Watch the Mo' Modal Rhythm online guitar lesson by Jeff Scheetz from Blues-Rock: Secret Sauce
This rhythm lick is also based on the A Dorian scale. How do you base rhythm on a scale you say? Well, A Dorian is the second mode of G Major, so if we're playing chords that are in the key of G Major, then our "tonal center" is A, so we'll base our solo around A Dorian.
Think of the tonal center as the main chord that you hear, or the underlying note that we could be playing during the whole progression.