Knowledge Representation

John R. Anderson
Christian Lebiere


The function of this chapter is to specify how knowledge is represented in ACT-R. However, to do this we also need to specify something about how ACT-R processes this knowledge. The process assumptions in ACT-R can be organized according into the 2x2x2 system illustrated in the following table. There are assumptions about how the system performs and how the system learns. Within each topic one can distinguish between assumptions that are relevant to declarative memory and assumptions that are relevant to procedural memory. One can further divide the assumptions according to whether they apply to ACT-R at the symbolic level or the subsymbolic level. This and the next two chapters will discuss all aspects of the theory (i.e., all 8 cells in the table). The table indicates where each aspect of the theory is discussed. The table also indicates the units in the ACT-R Web tutorial that correspond to these topics. In our ACT-R classes we typically devote 1 week to each tutorial unit. There are 9 tutorial units. Students appear to require all 9 weeks (at 5-10 hours a week) to come to mastery of the material. So a word to the wise: the material in these chapters is not to be taken lightly.

Assumption of the ACT-R Theory

Declarative Procedural
Symbol Chapter 2 Chapter 2
Tutorial Units 1 & 2 Tutorial Units 1 & 2
Subsymbolic Chapter 3 Chapter 3
Tutorial Units 4 & 5 Tutorial Unit 3

Declarative Procedural
Symbol Chapter 4 & 5 Chapter 4
Tutorial Unit 6 & 9 Tutorial Unit 8
Subsymbolic Chapter 4 Chapter 4
Tutorial Unit 6 Tutorial Unit 7

As can be seen from the table, this chapter on knowledge representation will also discuss the performance assumptions of ACT-R at the symbolic level. This is because assumptions about how knowledge is represented are not meaningful until they are tied to how the knowledge will be used. The symbolic performance assumptions in this chapter are not the only consequences of the ACT-R representational assumptions. They are just the most immediate consequences. These representational assumptions will also serve as the framework for the other ACT-R process assumptions.

This chapter will contain sections to discuss the procedural-declarative distinction which is the most fundamental assumption in the ACT-R theory, the representation of declarative knowledge as chunks, the representation of procedural knowledge as productions, and the critical role of goal structures in organizing behavior. At all points in the development of the ACT-R theory we like to maintain an intimate relationship with data. Therefore, the chapter will end with a discussion of the Tower of Hanoi task and ACT-R model for it. This is a task that depends on performance assumptions at the symbolic level.


Addition Model

Tower of Hanoi

Model for Ruiz (1987)

Model for Anderson, Kushmerick, & Lebiere (1993)