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Baddeley & Hitch proposed their working memory model as an alternative to the short-term store in Atkinson & Shiffrin's 'multi-store' memory model(1968). The model proposed by Baddeley and Hitch has remained the dominant view in the field of working memory. However, alternative models are developing (see working memory) providing a different perspective on the working memory system.
The model of Baddeley & Hitch is composed of three main components; the 'central executive' (see executive system) which acts as supervisory system and controls the flow of information from and to its 'slave-systems': the 'phonological loop' and the 'visuo-spatial sketch-pad'. The slave systems are short-term storage systems dedicated to a content domain (verbal and visuo-spatial, respectively).
Baddeley & Hitch's argument for the distinction of two domain-specific slave systems in the model was derived from experimental findings with dual-task paradigms. Performance of two simultaneous tasks requiring the use of two separate perceptual domains (i.e. a visual and a verbal task) is nearly as efficient as performance of the tasks individually. In contrast, when a person tries to carry out two tasks simultaneously that use the same perceptual domain, performance is less efficient than when performing the tasks individually.
The phonological loop can be divided into two further sub-systems - the phonological store and the articulatory process. The phonological loop as a whole deals with phonological information. 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. 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 decay.
Four main findings provide evidence for the phonological loop:
- 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.
- The word length effect: Lists of short words are remembered better than lists of long words. This is explained by the fact that short words can be articulated faster, so that more words can be silently articulated before they decay. The model assumes that the phonological loop can maintain lists of words (or other verbal material) as long as their articulation duration does not exceed 2 seconds.
- 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, thereby leaving memory traces in the phonological loop to decay.
- The effect of irrelevant speech. Playing speech sounds, even from a foreign language, disrupts memory for verbal material. This is explained by assuming that speech sounds enter the phonological store automatically and interfere with memory traces of the list to be remembered.
However, some details of the findings are not easily explained by the original Baddeley & Hitch model. Criticisms of the phonological-loop component have been raised, for instance, by Dylan Jones (Jones, Macken, & Nicholls, 2004) and Nairne (2002).
The visuo-spatial sketchpad is concerned with visual or spatial tasks, such as remembering shapes and colours, or the location or speed of objects in space. It is also involved in tasks which involve planning of spatial movements, like planning one's way through a complex building. The visuo-spatial sketchpad can be further subdivided into a visual component, dealing primarily with objects and their visible features, such as shape, colour, and texture, and a spatial component, dealing with locations and movements in space.
Validity of the model
The strength of Baddeley & Hitch's model is its ability to integrate a large amount of findings on short-term and working memory. Additionally, the mechanisms of the slave systems, especially the phonological loop, has inspired a wealth of research in experimental psychology, neuropsychology, and cognitive neuroscience.
The concept of the 'central executive' has remained vague. It is conceived as a supervisory system that controls cognitive processes and intervenes when they go astray. It is responsible for coordinating the slave systems. Using the dual-task paradigm, Baddley and colleagues 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 (Baddeley & Della Sala, 1996). 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 (Miyake et al., 2000) and can be selectively impaired or spared by brain damage.
Recently, Baddeley (2000) has added a fourth component to the model, called the 'episodic buffer'. This component is a third storage system, dedicated to linking information across domains to form integrated units of visual, spatial, and verbal information (e.g., the memory of a story or a movie scene). The episodic buffer is also assumed to have links to long-term memory. 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. The episodic buffer has not been investigated extensively and its functions remain vague.
- Baddeley, A. D. (2000). The episodic buffer: a new component of working memory? Trends in Cognitive Science, 4, 417-423.
- Baddeley, A.D., & Hitch, G. (1974). Working memory. In G.H. Bower (Ed.), The psychology of learning and motivation: Advances in research and theory (Vol. 8, pp. 47--89). New York: Academic Press.
- Baddeley, A. D., & Della Sala, S. (1996). Working memory and executive control. Philosophical Transactions of the Royal Society of London, 351, 1397-1404.
- Baddeley, A. D., & Wilson, B. A. (2002). Prose recall and amnesia: implications for the structure of working memory. Neuropsychologia, 40, 1737-1743.
- Jones, D. M., Macken, W. J., & Nicholls, A. P. (2004). The phonological store of working memory: is it phonological and is it a store? Journal of Experimental Psychology: Learning, Memory, and Cognition, 30, 656-674.
- Miyake, A., Friedman, N. P., Emerson, M. J., Witzki, A. H., Howerter, A., & Wager, T. D. (2000). The unity and diversity of executive functions and their contributions to complex "frontal lobe" tasks: A latent variable analysis. Cognitive Psychology, 41, 49-100.
- Nairne, J. S. (2002). Remembering over the short-term: The case against the standard model. Annual Review of Psychology, 53, 53-81.
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