Verbal short-term memory

working memory model proposed by Baddeley and Hitch (1974). The phonological loop is assumed to consist of a phonological short-term store subject to rapid decay, and a subvocal rehearsal mechanism that can be used to maintain phonological representations within the store (Baddeley, 1986).

Auditory linguistic inputs have obligatory access to the phonological store, and other inputs may be recoded as verbal information. This simple model has proved capable of accommodating a great deal of experimental evidence from normal adult participants, children, and neuropsychological patients (see Baddeley, 1997, and Gathercole & Baddeley, 1993, for reviews).

Several findings have now become hallmarks of verbal short-term memory. The presence of a phonological similarity effect, the impaired recall for items that share similar phonological structure, is typically attributed to the confounding effects of decay of phonologically similar representations in the short-term store (e.g., Baddeley, 1966; Conrad & Hull, 1964). Listening to irrelevant speech (e.g., Colle & Welsh, 1976; Salamè & Baddeley, 1982), but not pulsed noise (Salamè & Baddeley, 1987), during serial recall tasks results in poorer retention suggesting that the irrelevant speech effect is due to the

obligatory access of phonological (but not other auditory) information to the short-term store. Temporal decay of the phonological representations in the short-term store is indicated by the word-length effect, the decreased recall accuracy for memory sequences with lengthy articulatory durations (e.g.,

Baddeley, Thomson, & Buchanan, 1975; Cowan, Saults, Winterowd, & Sherk, 1991).

Evidence for the importance of rehearsal processes is reflected in findings from a number of studies. For example, recall is reduced when rehearsal is prevented such as when individuals engage in articulatory suppression, the concurrent repetition of irrelevant sounds such as the-the-the (e.g., Baddeley, Lewis, & Vallar, 1984). Important developmental work suggests that the ability to rehearse items appears to emerge after about seven years of age (Cowan &

Kail, 1996; Gathercole & Adams, 1993; Gathercole, Adams, & Hitch, 1994).

Recently, a third component responsible for controlling timing mechanisms has been proposed to account for data pertaining to temporal grouping (e.g.,

Frankish, 1985, 1989; Hitch, Burgess, Towse, & Culpin, 1996) and the immediate recall of rhythmic tapping (e.g., Larson & Baddeley, 2003; Saito, 2001). Neuroimaging studies of short-term memory have identified

distinguishable regions supporting each of these functions including recoding, storage, maintenance of temporal order, and rehearsal processes (e.g.,

D’Esposito, Postle, Ballard, & Lease, 1999; Henson, Burgess, & Frith, 2000).

It is clear that verbal short-term memory does not operate in isolation, but within the context of a complex cognitive system. Several studies have

investigated the potential impact of other cognitive processes on verbal short-term memory. The support available from the knowledge base within long-short-term memory has been one such candidate process, and several lines of research point to the contribution of long-term knowledge to short-term memory performance.

Examples include the better recall of familiar (known) words than nonsense syllables, known as the lexicality effect (e.g., Hulme, Maughan, & Brown,

1991), and the better retention of words in sentences than unrelated words (e.g., Baddeley, Vallar, & Wilson, 1987). Other phenomena considered to reflect the contribution of long-term knowledge include better recall for the following: (1) sound sequences with a higher probability of occurring in the lexicon, known as the phonotactic frequency effect (e.g., Gathercole, Frankish, Pickering, &

Peaker, 1999; Munson, 2001); (2) words that are more frequently used, the word frequency effect (e.g., Hulme, Roodenrys, Schweickert, Brown, Martin, &

Stuart, 1997); (3) words for which it is easier to form a mental image, the imageability effect (e.g., Bourassa & Besner, 1994); and (4) nonwords high in

‘wordlikeness’ (more similar to known words), the wordlikeness effect (e.g., Gathercole, 1995; Gathercole, Willis, Emsilie, & Baddeley, 1991). One account of the role played by long-term knowledge is that of redintegration, the use of activated lexical representations to reconstruct incomplete representations held in short-term memory (Gathercole, et al., 1999; Gathercole, Pickering, Hall, &

Peaker, 2001; Hulme et al., 1997; Schweikert, 1993; Thorn, Gathercole, &

Frankish, 2005). It has been suggested also that long-term knowledge may enhance the stability and quality of phonological representations themselves (Thorn & Frankish, 2005; Thorn et al., 2005).

Articulation rate is another mechanism found to be associated with verbal short-term memory (e.g., Cowan, Wood, Wood, Keller, Nugent, & Keller, 1998;

Hulme & Tordoff, 1989) with developmental changes in articulation rate closely tied to changes in short-term memory (e.g., Henry, 1994; Hulme, Thomson, Muir, & Lawrence, 1984). Several researchers have reported superior recall for short- than long-duration words of matched syllable length (e.g., Baddeley et al., 1975; Hulme & Tordoff, 1989). It has been suggested that articulation rate may

limit rehearsal in short-term memory, which is assumed to be a real-time process similar to covert speech (Landauer, 1962). Articulation rate, and speech skills more generally, may limit recall success at output as well (e.g., Vance,

Stackhouse, & Wells, 2005; Wells, 1995). In contrast, Ferguson, Bowey, and Tilley (2002) have suggested that previous reports of the close association between articulation rate and memory span may be overestimated. These researchers argue that the common use of speech rate measures based on

multiple word repetition may have introduced confounding effects because these measures themselves impose a memory load. Contrary to previous findings, Ferguson et al. reported that single-word speech rate accounted for only a small proportion of the variance in memory span performance in a group of primary school children.

1.2.1.2 Visuospatial short-term memory. The most detailed account of