The Brain Orchestra

The Brain Orchestra, designed and implemented by researchers in the Synthetic, Perceptive, Emotive and Cognitive Systems group (SPECS) at the University Pompeu Fabra in Barcelona, takes as its input a series of physiological responses to visual stimuli, including EEG measures, heart rate, and skin conductance. Its output is something that could be described as musical (assuming your definition of "music" is relatively fuzzy).

From BBC News:

The performers launch sounds or affect their frequencies and modulations based on two well-characterised effects seen in EEGs: the steady-state visually evoked potential (SSVEP), and the so-called P300 signal.

Two of the performers were given a task to watch a screen in front of them, with flashing rows and columns of letters, and told to look for a particular letter. When expectation is fulfilled, 300 thousandths of a second later, a signal known as the P300 appears in the EEG.

In the Multimodal Brain Orchestra, the P300 signal is registered - with a dot demarcating it on the EEG trace projected to the audience, so that they can see the effect of the performer's thought - in turn launching a sound or recorded instrument.

Two more performers were given boxes with four lights flashing at different frequencies. The SSVEP is a brain signal that comes about when visual stimulus in the retina at a given frequency causes the brain to synchronize, so that frequency appears in the EEG.

Given a cue from the conductor, the performers switch their attention from one flashing frequency to another. One of them affects the volume of a given sound - known to influence the level of arousal in the circumplex model - and the other affects a certain modulation of that sound, which is known to influence the valence, how positive or negative the arousal is emotionally.