In 1985 the composer György Ligeti published some wonderful Etudes for piano. I regularly listen to them ever since. They were also the material (or at least a few bars of “Cordes Vides”) to test an idea for an expression editor: a system that was aimed to facilitate editing operations that are musically meaningful while at the same time amounting to perceptually sound results. The editor, named Expresso, was conceived as a calculus of expression. Quite an ambitious project that aimed to formally describe how different types of expression are linked to different types of musical structure.
The research was inspired by several researchers in the field of “musical expression”: Henry Shaffer, Eric F. Clarke, John Sloboda,Christhoper Longuet-Higgins, Johan Sundberg, Alf Gabrielsson (just to name a few European researchers), all of them contributed, along with quite a few fellow researchers all over the world, to an understanding on how "expressive nuances" in music performance make the difference between one and another performance. A recent paper by Luke Windsor (University of Leeds) summarizes and elaborates on this research. You might like it.
Windsor, W.L. et al. (2006). A structurally guided method for the decomposition of expression in music performance. The Journal of the Acoustical Society of America, 119(2), 1182. DOI: 10.1121/1.2146091