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Module 2: Intro to Waveforms (ICETAP)

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on 11 January 2015

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Transcript of Module 2: Intro to Waveforms (ICETAP)

EEG Module 2: Intro to Waveforms
Frequency: The number of cycles of a repeating signal per unit time (how closely-spaced the waves are). The frequency of an EEG is measured in Hertz (Hz), which means cycles (waves) per second.

Amplitude: The distance from a wave’s peak to its trough (wave height).

Real Life Example: Sound is a wave form with frequency and amplitude. A higher frequency wave has a higher pitch. A higher amplitude corresponds to a louder volume.
Frequency and Amplitude
How does a single speaker play the low rumbling frequencies from a bass guitar and the clashing high frequency cymbals at the same time?

It’s wave summation: the bass guitar’s signal gets added with the high pitched cymbals so that the speaker only has to play one wave.

The baseline of the green trace oscillates like the low-frequency blue wave, while the high-frequency yellow wave creates the small peaks and valleys.
Wave Addition and Subtraction
The EEG is a summation of several wave frequencies (frequency cutoff can vary slightly):
Beta (12 -– 30+ Hz)
Alpha (8 –- 12 Hz)
Theta (4 –- 8 Hz)
Delta (0 –- 4 Hz)

The pictures on the right show what each of these wave types would look like in its pure form at the 50 mm/sec sweep speed commonly used intraoperatively. However, you will not see a pure form intraoperatively since different waves ride on each other simultaneously.
Waves in the EEG
Sometimes subdivided:
“Fast beta”: 20-35 Hz
“Slow beta”: 12-20 Hz
Typically lower amplitude than the other waves
May be prominent in an alert and wakeful state and during REM sleep
May be predominant rhythm of wakeful relaxation
Prominence increases when eyes are closed
Becomes prominent with drowsiness and light sleep
Theta oscillations may be associated with recalled memory
Often larger amplitude than other waves
Characterizes deep sleep during non-REM sleep
Beta waves: 12 - 30+ Hz
Alpha waves: 8 –- 12 Hz
Theta waves: 4 –- 8 Hz
Delta waves: 0 –- 4 Hz
Gamma waves are higher frequency brain waves which are not shown on an intraoperative EEG
Gamma waves are implicated in determining consciousness but much is to be researched
Sometimes artifacts from electromyographic activity is seen in the gamma wave band
Gamma waves: 30-100 Hz
In an awake person, all four types of waves occur at the same time. How can we tell what component wave frequencies make up a raw EEG?
If we could separate out the beta, alpha, theta and delta waves so they were displayed separately along some arbitrary vertical axis, the EEG might look something like this.
Fourier analysis
The EEG at right is the sum of a large amplitude 2 Hz delta wave, a medium amplitude 8 Hz alpha wave, and a low amplitude 20 Hz beta wave.

Below it is the Fourier transform of the idealized EEG.
The X-axis changes from time to frequency
Spikes (orange arrows) indicate frequencies of the original signal
The size of each spike (power) is relative to the amplitude of that component’s waves
Idealized EEG as an example
Different Wave Frequencies Altogether
Back to the speaker analogy: if a speaker plays only one wave, how do your ears still hear all the separate frequencies?

Your hair cells in cochlea are tuned to different frequencies which deconstructs the sound into different frequency waves

A Fourier transform allows computers to do the same thing – It transforms complex signals into a simple trignometric waves - very useful feature when analyzing EEG waveforms.
Outro.. but it's just a beginning!
Raw EEG separated into different frequencies
Content by: Liz Whitlock and Andrew Park
Prezi by: Andrew Park
Full transcript