So in a previous post I discussed what EEG or brain waves are, now I will discuss how we interpret them. When you record EEG or brain waves it looks something like this:
So, that is what brain waves look like - but how do neuroscientists interpret the squiggly lines? To understand that you have to think in terms of frequency. When we say frequency - we essentially mean how many times per second a waveform or signal oscillates. If you think of televisions - most people know now that a 120 Hz television is better for watching sports than a 60 Hz television. Why is that? A 60 Hz television draws the image on the screen 60 times a second which seems like a lot. However, is something is moving very fast it actually might be moving at a rate such that you see blur or gaps in the signal because you need to draw is faster to see the complete motion. Thus, a 120 Hz television draws an image 120 times a second which removes the blur. Here is a video that explains the concept.
But what about brain waves? If you look at the EEG signal above you will see that the signal oscillates, or goes up and down, and thus it has a frequency. This stems from the fact that the firing of neurons is not constant but differs in intensity and amplitude over time - think of your heart rate speeding up and what that would look like on a ECG. We are starting to understand that different frequencies of neural activity reflect different neural processes. For example, when you are concentrating on solving a complex problem we see an increase in brain activity in the 4 to 8 Hz range over the front of the scalp, the prefrontal cortex. Note, most human brain activity is between 1 and 40 Hz. So, when we record frequency we use a mathematical technique called the fourier theorem to find out how much brain activity there is at each frequency (e.g., at 1 Hz, 2 Hz, 3 Hz - read more HERE).
So, the FFT (or Fast Fourier Transform) tells us how much activity there is at each frequency interval. If you take a snap shot of EEG, say over 1 minute, and run a FFT on the EEG data you get a "brain state" - the frequency spectra of the EEG signal. We, as neuroscientists, are starting to understand that different brain states reflect different things. For example, monks during meditation typically have a brain state in which we see more gamma (32+ Hz) activity (more on monks and EEG HERE). Typically, as opposed to talking about a specific frequency neuroscientists talk about frequency ranges: delta (1 to 4 Hz), theta (4 to 8 Hz), alpha (8 to 15Hz) beta (8 to 32 Hz), and gamma (32+ Hz). Wikipedia covers the topic reasonably well HERE.