Baddeley's model of working memory

The central executive is a flexible system responsible for the control and regulation of cognitive processes. It directs focus and targets information, making working memory and long-term memory work together. It can be thought of as a supervisory system that controls cognitive processes, making sure the short-term store is actively working, and intervenes when they go astray and prevents distractions.[4]

It has the following functions:

• updating and coding incoming information and replacing old information
• binding information from a number of sources into coherent episodes
• coordination of the slave systems
• shifting between tasks or retrieval strategies
• inhibition, suppressing dominant or automatic responses[4]
• selective attention

The central executive has two main systems: the visuo-spatial sketchpad, for visual information, and the phonological loop, for verbal information.[5]

Using the dual-task paradigm, Baddeley and Erses have found, for instance, that patients with Alzheimer's dementia are impaired when performing multiple tasks simultaneously, even when the difficulty of the individual tasks is adapted to their abilities.[6] Two tasks include a memory tasks and a tracking task. Individual actions are completed well, but as the Alzheimer's becomes more prominent in a patient, performing two or more actions becomes more and more difficult. This research has shown the deteriorating of the central executive in individuals with Alzheimer's.[7]

Recent research on executive functions suggests that the 'central' executive is not as central as conceived in the Baddeley & Hitch model. Rather, there seem to be separate executive functions that can vary largely independently between individuals and can be selectively impaired or spared by brain damage.[8]

Baddeley's model of the phonological loop

The phonological loop (or articulatory loop) as a whole deals with sound or phonological information. It consists of two parts: a short-term phonological store with auditory memory traces that are subject to rapid decay and an articulatory rehearsal component (sometimes called the articulatory loop) that can revive the memory traces.

Any auditory verbal information is assumed to enter automatically into the phonological store. Visually presented language can be transformed into phonological code by silent articulation and thereby be encoded into the phonological store. This transformation is facilitated by the articulatory control process. The phonological store acts as an "inner ear", remembering speech sounds in their temporal order, whilst the articulatory process acts as an "inner voice" and repeats the series of words (or other speech elements) on a loop to prevent them from decaying. The phonological loop may play a key role in the acquisition of vocabulary, particularly in the early childhood years.[9] It may also be vital for learning a second language.

Five main findings provide evidence for the phonological loop:

1. The effect of phonological similarity:
   Lists of words that sound similar are more difficult to remember than words that sound different. Semantic similarity (similarity of meaning) has comparatively little effect, supporting the assumption that verbal information is coded largely phonologically in working memory.[10]
2. The effect of articulatory suppression:
   Memory for verbal material is impaired when people are asked to say something irrelevant aloud. This is assumed to block the articulatory rehearsal process, leading memory traces in the phonological loop to decay.[11]
3. Transfer of information between codes:
   With visually presented items, adults usually name and sub-vocally rehearse them, so the information is transferred from a visual to an auditory encoding. Articulatory suppression prevents this transfer, and in that case the above-mentioned effect of phonological similarity is erased for visually presented items.[12]
4. Neuropsychological evidence:
   A defective phonological store explains the behavior of patients with a specific deficit in phonological short-term memory. Aphasic patients with developmental verbal dyspraxia are unable to set up the speech motor codes necessary for articulation, caused by a deficiency of the articulatory rehearsal process.[13]
5. On the other hand, patients with dysarthria, whose speech problems are secondary, show a normal capacity for rehearsal. This suggests that it is the subvocal rehearsing that is crucial.[14]

An accumulation of literature across decades has lent strong support to the theory of phonological STS. In a 1971 study, Stephen Madigan demonstrated that a larger recency effect is seen during forward serial recall when people are presented a list auditorally as opposed to visually. (A smaller effect is seen in backwards serial recall.)[15] In his study, auditory presentation led to greater recall of the most recently studied items. Catherine Penney expanded on this discovery to observe that modality effects can also be found in the case of free recall tasks.[16] In 1965, Dallett had discovered that this observed modality effect is greatly reduced by the addition of a "suffix" item to the presented list; this suffix is a distractor item that is not to be recalled.[17] Robert Greene utilized this observation in 1987 to discover that this suffix effect has a larger impact on lists learned auditorally as opposed to visually.[18] The culmination of all of these findings results in strong support of the theory that there is a short-term store that phonologically stores recently learned items. In addition, Bloom and Watkins found that the suffix effect is greatly diminished when the suffix is not interpreted as linguistic sound, which agrees with the phonological short term store theory as it would be largely unaffected by non-linguistic distractors.[19]

Alan Baddeley's theory of working memory has yet another aspect to which memory can be stored short term. The visuo-spatial sketchpad is the store that holds visual information for manipulation.[20] The visuo-spatial sketchpad is thought to be its own storage of working memory in that it does not interfere with the short term processes of the phonological loop. In research, it has been found that the visuo-spatial sketchpad can work simultaneously with the phonological loop to process both auditory and visual stimuli without either of the processes affecting the efficacy of the other.[21] Baddeley re-defined the theory of short-term memory as a working memory to explain this phenomenon. In the original theory of short-term memory, it is understood that a person only has one store of immediate information processing which could only hold a total of 7 items plus or minus two items to be stored in a very short period of time, sometimes a matter of seconds. The digit-span test is a perfect example of a measurement for classically defined short-term memory. Essentially, if one is not able to encode the 7 plus or minus two items within a few minutes by finding an existing association for the information to be transferred into long-term memory, then the information is lost and never encoded.[22]

However, visuo-spatial short-term memory can retain visual and/or spatial information over brief periods of time.[22] When this memory is in use, individuals are able to momentarily create and revisit a mental image that can be manipulated in complex or difficult tasks of spatial orientation. There are some who have disparities in the areas of the brain that allow for this to happen from different types of brain damage.[21] There can also be a misunderstanding here in the differences between transient memories such as the visual sensory memory. A transient memory is merely a fleeting type of sensory memory; therefore, as the visual sensory memory is a type of sensory memory, there is a store for the information, but the store lasts for only a second or so. A common effect of the visual sensory memory is that individuals may remember seeing things that weren't really there or not remembering particular things that were in their line of sight. The memory is only momentary, and if it isn't attended to within a matter of seconds, it is gone.[20]

There are two different pathways in the brain that control different functions of what is known inclusively as the visuo-spatial sketchpad. The sketchpad consists of the spatial short-term memory and the object memory. The spatial short-term memory is how one is able to learn and thus remember "where" they are in comparative representation to other objects. The object memory of the visuo-spatial sketchpad is essential in learning and remembering "what" an object is.[22] The differences between these two differing visual abilities is due in large part because of different pathways of each of the abilities in the brain. The visual pathway in the brain that detects spatial representation of a person to and within their environment is the dorsal stream. The visual pathway that determines objects shapes, sizes, colors and other definitive characteristics is called the ventral stream.[21] Each of these two streams runs independent of one another so that the visual system may process one without the other (like in brain damage for instance) or both simultaneously. The two streams do not depend on one another, so if one is functioning manipulatively, the other can still send its information through.

Logie has proposed that the visuo-spatial sketchpad can be further subdivided into two components:

1. The visual cache, which stores information about form and color.
2. The inner scribe, which deals with spatial and movement information. It also rehearses information in the visual cache and transfers information to the central executive.[23]

Three main findings provide evidence for the distinction between visual and spatial parts of the visuospatial sketchpad:

1. There is less interference between visual and spatial tasks than between two visual tasks or two spatial tasks.[24]
2. Brain damage can influence one of the components without influencing the other.[25]
3. Results from brain-imaging show that working memory tasks with visual objects activate mostly areas in the left hemisphere, whereas tasks with spatial information activate more areas in the right hemisphere.[26]

In 2000 Baddeley added a fourth component to the model, the episodic buffer. This component is a limited capacity passive system,[27] dedicated to linking information across domains to form integrated units of visual, spatial, and verbal information with time sequencing (or episodic chronological ordering[27]), such as the memory of a story or a movie scene. The episodic buffer is also assumed to have links to long-term memory and semantic meaning.[28]

"It acts as a buffer store, not only between the components of Working Memory, but also linking Working Memory to perception and Long-Term Memory".[27] Baddeley assumes that "retrieval from the buffer occurred through conscious awareness".[27] The episodic buffer allows individuals to use integrated units of information they already have to imagine new concepts. Since this is likely "an attention-demanding process...the buffer would depend heavily on the Central Executive".[27]

The main motivation for introducing this component was the observation that some (in particular, highly intelligent) patients with amnesia, who presumably have no ability to encode new information in long-term memory, nevertheless have good short-term recall of stories, recalling much more information than could be held in the phonological loop.[29] "The episodic buffer appears...capable of storing bound features and making them available to conscious awareness but not itself responsible for the process of binding".[30]

It is assumed that "conscious access to the phonological loop or sketchpad may operate via the buffer".[31] This is based on the assumption that both the visuo-spatial sketchpad and phonological loop act as minor buffers, combining information within their sensory area. The episodic buffer may also interact with smell and taste.[31]