50
CHAPTER 3:AN INTRODUCTION TO SCHEMATA, SCRIPTS AND CONNECTIONIST ARCHITECTURES 51
proportion of these were interpreted as being relevant to the object of the sentence. This study emphasized the idea that relational knowledge could not be modeled by a simple function of the amount of information available, but rather required more complex models such as semantic networks.
Further research into the semantic network model revealed that the mental representation of relationships was more complex than was previously realized. Coleman and Kay (1981) showed that verbs can have different types of relationships, and also create implications about the properties of the subject and object they can take. They give the example of the verb to lie, which they argue, implies three distinct properties of its subject and object. Firstly, it implies that the statement made is false.
Secondly, it implies that the subject believes the statement is false, and thirdly, it implies that the subject intended to deceive. This complexity cannot be easily expressed in a semantic network.
This type of evidence combined, in the mid 1970s, with a realization by researchers that explaining knowledge about events and other complex concepts required more than information about the attributes of objects. Even positing a hierarchy of relationships, as Gentner did, was not enough to solve the problem. Impelled by the work being done in linguistics by theorists such as Chomsky (1965) on recursive models in language rule encoding, researchers such as Rumelhart (1975) and Thorndyke (1977) proposed “story grammars” which were though to underlie the encoding and comprehension of stories, as stories usually which contain a complex set of relationships between objects. The pinnacle of this line of inquiry was the creation of two closely related theoretical constructs – the schema and the script.
3.1.1
Schemata (Rumelhart & Ortony, 1977)A schema is a recursive, organizing structure which encodes complex concepts by combining simple objects with the relationships that occur between them (Eysenck & Keane, 1995). Schemata contain a list of relations and variables on which these relations act. The relations can encode simple transitive verbs (such as sit or drink), or they can encode complex relations indicating causation and temporal relationships (such as cause or produce). The variables can contain either concepts (such as objects) or other schemata; however, before an object or schema can be assigned to a variable, it must satisfy some test condition. These tests prevent nonsense constructions such as eat the air. These variables contain default values, so that if enough information is not provided to assign a value to all the variables, a complete, workable schema is still available. For example, if a subject is given the sentence “Joe drank heavily”, which does not contain information about what Joe drank, the information is still available by virtue of the default value. In this example, most subjects would respond to the question “what did Joe drink” with alcohol, liquor and other related terms, as these pass the test to fit into the drinks heavily schema. It is unlikely that objects such as water or milk would fit that schema.
Schemata encode generic knowledge, which can be applied to a range of situations, provided those situations do not violate the variable fitting test conditions. For example, the relation CARRY can apply to Joe carried the bottle as well as it does to The ship carries the passenger, but not to The passenger carries the ship, as this violates the necessary test condition (which might be SUBJECT must be larger than OBJECT). The ability of schemata to encode generic situations allows for greater efficiency in memory, as few schemata can be used to encode a wide variety of situations. Also, the existence of default values of variables allows for optimal processing to continue even under conditions of reduced information. However, the information provided by the default values might be incorrect, as it is based upon previous experiences in similar situations (Rumelhart & Norman, 1981). This can lead to errors in behaviour or cognition, as the information used for decision making does not fit the environmental situation the person is facing.
CHAPTER 3:AN INTRODUCTION TO SCHEMATA, SCRIPTS AND CONNECTIONIST ARCHITECTURES 52
3.1.2
Scripts (Schank & Abelson, 1977)Schemata, as defined above, are capable only of encoding statements. They are clearly not suited to encode more complex, sequence dependent behaviours which are common in everyday living (such as posting a letter or having coffee). A hoary example of such a complex action is having a meal at a restaurant. Operating in this situation requires following a strict sequence of events: waiting to be seated, selecting from the menu, waiting for the meal, eating, waiting for the bill, paying and leaving.
In order to successfully carry out a trip to a restaurant, it is necessary not only to have knowledge of each step, but also of what order they should be performed in. It is also necessary to be able to identify the signs that one step is completed and that the next has commenced. Clearly, a schema alone cannot encode all of this information. Schank and Abelson propose a script, which is a structure that encodes a stereotypical sequence of events. Scripts are said to exist for sequences of events which are experienced frequently; for instance, most pedestrians have a crossing the road at a traffic light script, but scripts can also encode the far more obscure – long time fans of Star Trek probably have a script for getting a starship underway.
As the examples above show, scripts exist to co-ordinate knowledge and behaviour in order to achieve complex goals such as eating in a restaurant while not causing a scene. In order for a script to encode all this information, it is necessary for the script to encode not only the actions required, but also the relationship between behaviours (such as temporal order) and the required role-players (such as the waiter). Each script is represented by a series of variables referred to as role-slots (Schank & Abelson, 1977). Role slots can be filled either by a role (such as waiter) or by a schema (such as entering the restaurant, or ordering the food). Based on the specific situation at hand, role-slots will be filled by the objects and information present in the environment. For instance, the waiter role-slot might be filled by the teenager behind the counter wearing the restaurant’s logo on his t-shirt, or it could also, in a different situation, be filled by a tall, elegantly dressed middle-aged man holding a pen and notebook.
Not any object can fill a role-slot, however. Each role-slot has criteria which are applied to determine if a particular object is suitable for filling the role-slot (Schank & Abelson, 1977). These criteria are stored in long-term memory, and are acquired through personal experience and other forms of learning, such as observation. If a particular person has only been to five star restaurants, it is likely that for this person a youth behind a counter would not satisfy the criteria for waiter. These criteria ensure behaviours are kept appropriate to the environment, while ensuring storage efficiency by allowing adaptation of scripts to new but similar situations.
It seems that the purpose of scripts is to allow for algorithms of cognition and behaviour to be applied to achieve particular goals in an environment. These algorithms are applied only to situations which fit a set of criteria for suitability, through a mechanism which involves the use of perception (to asses the resources currently available in the environment) as well as previous knowledge (to provide criteria to compare against, as well as to determine if the available resources are similar enough to those required). By means of this mechanism, behaviour in environments can be kept appropriate to the situation, which in turn ensures that the required goals will be met. The inclusion of learning to modify the scripts allows for behaviours to remain goal directed (as the basic structure of the script remains unchanged), but still relevant to environments which change (by adapting the criteria for accepting an object or schema as a role-slot filler).
A satisfactory amount of evidence has been gathered to support the theory of scripts. The commonality of script content was demonstrated by Bower, Black & Turner (1979). They collected descriptions of restaurant eating experiences from a large, diverse group of subjects. Although there was a great deal of variation in terms of the participants, the types of restaurant they frequented and the restaurant experiences they had had, Bower et al discovered a common core of 15 ordered experiences which were common. While this study demonstrates the content of scripts, and the phenomenon of time order, Galambos & Rips (1982) demonstrated the effect of scripts on general mental processing. Firstly, they primed a script in their subjects by showing their subjects various photographs of restaurants. Then, they asked subjects to decide if particular actions were part of the experience of visiting a restaurant.
CHAPTER 3:AN INTRODUCTION TO SCHEMATA, SCRIPTS AND CONNECTIONIST ARCHITECTURES 53
Galambos and Rips found that subjects took significantly longer to make the decisions when the action was not part of the script, than when it was. This demonstrated the effect of scripts not only on regulating behaviours, but also on making particular types of information more available.