Comparing similarity of two simultaneously recorded EEGs in freq domain
You can express correlation in the frequency domain but the closest thing to what you are describing would be Coherence.
It returns a number between 0 and 1 per each frequency bin of the spectrum. The closest this number is to 1 the more &quot;similar&quot; the signals would be and you can average this number over the whole spectrum or over a frequency range.
Hope this helps.