Upside down s and algorithms  computational formalisms and theory 
Alan Dix 
This book is now out of print, but you can download the final chapter preprint (PDF, 384K)

The time delay as Internet signals cross the Atlantic is about 70 milliseconds, about the same time it takes for a nerve impulse to run from your finger to your brain. Parallels between computation and cognition run as far back as computers themselves. Although at first it feels as if the cold, abstruse, more formal aspects of computation are divorced from the rich ecology of the humancomputer interface, the two are intimately bound. Mathematics has also been part of this picture. Indeed the theory of computation predates digital computers themselves as mathematicians pondered the limits of human reasoning and computation.
There are a number of aspects of this interplay between computation, mathematics and the humancomputer interface.
First, understanding your raw material is essential in all design. Part of the material of humancomputer interaction is the computer itself. Theoretical and formal aspects of computing can help us understand the practical and theoretical limits of computer systems, and thus design around these limits.
Second, diagrams, drawings and models are an integral part of the design process. Formal notations can help us sketch the details of interaction, not just the surface appearance, of an interactive system and thus analyse and understand its properties before it is built. This is the area that is typically called 'formal methods' within HCI, and we'll look at an example of this in section 2.
Third, various techniques from mathematics, simple counting to sophisticated equations, may be used to reason about specific problems in HCI. In this book we see chapters including Fitts' Law, a logarithmic regression; information foraging theory. which involves differential equations; not to mention the heavy reliance on statistical modelling and analysis of virtually all quantitative empirical work.
Finally, the design artefact of HCI involves people and computers working together within a sociotechnical whole. Amongst the many political, social and emotional aspects of this interaction there is also an overall computational facet. The theory of computation has, from the beginning, spanned more than mere mechanical computation and conversely an understanding of digital computation can help us understand some of the complexity within rich organisational ecologies.
theory in HCI, formal methods, dialogue modelling,