Interactivity Studies means to me research that is focused on the aspect of our field that no other field or discipline focus on, that is, the actual interaction between humans and some form of computational artifacts. I am sure that many will argue against this statement and I am open to reconsider it when I am shown good examples of interactivity studies in the way I think about it.
So, here are some publications that I have been involved with over the years that are clearly examples of Interactivity Studies.
Lars-Erik Janlert and Erik Stolterman. 2010. Complex interaction. ACM Trans. Comput.-Hum. Interact. 17, 2, Article 8 (May 2010), 32 pages. DOI=10.1145/1746259.1746262 http://doi.acm.org/10.1145/1746259.1746262
Janlert, L-E. & Stolterman, E. (1997). The character of things. Design Studies Vol 18, No 3, July (1997), 297-314.
Youn-kyung Lim, Erik Stolterman, Heekyoung Jung, and Justin Donaldson. 2007. Interaction gestalt and the design of aesthetic interactions. In Proceedings of the 2007 conference on Designing pleasurable products and interfaces (DPPI '07). ACM, New York, NY, USA, 239-254.
Well, I thought I would have more examples, but maybe I was wrong. I have two more articles on the way with Lars/Erik Janlert that clearly would end up on this list but they are in progress still.