https://papers.nips.cc/paper/2246-expected-and-unexpected-uncertainty-ach-and-ne-in-the-neocortex.pdf (Publisher version)
Yu, A., & Dayan, P. (2003). Expected and Unexpected Uncertainty: ACh and NE in the Neocortex. In S. Becker, S. Thrun, & K. Obermayer (Eds.), Advances in Neural Information Processing Systems 15 (pp. 157-164). Cambridge, MA, USA: MIT Press.