Calculating alpha power density

Hello all,

I'm running an experiment on frontal brain asymmetry for which I need to measure the alpha power density in the left and right hemispheres (F7 and F8 electrodes). I'm quite a newbie at MATLAB so I'd appreciate any kind of insight into this. I've imported the data from the EEG signals into MATLAB. And I've filtered them using the Butterworth filter as follows:

N=4;
fs = 250;

%F7
%alpha

W3 = 16/fs;
W4 = 22/fs;
Wn_a = [W3 W4];
[e,f] = butter(N,Wn_a);
alphaF7 = filter(e,f,F7);

%F8
%alpha

W3k = 16/fs;
W4k = 22/fs;
Wn_ak = [W3k W4k];
[ek,fk] = butter(N,Wn_ak);
alphaF8 = filter(ek,fk,F8);

Now what function do I use to obtain the alpha power density? I'd appreciate any kind of insight or if someone has written a code for something similar before.