CHAPTER 6
Working Memory Capacity
6. 1 Introduction
Working memory (WM), previously also known as short-term memory (STM), denotes the memory capable of transient preservation of information. WM is functionally different from the memory that stores historical information which is often termed as long-term memory (LTM). Richards-Ward ( 1 996) called the storage aspect of WM as Short Term Store to firstly emphasise its storing function, and secondly to avoid confusion about STM and WM. The latter point is believed to be unnecessary anymore as STM is believed to be the storage component of WM (Engle et aI., 1 999). The relationship between WM and STM is clarified in the section 6.2 . The significance of WM as key aspect to understand human cognition can be seen from the large number of documented research, ranging from some of the earlier works such as Miller ( 1 956), Atkinson and Shiffrin ( 1 968), and Baddeley and Hitch ( 1 974), to more recent investigations and debates such as Bayliss, Jarrold, Gunn, and Baddeley (2003), Bunting, Conway, and Heitz (2004), Kane and Engle (2003), and Logie, Maylor, Sala, and Smith (2004). This great body of research works offers also a great variety of perspectives on the nature of WM. Section 6.3 explores some of the characteristics of WM from research literature.
From the known characteristics of WM, manifestation of WM can be derived. Section 6.4 lists manifestations of WM. Manifestations of WM remain at an abstract level. Some of the manifestations can be translated into implementation patterns.
Different from manifestations, implementation patterns are system-definable and system-dependent. Section 6.5 presents implementation patterns obtained from translation of manifestations of WM. The translation criteria are based on characteristics of hypermedia-based learning systems designed for conceptual learning as opposed to learning procedures. We used the term "target-systems" to mean such systems. The system described in chapter 14 is an example of target system. Section 6.6 provides a summary of this chapter.
6. 2 Short Term Memory and Working Memory
The idea of short term memory (STM) emerged in the debate in cognitive psychology about a different memory system than the long term memory (LTM) (Schacter, 2000). An important trigger of this debate was the fact that amnesic patients revealed intact abilities to remember immediate short string of digits despite of having difficulties in long term retention of these digits. Miller (1 956, 1 994) called this memory system the
Working Memory Capacity
. short term memory (STM) and found that individuals were capable of remembering 5 to 9 (7+-2) chunks of digit(s) in the STM.
The view with solo emphasis on the storing aspect of the transient memory was quickly discredited by:
1 . evidences showing the tests result o f STM correlated poorly with performance on higher level tasks such as reading comprehension (Turner and Engle, 1 989); and
2. evidences showing that simple maintenance in STM was a very poor way of learning (Baddeley and Hitch, 1 974).
More and more researchers saw that working memory also includes a processing component (Baddeley and Hitch, 1 974; Daneman and Carpenter, 1 980; Baddeley,
1 992; Salthouse et aI. , 1 989).
At the time of their writing, Engle, Tuholski, Laughlin, and Conway ( 1 999) pointed out three different views regarding the relationship between STM and WM :
1 . STM i s equivalent to WM.
2. WM is a subset of STM. 3 . STM i s a subset o f WM.
In order to study the relationship of WM and STM, Engle et al. ( 1 999) used eleven
memory tasks and two general fluid intelligence tests in their study. Their analysis showed that:
1 . STM and WM reflect separate but highly related constructs; and
2. WM has a strong connection to intelligence whereas STM does not.
By taking out the common variance of STM from WM, Engle et al. (1 999) found that the residual (of WM) was highly correlated to the fluid intelligence score. It could be interpreted that the part of WM, which is uncommon to STM, is related to the fluid intelligence. Engle et al. (1 999) therefore argued that WM consists of a storage component, namely the STM, and a processing component, which is called the Central Executive (Baddeley and Hitch, 1 974). The STM is not connected to fluid intelligence whereas the Central Executive is.
6. 3 Characteristics of Working Memory Capacity
Relevant literature of different perspectives of working memory is presented in this section.
6 . 3 . 1 Structure of Working Memory Capacity
With recognition of a processing component in working memory (WM), B addeley
( 1 992) studied the structural aspect of working memory and tried to understand it by
decomposing WM into its sub-components. The structure of working memory was
described as a control-slave system comprised of the Central Executive (controlling and processing component), Phonological Loop (slave component for verbal information), and a Visual-Spatial Sketch-Pad (slave component for graphical
Working Memory Capacity information). The Central Executive takes the role of monitoring and controlling the output of the two slave systems and selecting what is relevant for potential processing. The term "potential" is used here because not all processing are consciously-directed. Metaphors used for working memory include "blackboard of the mind" (Reddy, 1 980), "mental sketch-pad" (Baddeley, 1 986), and "on-line memory" (Goldman-Rakic, 1 987). Although these metaphors capture the essential idea of the transient storage capacity of working memory, they still suggestively emphasise the role of the storage functionality of the working memory. The Central Executive, which is conceptualised as very active and responsible for the selection, ihitiation, and termination of processing routines (e.g., selecting, encoding, storing, and retrieving), is still the least understood aspect of the working memory (Baddeley, 1 986).
6 . 3 . 2 Working Memory as Operation
Baddeley ( 1 986, 1 992) studied working memory (WM) with emphasis on its structure; others took a more pragmatic approach and focused on the operational aspect of working memory (e.g. Salthouse et aI., 1 989; Daneman, & Carpenter, 1 980).
Salthouse et al. ( 1 989) proposed that working memory consists of ( 1 ) a storage capacity sensitive to the number of items presented, and (2) an operational capacity sensitive to the number of operations performed on items. Daneman and Carpenter ( 1 980) espoused the same view.
Salthouse et al. ( 1 989) found that young adults have higher operational capacity than older adults especially among the highly capable participants. Little or no difference was reported on the storage capacity across the age differences. Therefore, Salthouse et al. ( 1 989) concluded, at that time, that it was the operational capacity that causes age-related WM decline. A further study of the operational efficiency of WM showed that it was not the operational capacity (number of operations allowed) that affected the efficiency of WM, but it was actually the speed of execution (e.g. comparison speed) that determined the performance of the overall system of WM (Salthouse &
Babcock, 1 99 1 ).
Daneman and Carpenter ( 1 980) also agreed that the speed of execution was the main factor contributing to individual difference in WM and WM related performances. However, they put forwards an explanation of how quantitative differences in WM capacity (storage) could influence the qualitative differences (the execution speed). They speculated that larger the storage capacity of WM, easier it is for chunking concepts and relations into higher order units. The higher order (abstract) units in turn have positive influence on the execution speed.
The emphasis on the execution speed of the processing c�mponent in WM had already broken away from pure structuralism and turned towards functionalism. There are however, more functionalist views on WM. Atkinson and Shiffrin ( 1 968) is an example.
Working Memory Capacity
6 . 3 . 3 Working Memory as Function
In addition to the storage capacity, Atkinson and Shiffrin ( 1 968) defined WM functionally as the gateway allowing information to be transferred to the long-term memory (LTM). This definition stresses the ability to channel the incoming isolated information (as received by our senses) to the semantically networked structure in the LTM. This involves a great degree of cognitive efforts such as interpretation, translation, association, memorisation, and so on. This functionality is comparable to the Central Execution unit mentioned above and essentially it transforms and transfers the messages from the short-term storage system into the long-term one. The transformation process invokes the formation of rules (data with operational application) from pure data (in the form of incoming messages) and the transferring process filters what rules and data are to be stored for long-term and what are to be discarded.
From this point of view, efficient WM contributes to better memorising into and
retrieval from L TM, and vice versa for poor WM.
6 . 3 . 4 Working Memory and Age
Several studies have shown that age-related performance of young children and old adults, compared with young adults, can be characterised by the inability to retain information in WM while simultaneously processing other information (Laumann 1 999; Case, 1 995; Verhaeghen & Salthouse, 1 997; Salthouse & Babcock, 1 99 1 ; Kausler, 1 99 1 ). Deficiencies in WM capacity result in different performances in a variety of tasks and can be seen in the difference in the score of WM test (Laumann 1 999). Examples of tasks affected include natural language use (comprehension, production, etc.), recognition of declarative memory, skill acquisition, new vocabulary learning and so on (Byme, 1 996).
Campbell and Charness ( 1990) also reported age-related difference in WM performance. They used arithmetic operations that required the participants to perform long sequence of calculations involving seven steps. Older adults committed more errors and responded slower than younger adults. Age-related differences in
error rates were greatest at the stages where the loads on WM were high. The loads on
WM were particularly higher in later steps of the sequence of calculations than in
earlier steps (Campbell and Charness, 1 990).
6 . 3. 5 Working Memory and Navigational Pattern
An empirical study by Huai (2000) showed that students with holistic learning style also have a significantly smaller short-term memory but have remarkably higher learning effect in the long run, whereas the students with serial learning style (highly
capable to follow and remember sequentially fixed information) have better short term memory but poorer learning result in the long run.
This point shows the intricate relationship between humans' inbuilt abilities and how different learning styles are adopted to circumvent any deficiencies in those abilities.
Working Memory Capacity The navigational strategies adopted by serial learners are linear whereas holists sometimes do better by jumping directly to complex concepts (Felder, 1 988).
6 . 3 . 6 Working Memory and the Fan E ffect
Fan effect is a name given to the observed phenomena that the time required to verify a proposition encoded in long-term memory increases with the number of total propositions committed to long term memory (Anderson 1 983a, 1 983b). A proposition is a statement in the form like "The LA WYER is in the PARK". The fan effect experimental procedure was described in Cantor and Engle ( 1 993). Participants, who involved in fan-effect experiments, are presented and required to learn simple propositions (statements). The number of other propositions sharing the same concepts (LAWYER and PARK) is experimentally manipulated, and is called the fan size of the proposition. For example, if there are large numbers of propositions that have the term "PARK", then the fan-sizes of these propositions are said to be large.
A verification test is then given. "The LAWYER is in the BANK?" is an example of questions in the verification test. Test result of Cantor and Engle ( 1 993) showed that response time and error rate were greater for propositions with greater fan-sizes than
propositions with smaller fan-sizes. This phenomenon is called the fan effect.
The fan effect seems contradictory at the first sight: experts, who have greater number of propositions (knowledge) in a domain, should be faster in verifying a proposition than novices whereas novices should spend longer time during the verification test. It appears that the experimentally-observed fan effect is inconsistent to the generally expected expert/novice performance. Cantor and Engle ( 1 993) used the mental-model concept to explain this contradiction.
Experts have already had elaborated links between the propositions and thus an integrated mental model representation. The integrated mental model has the information of all the propositions. It serves as a single unit of access during the verification test. Because accessing one mental model is faster than accessing a set of propositions individually, experts can therefore perform the verification task faster than novices. Therefore, in experts, no fan effect or negative fan effect appears (Cantor and Engle, 1 993)
Cantor and Engle ( 1 993) also point out that the constituent propositions must be closely related in order for the mental model to emerge. By using causal- and/or temporal-relationship, propositions can be combined into a single theme and therefore an integrated mental model. When proportions are unrelated, it is difficult for the development of a mental model, and therefore fan effect can still be observed.
One important concept during the discussion of fan effect, which is generally related to long-term memory, is activation (Anderson 1 983a, 1 983b). Activation of one concept causes automatic activation of associated concepts and the total amount of activation is limited. In other words, the greater number of associations activated, the lower their activation levels will be. That is, the more associations to share the limited amount of activations, the less each association can get. Only when the activation level exceeds a certain threshold, a concept can surface to consciousness.
Working Memory Capacity
Even though not directly connected, the concept of activation described by Anderson (1 983a, 1 983b) is in accordance with the findings of Mednick ( 1 962). In a study asking participants to do free-association, Mednick found that the greater associative response rate correlated to lower total number of associations. For example, a participant might answer "chair, study, dinner, wood, etc." very quickly when the target associative object is "table", the total number of items this participant answered is smaller than others who give answer slower. More details about Mednick's study are presented in chapter 8 along with associative divergent learning
Based on the Anderson's ( 1 983a, 1 983b) concept of activation, Cantor and Engle ( 1 993) hypothesized that the individual difference in total amount of activation could
be related to individual difference in work memory (WM) task. This is because of the
attention-switching feature in WM task.
In the WM task called OSP AN (e.g. Turner & Engle, 1 989; OSP AN is described in more detail in the chapter 10 about Web-OSP AN), a participant is first required to solve an arithmetic operation (e.g. Is 3 x 5 = 1 2), then presented a word to be remembered (e.g. Tree). Attention has to be switched from the operation task to the memory task. After seeing the word, another arithmetic operation is presented, followed by another word to be remembered. Cantor and Engle ( 1 993) viewed that when the participants were working on the operation, the word did not receive any attention (low activation level) and therefore needed to be re-activated when it needed to be written down. Therefore, Cantor and Engle ( 1 993) argued that the amount of available activations in LTM determined the capacity of WM.
Based on the equivalence of available activations in LTM and WM, Cantor and Engle ( 1 993) further hypothesized that high WM participants should be more capable of integrating the propositions into a mental model than low WM participants, therefore negative fan-effect should be observed in high WM participants whereas fan-effect should be observed in low WM participants.
In an experimental study using fan-effect material that varied in thematic relatedness (how easy the propositions were integrated into a mental model), Cantor and Engle ( 1 993) were able to demonstrate the correlations between high WM and negative fan effect, and 10w WM and fan-effect.
6 . 3 . 7 Working Memory and Attentional Control
Attention control has been mentioned as a function of the Central Executive in WM (Baddeley, 1 992). Engle et al. ( 1999) viewed WM as the content of short-term memory plus the limited-capacity controlled-attention processes associated with the Central Executive. Goode, Goddard, and Pascual-Leone (2002) also adopted the view of capacity plus attentional control view of WM, but they focused more on the inhabitation aspect of attentional control, i.e. the ability to ignore irrelevance or distraction.
Kane and Engle (2003) used a task, called the Stroop task, to show the relationship of
attention control and WM. The Stroop task requires the participants to read out the
.)
I
Working Memory Capacity colour of the text while at the same time, the text is also a name of a colour. If the colour of the text and the name of the colour in the text match, it is called a congruent
trial, and otherwise it is called an incongruent trial. In the task, the goal of the
participants is to 1 ) read out the colour of the text and 2) ignore the word. However, the task is designed in such a way that congruent trials outnumbered the incongruent trials. So it is easy for the participants to forget to keep attention on the second goal
and just rely on the text to answer. Kane and Engle (2003) showed that the high WM
participants made fewer errors and responded faster in the Stroop task than their low
WM counterparts. This result demonstrated that the high WM participants were more
able to control their attention than the low WM participants.
Logie et al. (2004) used the term perspective memory to refer to goal maintenance. One is said to have good perspective memory if one can resist distraction and
forgetting of one's goal. In consistence with the study of Kane and Engle (2003),
Logie et al. found high WM correlated to high perspective memory and low WM
correlated to low perspective memory. Logie et al. also showed that the failure of perspective memory (i.e. forgetting the intention to do things) in old age is due to
inefficiency in WM.
6 . 3 . 8 Working Memory and Interference
Bunting, Conway and Heitz (2004) argued against Cantor and Engle's (1 993) proposal to equate working memory to the total amount of available activation in long term memory. By replicating the experiment of Cantor and Engle ( 1 993) in addition to three more experiments with varying degrees of interference (e.g. multiple persons in same location, or same person in mUltiple locations), Bunting et al. (2004) showed and asserted that the ability to resist interference, not the limited amount of activation, was the critical determinant of individual differences in the fan effect. Interference
showed a higher detrimental effect on low WM participants than on high WM
participants.
The interference-resistance view of WM had also been reported in earlier studies. As new items enter into working memory, other items become harder to access, and the cognitive system becomes less efficient. This is called the displacement or interference (Byme, 1996; Baddeley, 1 986). Less tolerance of displacement or interference indicates sign of low working memory capacity.
6 . 3 . 9 Working Memory and Domain/Task Specificity
An important question is whether WM capacity is domain dependent or domain independent. Most of the evidences gathered indicate the latter.
Daneman and Carpenter ( 1 980) speculated that the individual difference in reading comprehension should be attributed to the difference in efficiency of processing component in WM, in particular the difference of efficiency in processing textual
information. Daneman and Carpenter devised a test of WM called the reading span task. In the reading span task, participants read groups of sentences aloud while simultaneously trying to remember the last word of each sentence. They argued that measure of working memory capacity (in this case, the score of reading span task) is
Working Memory Capacity task-specific (i.e. to reading task) because 1 ) the processing and storage components in working memory compete for limited resource, 2) good readers have better reading strategies that lead to less requirement on processing and hence more resources are