Keywords: Layered Protocols, Interface Design, Interaction Model

Perceptual Control Theory (PCT) is a general psychological theory based on the tenet, ?All behaviour is the control of perception?. Layered Protocol Theory (LPT) can be seen as PCT applied to the special case of communication between cooperating partners, both controlling their own perceptions. PCT and LPT can be applied to the design and analysis of human?machine interfaces, although LPT may be more tractable in many cases. LPT is discussed in the context of the analysis and redesign of the interaction between a pilot and a Control Display Unit (CDU) in an operational helicopter.