It was seen that resecting the fibres that connect the cortex and the thalamus resulted in a disappearance of the alpha waves- with a predominance of delta waves. What does this mean? Without those fibres, the cortex isn’t being activated by the thalamus. We saw that this connection was required for alpha waves. We cut this tract, and what do we get? Delta. Add up these facts and we can infer that delta waves are due to cortical excitation independent of thalamic stimulation. Pretty cool, right? We’ll probably revisit this concept a little later.

Now what about the beta waves? The concept is similar. We need to revisit the idea of synchronicity first. We know that beta waves are seen when we are awake and are being stimulated. This means there is a lot of information being processed at a time. Imagine you are standing in a busy intersection. This is what your brain feels like all the time when you are busy writing an exam or doing anything of significant mental activity. There are cars going right, left, towards you, away from you, there are people walking, there are buses, and there are motorcycles, etc. Let’s look at it like this: the bigger the vehicle, the higher the voltage. But the problem is, some vehicles are going one way and others are going the exact opposite way. What does this mean? They are asynchronous! If you were to add up all the electrical activity represented by vehicles and people at the intersection, the voltage wouldn’t be that much because there will be so much cancelling out. However- the frequency will be increased because there is just that much activity at the intersection. That’s why beta waves are really small but have a very high frequency. At this point you can actually make a generalisation-  asynchronicity leads to smaller voltages.

 Beta wave form

Beta wave form


So to give a short summary of where each wave is found, here is a small list:

  • Delta waves:
  1. Stupor
  2. General anaesthesia
  3. Deep sleep
  • Theta waves:
  1. Stress, frustration
  2. Infants
  • Alpha waves:           
  1. Relaxed states
  • Beta waves:
  1. Increased mental activity

Now, I’m sure you’ve learned a lot about waves of the EEG and how to interpret a reading. There is one small drawback, though. It was found through multiple studies that EEGs are not always perfect- you won’t always find a perfect set of alpha waves or a perfect set of beta waves. Because the electrical activity being picked up is by millions of neurons firing simultaneously, it is difficult to always say for sure what wave is what. But experienced neurologists, and maybe us someday, can be able to make out patterns in EEG that can help up diagnose various neurological disorders.

Author: Shruthi Sivakumar

Sources and citations

Hall, John E. “States of Brain Activity - Sleep, Brain Waves, Epilepsy, Psychoses.” Guyton and Hall Textbook of Medical Physiology, 12th ed., Elsevier Saunders, 2011, pp. 723–726.