Figure

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

3. Selective attention to certain stimuli while ignoring others.

4. Temporary activation of long-term memory.

Evidence

One task Baddeley has used to study the workings of the central executive is random generation of digits or letters. The basic idea is that close attention is needed on this task to avoid producing stereotyped (and non-random) sequences. Baddeley (1996; see also Baddeley, Emslie, Kolodny, & Duncan, 1998) reported a study in which the participants held between one and eight digits in short-term memory while trying to generate a random sequence of digits. It was assumed that the demands on the central executive would be greater as the number of digits to be remembered increased. As predicted, the randomness of the sequence produced on the generation task decreased as the digit memory load increased (see Figure 6.8).

Baddeley (1996) argued that performance on the random generation task might depend on the ability to switch retrieval plans rapidly and so avoid stereotyped responses. This hypothesis was tested as follows. The random digit generation task involved pressing numbered keys. This task was done on its own, or in combination with reciting the alphabet, counting from 1, or alternating numbers and letters (A 1 B 2 C 3 D 4.). Randomness on the random generation task was reduced by the alternation task, presumably because it required constant switching of retrieval plans. This suggests that rapid switching of retrieval plans is one of the functions of the central executive.

Towse (1998) has argued persuasively that random generation involves various processes, and so is not a pure central executive task. His participants were asked to produce random sequences using the numbers 110 or 1-15, and the relevant set of numbers was either visible in front of them or was not presented. Number generation was more random when the numbers were visible, and this was especially the case with the larger set of numbers. Thus, an important factor in random generation is the generation of the potential set of response alternatives, and this is easier when the alternatives are visible.

The notion that the central executive may play an important part in timesharing or distributing attention across two tasks was considered in a number of studies discussed by Baddeley (1996). One study involved patients with Alzheimer's disease, which involves progressive loss of mental powers and reduced central executive functioning. First of all, each participant's digit span was established. Then they were given several digit-span trials with that number of digits. Finally, they were given more digit-span trials combined with the task of placing a cross in each of a series of boxes arranged in an irregular pattern (dual-task condition). All the Alzheimer's patients showed a marked reduction in digit-span performance in the dual-task condition, but none of the normal controls did. These findings are consistent with the view that Alzheimer's patients have particular problems with the central executive function of distributing attention between two tasks.

Evaluation

There is growing evidence that the central executive is not unitary in the sense of forming a unified whole. For example, Eslinger and Damasio (1985) studied a former accountant, EVR, who had had a large cerebral tumour removed. He had a high IQ, and performed well on tests requiring reasoning, flexible hypothesis testing, and resistance to distraction and memory interference, suggesting that his central executive was essentially intact. However, he had very poor decision making and judgements (e.g., he would often take hours to decide where to eat). As a result, he was dismissed from various jobs. Presumably EVR's central executive was partially intact and partially damaged. This implies that the central executive is consists of two or more component systems. Such evidence is consistent with the growing body of evidence that the attentional system is not unitary (see Chapter 5).

Shah and Miyake (1996) studied the complexity of the central executive by presenting students with tests of verbal and spatial working memory. The verbal task was the reading span task (Daneman & Carpenter, 1980; see Chapter 12). In this task, the participants read a series of sentences and then recall the final word of each sentence. The reading span is the maximum number of sentences for which they can do this. There was also a spatial span task. The participants had to decide whether each of a set of letters was in normal or mirror-image orientation. After that, they had to indicate the direction in which the top of each letter had been pointing. The spatial span was the maximum number of letters for which they were able to do this.

The correlation between reading span and spatial span was a non-significant +.23, suggesting that verbal and spatial working memory are rather separate. Shah and Miyake's other findings supported this conclusion. Reading span correlated +.45 with verbal IQ, but only +.12 with spatial IQ. In contrast, spatial span correlated +.66 with spatial IQ, and only +.07 with verbal IQ. As Mackintosh (1998, p. 293) concluded, "Within the constraints of this study, and particularly the subject population studied [only university students],, verbal and spatial working-memory systems seem relatively independent." Shah and Miyake (1996) favoured a multiple-resource model, and this was developed by Shah and Miyake (1999).

Overall evaluation

There are several advantages of the working memory system over that of Atkinson and Shiffrin (1968). First, the working memory system is concerned with both active processing and transient storage of information, and so is involved in all complex cognitive tasks (e.g., language comprehension; see Chapter 12).

Second, the working memory model can explain the partial deficits of short-term memory that have been observed in brain-damaged patients. If brain damage affects only one of the three components of working memory, then selective deficits on short-term memory tasks would be expected.

Third, the working memory model incorporates verbal rehearsal as an optional process that within the phonological loop. This is more realistic than the enormous significance of rehearsal within the multi-store model of Atkinson and Shiffrin (1968).

On the negative side, the role played by the central executive remains unclear. The central executive has limited capacity, but it has proved hard to measure that capacity. It is claimed that the central executive is "modality-free" and used in numerous processing operations, but the precise constraints on its functioning are unknown. It has been assumed that the central executive is unitary, but this is becoming increasingly controversial (see Kimberg, D'Esposito, & Farah, 1998). Rather unfairly, Donald (1991, p. 327) argued as follows: "The 'central executive' is a hypothetical entity that sits atop the mountain of working memory and attention like some gigantic Buddha, an inscrutable, immaterial, omnipresent homunculus [miniature man], at whose busy desk the buck stops every time memory and attention theorists run out of alternatives."

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