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Working Memory: A Multi-Component Model
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Msc Human Cognitive Neuropsychology
The School of Philosophy, Psychology and Language Sciences,
University of Edinburgh
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2010
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Abstract
Previous studies have reported that combining two memory tasks or varying the level of cognitive
demand in a task does not cause disruption in healthy adults’ performance. To examine whether a dual
memory task would be more sensitive to the manipulation of cognitive load, a experiment was set in
which recall of digits sequences was combined with recall of visual patterns. The results suggested
that different levels of cognitive load do not affect the task being performed simultaneously, when
retaining sequences of digits in memory while encoding and retrieving visual patterns and vice-eversa. These data challenge the assumption of a single limited attentional capacity, which prevents a
trade-off of capacity between two concurrent tasks. However, this data can be easily explained by a
multi-component model of working memory.
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Table of Contents
1. Introduction .......................................................................................................................... 5
1.1 Working memory and its different approaches ................................................................ 5
1.2 The multi-component working memory model ................................................................ 8
1.3 Coordination operation in dual tasks .............................................................................. 16
1.4 Aims and predictions of this study ................................................................................. 18
2 Method ................................................................................................................................. 22
2 .1 Participants .................................................................................................................... 22
2.2 Stimuli ............................................................................................................................ 22
2.2.2 Serial Digits recall.................................................................................................... 22
2.2.3 Visual pattern recall ................................................................................................. 22
2.3 Procedure ........................................................................................................................ 23
3 Results .................................................................................................................................. 28
3.1 Digit delayed recall x visual pattern immediate recall…………………………… ….. 27
3.2 Visual pattern delayed recall x digit sequence immediate recall…………………………..27
4. Discussion ............................................................................................................................ 33
References ............................................................................................................................... 39
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1. Introduction
1.1 Working Memory and Its Different Approaches
Working memory refers to on-line cognition- the monitoring, processing and
maintenance of information (Shah and Miyake, 1999). It is comprised of the functional
components of cognition responsible for comprehending and forming mental
representations about immediate environment, retaining information about immediate past
experience, supporting acquisition of the new knowledge, problem-solving, and to
formulating and acting upon specific goals (Hitch & Baddeley, 2007). A current debate in
working memory research is about the nature of working memory. In order to determine
the nature of working memory it is a key point to understand how cognitive functions and
their impact on other forms of information are managed in the human brain. One focus of
discussion is whether working memory can be used as a general unitary concept (WMG)
or as a specific model (WMS). These different approaches have resulted in two research
traditions, each one with different perspectives on the nature, structure and functions of
working memory. According to the WMG (e.g. Cowan, 2001), working memory capacity
is limited by a general flexible attentional resource that can be shared, whereas in WMS
working memory is comprised of multiple independent domains and cognitive resources
can operate in parallel (Baddeley & Logie, 1999).
A way to tackle how working memory resources are allocated is through dual task
methodology. The ability of individuals to perform more than one task at the same time
has been one of the ongoing debates within the memory and attention literature. It is a
very ecologically valid test as in real life situations people find themselves holding
information in memory while performing a different task (e.g. talking while walking,
cooking breakfast or thinking about directions while driving). Different theorists diverge
in opinion about dual task ability.
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Within the theories in agreement with WMG model are the Time-Based ResourceSharing (TBRS) model (Barrouillet, Bernardin and Camos, 2004; Barrouilet & Camos,
2009) which proposes a general-purpose pool of limited resources that is shared between
distinct cognitive activities. To avoid the decay, the information needs to be refreshed by
attentional focus, otherwise it will be lost over a certain period of time. This pool of
resources is usually interpreted as attention and it serves as a ‘bottleneck’ that switches
between one activity to another in order to maintain the information during the dual task.
Cognitive load is defined as the proportion of time during which attention is being
captured by an activity, thereby impeding memory traces to decay (Barrouillet & Camos,
2009). The central attentional resource is thought to be shared between mental processes
regardless of the nature of the information involved. In this view, verbal and visuospatial
activities are assumed to compete for a common pool of domain-general limited resources,
resulting in interference between such activities when they are performed together.
Vergauwe, Barrouillet and Camos (2010) designed an experiment to examine the dual task
effects between different domains. The storage tasks were the letter span- a series of
consonants to be recalled (verbal storage) and the location span - a red square that
appeared in different locations on matrices (visuo-spatial storage). For the processing
tasks, two choice-reaction time tasks were used. For the verbal task, the participants were
presented a range of words and asked to judge whether or not they were animal names
(i.e., ‘elephant’). For the visual task, boxes were presented together with a horizontal line
(varying in length) and two squares dots (differently positioned in relation to the
horizontal line) and between the dots there was a gap. Participants were required to judge
whether or not the line could fit into the gap. The increasing cognitive load of both verbal
and visuospatial processing disrupted verbal storage and verbal recall was consistently
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poorer when it was combined with verbal processing than when it was combined with
visuo-spatial processing (Vergauwe, Barrouillet & Camos, 2010).
Visual recall performance (i.e. memory for coloured discs) has been shown to be
disrupted by concurrent visual activities such as a tone-pitch discrimination task.
Increasing the attentional demands on the discrimination task resulted in a larger dual task
decrement, indicating an existence of a common pool of domain-general resources.
Although increasing the cognitive load of either verbal or visuospatial processing
disrupted verbal storage, verbal recall performance was poorer when it was combined with
verbal processing than when it was combined with visual processing. It is assumed by the
authors (Vergauwe, Barrouillet & Camos, 2010) of the study that a domain-specific
interference is explained by the fact that verbal information is maintained by two
independent mechanisms: attentional refresher and articulatory rehearsal. In this case,
while visuospatial information interfered only with the attentional refresher, verbal
processing interfered with both mechanisms.
Another model that accounts for the unitary working memory is the model
proposed by Cowan (1999, 2001). In this view, working memory is an activation of items
from long-term memory. Different codes are processed according to the same principles.
Not only does the capacity-limited focus of attention contribute to recall, but also the time
or interference-limited sources of activation of long-term memory. Cowan (2001)
suggested that some information can also be held in the focus of attention while other is
stored passively. The information that is focus of attention can be adversely affected by a
capacity limit, although the information that is passively stored is constrained by temporal
limits and interference factor. The focus of attention shifts between different sources from
time to time and such an operation prevents the capacity of central storage to be
overloaded. Then, in the dual task it is possible to recall contents from one task and then
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shift attention towards another task, consequently activating memory representations of
this latter task and recalling the content of it. Morey and Cowan (2005) combined visual
and verbal stimuli. The visual task consisted of arrays of coloured-squares, then a second
array with one of the squares circled was presented and the participant judged if the
circled square was the same colour as it was in the first array presented. If the response
was positive, they should type the S key, if was not, they should type the D key. In one of
the trials participants rehearse the numbers in silence and in another they rehearsed aloud.
The addition of the spoken rehearsal produced dramatic interference; it was though that
attention was ‘called away’ from the visual task to engage in recalling the digits
preventing the encoding of the visual array. This interpretation was confirmed by a second
experiment (Morey and Cowan, 2005). In this second experiment they repeated the same
task but used articulatory suppression of unrelated items instead of rehearsal numbers and
found no effects on the visual task.
1.2 The Multi-Component Working Memory Model
The concept of Working Memory was developed and empirically tested by Hitch
and Baddeley (1974). This model comprises three temporary storage systems, namely, the
phonological loop, the visual sketch pad and the episodic buffer (with the latter being a
more general integrated storage system) and an attentional controller the central executive.
According to Baddeley and Hitch (2007) the phonological loop is involved in verbal
processes and is assumed to have a limited capacity available, which depends on the
amount of memory activation available. This subsystem has been fractionated into a
passive phonological store and an active rehearsal process. “The phonological store
maintains items through repetition or registers visually presented material within the store
by a process of articulatory naming” (Baddeley & Hitch, 2007, p.4). The rehearsal process
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is responsible for refreshing the decaying representations in the phonological store
(Baddeley, 2007).
The visual sketchpad stores and processes visuospatial information and is
comprised of a passive visual cache and an active spatially based system - the inner scribe
(Logie, 1995). The visual cache retains visual patterns while the inner scribe retains
sequences of movements. The episodic buffer forms an interface between the three
working memory subsystems and long-term memory. It binds perceptual information from
the subsystems and from long-term memory. The term ‘buffer’ has to do with how it
provides an interface between a number of different codes - visual, verbal, perceptual and
from long-term memory both episodic and semantic (Baddeley, 2007).
The central executive itself is not unified and is assumed to oversee a range of
control processes such as controlling the subsidiary systems, which allow for the selection
and implementation of strategies (Logie, Osaka & D’Esposito, 2007). It helps coordinate
combined tasks as well as dealing with processing (Duff and Logie, 2001). Furthermore,
the central executive is involved with some aspects of attention such as shifting attention
between tasks as well as distributing and focusing attention in a dual task. Other processes
that this system is involved with are monitoring and updating the representations of
working memory, inhibiting dominant responses and building information from the
subsidiary systems and from long-term memory (Baddeley, Baddeley, Bucks and
Wilcock, 2001).
The non-unitary nature of the model proposed by hitch and Baddeley (1974) is
supported by the dual task paradigm (Baddeley, Logie, Bressi, Della Sala & Spinnler,
1986). Experimental paradigms used by Baddeley and colleagues compared individual’s
performance in a single task condition to individual’s performance in a dual task
condition. This comparison only is possible because each participant score is defined
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before dual task being performed. Then dual task is set according to individual span (Della
Sala, Foley, Beschin, Allerhand & Logie, in press). The use of a span procedure
guarantees that the individual is exhausting his or her maximum capacity for a specific
task (Duff & Logie, 2001). In these dual task paradigms each one of the concurrent task is
thought to tap one of the two subsidiary systems which means that the tasks pertain each
one to one of each different domains. The nature of the two tasks performed must be taken
into consideration since both tasks need to rely on different resources in order to avoid
disruption in activity in dual tasking. Following this requirements, the dual task paradigm
can be performed by healthy adults with little decrement in dual task (Baddeley et al.,
1986, Cocchini, Logie, Della Sala, MacPherson & Baddeley, 2002; Logie, Cocchini, Della
Sala & Baddeley, 2004; Macpherson, Della Sala, Logie & wilcock, 2007, Sebastian,
Menor & Elousa, 2006; Duff & Logie, 2001). The fact that the dual tasks do not disrupt
the performance in the main task has led working memory to be associated with multiple
domain-specific systems. The codes used in the dual task are different and is suggested to
employ different form of resources. More specifically, digit recall is thought to involve the
phonological loop whereas tracking is thought to tap the visuo-spatial sketchpad
(Baddeley, 2007). Also, the use of selective interference paradigms have reinforced the
idea of a working memory system characterized by separated pool of resources, in that
tasks pertaining at the same domain are combined. The results showed that two tasks
pertaining to the same domain can not be supported, causing disruption on the tasks.
(Baddeley, Zuco & Baddeley, 1990).
In a study conducted by Baddeley et al. (1986) digit sequences recall was
presented
simultaneously with perceptuomotor tracking of a randomly moving target.
This test was devised to compare dual task performance within healthy young adults,
healthy aging adults, and Alzheimer’s disease (AD) patients. The dual task requirement
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resulted in about 15% drop in the performance of each task for young and healthy aging; a
decrement considered to be small if participants are under a very demanding task because
they are using their maximum capacity for each task. Only AD patients were penalised
and this was associated with a dysfunction in attentional control within the central
executive of working memory. From this point on, much attention has been paid to
research in how patients with Alzheimer’s disease perform under dual task conditions
which led to the idea for a separation in the attentional controller of working memory
(Baddley & Della Sala, 1996). AD patients presented a specific dual task coordination
function which reinforced the view that executive control in working memory cannot be
characterized by a unified system; in other words it is not general-purpose control
mechanism but it reflects more than one process in the healthy brain (Baddeley, 1996;
Baddeley, Emslie, Kolodny & Duncan, 1998; Baddeley et al., 2001).
A possible caveat in the paradigm tracking and digit recall was that a tracking task
was supported by a perceptuomotor system which could indicate a perceptuomotor system
separated from working memory and not a distinct system within working memory. To
examine this issue, Cocchini et al. (2002) compared the performance of young adults on
two different memory tasks, namely, digit recall (immediate verbal memory) and visual
pattern recall (immediate visuo-spatial memory). The results did not suggest consistent
reductions in performance in the verbal or in the visual when performed concurrently.
Immediate recall of numbers dropped around 8% when a visual pattern preload was
retained and an immediate recall of a visual pattern dropped by around 5% when a digit
preload was retained. Those effects were significant and were not predicted by a multicomponent model of working memory, although the decrements were considered small
when considered that both tasks were performed under a very demanding cognitive load;
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the large drop in dual-task performance that would be predicted by a model that assumes a
single general purpose memory system was not observed.
In this study, a preload procedure was utilised with the participant being required
to encode material (e.g. digit sequences) and after 15 seconds recall it. In the interval of 15
seconds, the participant should encode material, but for immediate recall. For this reason,
the immediate recall of content was compared to delayed recall of the same content. One
interesting result was observed in this experiment: when comparing performance on
delayed and immediate condition in dual task digit recall in the delayed condition was
poorer than at immediate recall - even if the participants were free to rehearse the digits
during the delay. To ensure that the preload task was not being encoded in the long term
memory, while participants were performing the interpolated task, articulatory suppression
was employed as a secondary task. In the articulatory suppression task participants were
asked to repeat aloud irrelevant content such as the word “go” for 15 seconds. Articulatory
suppression has proved to disrupt immediate verbal memory and due to this fact, it is
suggested that long-term memory does not support the retention of the verbal material.
Instead, it is assumed that the retention of digit sequences rely on subvocal rehearsal
within the phonological loop component of working memory, and repeating aloud a
random word affects the rehearsal which in turn prevents information decay. It is also
important to compare the size of disruption caused by the articulatory suppression to the
size of disruption caused by the dual task conducted in this experiment. The size of the
disruption caused by ariculatory suppression reflected how the disruption resulted from
the dual task cannot be considered robust (Cocchini et al., 2002). If it is expected that two
different codes are not being processed by different domains the disruption caused by a
dual task should be the same or similar regardless of the nature of the tasks being used in
the dual task. Indeed, the single resource model would require additional assumptions
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about how the current state of one task was maintained while attention was focused on the
other task and vice versa. The same pattern of results was found in a study combining two
memory tasks comparing the performance from three groups: healthy young adults, older
young adults and Alzheimer’s disease patients (Macpherson, et al., 2007). Healthy older
adults and young adults were not reliably impaired in the dual task; participants recalled
significantly more digits during delayed digit recall than digit recall combined with
articulatory suppression and visual patterns. Also, participants recalled fewer digits during
digit recall combined with visual pattern compared to digit recall combined with tracking,
indicating that a combination of two memories tasks is more demanding than combining a
memory task with a perceptuomotor task.
Arguably, if a single attentional resource is divided between two tasks then varying
the demand of one task should produce a significant effect on another secondary task. In a
single attentional working memory model if one task overloads memory resources then
there is less spare capacity to process the other task which could influence an individual’s
performance. Logie, et al. (2004) used the dual task paradigm in which the demand of the
one task was fixed while the demand of the other task was varied. The dual task was
consisted of digits recall and tracking. In this study they compared 3 groups: Alzheimer’s
patients, healthy adults and normal aging. The dual task performance in both conditions
(i.e., tracking fixed demand concurrently with varied demand of digits length and fixed
length of digits sequence concurrently with varied demand of tracking) was very similar to
that found for performance of each respective single task in the healthy groups. This
translates to the non effect of the second task upon the first task. Participants could not be
performing one task at ceiling; leaving extra capacity to support the other task since the
experiment was designed using the individual span for the fixed level which assured that
the individual was using his or her maximum capacity. Many levels of load were tested at
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the same level both below and above the level of maximum capacity. Even when the
demands of the concomitant tasks were much lower than individual ability level, a dual
task decrement was not found.
These results were replicated with another combination of tasks by Logie, Della
Sala, MacPherson and Cooper (2007). In this study, the differences between younger and
older participants were examined in a dual task combining digit recall with different speed
levels on a response time (RT) task. In this task participants were asked to press a
response key when an asterisk presented in the middle of the computer screen; the levels
of demand were defined according to the time between the response and the next stimulus.
Although dual task during encoding produced significant interference in memory
performance, digit recall performance did not change according to the different levels of
demand on the second task, which confirmed that increasing the level of one task does not
leave spare capacity or that a high demand second task disrupts the first task. The young
participants dual task performance was not influenced by the level of complexity of the
tasks - tasks being performed at very low demand or at very high demand for each
participant. The effect in encoding was consistent with previous experiments – a
perceptuomotor task can undermine the verbal storage task during encoding (e.g. NavehBenjamim, Guez & Marom, 2003).
Duff and Logie (2001) also tackled the evidence of a multi-component model in an
experiment using a processing and storage dual task combination. A multi-component
model supposes that in a task that involves processing and storage with the former being
dealt with by the phonological loop or sketchpad, while processing is dealt by the central
executive. In this experiment, the ‘working memory span’ was the measure, whereby the
participant read lists of sentences and decided if the sentence was coherent or not and
memorised the last word for each sentence (Daneman & Carpenter, 1980). As in this task
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the level of complexity increases in complexity and the final words of each sentence need
to be remembered in serial ordered recall, this task is assumed to involve different levels
of on-line processing and different levels of temporary memory load. A single attentional
model of working memory interprets this task as a trade off between memory and
processing tasks. That is, more complex sentences demand more working memory
resources leaving less spare capacity for retention of the final words. However, for a
multi-component model of working memory, the system responsible for processing and
the systems responsible for storing are considered as entirely separate entities. In this
particular experiment, before of performing the dual task, the participants performed both
tasks (checking the sentences meaning and recalling of list s of words) in a single
condition to determine an individual span for each task. Next, in a dual task condition,
participants were asked to perform the sentence checking and final sentence word
recalling. There was an increase in the overall performance on dual task which was not
above 40%. This decrement was associated with an overload on the central executive. In
addition, the speed allowed for encoding also increased when the number of words to be
remembered increased. The speed increasing for encode was not a feature of the single
task which makes it more likely that that a substantial dual task decrement in memory
span will be observed.
There was no evidence of a trade off between resources. If both tasks were being
performed at an individual’s maximum capacity, than each task should be utilizing 100%
of the supply of resources available for the task. In the case of a single attentional
capacity, when combining the two very demanding tasks, one of the tasks would be
prioritise and as a consequence performance in the other task would be undermined. The
fact that participants can perform two tasks in a dual task condition and achieve almost the
same result as when performing these tasks in a single task condition is difficult to
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conciliate with a single pool of resources. Showing that processing task and memory task
can be performed at the same time was interpreted as a temporary storage independent of
processing system. (Duff & Logie, 2001).
To explore a task switching mechanism operating in the dual task a second
experiment articulated a dual task in which processing task included items that were more
unrelated to the items included in the storage task. More specifically, the dual task
combined arithmetic verification and recalling presented words. If a task switching
mechanisms was operating in this dual task, then it would cost less for a single limited
capacity system if the items included in these tasks were more similar between each other
in comparison to each task including tasks less related between each other. The fact that
such an experiment resulted in a smaller decrement than in the previous experiment, in
which the material used on the tasks were related to each other, serves as evidence that the
dual task performance is not necessarily supported by a task switching account.
Conversely, more unrelated tasks are an advantage for the multi-component model as
there would a whole pool of resources for supporting each different task contents. In this
case, the purer the task (e.g. a task that involves just visual information rather than a task
that uses visual information but can also involve verbal coding) the better the dual task
performance will be.
1.3 Coordination Operation in Dual Tasks
One reasonable explanation for the modest decrement in performance on the dual
task can be offered by the possible different strategies used by participants for recalling
the items in the task. A study conducted by Logie, Della Sala, Laiacona, Chalmers, &
Wynn, (1996) showed that despite the fact that subjects were expected to use verbal
rehearsal and phonological coding in order to retain the items, they may use different
strategies to recall material which can interfere with the results of an experiment. For
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example, if subjects used a semantic strategy such as a visual mnemonic or a lexical
strategy to retain material this would undermine the performance as this subject is using
cognitive resources other than the phonological loop. As such, when performing this task
concurrently with a visual task there will be disruption in performance since the task is
relying on the same resource and consequently overusing its capacity.
Also, the drop in performance observed may be a part of a working memory
system that is needed to coordinate two systems operating simultaneously. One of the
functions that the central executive is responsible for is the dual task coordination of so
called ‘slave systems’ (Baddeley et al., 1986, Cocchini et al., 2004). The minimal effect
found in the dual task could be thought as a result of the central executive being required
for processing and for some storage function as well, leading this system to be overloaded.
However, how the operation of theses independent resources might be coordinated is still
is not clear yet.
An additional cost, observed in the dual memory tasks, relates to a coordination
function implemented when a person is encoding and retrieving material that is held
within specialized memory resources (Duff & Logie, 2001). “In the single-task condition,
the encoding and retrieval processes have to deal only with one kind of material (e.g.
verbal) and one type of memory resource (e.g. phonological loop). In the dual task
condition, the verbal material would be held in a verbal buffer and the visual material in a
visual buffer” (Cocchini et al., p.1093-1094). For instance, take the case of a preload
procedure with two memory tasks in which the preload task is A and intervening task is B.
The coordinating mechanism is involved with the encoding from buffer in task A, then
encoding and retrieval from the buffer from the task B, followed by the retrieval from the
task A buffer. In a dual task, performance depends on the efficiency of both the storage
mechanism and the coordination mechanism, which would require extra to the
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coordination device (Cocchini et al., 2002 ). Such a cost is due this swapping between
buffers. In agreement, Baddeley, Emmslie, Kolodny & Duncan (1998) suggested that
memory span tasks need encoding, retention and retrieval on a constant repetition over the
performance of the dual task. The memory task costs because is not just the phonological
loop that is involved in retention, but also executive resources are required for operating
the repeated encoding and retrieval processes.
Barnard (1999) contributed with an explanation for the coordination task saying
that working memory involves specific connections and flow between components
resources and multiple stores preserving information in different codes. In the model
proposed by Barnard (1999) the equivalent to a central executive is responsible for
fulfilling their executive functions in complex tasks as well as generating specific codes
(i.e., acoustic or visuo-spatial) according to the nature of the stimuli. This means seriating
or organizing the underlying content of thought into linguistic or spatial codes before
sending the information to the correspondent systems.
1.4 Aims and Predictions of this Study
Previous studies (Baddeley et al., 1986; Baddeley et al., 2001; Cocchini et al.,
2002; Logie et al., 2004; MacPherson et al., 2007) have showed that two tasks can be
performed with no decrement which is entirely compatible with a multi-component
working memory view. These results were found when two memory tasks were combined
or when levels of demand are manipulated. A remaining issue is whether the dual memory
task in addition to the manipulation of the cognitive level will reproduce the same pattern
of results as previous studies. Thus, the present study included the same paradigm in
which the dual memory task paradigms and were used such as the studies conducted by
Cocchini et al. (2002) and Macpherson et al. (2007). This paradigm was extended to the
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manipulation of cognitive levels of demand such as in the study conducted by Logie et al.
(2004).
According to a multi-component model, a trade-off should be observed only when
storage or processing of one activity involves the same domain information and different
cognitive loads in one of the tasks should not disrupt the other task. However, if a domaingeneral resource is shared between verbal and visuo-spatial activities recall performance
should be affected by changes in cognitive load. The prediction made for this study was
that increasing or decreasing of cognitive load would not interfere with recall of material
fixed at participant span (delayed recall).
To examine this prediction, an experiment was designed in which a dual memory
task was performed when one of the tasks was maintained at span while the other task has
its demand manipulated through five different levels of complexity, ranging from two
levels below the individual span to two levels above the individual span. However,
differently of the study conducted by Logie et al. (2004), the comparison made was among
young adults rather than among Alzheimer’s patients with healthy older adults and young
adults. The demand level on the fixed-demand task was set at each individual’s maximum
capacity before the dual task condition. The use of individual span indicates that the
individual is performing his or her maximum capacity and this prevent the possibility that
the scopes of the experiment were undermined by the fact that individual had spared
capacity to be spent on the second task. Therefore, the task is considerate at a perfect
level, since it is not too difficult which would lead to a floor performance and it is not too
easy, in this case leading to a ceiling performance.
For this study a preload procedure was used; participants were required to encode
and hold in memory the material for one task (preload task), followed by the presentation
of a different memory stimulus to be encoded and recalled immediately. Next participant
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was asked to recall the preload task. A preload procedure was used with the intent to
prevent that any adverse effect found is due to a dual task product and not caused by
competition for sensory input. In combining two memory tasks in the present experiment,
it was essential to prevent that competition for sensory input – for example, having o listen
to digits and watch a visual pattern simultaneously, could cause the encoding to fail.
According to Cocchini et al. (2002), two stimuli being encoded at the same time can
disrupt the task, only to be competing for the same input channels. Then a task
performance could be affected by this competition and not due to a lack of sufficient
resources to deal with the tasks. A preload procedure voids this competition.
The activity to tap into the visual domain was visual patterns. Tose were designed
based on the Visual Patterns Test –VPT (Della Sala, Gray, Baddeley, Allamano & Wilson,
1999). This test is assumed to measure the non-verbal aspect of working memory. The
non-verbal-memory aspects of working memory were divided into visual and spatial
components (Logie, 1995). Therefore, the VPT was designed to obtain a more focused
measure of the visual component of visuospatial memory, rather than measuring visual
and spatial memory in working memory. A study carried out by (Della Sala, et al., 1999)
showed that The correlation between the span of visual pattern and Corsi Block Test -test
that is thought to measure the spatial component of visuo-spatial memory, since it requires
memory for position of blocks (Corsi, 1972) - is low, suggesting that the tests were
measuring different functions. In the same study, the use of selective interference
paradigm showed that Corsi Block test and visual pattern test were disrupted when spatial
and visual secondary tasks, respectively, were performed during a delay interval. This
suggests that the tests were measuring different functions. In the same study, the use of
selective interference paradigm showed that Corsi Block test and visual pattern test were
disrupted when spatial and visual secondary tasks were performed during a delay interval,
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respectivally. This confirms that the visual pattern task is involved with the visual domain
and Corsi Block test was involved with the spatial doamin.
The other task utilised in this study was a digit span task; according to Baddeley
(2007) digit span is a task that requires working memory for its recall. For Baddley (2007)
digit span is assumed to be dealt by the verbal domain of working memory and
substantially confirmed by empirical evidence. It is thought to be heavily supported by
working memory due to the lack of effective strategies to support encoding (Logie et al.,
2007). Besides, sequences of digits do not involve any semantic or lexical information
which make it unlike to involve long-term memory (Baddeley, 2007; Logie et al, 2007).
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2 Method
2 .1 Participants
Participants were originally 32 native English-speakers university students and
they signed a consent form. This offered the explanation about the procedure and about
the rights of the participant.
2.2 Stimuli
2.2.2 Serial digits recall.
Participants listened to a list of digits recorded by a female native English speaker
at a rate of one digit per second. Immediately after presentation participants were asked to
recall the digits orally in the order of presentation. Following correct recall of two out of
three sequences, the sequence length was increased by one digit and the procedure
continued. No restrictions were imposed in time to recall. Participants were presented with
three sequences at each length, and testing ceased when the participant failed to recall at
least two out of the three sequences. Individual span was taken as the longest length of
sequence for which the participant recalled at least two out of three sequences correctly.
The dependent variable was the percentage of correctly recalled digits in the correct
position.
2.2.3 Visual pattern recall.
The visual pattern test consisted of matrices containing varying arrangements of
opened and filled squares. The matrices patterns were designed to be difficult to encode
verbally. The vast majority of the patterns contained squares of 2.5 x 2.5 cm in size with
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the exception of the 2 x 11-squared matrix that contained 2x2 cm squares due to space
limitations. These patterns were randomly-generated in squares filling (in black) half of
the squares. The participants were presented in a card containing a matrix for three
seconds. Next, the pattern was removed and participants were given a same size blank
matrix in which they had to mark with a cross the previously filled cells. Following the
correct recall of one out of three patterns at any given level; complexity was increased by
adding two squares at a time. The grids steadily progressed in size, from the smallest, a
2X2 matrix (with two filled cells) to the largest, 5X6 matrix (with 15 filled cells).
The
individual visual pattern span was taken as the mean of the last three patterns for which
filled squares were correctly recalled.
2.23 Procedure
The testing session took approximately 30 minutes. As soon as the participant
arrived they were asked to read the information sheet about the study and to sign the
consent form. The procedure was explained and the session finished after they completed
all the phases of the experiment.
Participants of this study were specifically asked to use subvocal rehearsal in the
retention of the digit sequences. For the visual pattern recall the instructions highlighted
the use of visual encoding. In the digits sequence recall task participants were encouraged
to recall the sequences in the right order but to recall the digits even if they could not
remember the exact sequence. This was made in order to preserve the right order of the
sequence. For example, if the participants were presented sequence containing the
numbers “three-four-five” but could not remember the number 5 they should say “three, I
do not remember, five”.
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The material was comprised of three different sets for visual pattern test: one for
the span checking, one for the fixed level and one for the varied level. The same was
devised for the digit span. None of the patterns or the sequences of numbers were
repeated.
Digit span and visual pattern test were assessed in three conditions: 1) single-task,
2) dual-task in a fixed level of difficulty and 3) dual-task in a varied level of difficulty. For
the dual task condition, participants were exposed to one task for delayed recall and the
second task for immediate recall. The number of patterns or sequences in the interpolated
task was relative to how many patterns could be fitted within 15 seconds. The number of
lists for each participant varied depending on the length of their digit span, and the
performance measure was the proportion of digits recalled correctly. Most of the time ,
participants had only one visual pattern or digit sequence as intervening task, as most of
participants had too long sequences length or too high complexity level of patterns to be
recalled within the 15 seconds available for the immediate task
The experiment comprised four phases. In the first phase of the experiment, digit
recall in a standard condition was assessed to determine their individual digit span. In the
second phase visual pattern recall in a standard condition was assessed to determine their
individual visual span. In the third phase the level of demand for the first task (e.g. digit
span) was fixed at individual span, whereas the demand of the second task (e.g. visual
pattern) was varied the five levels of demand. The fixed condition was for delayed recall
and the varied condition was for immediate recall for both of the tasks. There were five
different sequences at span to be combined different levels of the second task. For the
varied demand condition, five different levels of demand patterns comprising levels very
low level (i.e. Span-2), low level ( i.e. Span-1), standard (at Span), high (i.e. Span +1),
and very high (Span+2) – span refers to individual span for each person calculated in each
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case. For example: digit sequence at span was presented. While the sequence was being
held in memory a series of visual pattern at a given level were showed for immediate
recall during 15 seconds. In most of the cases only one visual pattern could be displayed
for encoding and immediate recall during the 15 seconds period.
After that, the
participant was asked to recall the sequence of numbers. In the fourth phase, the level of
demand for the first task (visual pattern) was fixed at span for each participant whereas the
demand of the second task (digit span) was varied through the five levels of demand in the
same way as was done in the third phase. The design of dual task is shown in detail in
Figure 1.
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Figure 1. Dual task structure.
Encoding
at span
Span -2
Recall
recall
15 seconds
Encoding
at span
Span -1
Recall
recall
15 seconds
Encoding
at span
At
Span
Recall
recall
15 seconds
Encoding
at span
Span+1
Recall
recall
15 seconds
Encoding
at span
Span+2
Recall
recall
15 seconds
Note: Five different structures for the task in the dual task.
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The order of the experiment phases were counterbalanced across subjects. Four
groups of participants were formed each group containing one quarter of the total number
of participants. Each group followed a different order of phases: Group 1) phase 1, phase
2, phase 3 and phase 4, Group 2) phase 2, phase 1, phase 3, and phase 4, Group 3) phase
1, phase 2, phase 4, phase 3, Group 4) phase 2, phase 1, phase 3, phase 4.
Recall for the digits task was recorded on an audio cassette for subsequent scoring.
In order to facilitate the process of correction of the visual pattern tasks each pattern had a
reference number and on the answer sheet the blanked patterns; each square was marked
with a number. Each participant had an answer sheet filled with the numbers corresponded
to the squares marked by the participant which were copied for subsequent correction. The
example of correction method is shown in Figure 2.
Figure 2.
1
2
3
4
Right answer: 2-3
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3 Results
The data of two participants could not be used, remaining then 30 participants,
comprised of 14 females and 16 males. Their mean age was 22.9 years old (SD = 2.6),
lowest age 20 and higher 29. They were paid a modest honorarium for taking part
As the order that participants was counterbalanced (the experiment could be taken
in four different orders). An one-way ANOVA was conducted to observe whether the
order that participants accomplished interfered on their performance on each condition.
The was no significant interaction between the four conditions and order: Digit delayed
recall [F (12, 104) =1.49 MSE = 606.55, p>.001]; digit immediate recall [F (12,104) =.97,
MSE = 328.85, p>.001]; visual pattern delayed recall [F (12,104) = 1.01, MSE = 470.75,
p>.001]; and visual pattern immediate recall [F(12,104) =.61, MSE = 460.75, p>.001],
indicating that participants were not beneficiating of a possible training effect.
The Table 1 shows the mean scores for digit span checking and for visual pattern
span checking.
Table 1.
Digit Span
6.86
Visual
Pattern
10.6
Overall scores for each participant in each condition were analysed and differences
among conditions were evaluated via analyses of variance (ANOVA).
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3.1 Digit delayed recall x visual pattern immediate recall
There was no significant effect of dual task demand for digits delayed recall [F
(4,116) =1.05, MSE = 637.6 p>.001] which means that maintaining digits in memory was
not disrupted by encoding and recalling visual patterns (see Figure 1). The effect size was
.035.
Relation between Time and Participants Scores
120
110
100
90
85.28
77.5
78.02
80
76.97
72.1
70
digit delayed
60
50
40
30
20
1
2
3
4
5
Figure 1. Digit sequence means scores for delayed condition, interpolated by visual
pattern immediate recall. The trend for delayed digit recall is reliably constant, while
the visual pattern varied through the five different levels of demand.
Figure 2 shows a significant decrement on the performance of visual pattern as the
levels of demand increased [F (4,116) = 1.38, MSE = 308.82, p <.001]. Participants
showed a total drop in performance of 9.7% between the lowest (81.6%) demand and the
highest (91.3%) demand conditions. The effect size was 0.367, indicating that this results
can be extended to a bigger size of population. The effect size was .046.
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Relation between Time and Participants Scores
120
110
100
87.51
91.35
85.83
83.43
90
80
81.59
70
visual pattern
immediate
recall
60
50
40
30
20
1
2
3
4
5
Figure 2. Means scores for visual pattern immediate recall. Increasing the the level of visual
pattern produced a decrement in participants perfornce in this task.
3.2 Visual pattern delayed recall x digit sequence immediate recall
There was no significant effect of dual task for visual pattern delayed recall [F
(4,116) =.94, MSE = 461.4, p>.001] which means that maintaining visual patterns in
memory was not disrupted by encoding and recalling sequences of digits. It can be
observed in Figure 3 that the trend for visual pattern delayed recall is reliably constant,
whereas the digit immediate recall trend varied through the five different levels of
demand. The effect size was 0.32.
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Relation between Time and Participants Scores
120
110
100
90
80.36
80
80.32
79.67
73.59
72.73
70
visual pattern delayed
recall
60
50
40
30
20
1
2
3
4
5
Figure 3. Visual pattern means scores for delayed condition, interpolated by visual digits
sequences immediate recall. The trend for delayed visual patter recall is reliably constant,
while the visual patterns varied through the five different levels of demand.
There was a significant decrement on the performance as the level of demand on
digit sequences increased [F (4116) = 16.8, MSE = 327.79, p <.001]. Participants showed
a total drop in performance of 32.5% between the lowest (66.2%) demand and the highest
(98.7%) demand conditions (see Figure 4).
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Relation between Time and Participants Scores
120
110
100
98.88
98.66
90
89.03
83.03
80
70
66.16
digit sequences
immediate recall
60
50
40
30
20
1
2
3
4
5
Figure 4. Means scores for digit sequences immediate recall. Increasing the the level of digit
sequences produced a decrement in participants perfornce in this task.
It can be observed from the trends of Figure 1 and Figure 2 that the trends are not
showing opposite directions, showing no evidence for a trade off between the two tasks
presented here. The same can be verified from Figure 3 and Figure 4.
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4. Discussion
Previous studies have shown that two concurrent memory tasks can be performed
by healthy adults with minimal impact on the performance on either task (Cocchini, 2002;
MacPherson, 2004). Also, previous studies have shown that varying the demand of one of
the tasks involved in the dual task doesn’t affect the performance in those tasks (Logie,
2004). These findings seem to indicate the operation of distinct, specialized cognitive
resources for each task. However, there is still an ongoing debate about whether or not
working memory is characterized by multiple specialized systems or by a non-specialized
indistinct system. The dual task is a reliable way to tackle this issue and varying the
demand of the cognitive load on a dual task experiment allows exploring in detail how
resources interact with the demand from cognitive tasks (Logie et al., 2004; Logie et al.,
2007). The present study aimed to examine how resources are allocated during the dual
task when the demands of one of the tasks are manipulated. The population chosen to be
part of the experiment was healthy adults since most of literature concerning the dual task
is related to Alzheimer’s disease. It is equally as important to conduct more studies to
explore dual task capacity in the healthy brain.
The results reported in the present study show that varying the level of complexity
of one task did not affect the performance in the concurrent task. The experiment showed
an overall reduction in performance for participants with increasing task demand.
However, no evidence was found that manipulating the load of one task affected
performance on the other task. These findings are difficult to conciliate with a single
general-limited capacity, since this would be very sensitive to a manipulation of the
cognitive demand.
This is in accordance with a multiple-component model of working memory
(Baddeley, 2007) and this model can offer a very reasonable interpretation of the data
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found here. Digit span task is processed by the phonological loop and the visual patterns
by the visual sketchpad. As both systems have distinct resources available for each task,
the loads used for processing one activity did not affect the processing for the other
activity.
More specifically, the prediction for the operation of the tasks presented in this
experiment is that when participants were holding digit sequences in their memory as well
as Vergauwe al. (2010) encoding and recalling visual patterns bellow they span, they had
the capacity for verbal material being exhausted while the capacity for visual material
processing was not being totally used. This is explained by the fact that performance is
below span and there was still residual capacity spared, which made that participants
performed quite well in these tasks having very high scores. When the level of demand
was set at span a non significant small decrement was observed in their performance,
which can be explained by two very demanding tasks being performed at the same time
and the additional cost needed for a coordination operation between those tasks (Cocchini
et al, 2002; Duff & Logie, 2001; Baddeley et al, 1998). Then, the levels of visual
performance immediate recall were increased. This made the scores decrease, as the load
was higher than visual sketch pad could support. The resources were not enough to deal
with the demanding task but there were no sign that this affected the performance in the
verbal task, suggesting that each system conserve their own pool of resources. There was
no sign of a trade off between those verbal and visual processing Vergauwe al. (2010)
suggesting that. This pattern of results is very similar to that found in previous studies
(Baddeley et al., 1986, Cocchini, et al., 2002; Logie, et al., 2004; Macpherson, et al., 2007,
Sebastian et al., 2006; Duff & Logie, 2001). Cochini et al and Macpherson et al. (2004)
proposed that people can perform two memory tasks without decrement in the tasks is
possible. In addition, when Logie et al. (2004) and Logie et al. (2007) manipulated the
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demand of tasks in the dual task no decrement on the coupled task was found; this seems
to reflect the fact that two different activities utilised independent resource pools.
An interesting factor that appeared was regarding the trend found on the visual
patterns immediate recall task. Those did not show the sharp decrease showed by digit
sequences immediate recall. This could be explained by some of the more complex
patterns in the immediate condition recall being more difficult to recall than the other
pattern across the task, making participants achieving same proportion in the scores than
they achieved in the less complex patterns.
It could be argued that participants were not performing at their maximum span
when their individual span was checked and they were putting more effort when
performing the dual task. However, the means for digit sequence was similar to Cocchini
et al (2002) and the means for visual pattern were higher than the ones found Cocchini et
al., 2004. The results cannot be explained by participants not performing at their
maximum capacity (e.g. the span checking procedure were not sensitive due to
participants putting more effort on dual ask), since the results showed that performance
was constant and the increase of the task demand disrupted as the task demand increased.
Vergauwe al. (2010) also varied the demands of two dual tasks combining visual
storage and processing tasks and two verbal storage and processing tasks, although
contrary to other studies and the experiment reported here, it was observed decrement in
participants performance. These divergent findings may be regarded to differences
between the procedures that were used by different authors. The tasks included in the dual
need to draw upon different domains of working memory. The tasks used by Vergauwe et
al. (2010) are said to be supported by different domains. However semantic judgement, for
example, is prone to involve visual content (some participants use metal imagery to
remember the elements for semantic judgement and use verbal coding to support visual
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task, when retaining the name of the colours for example ain colours). Those tests are not
empirically proved to use the codes they are intending to. Conversely, the Digit Span Task
is considered to measure verbal capacity and empirical bases support it. This task was
validated as a technique that measures short working memory and was exhausted tested
and confirmed as depending on the phonological loop component of working memory (for
a review see Baddeley, 2007). Besides, visual patterns were proven to be an important tool
to measure visual memory (Della Sala et al., 1999; Logie et al., 1990).
Morey and Cowan (2005) further do not use tasks that seem to be purely
measuring they domain intent to (a task is said to measure visual capacity, but it involves
other stimuli, other codes, such as verbal). For instance, the task used for Cowan and
Morey (2005) involved colours and the some participants could easily be using verbal
coding to support this visual task.
The current experiment procedure was designed to avoid any kind of
contamination from stimuli that would require cognitive processes that not the ones being
required by the tasks. This is what did not happen in the Cowan and Morey (2005), they
presented a cross pattern in between two tasks to signalise the start of another task and
participants had to pres keys with the letters experiment. Yet, these authors do not titrate
the task for individuals’ performance. The disruption caused by the dual task condition
could mean that that level of difficulty in one of the tasks is above individual level and the
disruption is being caused simply due to overload of the individual limited capacity for
each specific domain.
Cowan’s (2001) theory may interpret the results reported here in terms of a single
pool of resources, with the preload task causing temporary activation that gradually
decayed the memory preload involves temporary activation that gradually decays while
the attentional system is focused on the interpolated task. During delayed recall, attention
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is then focused on retrieving items from decaying traces. So participants held a preload
content (verbal or visual) in the passively held store while the contents in the intervening
task were being focused by attention. However Cowan’s theory would find some difficulty
to explain how individual performing a task in a very low level (two below their span) did
not left extra capacity to be used on the task that was demanding more capacity to be dealt
with the second task In this case, the task one or two levels above individuals span. If a
trade off between two tasks within a single limited capacity model was to be considered
the results pattern should show opposite trends which means that as performance in one
task increased performance in the other task would decrease. On the contrary, the data
suggest a multi-component working memory model.
The multi-component working memory model is also supported by studies related
to individual differences. In a study examining individual ability in verbal and spatial
processing and storage complex span tasks, Bayliss, Jarrold, Gunn and Baddeley (2003)
found that individual differences in processing efficiency and storage capacity each
account for unique variance in complex span tasks performance. This means that
processing and storage are independent, since they offer independent constraints on
complex span tasks performance. Accordingly, Hitch and Hutton (2001) found that
arithmetic and verbal tasks were explored in children and the results showed that reading
span and operation span accounted for unique variance in reading attainment, reflecting a
separation of resources for tow different domains
A possible caveat for the current experiment is that the visual patterns used for the
visual task were not validated before, therefore, the same experiment redesign taking into
consideration these particularities in future experiment. Also, as the experiment was made
on paper and the time was controlled by the experimenter, it does not offer the same time
accuracy time as a computer-paced programme could offer. However, it does not
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invalidate the findings provided by this experiment. A more accurate measure of time,
using a computer paced program would provide a more reliable study. Thus, a possibility
for a future study is to reproduce this experiment, although in this new study using a
validate pattern test (i.e. the visual Pattern test- reference) and a computer-paced program.
In conclusion, the experiment reported that when two tasks are performed when
one of the tasks is set at a fixed level and the other task has its demand manipulated, there
was no trade off between two different activities (i.e., visual and verbal). The results
found in this study have important implications for the theoretical matters of working
memory, which is the definition about if working memory is better characterised as
comprising a single limited capacity or independent pool of resources that can operate in
parallel. The fact that the level of one task does not place a constraint on the other task
reflects the operation of domain-specific memory systems with independent pools of
resources - as predicted by the multi-component working memory model. Conversely,
theories that assume a single general-limited capacity would find difficulty in explaining
why the demand manipulation of one task did not produce any effect in the other task. The
present study added more evidence about distinct systems containing independent pool of
resources that can operate in parallel.
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