Frames and scripts, as knowledge representation schemes take into account context and relationships. These techniques also provide a useful formulation for representing more complex structures such as objects, scenes and multiple-sentence stories. One of the key ideas of the script approach is to reduce a sentence or story to a set of semantic primitives using a formalism called conceptual dependency (CD).
Conceptual dependency (CD) is a theory of natural language processing which mainly deals with representation of semantics of a language.
The main motivation for the development of CD as a knowledge representation techniques are given below:
1. To construct computer programs which can understand natural language,
2. To make inferences from the statements and also to identify conditions in which two sentences can have similar meaning,
3. To provide facilities for the system to take part in dialogues and answer questions,
4. To provide a necessary plank that sentences in one language can be easily translated into other languages, and
5. To provide a means of representation which are language independent.
Knowledge is represented in CD by elements what are called as conceptual structures. What forms the basis of CD representation is that for two sentences which have identical meaning there must be only one representation and implicitly packed information must be explicitly stated. In order that knowledge is represented in CD form, certain primitive actions have been developed.
Primitive CD Actions:
1. ATRANS – Transfer of abstract relationship (e.g., give)
2. PTRANS – Transfer of physical location of an object (e.g., go)
3.. PROPEL – Application of physical force of an object (e.g., throw)
4. MOVE – Movement of body part of an animal by the animal (e.g., kick)
5. GRASP – Grasping of an object by an actor (e.g., hold)
6. INGEST – Taking of an object by an animal to the inside of that animal (e.g., drink, eat)
7. EXPEL – Expulsion of an object from inside the body by an animal to the world (e.g., cry)
8. MTRANS – Transfer of mental information between animals or within an animal (e.g., tell)
9. MBUILD – Construction of a new information from an old information (e.g., decide)
10. SPEAK – Action of producing sound (e.g., say)
11. ATTEND – Focusing a sense organ towards a stimulus (e.g., listen)
Apart from the primitive CD actions we may use the following six categories of objects:
Only physical objects are physical producers.
Actions are done by an actor to an object (a person who performs some action is known as actor).
Every action takes place at some locations and serves as source and destination.
An action can take place at a particular location at a given specified time. The time can be represented on an absolute scale or relative scale.
These serve as modifiers of actions, the actor PROPEL has a speed factor associated with it which is an action aider.
Serve as aides of picture producers. Every object which serves as a PP, needs certain characteristics by which they are defined. PA’s practically serve PP’s by defining the characteristics.
There are certain rules which the conceptual categories of types of objects discussed can be combined.
CD models provide the following advantages for representing knowledge:
1. The ACT primitives help in representing wide knowledge in a succint way. To illustrate this, consider the following verbs. These are verbs which correspond to transfer of mental information.
In CD representation all these are represented using a single ACT primitive MTRANS. They are not represented individually as given. Similarly, different verbs which indicate various activities are clubbed under unique ACT primitives, thereby reducing the number of inference rules.
The main goal of CD representation is to make explicit of what is implicit. That is why, every statement which is made has not only one actor and object but time and location, source and destination.
The following set of conceptual tenses make usage of CD more precise:
o – object case relationship
r – recipient case relationship
p – past
f – future
t – transition
2. Many inferences are already contained in the representation itself. This is explained with the help of an example.
CD brought forward the notion of language independence because all ACTs are language-independent primitives.
CD is a special purpose extension of semantic networks in which specific primitives were used in building representations. It still remains today as a fundamental knowledge representation structure in natural language processing system.