Chapter 6
Representation and Control
Associative Nets and Frame Systems

As we have noted, Artificial Intelligence focuses on representation and control (systematic methods of coding what experts know about domains and stategies)

Issues to deal with:

Models of the world

Representations in general

We have seen some simple representations of knowledge - how can we represent the knowledge so that we can use desired operations?

Knowledge Representation and Expert Systems

What can expert systems bring to bear on these exponentially hard problems? Knowledge

Problem: this knowledge is hard to formalize - more "experience" than theory

How to represent knowledge so it can be accessed and used...

"The design of the inference engine (control) will usually determine both when representations of knowledge are applied and how their application is controlled."

Knowledge structures

knowledge: data about a particular domain

structures: organization

(e.g., schema ... frames, scripts )

Barlett (1932) Minsky(1975) Schank(1977)

psych AI

Chapter 6 : Associative Nets and Frame Systems

Graphs, Trees and Networks

Graphs: Figure 6.1

Networks: (labels on links)

Causal Network: Figure 6.2

Trees: Figure 6.3

Semantic nets and Associative nets: Figure 6.4

A representation is a set of syntactic and semantic conventions that make it possible to describe things.

- syntax specifies the symbols that may be used and the ways the symbols may be arranged

- semantics specifies how meaning is embodied in the symbols and the arrangements allowed

Semantics distinguishes semantic nets from ordinary nets

Semantic Nets: first structured representations

(Quillian, 1968)

Information is represented as a set of nodes connected by a set of label arcs which represent relationships

E.g.,  			LISP implementation				Graphically
ATOM    		propertylist
Marcus    		(instance Man)
chair			(ISA furniture)
my-chair		(instance chair)
each property is a one-way link

Semantic Nets and problem solving

Spreading Activation - "intersection search"

find relationships among objects by 'spreading activation' out from each node and see where activation meets

(connectionism and neural nets)

Also used for representing sentences

(e.g. ATN's Augmented Transition Networks)

Restrictions on way info is represented?

- in principle, none ... but

e.g. Mychair can be wet but concept chair cannot

e.g. Size of American population differs from size of a man 5'11"

e.g. What is best organization & what links in net? (Object-oriented design)

(question of what hierarchical representation emphasises important relationships)

inheritance, demons (a.v.), defaults - led to frames

Frames - Minsky, 1975

share the notion that complex entities can be described as a collection of attributes and associated values

Associative memory

OBJECT ATTRIBUTE VALUE

common approach: associate with object with 'property list'

(list of (attribute value) pairs)

Expert System 'shells' and recent AI languages (KRL, FRL) provide 'slot-and-filler' structures as part of the language.

The programmer need not be concerned with implementation

Structures developed to represent knowledge so can be used to solve problems... thus they are associated with particular inference mechanisms .

frame - a collection of semantic net nodes and slots that together describe a stereotyped object, act, or event

psychological insight -

*intelligent agents have available in memory a large collection of structures representing previous experiences with objects, locations, situations, and people

*analysis of new experiences draw on past experiences

*concept/implementation: a frame describes a class of objects using a collection of slots that describe aspects of the objects

slots can have

  • frames contain information about 'important' aspects of the object or situations they describe (expectations)

  • frames contain attributes that must be true of objects - used to build a description

  • frames describe typical instances

    Use: make it easy to infer as yet unobserved facts about a new situation

    Instantiation: how match "best"

    keywords (how many times)

    events

    Uses: frames in natural language processing

    Schank & Abelson - scripts

    Frames very easily led to Object-oriented programming (simula 1967)