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.