OK, I think I understand. I answered a similar question in this thread: http://forum.choosemuse.com/forum/developer-forum/3139-the-number-of-fft-data
Quoting from that answer:
"To get the frequency resolution for the bins, you can divide the sampling rate by the FFT length, so in the case of Muse: 220/256 ~ 0.86Hz/bin
So, the zeroth index of the FFT array represents 0Hz, the next index represents 0-0.86Hz, and so on up to 128*0.86 = 110Hz, which is the maximum frequency that our FFT with its 220Hz sampling rate can detect."
Also, like I said, there is no such thing as power in uV. So there isn't really a way to have (frequency, power in uV) value pairs, because the second number doesn't make sense. The /muse/elements/raw_fftX arrays contain the power values for each frequency, but they are in units of 10*log(PSD), not uV. So, you can definitely have (frequency, power) value pairs, it's just that the power values are fundamentally not in uV.
For instance, you could create a set of (frequency, power) value pairs like so: (i*0.86, /muse/elements/raw_fft0[i]) where i is the index of some element in the /muse/elements/raw_fft0 array. That gives you each frequency with its corresponding power.
/muse/elements/low_freqs_absolute - This is the absolute power of frequencies from 1-8Hz. So, we calculate the power for the whole spectrum and take only the sum of the values from 1-8Hz and output them here.
/muse/elements/alpha_absolute - This is the absolute power of the Alpha band, which we define as 9-13Hz. Same idea as the low_freqs_absolute, but for a different band of frequencies.
/muse/elements/alpha_session_score - This is not a measure of power. It is the output of a special scoring function that we describe on the developer website:
"Session scores gives values between 0 and 1 by comparing the user's current band power value to an average of their recent historical values of that same band. For example, if the user's historical alpha power has been between 0.2 and 0.4, and their current alpha power is 0.3, then their session score will be 0.5 since 0.3 is halfway between 0.2 and 0.4."
/muse/elements/alpha_relative - This is the relative power of the Alpha band. So, the absolute power of the alpha band as calculated before, but then divided by the sum of the power in the rest of the spectrum. Basically, what percentage of the signal's power is in the Alpha band, rather than an absolute number.
The developer site documents what these paths contain, please take a look there: https://sites.google.com/a/interaxon.ca/muse-developer-site/museio/osc-paths/osc-paths---v3-6-0