Calculation of absolute band powers


#1

How is the absolute band power calculated? Can you people also put some light on what absolute power mean here. Are there any smoothing functions used for it.
I know that the raw EEG values are used to calculate the FFT across 129 bins. But it seems that the values are converted or transformed before executing the FFT.
I tried doing a FFT using 256 raw EEG values just before a raw_fft0 value is encountered in the data. But, the FFT values don’t match with raw_fft values! Is there a reason or step that I am missing. Also, I would like to know about PSD, what does it mean, how to interpret it, and what does it tell us about the person’s brain?

Can the absolute band power be plotted against time? Do I need to take an antilog of the values, as the muse OSC descriptions says that it is log of power!

Help is very much appreciated.

Thanks.


#2

Hi chhetri,

The absolute band powers are the sum of the PSD values in a certain range. For delta for example, this would be the sum of the PSD coefficients in the range from 1-4Hz.

The PSD values are calculated using a FFT on a 256 sample window with 90% and a Hamming window on top. The sampling rate is 220Hz.

Power Spectral Density (PSD) is a measure of power. So it basically indicates how much power is contained in certain frequencies. Very broadly speaking, the amount of power in different frequency bands and how it changes over time is a reflection of brain activity changing in some way, since the brain is the source of the signal. For a detailed look into all things EEG and BCI, I suggest you check out our Intro to Brains section on the Muse dev site.

Absolute band power can definitely be plotted over time. You can do it in MuseLab if you want to try it out! Just make a Scrolling Line Graph and assign one of the absolute band power ranges to it. You do not need to take the inverse log of the values, you can just graph them as they are.


#3

So if i have an absolute power of say 800 alpha and my friend has an absolute power of 810 alpha - how can i practically interpret it?