Baddeley and Hitch emphasise that their short term memory system is essentially an active, working system, not just a ‘route’ to long term memory as Atkinson and Shiffrin (1968) considered it. Each of the three components have limited capacity and can work independently from each other. In this, two tasks can only be successfully be performed together if they use the different components. On the basis of this assumption, much research has been carried out on working memory using dual task studies. These studies require participants to perform a task, which primarily utilises either the phonological loop or the visuo-spatial sketchpad. Distracting tasks are then required to be performed concurrently, utilising each component of working memory. The effects of these concurrent tasks are then assessed. The reading span task (Daneman and Carpenter, 1980) is also a widely used experimental technique used to explore working memory. As with other tasks in this area, the reading span task requires the participant to maintain some information in memory, whilst simultaneously manipulating other data.
The most commonly researched area of working memory is the phonological loop. The phonological loop consists a passive store which holds speech-based information, all be it for very limited times before it decays, and an articulatory control process through sub-vocal rehearsal, prevents this decay and is also responsible for converting written stimuli into its phonological form for storage. It is sometimes referred to as an inner ear.
Reasons for the intense investigation in this area are varied, but include the ease with which the presentation of both visual and auditory words can be controlled. Also the phonological can be accessed both directly, with auditory presentation, and indirectly with a visual representation. Some brain damaged patients present with one component damaged, and the other not, lending themselves as ideal candidates for studies in this area.
Due to the levels of research in this area, several theories have been proposed, evaluated and verified, in this supporting the notion of the existence of the loop.
Baddeley, Thompson and Buchanan (1975) found that an individual’s ability to recall a sequence of words was poorer for long words than for short, the word length effect. The working memory model accounts for this in that the longer the word, the longer it takes to articulate, preventing the rehearsal and storage of other words.
In tasks designed to explore the articulatory suppression effect, in addition to recalling presented words, Participants are required to perform a distractor - repeating the word ‘the’ or the digits 1 to 8, while doing the main task. The number of items recalled falls when the distractor task is employed. This is explained in terms of working memory, in that the distractor articulated material fills the articulatory loop, preventing rehearsal of the information that is to be remembered. This also eliminates in word length effect as it is hugely dependent on the rate of articulation of the to be remembered items.
With the irrelevant speech effect, it was found that speech-like sounds, regardless of language, speed, normal or reversed speech, presented concurrently with the to be remembered items in a recall task, impaired the ability of participants to perform the task. This led to the theory that speech sounds have select access to the phonological store, and can disrupt the memory trace for items to be recalled.
The visuo-spatial sketchpad has been studied to a lesser extent. It is used in the temporary storage and manipulation if spatial and visual information. It is very much independent of the phonological loop, and so can make spatial based decision whilst the loop is active. It is again comprised of a passive store and an active control process, preventing decay through the rehearsal process. What research has been carried out on the visuo-spatial sketchpad, has also employed the dual-task methodology. One study by Baddeley et al, (1975) used the Brooks matrix task, where participant imagined a 4x4 matrix. The locations of digits within this matrix were then described to them, by means that were easy to visualise, or nonsensical. the participants were then asked to recall the instructions. This task was combined with the pursuit rotar task (were the participant is required to track a light moving around a circular track). In this case, recall in the easily visualised task was greatly impaired, but was not impacted in the event of nonsensical instructions. The model accounts for this effect in that the pursuit motor task also involves visual perception, thus interfering with performance on the easily visualised Brook’s matrix task.
Baddeley (2000) himself felt it necessary to add an additional component to his original model (1974, 1990). The added component was the episodic buffer. To summarise, the model initially identified had the central executive, involved with general processing, and both slave systems, the visuo-spatial sketchpad and phonological loop to permit the processing and temporary storage of specific information only. The concept of the episodic buffer was to act as a general storage system were several kinds of information can be combined.
Finally, in Baddeley’s (1990) model, there is the central executive, resembling an attentional system, is the most significant and versatile component of working memory. However this is also the least well researched part of the model. As Baddeley (1996) later acknowledged the
initial concept of the central executive was very vague, essentially combing many complex tasks such as planning, strategy selection, retrieval checking etc. The main functions of the central executive have been defined as switching of retrieval plans, time sharing in dual-task studies, attending to particular stimuli whilst ignoring others and short term activation of short term memory.
In terms of neuropsychological support for the working memory model, the evidence for the phonological loop is most convincing. Some brain-damaged patients with very poor short term memories do not exhibit either the word length or phonological similarity effects. this could indicate a damage to the phonological store. patients who have lost the ability to speak, but still possess language processing skills do show both word length and phonological similarity effects. An fMRI study (Henson, Burgess and Frith, 2000) carried out on healthy volunteers broadly supported previous scans on brain damaged patients, where separate areas of the barin were involved with storage and rehearsal.
Overall the working memory model has both advantages and disadvantages over the original short term memory system proposed by Atkinson and Shiffrin (1968). Whereas short term memory is essentially passive, with attended information moving through it, from sensory memory to long term memory, the working memory provides a model with both active processing and passive storage mechanisms, more conducive to the complex ongoing tasks such as conversation.
The tripartite nature of working memory would explain how some brain damaged patients display a partial deficit in short term memory. If the damage effects only one of the three components then the selective deficit would be expected.
On the negative side, more recent research has argued that the visuo-spatial component of working memory, should be further sub-divided. Logie (1995) proposed 2 different areas, the visual cache, storing visual information such as form and colour, and the inner scribe, dealing with spatial and movement information. This is backed by the findings of Baddeley and Lieberman, 1980) who found when visual and spatial tasks are performed together, little interference occurs. Some brain damage patients appear to display damage to the visual component but not to the spatial area. What remains to be determined is to discern how information and processing from both components can be integrated in tasks were they are used in combination.
The role played by the central executive in working memory is largely unclear. By definition it is of limited capacity, but attempts to measure this capacity have been unsuccessful. It is claimed that the central executive is used for various processing operations but the limitations on its functioning are still unknown. The central executive was also presumed unitary, but as further research is completed, it may be comprised of several different executive mechanisms. As admitted by Baddeley and Hitch (2000), “the central executive is the least understood component” of the working memory model, and until more is known, the working memory model appears incomplete.
References
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Baddeley, A. D. (1986). Working Memory. Oxford University Press, New York.
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Logie, R.H. (1995). Visuo-spatial working memory. Hove, UK: Psychology Press.
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