Why couldnt we consider physical gestures and emotional states as musical instruments themselves?  

Physical gestures, emotional states, environmental feedback are a part of the holistic experience of 'live' musical performance. A chief complaint of mainstream audiences is that, as a performance instrument, the computer has a narrow range of expression. (Perhaps they have never seen Eric Lyon in action) If the computer (as performance instrument) were more robust, more responsive, would this engender a more intense experience of 'live' performance'? When a clarinetist's instrument heats up from his breath, the wood becomes warm and more responsive. His instrument atunes to its enviroment. When a trumpeter's lips purse to reach three octaves above middle C, he is functioning at the peak of his physical abilities. As well as complete mental alignment, there is intimate physical connectivity. Can the computer be made more aware of its environment?  

From the original post, beyond cognition and brain wave mapping (David Rosenboom et al) I think the real problem is sonification. I think we would come back to the problem of performer/computer interaction and the lackluster look and feel of said interaction. I can just see the soloist on stage with probes attached to their foreheads 'thinking' emotions. I think that until computers can sucessfully pass the Turing test, we should leave the 'feeling' to the humans.  

There seems to be an assumption here that the emotion the performer is experiencing during performance is the same as the one he or she is trying to convey, and I don't think that to be at all true. If B.B. King plays at Carnegie Hall, he's probably elated and excited to be there. The fact that he can nonetheless sing an achingly sad blues is due to his having experienced that emotion before and spent years conveying it in performance. An excellent performer attains consistency in performance quality by cultivating a degree of independence between the feeling being expressed and that being experienced at the time. If we detect the feelings of the performer at the time, that does not mean that it necessarily represents the feeling of the piece being performed. However, it does open up a new avenue of expression and even improvisation based on self-manipulation of one's emotional state.  

In short, I don't believe in any interest of musicians or the audience in an emotion-aware computer/software, but a lot in emotion-aware performers. In fact there is a lack thereof. For emotion-aware computer music we need to interact by means of musical interfaces. That is not only a matter of mastering technical difficulties, but also a matter of taste. I made good experiences with simple fader boxes as well as interfaces to existing instruments. My main interest at the moment would be to find a sensor for muscle tension, as body tension is one of the strongest though almost invisible emotional expressions...