ACT-R and the basic-level activation

2009-5-22노홍찬

ACT-R 5.0: An Integrated Theory of the Mind J.R. Anderson et al., Psychological Review, 2004, 633

◦ ACT-R architecture overview◦ The perceptual-Motor System◦ The Goal Module◦ The Declarative Memory Module◦ Procedural Memory

Reflections of the Environment in memory J.R. Anderson et al., Psychological Science, 1991, 374

◦ Form of the memory functions◦ Environmental Explanation◦ Formulating the Effects of Practice and Retention

Content

ACT-R◦ Adaptive Control of Thought-Rational, first proposed

in 1998 Motivation of ACT-R

◦ An image of the mind as a disconnected set of mental specialties.

◦ “how is it all put back together?” Goals

◦ Producing a theory that is capable of attacking real-world problems that is capable of integrating the mass of data from

cognitive neuroscience methods like brain imaging

Why ACT-R?

visual module ◦ for identifying objects in the visual field

manual module◦ for controlling the hands

goal module ◦ keeping track of current goals and intentions.

production module◦ coordination in the behavior of other modules◦ only respond to a limited amount of information in

the buffers◦ recognize patterns in these buffers and make

changes to these buffers serial vs parallel processing

◦ the content of any buffer is limited to a single declarative unit of knowledge, called a chunk

◦ only a single production is selected at each cycle to fire. In this

ACT-R 5.0 Architecture

Two separate modules for the visual module◦ Visual-location module with a buffer

Where is the object?◦ Visual-object module with a buffer

What is the object? Object identifying process

◦ Request for where system with a series of constraints◦ Where system returns a chunk representing a location meeting the

constraints◦ Request for what system with the chunk representing the visual location◦ What system shifts attention to the location◦ What system generates a declarative memory chunk representing the

object Dual tasks for motor modules

◦ Ex) visual motor, manual motor, …◦ For the same modules, serial execution◦ For the different modules, parallel execution

The perceptual-Motor System

Responsibility◦ in the absence of supporting external stimuli

keeping track of what intentions are Keeping a representation of a set of subgoals Keeping track of problem state

Goal Module

Cognitive core of ACT-R along with procedural system

Activation of a chunk (Ai)◦ Base level activation (Bi)

Reflect general the memory’s usefulness in the past◦ Associative activation (∑WjSji)

Reflect its relevance to the current context Wj reflect the attentional weighting of each element (elem

j) of current goal Sji are the strengths of association from each element

(elem j) to chunk I◦ Retrieval probability and the latency is determined by Ai

The Declarative Memory Module

Base level activation Bi of chunk i◦ reflects the log odds an item will reoccur as a function of how it has

appeared in the past.

◦ where frequency of the retrieval of the chunk is n jth retrieval of the chunk represents jth element of the series tj represents the elapsed time since the jth retrieval of the chun many applications suggests the value of the parameter d as 0.5

Bi has been suggested by the author’s previous work The most successfully and frequently used part of the ACT–

R theory.

Base level activation (Bi)

The attentional weighting Wj◦ Wj is 1/n

where n is the number of elements consisting chunk i The strength of association Sji

◦ Sji is S – ln(fanj) Where

fanj is the number of chunks associated to element j S is a parameter, which is estimated as about 2 in many applications

Ex) A hippie was in the park Each oval is a chunk Each element has the same

attentional weight as 1/3 Hippie in park

Wj is 1 or 3 for each element

Associative activation (∑WjSji)

Retrieval probability◦ Almost the same as Ai◦ just transformed by sigmoid function

◦ Where ζ is the threshold that is the minimum Bi for the retrieval to begin S is a parameter whose role is the transform noise

0.4 for many applications Latency of the retrieval

◦ just the same as the value of Ai without log function◦ F is the latency factor

Retrieval probability & latency

Activation level calculated by the activation equation

Real retrieval time vs estimated retrieval time◦ The retrieval time is estimated by

the activation level presented above

The correlation between the real and estimated time◦ 0.986 ◦ nearly no dependency with the

parameters

Hippie experiment for Activation theory

The production system ◦ can detect the patterns that appear in these buffers and

decide what to do next Because of the seriality in production rule execution,

only one can be selected◦ the one with the highest utility

where ◦ Pi is an estimate of the probability that if production i is chosen

the current goal will be achieved, ◦ G is the value of that current goal, ◦ Ci is an estimate of the cost◦ Pi and Ci are learned from experience with that production rule.

Procedural memory

Anderson et al, Reflections of the Environment in memory, 1991◦ Gave the foundation of base-level learning equation to ACT-R theory

가정◦ 인간의 기억 메커니즘은 인간의 진화과정을 통해 환경적인 조건에 최적으로

반응하도록 적응해 왔다 . 기존의 연구들은 사람에 대해 적절한 입력을 주고 사람들의 행동을

관찰함으로써 기억의 메커니즘을 찾아내려고 시도함◦ 이와는 반대로 인간의 행동이 환경적인 조건에 최적으로 적응하도록 진화해왔다는

가정하에 , 환경적인 조건들을 관찰함으로써 인간의 기억 메커니즘을 밝히려는 시도를 수행함

기존의 연구들은 retention function 과 practice function, spacing effect 에 대해서 부분적인 설명이 가능할 뿐 , 모두를 다 설명하지 못함◦ Anderson 은 환경적인 조건을 관찰하고 이를 인간의 기억 메커니즘을 설계하는데

적용함으로써 위 3 가지 effect 를 모두 성공적으로 설명할 수 있는 theory 를 주장

Base-level activation in more detail

Exponential function

Retention function

Power function

Retention function

Practice Function

Spacing effects

Odds◦ If the probability of event i’s happening is p

the odds is defined as p/(1-p) Ranging from 0 to infinity

환경에서의 needs odds가 과거의 기억 인출 기록에 의해서 어떻게 영향을 받는지 알아봄

3 environmental sets◦ New york times headlines

1986101 ~ 19871231 Checked each word occurrence

Every time a word appears in the text, it’s a request for the reader to retrieve the word◦ CHILDES database

Related to children’s verbal interactions Every time someone says a word to a child, it’s a request on the child to retrieve the word’s

meaning◦ Mail messages of the author

198503 ~ 198912 Every time the author receives a message a certain person, it’s a request for the author to

retrieve the memory of the sender

Environmental Explanation

Recency effect

Environmental Explanation

Frequency effect

Environmental Explanation

Spacing effect

Environmental Explanation

Basic assumptions from the environmental experiments◦ The strengths from individual presentations sum

to produce a total strength (frequency effect)◦ Strengths of individual presentations decay as a

power function of the time (recency effect)◦ The exponent of the power function for decay of

each presentation decreases as a function of time since previous presentation (spacing effect)

The provided mathematical formula

Comparison between estimation and real one

Base-level activation vs Cache algorithms◦ Base-level activation

Frequency effect 와 recency effect 의 결합 Power function 의 활용

◦ Cache algorithms Need odds vs cache replacement policy LRU, LFU, LRFU (1999)

LRU: only recency effect 고려 LFU: only frequency effect 고려 LRFU: recency effect + frequency effect

not with power function

Discussion

Chunking using ACT-R vs association rule mining◦ Association rule mining

Offline algorithm◦ Chunking with ACT-R

Online algorithm

Discussion