The development of the visuospatial sketchpad has received less attention than the phonological loop. One task used to study it assesses immediate memory for a sequence of spatial locations and is known as Corsi Blocks. The apparatus consists of a set of identical cubes arranged irregularly on a flat horizontal board. On each trial the experimenter touches a random subset of the blocks one at a time and the participant's task is to recall the sequence of locations by pointing. Span is assessed in the normal way by increasing the number of blocks touched until accurate recall is no longer possible. Neuropsychological evidence indicates a double dissociation between Corsi Blocks span and auditory-verbal memory span, consistent with the idea that they reflect different subsystems (De Renzi & Nichelli, 1975). Other evidence is consistent with this interpretation. For example, order of report has a large effect on digit span, such that backward span is lower than forward span, but there is no effect of order of report on Corsi span (Isaacs & Vargha-Khadem, 1989). Another type of evidence is the absence of correlations between individual differences in digit span and Corsi span, as shown in a recent study of 5- and 8-year-olds by Pickering, Gathercole, and Peaker (1998). However, as regards developmental change, Corsi span increases at about the same rate as digit span (Isaacs & Vargha-Khadem, 1989). This similarity is not inconsistent with the hypothesis that the two tasks tap different systems. However, it does suggest that a common factor is involved in developmental change. One proponent of such a view is Kail (1988), who found parallel developmental profiles for speed of processing in a range of tasks and interpreted them in terms of a common, global mechanism.
Another task used to study the development of the visuospatial sketchpad is immediate memory for sequences of pictorial stimuli. Pictures are interesting because they can be encoded visually and verbally. In adults, immediate memory for a series of pictured items is sensitive to word length and phonemic similarity of their names and is disrupted by articulatory suppression (Schiano & Watkins, 1981). This indicates a tendency to recode pictures verbally, using the phonological loop. However, in a similar task Hitch, Halliday, and Littler (1993) found no effect of word length of picture names in 4-year-olds. The word-length effect for pictures emerged progressively with age and became equivalent to its counterpart for verbal materials by age 11. The same developmental pattern was found for effects of phonemic similarity of picture names
(Hitch, Halliday, Schaafstal & Schraagen, 1988). Further studies have shown that young children are sensitive to the visual characteristics of the pictures (Hitch et al., 1988; Hitch, Woodin & Baker, 1989). Thus, Hitch et al. (1989) found that younger children had poorer memory for visually similar pictures, whereas older children were unaffected by visual similarity. Hitch et al. also found that when older children were required to perform articulatory suppression, they became sensitive to visual similarity. Thus, older children make use of the visuospatial sketchpad when their tendency to use verbal recoding is disrupted, which is consistent with adult data described earlier (Walker et al., 1993).
In theoretical terms, therefore, there seems to be a marked developmental change in how subsystems of working memory are used to remember nameable pictures. At around age 5, children primarily rely upon the visuospatial sketchpad, whereas only a few years later, they clearly prefer to use verbal recoding. The reasons for such a major qualitative change are not immediately obvious. One interpretation is that development of the functional capacity of the phonological loop makes verbal recoding of pictures an increasingly effective strategy. However, this explanation seems inadequate on the evidence that adults persist in using verbal recoding even when it impairs their performance. For example, Brandimonte, Hitch, and Bishop (1992) investigated adults' ability to manipulate mental images of recently presented visual stimuli. Performance was critically dependent on memory for the visual appearance of the stimuli. Nevertheless, participants showed a strong tendency to engage in verbal recoding, as evidenced by the improvement in their imagery performance when verbal recoding was disrupted by articu-latory suppression. This rare instance of performance improving under dual-task conditions has been replicated on a number of occasions (Brandimonte, Hitch & Bishop, 1992; Hitch, Brandimonte & Walker, 1995). We note that the verbal overshadowing of memory for faces in adults (Schooler & Engstler-Schooler, 1990) may be a related phenomenon. A more plausible reason why verbal recoding becomes pervasive as development proceeds is that it gives access to the vast representational capacity of the language system. Normally, such recoding will be useful, but occasionally, as when memory for detailed visual appearance is required, it is disadvantageous.
However, such an account says nothing about what function the sketchpad serves in cognitive development. Unfortunately, we are far from answering this important question. Useful clues are provided by recent studies of children with Williams's syndrome, a genetic disorder accompanied by deficits in visuospatial and numerical skills but relatively normal language skills. Williams's children are markedly impaired on the Corsi Blocks task but relatively spared on digit span (Wang & Bellugi, 1994; Jarrold, Baddeley & Hewes, 1999). In contrast, children with Down's syndrome show the converse pattern. These observations suggest that the sketchpad may be necessary for the normal development of visuospatial and numerical skills. We note also that genetic dissociation lends further support to the hypothesis that the development of working memory involves separable components.
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