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The distinction between short-term memory (STM) and working memory (WM) is important. STM refers to a cognitive system that is used for holding sensory events, movements, and cognitive information, such as digits, words, names, or other items for a brief period of time (Kolb and Wishaw, 2009). It has been suggested that an average person can store between 5

and 9 chunks of information in their STM, this is referred to as The Magical Number Seven, Plus or Minus Two (Miller, 1956). Although they are conceptually different, the use of the terms STM and WM in literature is not always strict. STM and WM are different theoretical concepts that are assumed to reflect different cognitive functions. At a superficial level, working memory seems functionally indistinguishable from short-term memory, which is to temporarily store information in an activated state. However, short-term memory is a subset of working memory, (i.e. working memory = short-term memory + attention).

WM is a cognitive temporary storage system with a limited capacity that is responsible for holding information for processing and the maintenance plus manipulation of information and requires reactivation in order to avoid rapid decay of information. There have been several models developed to explain the ability to temporarily store information and use it since Miller’s (1956) important work on The Magic number 7 (plus or minus two). Most models of working memory agree on the fundamental processes of working memory of encoding, maintenance and retrieval. However, models differ in the way in which these processes are applied. Working memory models can be divided into two categories 1) models that view working memory as a complete system in itself with connections to other systems (e.g. Baddeley & Hitch, 1974; Baddeley, 2012), 2) models that conceptualise working memory as part of a larger cognitive architecture that incorporates several aspects of higher order cognition (e.g. Cowan, 1999; 2001; Cowan, Elliot, Saults, Morey, Mattox,

Hismjatullina, & Conway, 2005).

The most commonly and frequently used and referred to model of WM is that presented by Baddeley, the multiple component model (Baddeley, 1986; 1992; Baddeley & Hitch, 1974) (Figure 1). The model is a three-component model of working memory that is comprised of the ‘central executive’ an attentional control system, concerned with information control and monitoring information processing, and two subsidiary slave systems supporting the central executive, the ‘phonological loop’ and the ‘visuospatial sketchpad’ (Fig. 1). The phonological loop holds the verbal information by using a temporary storage system and refreshes the information by using a subvocal rehearsal system. The visuospatial sketchpad serves the function of integrating spatial, visual, and possibly kinaesthetic information into a unified representation which may be temporarily stored and manipulated (Baddeley, 2003). The central executive is also assumed to have several components including focusing, dividing and switching attention (Baddeley, 1996; 2002; Baddeley, Emslie, Kolodny, & Duncan,

1998). In 2000 Baddeley introduced a fourth component to his working memory system, namely the ‘‘episodic buffer’’. This component is assumed to be a limited capacity system that depends heavily on executive processing and capable of binding together information from a number of different sources. While the central executive is concerned with attentional control, the episodic buffer is principally concerned with the storage of information. The episodic buffer is episodic in the sense that it holds information into chunks or episodes and it is a buffer in the sense of providing a way of combining information from different

modalities into a single multi-faceted code. Baddeley argued that the episodic buffer is controlled by the central executive, which is able to retrieve information from the store in the form of conscious awareness, or reflect on that information and, where necessary, manipulate and modify it (Baddeley, 2000; 2003).

Figure 1 Baddeley’s working memory model.

While Baddeley’s WM model is the most frequently used model, some prefer the alternative framework to working memory provided by Cowan, the Embedded Process Model (Figure 2) (Cowan, 1999; 2001; Cowan, Elliot, Saults, Morey, Mattox, Hismjatullina, & Conway, 2005). The Embedded Process Model of working memory rely on five principles.

1. That working memory information comes from hierarchically arranged processes consisting of long-term memory, the subset of long-term memory that is currently activated, and the subset of the activated component of memory that is currently in the focus of attention and awareness.

2. The focus of attention is capacity limited whereby four chunks can be held in memory at any one time and activation is time limited, that activation of information within working memory will decay unless reactivated through rehearsal.

3. The focus of attention is controlled conjointly by voluntary processes (a central executive system) and an involuntary process (the attentional orienting system). 4. Stimuli with physical features that have remained unchanged over time and are of no

key importance to the individual, still activate some features of memory, but they do not elicit awareness.

5. Awareness influences processing. In memory it allows new episodic representations to be available for explicit recall (Cowan, 1999; Cowan, Day, Saults, & Keller, 1992).

Unlike Baddeley’s WM model, Cowan accounts for a limit in the capacity of attentional focus across the areas of active long-term memory. Cowan considered working memory to be part of short-term memory and long-term memory, and that representations in working memory are a subset of representations in long-term memory (Cowan,

Figure 2 Cowan’s working memory model.