Figure

Percent recall as a function of learning instructions (visual imagery vs. rote rehearsal) and of interference (dynamic visual noise or irrelevant speech). Data from Quinn and McConnell (1996).

• The inner scribe, which deals with spatial and movement information. It rehearses information in the visual cache, transfers information from the visual cache to the central executive, and is involved in the planning and execution of body and limb movements.

Evidence consistent with this theory was reported by Beschin, Cocchini, Della Sala, and Logie (1997). They studied a man, NL, who had suffered a stroke. He found it very hard to describe details from the left side of scenes in visual imagery, a condition known as unilateral representational neglect. However, NL had no problems with perceiving the left side of scenes, so his visual perceptual system was essentially intact. A key finding was that he performed very poorly on tasks thought to require use of the visuo-spatial sketchpad, unless stimulus support in the form of a drawing or other physical stimulus was available. According to Beschin et al. (1997), NL may have sustained damage to the visual cache, so he could only create impoverished mental representations of objects and scenes. Stimulus support was very valuable to NL, because it allowed him to use his intact visual perceptual skills to compensate for the deficient internal representations.

How useful is the visuo-spatial sketchpad in everyday life? Some suggestions about its uses were put forward by Baddeley (1997, p. 82):

The spatial system is important for geographical orientation, and for planning spatial tasks. Indeed, tasks involving visuo-spatial manipulation, have tended to be used as selection tools for professions... such as engineering and architecture.

There may be important links between the visuo-spatial sketchpad and the spatial medium identified by Kosslyn (e.g., 1983). The spatial medium is used for manipulating visual images, and shares some features with Baddeley's visuo-spatial sketch pad (Brandimonte, Hitch, & Bishop, 1992; see also Chapter 9).

Evaluation

Is there is a single visuo-spatial sketchpad or separate visual and spatial systems? The evidence favours the notion of separate systems. Baddeley and Lieberman's (1980) finding that the maintenance of spatial information in working memory was not disrupted by a concurrent visual task is consistent with the notion of separate components. Intriguing evidence from a brain-damaged patient (LH), who had been involved in a road accident, was reported by Farah, Hammond, Levine, and Calvanio (1988). He performed much better on tasks involving spatial processing than on tasks involving the visual aspects of imagery (e.g., judging the relative sizes of animals). This evidence is also consistent with the notion of separate visual and spatial systems.

There is also relevant neurophysiological evidence. Smith and Jonides (1997) carried out an ingenious study in which two visual stimuli were presented together, followed by a probe stimulus. The participants had to decide either whether the probe was in the same location as one of the initial stimuli (spatial task) or had the same form (visual task). The stimuli were identical in the two tasks, but there were clear differences in brain activity as revealed by PET. Regions in the right hemisphere (prefrontal cortex; premotor cortex; occipital cortex; and parietal cortex) became active during the spatial task. In contrast, the visual task produced activation in the left hemisphere, especially the parietal cortex and the inferotemporal cortex.

In spite of the evidence discussed here, visual and spatial information becomes interlinked in many situations. This makes the notion of a combined system more attractive (J.Towse, personal communication).

Central executive

The central executive, which resembles an attentional system, is the most important and versatile component of the working memory system. However, as Baddeley (1996, p. 6) admitted, "our initial specification of the central executive was so vague as to serve as little more than a ragbag into which could be stuffed all the complex strategy selection, planning, and retrieval checking that clearly goes on when subjects perform even the apparently simple digit span task."

Baddeley (1996) argued that damage to the frontal lobes of the cortex can cause impairments to the central executive. Rylander (1939, p. 20) described the classical frontal syndrome as involving "disturbed attention, increased distractibility, a difficulty in grasping the whole of a complicated state of affairs...well able to work along old routine lines.cannot learn to master new types of task, in new situations." Thus, patients with the frontal system damaged behave as if they lacked a control system allowing them to direct, and to re-direct, their processing resources appropriately. Such patients are said to suffer from dysexecutive syndrome (Baddeley, 1996).

It would not be useful to define the central executive as the system that resides in the frontal lobes. As Baddeley (1996, p. 7) pointed out, "If we identify the central executive exclusively with frontal function, then we might well find ourselves excluding from the central executive processes that are clearly executive in nature, simply because they prove not to be frontally located." Baddeley's (1996) preferred strategy is to identify and assess the major functions of the central executive, such as the following:

1. Switching of retrieval plans.

2. Timesharing in dual-task studies.

Randomness of digit generation (greater redundancy means reduced randomness) as a function of concurrent digit memory load. Data from Baddeley (1996).

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