A MEMORY SYSTEM FOR A SITUATED DESIGN AGENT BASED
ON CONSTRUCTIVE MEMORY
PAK-SAN LIEW AND JOHN S. GERO
Key Centre of Design Computing and Cognition
University of Sydney NSW 2006
Australia
{paksan, john}@arch.usyd.edu.au
Abstract. Situated design agents require new types of memory. The design of a memory system for use in such a design agent, based on the notion of constructive memory is proposed. The specification of the memory system is derived from the architecture of a situated design agent. Cognitive studies of the human memory system provide the functional and behavioural characterization of the memory system.
Introduction
This paper outlines the conceptual architecture of a memory system for a situated design agent based on the paradigm of constructive memory in design (Gero, 1998; 1999a; 1999b). The incorporation of this memory system facilitates the creation of a computational design system with performance that matches normal design systems and has the ability to operate in an environment for which the system has not been directly programmed. Through a set of learning capabilities and the interaction with the environment, the agent is able to learn from experiences and ground what it knows through these experiences. This is a fundamentally different approach to the programmer encoding into the agent all the knowledge a priori, which is the approach adopted when using expert systems.
The Constructive Memory Paradigm
Memory construction occurs whenever a design agent uses a past experience in the current design environment in a situated manner. Any information available in the current design environment is used as cues in this construction process. Associated memories are constructed and grounded
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LIEW, P.S. AND GERO, J.S. according to the current interactions between the agent and the environment.
This behaviour of a constructive memory system within a situated environment has the following characteristics:
memory operates as a dynamic process, it is not a static imprint to be stored in a specific location and retrieved for used later;
the operations of the memory system are not predefined, they are influenced by the situatedness of designing;
construction of memories do not rely on the exact matching between what the agent has in the memory system and the current designing environment; and
memories that are constructed may not match the original experience exactly as it was first experienced, but change according to when, where and what the memory system is cued with.
The Memory System
The various components of the memory system are derived from cognitive studies of the human memory system. In particular the following models of human memory are adopted:
modal model of human memory system (Atkinson & Shiffrin, 1968);
multiple-component model of long-term memory (Baddeley, 1999); and
multiple-component workspace model of the human working memory (Logie,
0 1
1995).
illustrates the architecture of the proposed memory system for the situated design agent. The paths in 0 are shared between processes to avoid overcrowding the diagram. They can be taken as directed association between two entities, i.e. information or instructions flow in the indicated direction.
Working Memory
The working memory is a workspace for reflective and reactive processes where explicit design-based reasoning occurs. It is also the working environment where executive control functions are performed. These processes selectively alter and/or add information to the working memory based on the content of the LTM
(14)
[the numbers in parentheses refer to the paths in Figure 1] or STM
(12)
. Information within the working memory are combined with the stored knowledge and experiences, manipulated, interpreted and recombined to develop new knowledge, assist
1
The functional components of the architecture do not imply that different types of memory reside in different location within the agent. The boundaries between different forms of memory are there to highlight the functions of different type of memories. All these memories are used to construct a design situation.
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A MEMORY SYSTEM FOR A SITUATED DESIGN AGENT BASED ON… learning, form goals, and support interaction with the external environment.
Knowledge and experiences accumulated are available for use in the immediate task and once activated, they can then be manipulated for extraction of novel information that had not been stored explicitly in the past. The behaviours of these operations are influenced by: the external environment; the interactions between the agent and the environment; and the internal state of the agent.
The reasoning that occurs within the working memory is design related. It includes synthesis processes such as analogy, combination, mutation and designing from first principles (Rosenman & Gero, 1993) and reformulation processes (Gero, 1998) that deal with re-parameterization and parameter expansions. These operations of the working memory (explicit reasoning) are based on three main components:
the executive control
: where the hub of the agent control processes is located;
the STM and LTM: where knowledge and experiences of the agent resides ((14)and
(12)); and
the cache
: that provides the buffering for the reasoning processes (11).
Implicit Memory
CONCEPTOR
Working Memory
2
5
8
Executive
Control
11
14
13
Procedural
Memory
EFFECTOR
4
PERCEPTOR
9
Cache 15
Priming
Conditioning
12
3
6
Short-Term Memory
19
Explicit Memory
1
SENSOR
7
Slot 1
Slot 2
Slot 3
16
Episodic
Memory
20
Semantic
Memory
10
17
Slot r Long-Term Memory
ENVIRONMENT
18
MEMORY SYSTEM
Conceptual architecture of a computational memory system (numbered paths are indicated as
(#) in the text where # is the number as shown in the diagram)
The control processes within the executive control are involved in the organization, coordination, manipulation, movement and comparison of information and knowledge (derived from the environment or from the explicit memory) for design related reasoning. These processes deal with:
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LIEW, P.S. AND GERO, J.S. strategy selection and hypothesis-testing procedures associated with concept formation problems during conception, perception, hypothesizing, expectation and devising (Gero & Fujii, 2000); the coding of percepts during perception where related stored knowledge and experiences from LTM or STM are activated and constructed within the working memory to allow the interpretation of current percepts in the current design session; and tasks where there is a clear solution (reactive process) based on the current bias.
Inputs arrive from: the conceptor (8) and perceptor (9) during conception; the perceptor (9) during perception; the LTM (14) and STM (12) when they play host to activated representations and procedures from them.
All of the above inputs are subjected to biases (13) before they are processed within the working memory system. The bi-directional path (13) between the priming component and the working memory models the effect of using the default bias or the setting of a predefined bias.
Short-Term Memory
The structure of the short-term memory (STM) is modeled as a short-term, fixed-size buffer (0) that captures the limited-capacity characteristics of the
STM. The content of the buffer is constructed from the memory trace that enters the STM. The path (17) from the buffer to the LTM indicates the build up of a long-term trace due to the memory trace’s level-of-processing.
The slots in 0 are used to indicate the notion of the relative level-ofprocessing of the various traces within the buffer.
Both STM and LTM are used in the construction of memories in the working memory during the reflective and reactive processes ((12) and
(14)). The differences between memory traces residing in the STM and LTM lie in the fact that memories in the STM are not as readily recalled in the memory construction process as compared to memory traces in LTM.
The STM serves as a gateway into and out of the LTM. The path into the
LTM occurs when memory traces are transferred from the working memory to the LTM through the STM. As soon as a design situation or design experience has been constructed and served its purpose within the working memory, it is transferred into the STM as a new memory trace (12). Memory traces in the STM are integrated into the LTM gradually through their levelof-processing (17). STM acting as a gateway out of the LTM occurs when memory traces in the LTM that are not grounded frequently are transferred into the STM (16) and out of the memory system. These two actions model the phenomena of long-term learning and the phenomena of “use it or lose it” in human memory system.
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A MEMORY SYSTEM FOR A SITUATED DESIGN AGENT BASED ON…
The amount and form of memory traces that are transferred from the
STM to LTM (17) are primarily a function of their level-of-processing. For a memory trace to be transferred to long-term memory, it must be go through a series of encoding levels. Active processing of a memory trace through its frequent association with existing memory traces within the LTM and its grounding according to (matching with) the current design situation, produce durable short-term memories. As the durability of a memory trace increases, its level of encoding increases. The increase in level-of-processing of a specific memory trace facilitates its inclusion into the LTM. This simple formulation captures the important characteristic of long-term learning in which a process of encoding material in terms of prior experiences and grounded usage produces comparatively durable and readily retrievable memory traces.
Long-Term Memory
The long-term memory is a relatively permanent repository for memory traces. Memory traces stored here do not decay and disappear as readily as those in the STM. New memory traces about design situations and design experiences are encoded into the LTM after they have been processed in the working memory and STM. To influence subsequent explicit behaviour, the memories within the LTM and STM are constructed and brought back into the working memory where it becomes part of the reasoning process.
Long-term memory refers to all effects of prior experiences and knowledge on subsequent behaviour of the situated design agent. This behaviour is classified as reflective
, reactive
or reflexive
. Explicit memory systems provide memories for reasoning involving the working memory
(reflective and reactive processes) while implicit memory systems deal with memories that affect the reflexive behaviour of the agent (the reflexive processes).
Explicit Memories
The reflective and reactive aspects of the situated design agent are based on explicit memory. Explicit memory consists of a semantic and an episodic memory subsystem. Semantic memory handles design knowledge while the episodic memory deals with the memories of design situations and design experiences.
Requests for explicit memories and transfer of the constructed memories occur through process (14). These operations are subjected to the bias activated at the current point in time (13). Explicit memories that are not
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LIEW, P.S. AND GERO, J.S. subsequently grounded are transferred to the STM before they are purged from the system (16). On the other hand, memories in the STM that have passed through a series of processing (level-of-processing) are transferred to explicit memory (17) for longer retention.
Episodic memory : deals with the storage and construction of memory traces related to design situations and design experiences. Once a new design situation or a design experience has been constructed and used within the working memory, it is stored in the STM as a new experience where it is subjected to several levels of processing before it is transferred into the episodic memory
(17)
. The content of the episodic memory is also subjected to generalization
(20)
. A common set of design experiences is compiled into a general design rule and the original experiences may be removed from the system.
Semantic memory: deals with general principles and refers to the acquisition, storage and usage of design knowledge. These include both general and specific knowledge related to designing. General design knowledge is domain independent and includes knowledge such as those are based on the generic FBS framework. Specific design knowledge is that which deals with a particular domain such as knowledge related to Design for Assembly and
Design Simplification. All of this knowledge is grounded according to the agent’s current internal state in relation to the current environment and is subjected to addition, modification, updating, replacement and removal as the agent learns new knowledge. Various possibilities for these operations include: knowledge gained through explicit learning processes that occurs during reflective and reactive process (14); knowledge (without contextual information) generalized from a common set of design experiences to form semantic knowledge that describes the underlying principles of the experiences (20); and knowledge transfer from implicit memory after it has been learned implicitly (19).
Implicit Memory
The implicit memory component captures knowledge expressed through performance rather than through recall and adaptation (semantic memory) or construction (episodic memory). Experiences are accumulated in behavioural changes. The reflexive behaviour of the agent is based on the implicit memory system. Implicit memories are taken as memories that are not used explicitly within the working memory during reasoning. These memories are information acquired previously that influences the agent’s
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A MEMORY SYSTEM FOR A SITUATED DESIGN AGENT BASED ON… behaviour or reasoning process either directly or indirectly.
Procedural memory:
contains process knowledge on how to perform a specific design task. This knowledge about processes can be taken as the design skill that an agent has. With practice, these skills can be improved.
The improvement is retained (remembered) from one practice session to the next using the feedback from the working memory
(18)
even though the information remembered is used in a reflexive manner later (
(3)
via
(10)
).
If necessary, the knowledge learned is used together with those in the semantic memory if its application into the current situation needs to be considered together with other knowledge.
Priming: is the process that activates (biases) one or more existing memory traces by a stimulus (the priming stimulus) presented to the working memory system
(13)
and it influences the subsequent perception
(9)
and reasoning process
(8)
. The priming stimulus originates from the memory traces contained within the LTM or STM (
(15) and
(19)
) and it allows activated memories in the STM or LTM to be constructed into the working memory more easily after the priming experience than before (
(12)
,
(14)
).
Conditioning:
provides the reflexive behaviour to the agent by matching the pre-conditions of the rules it contains with the conditions of the environment and applying those rules when applicable. The agent’s behaviour is affected directly by this reflexive action in the reflexive operation (
(7)
,
(10)
,
(3)
,
(1)
). Common reactive operations that are frequently used are transferred to the conditioning component
(18)
so that they can behave in a reflexive manner without explicit reasoning in the working memory. The learning of new rules either by creating entirely new rules or by modifying/replacing existing rules will result in the creation of an artificial grammar. This occurs through an implicit learning process such as classical conditioning (Pavlov, 1960) or operant conditioning (Skinner,
1938).
Discussion
This paper has proposed a theoretical framework for the design of a constructive memory system for use in a situated design agent. The next phase of the research entails the implementation of the ideas presented here.
The interactive activation and competition (IAC) network (McClelland,
1981) has been identified as a possible mechanism to implement the characteristics of the memory system. Once the memory system has been
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LIEW, P.S. AND GERO, J.S. implemented it can be used to further test the concepts of situated design agents.
Acknowledgements
This research is supported by a University of Sydney Postgraduate Research Award and a
University of Sydney Sesqui R&D grant.
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