Alain Destexhe from the Unité de Neurosciences, Information & Complexité at the CNRS near Paris produced music from the neuronal activity of a cat. By assigning spikes of activity from certain neurons to specific notes, he was able to sonify the animal's mental activity.
Waking cacophony, sleeping harmony
It is interesting to see how repetition of single notes or short phrases rises more readily during REM sleep than in wakefulness or even shallow sleep. Might it be because during dreams, we are more inclined to go with the flow and accept absurdities as reality, than if we are lucid?
Though not the point of the experiment, the sonification helps to illustrate the difference between mental states. The music itself does not go any further on its own. The research was published in The Journal of Neuroscience in June, 1999.