COMPUTER - Stackscb815km8173/cb815... · 2015. 10. 19. · COMPUTER SIMULATION OF SHORT-TERM...
Transcript of COMPUTER - Stackscb815km8173/cb815... · 2015. 10. 19. · COMPUTER SIMULATION OF SHORT-TERM...
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Kenneth R. Laughery
State University of New York at Buffalo
Kenneth R. Laughery
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COMPUTER SIMULATION OF SHORT-TERM MEMORY:
A COMPONENT-DECAY MODEL
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COMPUTER SIMULATION OF SHORT-TERM MEMORY:1
A COMPONENT- DECAY MODEL
2Kenneth R. Laughery
State University of New York at Buffalo
Prepublication draft of a chapter to appear in
J. T. Spence and G. H. Bower (Eds.), The Psychologyof Learning and Motivation: Advances in Researchand Theory. Vol. 111. New York: Academic Press.
This research was supported by Research Grant No. MH-11595 from theNational Institute of Mental Health, United States Public Health Service.
2Contributions to the development of the short-term memory model have beenmade by a number of people associated with the author during the past twoyears. Special thanks go to Patricia Fiero, Richard S. Cimbalo, Allen L.Pinkus, James C. Fell and Gilbert J. Harris.
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TABLE OF CONTENTS
I. Introduction
11. The Model - An overview111. The Model - A Detailed Description 8
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A. Basic Units of InformationB. Basic Forgetting Mechanism 14
14C. The Memory Structures1. Long-term Memory 162. Short-term Memory l8
24D. Memory Processes '"*1. Noticing2. Item Recognition 313. Updating Item in STM 374. Checking Environment 425. Rehearsal6. Links and Order Information 507. Using Links 528. Responding 5£9. Recoding 55
E. Types of Errors that the Model Can Make 63
IV. A Sample Simulation 67
V. Some Simulation Results 79
A. Simulated Study Number 1: A General Experiment 79
B. Simulated Study Number 2: High vs. Low Auditory 86Similarity
VI. Discussion and Conclusions 89
A. Some Possible Extensions 9°1. Interitem Time Distributions During Input 902. Split Span Studies 923. Relative Recency Judgments 944. Free Recall 955. Delayed Recall 96
B. Some Possible Revisions 98
VII. References 103
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FIGURE
LIST OF FIGURES
1. General Model of Human Memory System 2
2. Family of Curves Describing Decay Functions 15
(this figure shows 15 of the 30 decay
functions in the model)
3. Long-term Memory Structure 17
4. Short-term Memory Structure 19
5. Short-term Memory Structure for a Single Item 20
6. Subject Executive Routine - SI 277. Store Basic Component Routine - S2O 298. Short-term Memory Structure After Inputing 32
First Item (Letter X)
9. Net Sorting Routine - S4 34(Recognition or Discrimination Process)
10. Store and Update in Short-term Memory 39
Routine - S5U. Interitem Activity Routine - SlO12. Rehearsal Routine - SlOO 4613. Recoding Information in Long-term Memory 59
14. Serial Position Curves for 8 and 10 Item Sequences 87
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I . INTRODUCTION
The past few years have witnessed the development of several models
of human memory. Examples are Atkinson & Shiffrin (1968), Bower (1967),
Sperling (1967) , Waugh & Norman (1965) and Wickelgren & Norman (1966) . Themost generally accepted concept in the theories is that memory can best be
represented by three separate storage systems. The three parts of this
general model of memory are illustrated in Fig. 1.
The first of these systems is the sensory storage or very-short- term
memory, and is viewed as a peripheral or perceptual storage. There is
general agreement that information stored in this type of memory decays
with time and is lost in a matter of a few seconds or less (Averbach &
Coriell, 1961; Glucksberg, 1965; Sperling, 1960). Another type of memory,
short-term memory (STM), is the focus of these various models. Despite
general agreement that information is lost from STM (as opposed to loss
of access to the information) , there are two views as to the nature ofthis loss. One view is that information is lost as a result of decay; an
alternative is that STM has a limited capacity so that items are lost by
being replaced by new items entering the system. Choosing between these
alternative postulates is almost a matter of theoretical style, since the
issue has not lent itself to any clear-cut experimental resolution. Long-
term memory (LTM), the third type of storage, received relatively little
attention in the theoretical efforts cited above. It would appear, however,
that there is general agreement as to why information is not always recalled
perfectly from this permanent storage. The widely accepted view is that
what is lost is access to the information and not the information itself.
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The work reported in this paper represents an attempt to construct
a computer simulation model of human memory. It is a theory of the
average S. 3 The model represents the human at an information processing
level of description. At this level the human is viewed as having avail-
able a variety of processing mechanisms which can be employed to operate
upon information. The source of the information may be the external en-
vironment, such as a display device, or the internal environment (memory)
Like the other models already mentioned, a system consisting of the three
storage mechanisms is envisioned with the main focus on STM. The model
differs from the others, however, in one important respect: It describes
in complete detail a set of structures and processes that are intended
3As a rule, simulation models have either focused upon the behavior of an
individual S (e.g., Feldman, 1963; Laughery & Gregg,1962) or upon the
performance of an "average S" (e.g., Feigenbaum,1963). Clearly, these
are two separate approaches to theory construction. In addition to the
theoretical inclinations of the modeler, the decision as to which approach
to take is frequently determined by the nature of the task environment.
The tactic of building models for individual Ss (and hopefully finding
ways of pulling them together into a general model that includesappro-
priate parameters for representing the various individuals) has usually
been employed for simulating problem solving kinds of behavior. In these
tasks one can collect protocol data which provides a rich source of
information regarding behavioral processes. On the other hand, attempts
to construct models of the "average S" have generally involved tasks
which preclude getting protocols (the task would be grossly disrupted by
the procedure). The memory-span task is an example of a situation where
protocols would have a disrupting effect (except for post-sequence verbal
reports) .
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to represent certain aspects of human memory.
While the model to be described purports to deal with the human
memory system, it is actually a simulation of performance in a particular
task. The task is the standard memory-span procedure, where a sequence
of items (such as digits) is presented, following which j5 reproduces as
many of the items as he can remember. Furthermore, the task is limited
to a vocabulary of 36 items; the 10 digits, 0-9, and the 26 letters of
the alphabet.
Although the model is presently limited to the memory- span procedure
and a 36 item vocabulary, it will be argued later that the model can be
expanded to simulate performance in many other memory tasks. Such exten-
sions will not require a restatement of the basic memory structures or
processes, but rather will involve only the addition of task-dependent
structures and processes to allow the model to perform in the different
situations. This distinction between basic and task-dependent structures
and processes is crucial. Indeed, one of the theses of this effort is
that a relatively few basic memory structures and processes can account
for performance on a variety of memory tasks-- the degrees of freedom in the
model are not larger than the points of fit.
One problem that has plagued computer simulation theorists is com-
municating to others the structures and processes that make up the model.
These models usually deal with relatively complex tasks, so the models
themselves have been complex. One issue is the level of description at
which the model should be presented. Presenting the program instructions
usually provides little help in understanding the theoretical concepts
underlying the model. On the other hand, attempts to describe the model
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verbally generally result in some loss of precision. A compromise is to
describe the processes in the model in terms of flow charts which capture
the formal qualities of the model while communicating the logical com-
plexity .Another communication problem is the integration of the various parts
of the model. There are many parts of a simulation model, and it is im-
portant to understand how they fit together. A linear presentation of
these parts plays considerable havoc with the reader's intellectual
digestive system. A more appropriate procedure is to make two or three
descriptive passes through the model with each successive presentation
providing a more detailed description. The following model presentation
consists of an overview, a detailed description and an example of the
model's performance (a trace) on a specific sequence of items.
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11. THE MODEL - AN OVERVIEW
In this section a general description of the model will be presented.
As already mentioned, the model represents performance in a memory- span
task where the item vocabulary consists of the 36 digits and letters.
The digits and letters, however, are not the basic units of information
in the model. The basic information units or components are a set of
visual and auditory features that define the visual and auditory dimen-
sions of the vocabulary items
Two types of memory, STM and LTM, are represented (the current version
of the model does not include a very-short- term memory). The LTM contains
the visual and auditory definitions of the vocabulary items and represents
S's permanent knowledge about the items. The STM is a series of memory
structures, each holding information about an individual item. This item
information includes the names of its auditory components, the times at
which the components were stored, and functions describing the decay of
the components
The basic forgetting mechanism in the model is a decay process. This
process is described by an exponential function which defines the probabilit
of retrieving a component as a function of length of time it has been in
STM. The components decay independently.
The flow of events in the model is as follows
1. A set of components (visual or auditory-representing the
presentation mode) are presented to S.
2. The S notices ("sees" or "hears") the set of components.
3. The S searches LTM and finds an item whose components match the
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input (the item is recognized)
4. An STM structure is set up for the item with a substructure for
each auditory component. Each substructure contains the component's
name, the time it was stored and a function describing its decay.
5. If the next item has not been presented, the items already in STM
are rehearsed during the interitem interval. Rehearsal consists
of retrieving components of an item from STM, recognizing the item
by finding a match in LTM, and then updating the component sub-
structures in STM. Updating involves resetting the time and
changing the decay function so that the component decays at a
slower rate.
6. If after all items are rehearsed there is still time available
(a new item has not been presented), an attempt is made to recode
groups of items that are consistent with certain recoding criteria.
7. At specific points during the above activity S checks if a new
item has been presented, and if it has the process reverts back
to step 2.
8. When a special "respond" signal appears and is recognized (a 37th
vocabulary item) , control is transferred to a respond process
that attempts to recall (and output) the items.
9. When recall of a sequence is completed, the STM structures repre-
senting that sequence are erased. A new sequence is then begun
with nothing in STM. Thus, the model does not deal with proactive
or retroactive interference between sequences.
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111. THE MODEL- -A DETAILED DESCRIPTION
Two major parts of the simulation program can be distinguished-
data structures and routines. The data structures represent the long-
term and short-term memories. The routines represent the processes
available for operating on memory. These processes include taking in
information from the environment, storing it in memory, retrieving it
from memory, and outputting information to the environment. Other
routines represent processes such as rehearsal and recoding or chunking
Beyond this distinction between data structures and routines, two
basic concepts are fundamental to understanding the model. These model
concepts are a simulated clock and a window. The clock is essentially a
cumulative record of the time required by the various memory processes.
Underlying this concept is the assumption that the human is basically a
serial processor; he can perform only one cognitive process at a time,
and all processes require some amount of time to be carried out. With
the occurrence of each process, the clock is incremented by an amount
of time associated with that process. The time base is milliseconds.
The window represents the visual display or the tape recorder through
which information is presented to S. While the data structures and
routines are Intended to represent the human subject in a particular task,
the program also includes data structures and routines that are intended
to represent the experimenter. "Experimenter" refers to the human in
charge of the experiment as well as those pieces of equipment that are
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part of the laboratory situation. In the model the window is a cell or
series of cells into which and from which the experimenter routines can
place and remove information. The experimenter routines monitor the
simulated clock and at appropriate times put information into or take
information out of the window. The S also monitors the window, and through
it the information is "seen" or "heard."
Given these two concepts, the model can now be presented in detail.
The presentation is divided into five sections; the basic information
units, the forgetting mechanism, the memory structures, the memory processes,
and types of errors the model can make.
A. Basic Units of Information
In addition to the 10 digits and 26 letters, there is a 37th item in
the model's vocabulary which is a signal to S that the sequence is complete
and it is time to recall. It represents an auditory or visual signal
occurring at the end of the input sequence
While the digits, letters and special symbol make up the vocabulary
of the simulated S, these are not the basic units of information with
which the model deals. Rather, the information units are visual and
auditory components or features that can be used to define the visual
and auditory dimensions of the vocabulary items. The auditory components
4No discussion of the experimenter data structures and routines will
be given since this part of the program contributes little to under-
standing the model.
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are named Pi, P2, , P43, and their definitions are presented inTable 1. The visual information components in the model are a set of 21
basic line segments and line relationships which include elements for
describing a standard version of the digits and letters. These visual
components are named VI, V2, , V2l, and their definitions are presented in Table 11.
Each of the vocabulary items, digits and letters, can be uniquely
defined by some combination of either the auditory components (which de-
scribe how the item sounds) or the visual components (which describe
what the item looks like) . These auditory and visual descriptions arepresented in Table 111. The visual and auditory descriptions of the
respond signal are special symbols (V24 and P44) that are not part of thebasic sets in Tables I and 11.
While the types of information units that can be stored in memory
involves a basic assumption about the memory system, the units that
actually do get stored is probably a function of the task environment.
It will be assumed in this model that only auditory components are stored
in and retrieved from STM. This assumption seems appropriate for repre-
senting performance on a memory-span task involving digits and letters.
5The standard version is the digits and letters produced by an IndustrialElectronic Engineering Company Binaview display. The reason for selectingthis version was that Binaview displays were used in all our experimentsinvolving a visual presentation.
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Table I.
Basic Auditory Components
Component Name
PIP2P3P4P5P6P7P8P9PlOPllPl2Pl3Pl4Pl5Pl6Pl7PlBPl9P2OP2lP22P23P24P25P26P27P2BP29P3OP3lP32P33P34P35P36P37P3BP39P4OP4lP42P43
Phonetic Symbol
Pbtdkgfvc5
z/3vd3hmnQii
cmaoovvA
3-erwj1rciaxoiauou
Example
£aYbaytipdipcallgonefatvatthinthensuezooshoevisionchurchJLudgehatmayniplungeatitvacationpensatfatherallnotationpullpoolaboveaboveworkerworkerweyeslawrobaimhighboyloudopen
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Tabic 11.
Description of Basic Visual Components
CharacteristicMane
VI
V2
V3
V4
V5
V6
V7
V8
V9
VlO
Vll
Vl2
Vl3
Vl4
Vl6
Vl7
VlB
Vl9
V2O
V2l
CharacteristicDescription
Vertical Left
Vertical Center
Vertical Right
Horizontal Top
Horizontal Middle
Horizontal Bottom
Full Positive Sloping Line
Pull Negative Sloping Line
Full Curve, Closed
Half Curve, Closed, Top
Pull Curve, Open Right
Full Curve, Open Left
Full Curve, Open Top
Half Curve, Open Left, Top
Half Curve, Open Right, Top
Half Curve, Closed, Bottom
Intersection
Two Lines Sloping Same Way
Part Positive Sloping Line
Part Negative Sloping Line
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Table 111.
Auditory and Visual Definitions ofVocabulary Items
ocabulary AuditoryDefinitions
VisualDefinitionsItems
ABCDEFGHIJXLMN0PQRSTUVwXVZ0I23456789
Respond Signal
P39P2.P21P11.P21P4.P21P2lP24.P7P16.P21P23.P15P4OP16.P39P5.P39P24.P37P24.P18P24.P19P43P1.P21P5.P36.P30P26.P33P24.P11P3.P21P36.P30P8.P21P4.P31.P2.P37.P36.P30P24.P5.PHP35.P40P12.P21P12.P21.P38.P28P35.P31.P19P3.P30P9.P38.P21P7.P28.P38P7.P40.P8P11,P22,P5,PUP11,P24,P8,P32,P19P39.P3P19.P40.P19P44
V5.V7.V8V1.V14.V15VllV1.V12V1.V4.V5.V6V1.V4.V5V5.V11V1.V3.V5V2,V4 ,V6V4.V13V1.V20.V21VI,V6V1.V3.V20.V21V1.V3.V8V 9V1.V14V9.V11.V21V1.V14.V21V15.V16V2.V4Vl3V7.VBV7,V8,V19,V20,V2V7.V8.V18V2.V20.V21V4.V6.V7V11.V12V 2V6.V12V14.V15V2.V5.V18.V20V1.V4.V15V17.V20V4.V7V10.V17V3.V16V24
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For other tasks, however, other types of units (e.g., visual or speech-
motor) may be appropriate.
The basic forgetting postulate in this model is that informationunits in STM decay in time. Clearly this is a basic assumption about
the memory system. Actually, the model contains thirty different ratesof decay that describe how information is lost. Each rate is described
by an exponential equation giving the probability that a unit of informa-
tion, a component, can be retrieved as a function of the length of timethat it has been stored in memory. The form of these equations is
where p is the probability that the component is retrieved, t is the
length of time the component has been in store, and A, B, C, are free
parameters .In the present model the C parameter, which represents the asymptote
is assumed to be zero; the A parameter, the probability of retrieval ofthe unit at time t = 0, is assumed to be 1. These assumptions leave thedecay rate, B, as the free parameter describing the decay function.
A subset of the family of curves, or decay rates, used in the modelare shown in Fig. 2 along with the value of the B parameter for each ofthe curves. The D's are the names of the routines that actually carry
out the calculations for that particular decay rate. The manner in whichthis process is incorporated into the model will be described later.
It is assumed that human memory can best be represented by a system
B. Basic Forgetting Mechanism
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C. The Memory Structures
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consisting of three separate storage mechanisms: very-short- term memory,
STM and LTM. For the model presented here, only STM and LTM are represented
Some additional comments will be made later regarding the development of
the model to include a very- short- term memory.
The LTM is represented by a structure which contains information that
is permanently available to S. In order to simulate the human in a memory-
span task consisting of sequences of digits and letters, LTM contains in-
formation about digits and letters that is relevant to this particular
task. In the model this information includes: (a) the visual components
that describe what the item looks like; (b) the auditory components that
describe how the item sounds; and (c) some information about how the item
may be combined with other items for recoding into larger chunks. This
recoding information will be described in Section 111, D, 9 where recoding
processes are discussed.
The LTM structure is a modified discrimination net (Feigenbaum, 1963)
The nature of the structure is shown in Fig. 3. It consists of a list
(LO) containing the names of all items in the vocabulary (LI--L37) . Each
In the model the vocabulary items are coded as Ll-L37, where Ll = A,
L2= B, L36 = 9 and L37 = the special respond signal. The reasonfor this code, as opposed to referring to them by their usual symbols
(A,B,C, etc.) is simply that the computer programming language (IPL-5)
does not permit the standard symbols to be used In this fashion.
1. Long-term Memory
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AUDITORY COMPONENTSP39
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VISUAL
COMPONENTSV5V7VB
■— AUDITORYCOMPONENTS
P2P?l
VISUAL COMPONENTSVIVI4Vl5
AUOITORY COMPONENTSPI9
P4O
Pl 9
VISUAL COMPONENTSV 3VlB
L37(output signal)
AUDITORY COMPONENTSP44
VISUAL COMPONENTSV24
Flgur*- 3. I.otik-Uttii Memory Structure
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item is in turn the name of a list that contains the names of two sub-
lists. The first sublist contains the auditory components that describe
the sound of the item's name. The second sublist contains the visual
components that describe what the item looks like. Hence, this list
structure, consisting of the main list of vocabulary items each of which
has two sublists containing the auditory and visual components describing
the item, represents the LTM. There is a third sublist containing
information about recoding, but discussion of that portion of LTM will be
deferred to a later section.
In this model STM has an unlimited capacity. The STM structure
consists of a number of memory cells
which holds all information about an
The cells are connected by links (to
order information. Actually, each M
(see Fig. 4) , ft. —M , each ofindividual item (a digit or letter) .be described later) which represent
location is not a single cell but a
large number of memory cells which are organized into a list structure.
The M location contains the name of the memory structure which in turn
contains all of the information about that particular item. The memory
structure for an item is shown in Fig. 5.
In Fig. 5 the name of the structure is a list of the auditory com-
An important point here is that the number of cells, n, is not aparameter of the model, but the number of items to be remembered. Thatis, the model simply uses as many memory cells as needed to storeinformation about the items.
2. Short-term Memory
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i.
ITEMMn
Figure 4. Short-term Memory Structure
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Figure 5. Short-term Memory Structure for a Single Item
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v.
ponents that describe the item. More precisely, it is a list of names
of sublists each of which contains three types of information: (1) the
name of the auditory component, a P unit; (2) the clock time at which
that component was stored in STM; and (3) a decay function, a D, speci-
fying the exponential relationship between the probability of retrieving
the component and the length of time the component has been in a store.
At this point, two important assumptions of the model should be noted:
First only the auditory components of an item are assumed to bestored
in STM; and second, the decay of the individual components of an item
is assumed to occur independently.
In the initial conception of the model both auditory and visual
components were stored in STM. A number of studies have shown that the
auditory dimensions of items plays an important role in STM (e.g.,
Cimbalo & Laughery, 1967; Conrad, 1964; Laughery, 1963; Laughery &
Pinkus, 1966; Wickelgren, 1965a). However, other evidence has appeared
which indicates that in simulating performance in a memory-span task for
digits and letters, visual components should not be a part of the
information in STM (Baddeley, 1966; Cimbalo & Laughery,1967; Laughery,
Harris & Ulbricht, 1967). For example, interference and confusions
among items being recalled seem to follow their auditory, not their visual
similarities. As a result of these studies the model was modified so that
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only auditory components are a part of STM.
Another line of evidence is relevant to the question of "What is
stored in STM?" Hintzman (1965, 1967) has reported data which indicate
that speech-motor or articulatory components may also be important. Some
comments on this alternative and the possibility of "distinctive features
will be made later in the paper.
The second characteristic of STM in the model to be noted concerns
the independent decay of the auditory components. As indicated in Fig. 5
each component has a decay function specifically associated with that
particular component. The decay function may be the same or different
for various components. However, it is assumed that the rate at which a
component decays does not depend upon the nature of the other components
nor upon the specific decay functions assigned to other components.
Bower (1967) suggested two alternative possibilities as to how com-
ponents might be forgotten. The first alternative was called "hierarchical
loss of components." The idea here was that the various components could
be ordered in importance and the retention of a component would vary
directly with its importance. Furthermore, components at one level in
the hierarchy would not be lost until the components at the lower levels
BAs8As mentioned earlier, the decision to use only auditory components inSTM is task dependent. It is not suggested that visual, articulatory
or some other dimension of information has no role in STM; but rather,
in this particular task - the memory span for digits and letters - onlythe auditory dimension is relevant.
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had been lost. The second alternative suggested by Bower was called "in-
dependent loss of components." Here, the components are equally important
and show equal resistance to forgetting. He assumed that the components
in the independent loss alternative are forgotten at the same rate.
The process suggested here is independent loss but with possibly dif-
ferent rates for the several components. In other words, some will decay
more quickly than others but this rate is independent of the rate at which
others decay. Incidentally, the decay process does not describe the pro-
bability that a component is lost forever. Rather, it describes the
retrieval probability at a particular time. For example, if a component
is not retrieved during an attempted rehearsal, it may still be retrieved
during a later rehearsal or recall (of course, at a later time the pro-
bability would be even lower) .Another issue raised by Bower (1967) regards the value of the com-
ponent after its initial value has been forgotten. Bower offered two
possibilities: the value reverts to a null state (it simply goes away);
or, the forgetting consists of replacing the original value of the component
by some incorrect, non-null state. The null-state idea is similar to the
decay notion of forgetting while replacement is more consistent with
interference. In the present model when a component is forgotten it reverts
to the null state
Finally, it seems appropriate at this point to note an alternative
forgetting mechanism that was considered in the initial formulation of
the model. Instead of having the decay function represent the probability
of retrieving a component as a function of how long it has been in memory,
the function could represent the relationship between strength of the
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component and the time in store. By defining a strength threshold below
which the component is not retrieved and above which it is retrieved,9
the same all-or-none retrieval of the component would be represented.
The routines representing the memory processes will now be described
These are the processes available to S for taking in information from
the environment, storing information in memory, retrieving information
from memory, and outputting information to the environment. Flow charts
will be used to communicate the flow of events in the model. The aim
here is to indicate what the model says about these memory processes.
After this description of the processes, a sample simulation will be
presented.
The routines that comprise the simulation program are hierarchially
organized. A general routine at the top of this hierarchy is referred
to as the Model Executive Routine (El) , and it directs the overall flow
qThe introduction of retrieval thresholds would bring the model intocontact with the recent work on decision processes in STM (the theoryof signal detectability) by Wickelgren & Norman (1966). However, theapplication of decision theory to memory does not require a continuousstrength assumption since Bembach (1967) has proposed a model basedupon decision theory that assumes a finite-state memory (two states--available and unavailable). We intend to explore these alternativesin future versions of the model.
D. Memory Processes
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L
of events in the model. This routine executes a series of subroutines
which in turn have subroutines, and so forth. The Model Executive Routine
contains subroutines which represent the experimenter, the equipment and
the subject. The experimenter routines are responsible for setting up
the sequences according to specified criteria (e.g., the types of items,
digits or letters, in the sequences and the rate at which they are to be
presented), for monitoring the clock, and for placing items into the win-
dow and removing items from the window at the appropriate times. In
short, these routines conduct the experiment.
A number of model routines that represent S will be described in
detail, and Table IV presents the program name and a descriptive title
of each. The routine with which this discussion will begin is a sub-
routine of El and is referred to as the Subject Executive Routine (SI).
The flow chart in Fig. 6 presents SI. At the time this routine is
entered the simulated clock has been set to zero and an experimenter
routine has put the first item into the window. The SI routine is
entered at the beginning of a new sequence of items and, as can be seen
from the flow chart, exited when that sequence has been output by S.
Before each new sequence is presented to the simulated S, the clock is
reset to zero and the memory structures set up for the previous sequence
are erased. This procedure is essentially a present boundary condition
of the model and precludes its representing any proactive or retroactive
interference that may occur between sequences. It is therefore, a model
of performance on single "representative" sequence
An important characteristic of the model concerns the nature of
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Table IV.
Program Name and Descriptive Title of Model Routine
Program Name
El
SI
S2
S3
S4
S5
S6
SlO
Sll
S2O
522
S4O
S4l
542
S5O
552
SlOO
SlOl
SlO5
SllO
Descriptive Title
Model Executive
Subject Executive
Input Stimulus
Is a New Item in Window?
Net Sorting
Store and Update in Short-term Memory
Respond
Interitem Activity
Find Location (M) of Next Short-term
Memory Structure
Store Basic Component
Store Link Substructure
Select Item
Generate List of Consistent Items
Generate List of Items Consistent With
Most Components
Retrieve Auditory Components of Item
from Dictionary
Update
Rehearsal
Retrieve Remembered Components of an Item
Is Rehearsal Continued?
Recoding
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Figure 6. Subject Executive Routine - SI
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the item information that has been placed in the window by the experi-
menter routines. This information is the visual or auditory components
which describe the item being presented. If the simulated experiment
involves a visual presentation mode, the window will contain some com-
bination of visual components (V's). If the presentation mode is
auditory, some combination of auditory components (P's) will be in the
window. For example, if the first item is the letter X and the pre-
sentation mode is auditory, the window will contain P24, P5 and Pll
(see Table III) .1. Noticing
From Fig. 6 it can be seen that the first process is for S to
notice or take in the information in the window. What happens depends
upon the mode of presentation being simulated. If the presentation is
auditory (the components are P's), a STM structure is set up in Ml for
each of the components of the first item. If the presentation is visual
this structure is not set up immediately since visual components are not
stored in STM. As indicated in Fig. 6, the Input Stimulus Routine, S2,
is responsible for carrying out this input process.
With auditory presentation the STM structure is set up in the fol-
lowing fashion. In serial order, each of the components is copied from
the window and combined with a time
list. The name of the list is then
the structure stored in Ml (see Fig
individual component is carried out
tag and decay function to make up a
added to another list whose name is
5) . The storing in memory of eachby a Store Basic Component Routine,
S2O. The flow chart for S2O is presented in Fig. 7. This process works
in a straightforward manner: a new sublist which will represent this
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if/ure 7. Store K'ihlc Component Koutlnc - S2Oi'
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component is generated; the name of the component is taken from the window
and placed on this sublist; the value of the simulated clock at this
instant is copied and added to this sublist; and a decay function asso-
ciated with this particular component is retrieved and added to this
sublist. The Store Basic Component Routine is a process that takes time
(a time-charge process). Consequently, at the end of the process an
increment of time is added to the clock. This time increment is one of
the basic time parameters in the model. In most of the simulation runs
carried out to date a value of 10 milliseconds has been assigned to this
parameter. (The reasons for using 10 milliseconds will be discussed
later.)
The manner in which decay functions are associated with specific
components is also straightforward. The model has a dictionary containing
the various auditory and visual components and an associated decay rate
(B constant) which describes the decay for that component when it is first
placed into STM. (In the current version of the model, of course, the
visual components do not get put into the STM) . Hence, the Store BasicComponent Routine simply retrieves from the dictionary the decay function
associated with the particular component and adds it to the sublist. As
indicated in Fig. 2 the model contains a number of decay functions which
differ in terms of the decay rate (the B constant) . Actually, any one offive different decay processes, those with B values of 0.107, 0.129, 0.160,
0.208 and 0.269, have been used as the initial values of the decay process
for the different components. The reason for assigning different decay
rates to different components is based upon some ideas expressed by
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Wickelgren (1965) . Wickelgren noted that intrusion errors in the memory-span task tend to have an auditory component (a phoneme) in common with
the correct item. He further noted that some auditory components account
for more of these acoustically similar intrusions than others, and that
this difference was related to the pronunciation time of the phoneme.
Using this idea, the auditory components in this model were classified into
five categories according to their pronunciation time as reported by
Fletcher (1953) . The decay rates were then associated with the variouscomponents in the dictionary such that the longer the pronunciation time
of the component the slower the decay rate.
Once the sublist representing the first component has been set up and
added to the STM structure, the second component is copied from the window,
a similar component substructure is set up, and this substructure added
to the STM. This process repeats for each of the auditory components in
the window. When all of the components representing the first item have
been put into STM, S2 terminates. For example, if the first item was an
X in the auditory presentation mode, the STM structure will contain the
information indicated in Fig. 8. The
different component substructures, 0,
of the processing as well as the fact
added to the clock at the end of each
time tags associated with the
10 and 20, reflect the serial nature
that the time increment that was
execution of the Store Basic
Component Routine, S2O. Also, because the time increment is added to the
clock at the end of the process, the value of the clock would be 30 when
the Input Stimulus Routine, S2, is finished.
2. Item Recognition
The only thing the model has simulated thus far is S noticing the
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STRUCTURENAMEMl
COMPONENTSUBSTRUCTURE
COMPONENTSUBSTRUCTURE
COMPONENTSUBSTRUCTURE
Figure 8. Short-term Memory Structure After Imputing First item
(Letter X)
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k
item in the window. This noticing does not include what would normally
be thought of as S recognizing the item. He has merely "heard" the item.
The next step in the model (the second box in Fig. 6) is the Net Sorting
Routine and represents a recognition or discrimination process. In gen-
eral the process consists of taking a set of components and retrieving
from the discrimination net (LTM) a vocabulary item on the basis of these
information components. The flow chart in Fig. 9 describes the Net
Sorting Routine, S4. There are two inputs to this routine: a set of
components (V's and/or P's); and a signal indicating if no response, a
blank, is a legitimate alternative in the retrieval process. The out-
put of the Net Sorting Routine is the name of a vocabulary item if it is
not a blank or an appropriate signal if it is a blank. The reason for
the input signal regarding the legitimacy of a blank is to allow the
model to represent various task-dependent processes. For example, in
the response phase of the task, when S is attempting to recall the sequence
of items, the task may require that he output as many items as were input.
In this situation the Respond Routine, which also uses this Net Sorting
Routine, would signal that a blank is not a legitimate alternative, and
the model, like S, must guess (if the item ia not remembered). In other
parts of the model using the Net Sorting Routine, such as the rehearsal
process, a blank may be legitimate alternative.
From Fig. 9, the first step in the Net Sorting Routine isa sub-
routine, S4l, which generates a list of all items in the net that are
consistent with the input components. The consistency rulehere is that
if an item contains all of the components in the input it is considered
consistent. In the example where the first item isthe letter X and the
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Figure 9. Net Sorting Routine - S4(Recognition or Discrimination Process)
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input components are the three auditory components which make up the
letter X, the S4l routine would generate a list that contained only the
letter X. However, if the item was the letter E (whose auditory descrip
tion consists of the single phoneme P2l) the S4l routine would generate
a list of items consisting of all of the letters or digits that contain
this particular phoneme (BCDEGPTVZ3) .The second step in the Net Sorting Routine asks the question "Is the
list of consistent items empty?" It is possible under certain conditions,
which will be described later, that the input to the routine may be a
set of components that is not consistent with any vocabulary items. When
such a situation occurs the list of items generated by S4l is empty, and
the model goes to routine 542 which generates a list of items that match
the greatest number of input components. For example, if there are three
input components, and no item is consistent with all of them, 542 searches
for items that are consistent with any two of the input components and
puts them on the list. If none of the items are consistent with two
components, 542 gathers up those items consistent with any one component.
At this point a list of consistent items will have been generated
and the model proceeds to the question "Is there only one item on the
list?" If the answer is YES, the input components will have defined a
unique item, and that item would be output by S4. Since this recognition
process (Net Sorting) is one of the basic processes in the model, it has
an associated time charge. This time increment is another parameter of
the model and is added to the clock value as the last step in the S4
routine. In most of the simulation runs to date a value of 100 milli-
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36
seconds has been assigned to this time parameter. (The basis for using
100 milliseconds will be discussed later.)
Returning to Fig. 9, if the answer to the question "Is there only
one item on the list?" is NO, another question is asked "Does the set
of input components exactly match a single class of components of any
vocabulary item?" For example, if the iraput components were P24 and Pll,
the list of consistent items generated by S4l would contain both S and X.
The two imput components, however, exactly match the auditory description
of S but not X. Hence, the answer to the question would be YES and the
letter S would be output. Similarly, if the input component was P2l,
the items BCDEGPTVZ3 would be on the list. The output would be the letter
E because its auditory description exactly matches the single input com-
ponent (this rule creates a particular problem for the model which will
be described later) . Note that an exact mntch with a single set of com-ponents (auditory or visual) of an item is a second way that the input
components define a unique vocabulary item.
The remainder of the Net Sorting Routine deals with the situation
where the input components do not define a unique item, the NO branch
from the last question. At this point a Select Item Routine, S4O, is
executed. Since the procedures in S4O are straightforward, its flow
chart will not be presented. Essentially what happens is the following.
If the input indicates that a blank output is legitimate, and if the
list of consistent items contains 10 or more (an arbitrary number at this
stage in the development of the model), a blank signal will be output.
On the other hand, if there are 9 or less items on the list, or a blank
is not a legitimate alternative, the S4O subroutine selects one at random
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The last step in this branch of routine S4 is to increment the clock
by a time value associated with this phase of the process. The time in-
crement added under these conditions is not the same parameter as the
time increment when the imput components define a unique item (the value
of that time increment was 100 milliseconds). When a unique item is not
defined by the input, some sort of decision process (represented by S4O)
is presumably added which will cause the overall process to take longer.
Hence, the value of the time increment in the branch of the routine con-
taining the additional selection process will be longer than the time
charge when a unique item is defined. Various times have been assigned
to this parameter in the simulation runs to date, with 300 milliseconds
being a typical value.
After the Net Sorting Routine, the next step in the Subject Executive
Routine, Fig. 6, is the question "Was the item the respond signal?"
(Remember that the respond signal is one of the items in the discrimination
net and it will be uniquely defined when theappropriate component appears
in the window.) If the answer to this question is YES,control will be
transferred to the Respond Routine, S6. Theprocedures in the Respond
Routine will be discussed later. If the output of the Net Sorting Routine
is not the respond signal, control will be transferred tothe Store and
Update in Short-term Memory Routine, S5.
In this branch of the Subject Executive Routinethe possibility that
the output from the Net Sorting Routine was a blank isnot considered. The
reason is that in this branch there is no possibility thatthe blank
alternative would have been selected. The execution ofS4 has occurred
3. Updating Item in STM
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immediately after a new item has appeared in the window, and as long as
the item is in the window and S notices it he will not fail to recognize
it. In other words, all of the components of the item are available as
input to the Net Sorting Routine and thus leads to recgonizing a unique
and appropriate item. The possibility of a blank output occurs under
other conditions which will be described shortly.
When an item has been retrieved from the discrimination net, its
auditory components are added to the STM structure for that Item by the
S5 routine. If these components are already stored in the STM structure
(as they would have been in this case by the S2 routine) , then the informa-tion regarding these components will simply be updated.
The manner in which the Store and Update in Short-term Memory Routine,
S5, proceeds is shown in Fig. 10. The inputs to this routine are the
location of the STM structure where information about the item is being
stored (one of the M' s) and the name of the item. In the example mentioned
earlier if S5 is updating the first item in the sequence, X, these inputs
would be Ml and X (actually, this latter input will be L24, the programcode for X) .
From Fig. 10, the first thing that happens in S5 is the execution of
a subroutine, SSO, which takes the name of the item to a dictionary and
retrieves the auditory components that make up that dimension of the item
(for the letter S these are P24, P5 and Pll) . The next step is a question"Is there another component?" which is simply the first step in a loop.
Each of the components on the list generated by SSO are taken one at a
time and dealt with. After all of the components have been handled the
answer to this question is NO and the routine is exited. Initially, the
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Figure 10. Store and Update in Short-term Memory Routine - S5
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answer to the question is YES since none of the components have yet been
considered. The first component is then taken from the list and the STM
structure scanned to determine if this component already exists in the
structure. In the present example the auditory components of the letter
X will have already been stored in the STM structure named in Ml, and the
answer to this question will be YES. There are other circumstances, how-
ever, where the answer may be NO. For example, if the presentation mode
in the experiment is visual, the Input Stimulus Routine, S2, does not set
up the STM structure for the item, since the auditory cues are not yet
available. Indeed, when the presentation is visual, the item must be
recognized in LTM and the auditory cues retrieved before anything can be
stored in the STM structure. Under this condition auditory components
of the new item would then be set up along with time tags and decay functions
in the STM structure by routine S2O in the manner previously described.
Note that when the presentation is visual, the auditory components stored
in STM represent a secondary code of the imput stimulus (cf. Bower, 1967).
However, when the presentation is auditory the stored components represent
the primary code of the stimulus. There are other situations in which
the components may not already exist in the memory structure. These
situations arise during the rehearsal and will be mentioned when that
process is discussed.
If the component already exists in the memory structure, control is
transferred to the Update Routine, 552. The Update Routine makes two
changes in the memory for that component. First, it replaces the time tag
in the component substructure with a copy of the current value of the clock.
In other words, the decay of the component starts anew. The second change
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concerns the nature of the decay function. As indicated in Fig. 2, there
is a family of decay functions in the model such that the various functions
differ in terms of the rate of decay. There are 30 different functions in
all. When a component is updated its decay function is replaced by a func-
tion whose rate of decay is the next slowest in the set. This change repre
sents learning in the model.
There is an additional point worth noting about the change in the
decay rate that occurs during updating. As mentioned, the Updating Routine
simply selects the function with the next slowest rate. This updating pro-
cedure is also going to occur duringrehearsal, which means that the decay
function for a component may be updated many times. However, the amount of
change between functions is not constant. The differences between the slower
decay rates is less than between the faster rates.In fact, these differ-
ences are described by an exponential function also. The result of this
procedure is that the first rehearsal contributes more to remembering an item
than the second rehearsal, which contributes more than the thirdrehearsal,
etc. This change in decay rates representsa fundamental assumption about
the memory system
The Update Routine is another ofthe basic processes in the model.
Like the other basic processes, each time theUpdate Routine is executed
a time increment is added to the clock. Sincethis process is in some
sense a storage process, the time chargeassigned to it in the runs obtained
to date has been the same as the time for the StoreBasic Component Routine,
S2O. in most of the runs the value has been 10 milliseconds.
When the S5 routine is complete, apoint has been reached where the
simulated S has noticed the item in the window,recognized it as something
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with which he is familiar and set up an appropriate STM structure for the
item. In the case where the letter X was presented and was the first item
in the sequence, the time on the clock at the end of S5 would read 160
milliseconds (30 milliseconds for the subroutines executed by S2, 100
milliseconds for the net sort in S4, and another 30 milliseconds for the
updating in S5) . As indicated in the Subject Executive Routine, Fig. 6,the next step in the process is routine S3 which asks the question "Is
a new item in the window?"
At this point in the discussion there are another two characteristics
of the model that must be clarified: When and how S monitors the window;
and, the manner in which the experimenter routines interrupt to make
changes in the window. Earlier the point was made that Sis viewed as a
serial processor. With respect to noticing new items in the window, there
are at least two procedures which might be adopted that seem consistent
with the serial assumption. The first of these procedures assumes that
S is interrupted, regardless of what he is doing, at that instant when a
new item is placed in the window. The second procedure assumes that S
actively checks the window at frequent intervals. In the second alterna-
tive, S's sequence of behavior would be interruptable only at these check-
points. The procedure adopted is the second, and the rule for when these
checkpoints will occur is that S will look for a new item in the window
every time he has finished processing a single vocabulary item. This
processing may involve taking in and storing a new item or rehearsing items
that have already occurred. Thus, in Fig. 6, the window will be checked
after the S5 routine has been executed.
4. Checking Environment
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i.
The second point concerns when the experimenter routines make changes
in the window. The procedure is quite straightforward. Each timeS
is about to check the window to see if a new item hasoccurred, an experi-
menter routine is first executed which checks to see if any window changes
should have occurred. If so, the changes are made. More precisely, this
experimenter subroutine is executed as a first step in routine S3
The question as to how S knows that an item in the window is new is
essentially finessed in the present model. In short, a signal is output
by the experimenter routine indicating whether or not the item inthe
window represents a change since the last timeS monitored the window.
Routine S3 simply senses this signal. Also, routine S3 is a time charge
process and has a time parameter indicatingthe value added to the clock
at the end of the routine (10 milliseconds in most runs) .The next step in the Subject Executive
Routine depends upon whether
or not a new item has occurred in the window. If thepresentation rate
in the experiment is very fast(e.g., 10 items per second), the answer to
this question would be YES and the simulationwould branch back to the
first box in Fig. 6, Input Stimulus,S2. If, on the other hand, the
interitem interval is longer than the time consumedby the basic time
charge processes so far executed (160 milliseconds in thecurrent example) ,
control will be transferred to the Interitem Activity Routine, SlO.
The interitem Activity Routine is anattempt to simulate what S does
between taking in one item andwaiting for the next item to appear in the
window, in this model there are two types ofactivities that occur in this
interval, rehearsal and recoding. The general procedureduring the Intcr-
5. Rehearsal
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item period is shown in Fig. 11. First, the model attempts to rehearse
all of the items that have occurred thus far, routine SIOO. The number
of items rehearsed in this model is a function of the time available for
rehearsal. If there is still time remaining after all items have been
rehearsed (a new item has not yet occurred in the window) , an attemptwill be made to recode or chunk items into larger units, routine SllO.
If after the recoding process has been executed time still remains, con-
trol will branch back to the Rehearsal Routine again. In both the Rehearsal
and Recoding Routines the simulated j> checks the window to see if a new
item has occurred each time an item has been rehearsed or considered in
the recoding process.
The nature of the rehearsal process, routine SlOO, is described by
the flow chart in Fig. 12. The first step in the process is to go to
the location of the first STM structure, Ml. In this model the simulated
S^ always knows the location of the first item--he may not remember what
the first item is, but he knows where to look for it. This knowledge
makes Ml an anchor point (Feigenbaum & Simon, 1962) . The next step isto execute routine SlOl which examines the memory structure in Ml and
retrieves all of the components that can be remembered. The procedure
in SlOl is to consider the components one at a time and determine which
This time-based procedure is different from the procedure proposed byAtkinson & Shiffrin (1968) . They propose a rehearsal buffer with acapacity to regulate the number of Items rehearsed.
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lM X ..re 11. Interitem Activity Routine - St()
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Figure 12. Rehearsal Routine - SI OO
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47
can be remembered. This determination is made by subtracting the time
value associated with the component from the current value of the clock,
which results in the length of time the component has been in memory
since being stored or last updated. This time value is then used by the
exponential decay function associated with the component to compute a
probability that the component is remembered. The process then generates
a random number between 0 and 1, and on the basis of the random number
decides that the component is or is not remembered. For example, if the
item has been in store 1 sec. and the probability of retrieval computed
by the decay function is .80 and the random number generated is .65, the
component will be remembered. If the random number is .88, the component
would not be remembered. The SlOl routine is the basic retrieval process
in the model. Each time it is executed a time increment is added to the
clock The value of this parameter in most runs has been 10 milliseconds.
The remembered set of components output by SlOl is then provided as
input to the Net Sorting Routine, S4 in Fig. 9, which attempts to retrieve
an item that is consistent with these components. As indicated in the
third block in Fig. 12, a blank is a legitimate alternative in the Rehearsal
Routine. In the example, the SlOl routine would have retrieved some number
of auditory components of the letter X from Ml (probably all of them since
very little time has lapsed since the updating) and sorted them in the
discrimination net. The next question is whether or not the output of
the S4 routine is a blank.
From Fig. 12, if the result is not a blank (i.e., an item is retrieved
from the net), the auditory components which make up the retrieved item
are added to or updated in the short-term memory structure by routine S5.
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48
If the S4 output is a blank the S5 process is skipped.
The next step in the rehearsal process is to execute the Find Location
of Next Short-term Memory Structure Routine, Sll. Having rehearsed the
first item in the sequence, X, the model will now attempt to find the item
that followed X in the sequence. Since in the example no item has yet
occurred following X, a discussion of Sll will be delayed briefly. It is
sufficient to note here that Sll is capable of sensing the fact that the
last item rehearsed was the last item presented.
At this point in the Rehearsal Routine the simulated S has finished
rehearsing a single item. Consistent with the rule cited earlier, the
window is now checked to determine if a new item has occurred. The S3
routine executes the experimenter subroutine which checks the window and
makes any appropriate changes. If a change does occur, the S3 routineoutputs a YES and the Rehearsal Routine is terminated. Control is thentransferred back to the SlO routine, Fig. 11, which asks the question "Is
activity continued?" The fact that a new item has appeared in the window
results in a NO answer to the question which in turn causes the Interitem
Activity Routine, SlO, to be exited. Control then reverts to the SubjectExecutive Routine, Fig. 6, which executes the S2 routine taking in thenew item.
Back at the Rehearsal Routine, Fig. 12, if a new item has not appearedin the window the question "Is rehearsal continued?" is asked by routine
SlO5. The answer to this question is determined in a straightforwardfashion. If any of the items presented so far have not been rehearsed(a fact which Sll has determined), routine SlO5 outputs a YES. Control
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in the Rehearsal Routine then transfers back to the SlOl routine which
attempts to retrieve the components of the next item to be rehearsed. In
this fashion the Rehearsal Routine continues to rehearse items until one
of two conditions occur; a new item appears in the window, or all of the
items presented so far have been rehearsed. If all items have been re-
hearsed the Rehearsal Routine is exited. Control is transferred back to
the Interitem Activity Routine, Fig. 11, and the question "Is activity
continued?" is asked. Since rehearsal is finished but no new item has
occurred in the window, the answer to the question will be YES and the
Recoding Routine, SllO, will be executed.
In the example where only the letter X has been presented, the Re-
hearsal Routine is exited immediately after rehearsing the X. If no new
item has yet occurred in the window the Recoding Routine is executed.
The Recoding Routine, which will be described in detail later, notes
that only one item has been presented and is exited quickly. If a new
item still has not appeared in the window, control is transferred back
to the Rehearsal Routine which again rehearses the letter X. The result
of this procedure Is that the letter X continues to be rehearsed until
a sufficient amount of time passes and a new item occurs in the window.
When the new item appears control is transferred back to the Subject
Executive Routine which, as described above, takes in the new item by
executing routine S2.
The S2 routine now creates a new memory structure whose name is
stored in M2. Since the presentation is auditory, component substructures
are set up for each auditory component of the new item. Each of these
substructures contains the name of a component, the clock value when that
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50
component was stored and an associated decay function. At this point the
last characteristic of the STM structure will be described.
6. Links and Order Information
Referring back to Fig. 5, it can be seen that in addition to the
substructures for each of the auditory components making up the item,
there is another substructure that is a
This substructure is referred to in the
substructure contains information about
were presented. The manner in which it
part of the memory for the item,
model as the link. The link
the order in which the items
works is quite straightforward.
Each time a new item occurs the S2 routine, in addition to setting up the
auditory component substructures for the new item, goes back to the structurefor the previous item and adds to it information about where the next item
is being stored. Specifically, the information added to the structureincludes the location of the new structure (in this case, M2 ), a time tagwhich is the current value of the clock, and a decay function which defines
the rate at which the link will be lost from memory. This link information
is added to the memory structure by a separate subroutine (522) of S2 inmuch the same fashion that subroutine S2O (see Fig. 7) creates substructuresfor the component information. The 522 subroutine is considered a basicprocess in the model and has the same time parameter as the S2O subroutine.In other words, this single parameter defines the time charge for creatingcomponent substructures in memory (S2O) , creating link substructures inmemory (522) and updating these substructures (552) .
From the above discussion, it can be seen that the model distinguishesitem information from order information. This distinction obviously providesa mechanism that will allow the model to produce one of the most common
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51
11findings of the memory-span procedure—the order error. An order
error occurs when S recalls the correct items but in the wrong sequence.
The distinction between order and item information is not new; both Brown
(1959) and Crossman (1960) made the distinction to account for the order
error. The decision to introduce a decay process to account for the loss
of order information is supported by results reported by Wickelgren (1967) .
are alternative mechanisms that might be considered to account
for order errors. One possibility is simply to store an input time tag
with the item information (Yntema & Trask, 1963) . This time tag wouldrepresent the time when the item was input and would not be changed by
rehearsal. The recall procedure would involve scanning the cells and
outputting the items on the basis of the input time tags. The
availability of these tags might decay, thus producing the possibility
of order errors. Conrad (1965) offers a second alternative which is
simply to have the items stored in an ordered set of bins (memory
cells) . He argues that transposition errors can be accounted for onthe basis of item errors substituting for each other. His argument,
however, requires two additional assumptions: (1) can scan the
memory cells containing the items in order (which reflects the input
order); and (2) when an item is not remembered, a substitute will be
selected from a set of alternatives that is relatively small and consists
primarily of recent items. (Incidentally, it will become clear later
in the description of the Sll routine that the fact that the items are
stored in a sequence of cells called Ml, M2, etc. has nothing to dowith the order information. The cells could be filled randomly
(M5, M2, MB, etc.) --so long as the location of a new item is linked tothe memory structure of the previous item.)
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Returning to the Subject Executive Routine, Fig. 6. After the new
item is input, its components are sorted through the discrimination net by
routine S4. The output of this routine is the name of the new item, and
the question is asked "Is this item the respond signal?" If the answer
is NO, the S5 routine updates all of the auditory components in the memory
structure for the new item, stored in M2. At this point the window is
checked to see if another new item has occurred, and, unless the pre-
sentation rate is quite fast, the answer is NO. The Interitem Activity
Routine, SlO, is then executed again.
Rehearsal Routine, SlOO (see Fig. 12)
SlO begins by executing the
Rehearsal starts with the structure
in Ml which for this example consists of the components of the letter X.
The SlOl routine retrieves as many of these components as can be remembered
and these in turn are sorted through the net by S4. If the X is remembered,
the components in the memory structure are updated by S5. Control is then
transferred to the Find Location of Next STM Structure Routine, Sll, which
can now be described in detail.7. Using Links
The Sll routine is capable of doing three things. First, it looks
at the memory structure for the item just rehearsed and attempts to find
the link information. If there is no link structure associated with thisitem, the Sll routine outputs a signal indicating that the item just re-
hearsed is the last item taken in (the situation that was mentioned earlier)
If a link structure does exist, Sll proceeds to its second step; namely,
it attempts to retrieve the location where the memory structure for thenext item is stored, it does this by going to the link structure and re-
trieving the time tag, subtracting this time tag from the present value
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of the clock to determine how long the link information has been in memory,
using this time in the decay function to calculate a probability of re-
trieval generating a random number and deciding whether or not the next
location (M 2in the example) is remembered. (Notice that the specific
name of the next location, M2, has nothing to do with whether or not it
is remembered.) If it is remembered, the location is output and the link
structure is updated. The updating is much the same as updating one of
the component substructures. The time tag is reset to the current value
of the clock, and a new decay function representing a slower decay rate
is placed in the link structure. The third phase of the Sll routine
occurs when a link structure exists but its name is not remembered.
Under these circumstances all the STM locations, M's, containing memory
structures are scanned, and those that have not yet been rehearsed during
the current rehearsal process are retrieved. One of these locations is
then selected on a random basis. In this phase of Sll no updating occurs;
also no new link is formed between the previous item and the randomly
selected location.
The last step in the Sll routine is a time charge. Actually, there
are two separate time charges in this routine. Regardless of which of
the three alternatives (no link, retrieve link, pick link randomly)
dictates the results of Sll, a basic time increment representing the re-
trieval process is added. The time parameter which defines the increment
is the same as the one used by routine SlOl (which has a value of 10
milliseconds in runs to date). In other words, the basic retrieval process
is viewed as requiring the same amount of time regardless of whether
component or link information is being retrieved. If, however, the link
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exists and is retrieved (the second alternative), an additionnl time charge
for updating the link is added. This time charge is the same as the charge
for the Update Routine, 552 (also 10 milliseconds to date).
Most of the basic processes in the model have now been described.
As one might imagine, the model continues the cycle of taking in a new
item (which includes setting up its component substructures and the link
substructure on the previous item) , sorting the new item through the dis-crimination net and updating it, and then rehearsing the items that have
already occurred, always starting with the first item in the sequence (the
item stored in Ml) . After taking in each new item and after rehearsingeach of the previous items, the window is checked. When another new item
occurs, the procedure starts over. This sequence of events (except forattempts to recode which are yet to be discussed) is continued until the new
item is the signal to respond. When the respond signal occurs control is
transferred to the Respond Routine, S6.
8. Responding
The Respond Routine is essentially the same as the Rehearsal Routine
except that an additional time charge is made after each item is retrievedfrom the net. The additional time charge represents the time it takes Sto actually output the item (say it aloud or write it on paper) . Thisrespond time charge is another time parameter in the model, and in the runs
to date a value of 500 milliseconds has been used.There is an additional characteristic of the Respond Routine that
might be mentioned. The Rehearsal Routine allows for a blank item to beoutput by the Net Sorting Routine, S4. The Respond Routine contains an
option for allowing or not allowing a blank. Computer runs where blanks
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are not permitted, represent an experimental situation where j> knows how
many items were input and is required to output the same number (if he
does not remember an item, he must guess). Where blanks are allowed the
model is presumably simulating a situation where S can output as many
items (more or less than were input) as he chooses.
Since the Respond Routine is the last step in the Subject Executive
Routine Fig. 5, the completion of S6 also causes the SI routine to be
exited. Control is then transferred back to the Model Executive Routine,
El which may do several things. If all of the sequences have been
simulated for this run, the program may simply be terminated. If there
are additional sequences to be simulated, El resets the value of the
clock to zero, erases all of the information from the memory cells rendering
them empty, sets up the next sequence to be simulated, and again executes
the Subject Executive Routine.
9. Recoding
The one remaining part of the model that has not yet been discussed
is the Recoding Routine, SllO. The concept of recoding or chunking was
first introduced by Miller (1956). It refers to the process of combining
several items into a "larger unit" which can then be processed as a
single entity. Miller has shown that the memory span for items is greater
when the items are chunked. At the current stage of the model's development,
routine SllO has not been debugged completely, and in fact has not been
included in any of the simulation runs to be discussed in this paper.
Nevertheless, some of the basic ideas as to how the recoding process fits
into the model have been developed.
In considering the nature of the recoding process, it is immediately
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obvious that some rule must exist for defining the chunk. While j>s
undoubtedly combine items into a chunk according to a great many rules,
two have been considered in the context of the present model. The first
rule is based upon meaningfulness. A meaningful chunk can be formed when
a string of items can be combined into a unit with which S is already
familiar. An obvious example is a string of letters that form a familiar
word. Another example is the sequence of digits 1492 which has meaning
to most Ss.
A second rule for defining a chunk in the model is based upon pro-
nounceability. A sequence of items can be combined into a single unit if
the string has a pronounceable name. For example, the sequence of lettersDUP can be recoded into a pronounceable chunk and processed as a single
unit. Note that words are consistent with both the pronounceability and
meaningfulness rules for recoding. Not all meaningful combinations, how-
ever, are readily formed into pronounceable chunks. Sequences such as
FBI and IBM illustrate this point.
The first effort to develop a chunking process in the model focuses
upon the pronounceability rule. That is, a set of procedures have been
spelled out that permit the model to recode strings of letters or digits
into a pronounceable chunk. This Recoding Routine, SllO, is executed by
the Interitem Activity Routine, Fig. 11, after all of the items presented
thus far have been rehearsed. Two pronounceability rules are employed
to define a recodeable sequence of items: First, any set of three digits
can be recoded into a pronounceable chunk; and second any set of three
letters where the first and third are consonants and the second is a
vowel (i.e., any nonsense syllable) can be recoded into a pronounceable
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chunk .When the Recoding Routine is executed the first thing that happens
is that the items in the STM are retrieved one at a time starting with
the item in Ml. The retrieval process here is the same as in the Rehearsal
or Respond Routines; namely, the SlOl routine retrieves the remembered
components, the S4 routine sorts the components through the net and outputs
a vocabulary item, and the S5 routine updates the memory structure. When
the items in Ml, M2, and M3have been retrieved, the recoding routine
examines them to see if they satisfy either recoding rule (i.e., they
are all digits or they form a CVC) . Incidentally, after retrieving eachof these items the window is checked to see if a new item has yet appeared.
If a new item does appear, the recoding process is immediately interrupted,
and control reverts back to the S2 routine which will then process the new
item. If either rule 1 or 2 is satisfied by the string of three items,
the three items are recoded into a pronounceable unit. If neither rule
is satisfied (e.g., all three items were consonants) the Recoding Routine
moves ahead and picks up the item in M4. Now the items contained in M2,
M 3and M4are examined to determine if they can be recoded. In this
fashion the Recoding Routine continues to consider all combinations of
three items until it finds a recodeable set (at which point it goes ahead
and recodes the items into a chunk) , or until it has considered all of
the items presented thus far, or until a new item occurs in the window
and interrupts the process.
What does it mean in the model to recode a set of three items into
a chunk? A number of things happen. The model first goes to LTM for
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each of the three items and retrieves a set of auditory components which
represents the contribution the item makes to the name of the new chunk.The reader may return to Fig. 3 and recall the remarks describing the
information in LTM. It was mentioned that a third sublist associated witheach item in LTM contains information about recoding. Figure 13 presentsan example of one of these sublists (for the letter A). Two types of
information are contained in the recoding sublist. The first is a symbolwhich indicates that the item is a digit, vowel or consonant; from this theRecoding Routine gets its information to determine if a sequence of threeitems is recodeable. The second type of information in the substructure isthe auditory components that this item contributes to the chunk.
To understand the value of this information it is necessary to observewhat the model is attempting to do. The Recoding Routine is collectinga set of auditory components for each of the three items which it will thenput together to form a set of auditory components that will describe thesound of the name of the new chunk. For example, if the sequence of itemsis DUP, the Recoding Routine goes to the recoding sublist for item D andretrieves those auditory components contributed by the letter D when itappears as the first letter in a chunk. Next, the Recoding Routine goesto the receding sublist for the letter U and retrieves the auditory com-ponents that the letter U contributes to the chunk when it is in thesecond position. Finally, it goes to the recoding sublist for P andretrieves the auditory components contributed by P when it is in thethird position. Note that vowels only contribute to forming new chunkswhen they are in the second position (a CVC) . Hence, the sublist for theletter Am Fig. 13 contains only information that concerns A' s contribution
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LO
AUDITORY COMPONENTSP39
VISUAL COMPONENTSVSV 7V 8
RECODING INFORMATION
TYPE OF ITEMDIGIT, VOWEL OR CONSONANT
AUDITORY COMPONENTS CONTRIBUTED TO CHUNK
2nd POSITIONP25
Figure 13. Recoding Information in Long-term Memory
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J
when it is in the second position. For consonants, this sublist is
broken into two parts, one containing information for when the consonant
is in the first position and another containing information for the third12position. The sublists for digits, on the other hand, contain information
regarding the auditory components contributed by the digit when it is in
any one of the three positions.
The specific set of auditory components that the Recoding Routine
gathers up for DUP is P4, P3O and Pi. This set of auditory components
defines the name of the new chunk. The question now is "What is done
with the chunk?" The answer is that several things occur. First, a new
structure in the LTM is established that represents this chunk. If this
is the first chunk formed from the sequence of items, it represents the
38th item in LTM. Unlike the other items in LTM which have three sub-
structures (visual, auditory and recoding information), the structure
representing the chunk has only an auditory substructure. Naturally
this substructure contains the names of the chunk's auditory components.
A second set of events concerns changes in the STM brought about by the
recoding process. Without going into detail, a memory structure repre-
senting the chunk is added to the location containing the memory structure
12The acoustic contribution to the chunk made by a consonant in the firstor third position may be a function of the vowel that appears in thesecond position. However, for this initial formulation of the recodingprocess such effects are being ignored.
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for the first item in the chunk. For example, if DUP were the firstthree
items in the sequence, a memory structure for the new chunk is added to
the structure stored in Ml, which is where the information about D is
located. The chunking structure does not replace the structure for the
letter D, but instead becomes another part of the overall structure in Ml.
The details of how this structure is set up will not be presented here. It
is sufficient to note that the memory structure for the chunk contains
substructures for each of the auditory components which in turn contain the
name of the component, a time tag as to when it was stored, and a decay
function describing its rate of loss. In addition, this memory structure
for the chunk contains link information which connects it to the location
of the next item in the sequence following the last item of the chunk
(in this example, M4)
It should now be possible to see how the recoding process can bring
about an improvement in the overall performance on the task. When the
model returns to the memory structure at a later time to rehearse orto
retrieve the items for responding, it starts with Ml and attempts to
retrieve the chunk. It does this by retrieving the auditory components
of the chunk that are remembered, sorting them through the net and rec-
ognizing the chunk (which is now stored in the net structure representing
LTM), updating the components of the chunk and then going to the link
structure to find the location of the next item. If successful, it finds
M 4and proceeds in the usual
fashion,
make up the chunk is less than the sum
individual items, and since in dealing
memory locations as when the items are
Since the number of components that
of the components that make up the
with a chunk there is no search for
retrieved individually (routine Sll)
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the time required to rehearse the chunk is considerably less than the
time required to rehearse, on an individual basis, the items that make
up the chunk. Thus, the existence of chunks adds considerably to the
efficiency of the overall processing. Of course, during the Respond
Routine it is necessary to decode the chunk into its three items. This
procedure has not been worked out in detail in the present version of the
model. It seems reasonable to propose a process where the auditory
components of this chunk are taken as input, the LTM scanned and items
retrieved that are consistent with these components. Such a procedure