Health and Stroke

How Does EEG Work?

Learn exactly how EEGs work

Jan 30, 2023

Zeit Medical

EEG, or electroencephalography, is a procedure that records the electrical activity of the brain. This is done by placing multiple electrodes on the scalp. The recordings can then be analyzed to see the brain's electrical activity, this can also help in diagnosing certain problems, for example: EEG can show signs of seizures or other brain injuries such as stroke.

In this blog post, we will discuss how  EEG works and what insight it can give related to brain health.

What EEG is Measuring at a Cellular Level

The brain is made up primarily of neurons, which are cells that communicate with each other through electrical impulses. An electrical impulse occurs when there is a change in the voltage across the cell membrane. This change in voltage causes ion channels to open, which allows ions to flow into or out of the neuron.

This flow of ions creates a tiny electrical current, but because there are billions of neurons, the sum of these minute electrical currents can be measured as small changes even on the outside of the head. By placing electrodes directly on the skin of the head these small charges can be converted into data seen on a screen. The strength of the electrical signal and the rate at which it is produced can give insight into the function of the brain.

What Clinicians Are Looking for with EEG

There are two primary measurements used  when interpreting an EEG:

  • Frequency is measured in Hertz (Hz). It is defined as the number of times per second that an event occurs. In the context of EEG, it is the number of times per second that a waveform repeats itself.
  • Amplitude is measured in microvolts (µV) and is defined as the height of the waveform.

Clinicians are looking for three main things when they review an EEG:

  • Type of waveform or rhythm: There are four main rhythms which are commonly associated with a specific frequency ranges: delta (0.5 to 4Hz); theta (4 to 7Hz); alpha (8 to 12Hz); and beta (13 to 30Hz).
  • Location of the waveform (i.e. which electrode exhibits changes vs which electrodes don’t): The location of the waveform can give insight into which part of the brain is being affected.
  • Amplitude of the waveform: The amplitude can give insight into the amount of neural activity that is taking place. The more neural activity the higher is the amplitude of the electrical signal we can pick up outside the brain.

Brain Waves Measured by EEG

Four main types of rhythms have been defined for EEG:

  • Delta rhythm (0.5 to 4Hz): These are very-low-frequency waves that are produced when a person is in deep sleep. A stronger rhythm indicates deeper sleep.
  • Theta rhythm (4 to 7Hz): These are low-frequency waves that are produced when a person is drowsy, falling asleep, or in the lighter sleep stages.
  • Alpha rhythm (8 to 12Hz): These are mid-frequency waves that are produced when a person is awake but with closed eyes such as during mental relaxation or meditation. It becomes less prominent with eye opening or mental work.
  • Beta rhythm (13 to 30Hz): These are high-frequency waves that are produced when a person is alert and focused. These waves diminish with sleep medications, mental relaxation, drowsiness, and sleep.

Each brain wave rhythm can be seen in different settings, but often they are all overlapping. The strength or amplitude of each rhythm however can give clues on what is happening in the brain,  and the absence or presence of certain waves can show if there is pathology occuring. For example, a person that is awake but with eyes closed would expect to exhibit alpha waves.

What This Means for Diagnosing Brain Impairments

EEG is useful in monitoring the brain for changes in blood flow, or ischemia, which can impact the brain's ability to produce electrical impulses at neuronal synapses. When someone has a stroke, heart attack, or another condition that affects blood flow, the brain tissues will experience a reduction in blood flow. Detecting ischemic brain tissue quickly to get treatment as soon as possible can improve an individual's outcomes.

Following a major event, some tissues will still have just enough energy to survive for a short while, yet not enough to communicate or fully function (illustrated by a measurable reduction in frequency and amplitude). If blood flow, oxygen, and glucose can be restored efficiently this can prevent unnecessary irreversible damage. If it is not addressed quickly, it can result in cell death.

The Value of EEG

New EEG technology is paving the way to better detection of brain-related changes, whether from stroke, TIA, seizure, and beyond. When brain tissue is at risk, an EEG can provide a quick insight into how focal and global areas of the brain are functioning to help someone take rapid and informed action. EEGs are crucial to doctors in making certain diagnoses in real time.

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