Sensitivity (d') to auditory and visual signals as a function of concurrent imagery modality (auditory vs. visual). Adapted from Segal and Fusella (1970).

The main limitation of the study by Bourke et al. (1996) is that it did not clarify the nature of the central capacity. As they admitted (1996, p. 544).

The general factor may be a limited pool of processing resource that needs to be invested for a task to be performed. It may be a limited central executive that coordinates or monitors other processes and is limited in how much it can deal with at one time. It may also represent a general limit of the entire cognitive system on the amount of information that can be processed at a given time. The method developed here deals only with the existence of a general factor in dual-task decrements, not its nature.


Central capacity theories cannot explain all the findings. According to such theories, the crucial determinant of dual-task performance is the difficulty level of the two tasks, with difficulty being defined in terms of the demands placed on the resoures of the central capacity. However, the effects of task difficulty are often swamped by those of task similarity. For example, Segal and Fusella (1970) combined image construction (visual or auditory) with signal detection (visual or auditory). The auditory image task impaired detection of auditory signals more than did the visual task (see Figure 5.9), suggesting that the auditory image task was more demanding than the visual image task. However, the auditory image task was less disruptive than the visual image task when each task was combined with a task requiring detection of visual signals, suggesting the opposite conclusion. In this study, task similarity was clearly a much more important factor than task difficulty.

Allport (1989, p. 647) argued that such findings, "point to a multiplicity of attentional functions, dependent on a multiplicity of specialised subsystems. No one of these subsystems appears uniquely

'central'." It is possible to "explain" dual-task performance by assuming that the resources of some central capacity have been exceeded, and to account for a lack of interference by assuming that the two tasks did not exceed those resources. However, in the absence of any independent assessment of central processing capacity, this is simply a re-description of the findings rather than an explanation.

Modular theories

The views of central capacity theorists can be compared with those of cognitive neuropsychologists, who assume that the processing system is modular (i.e., consisting of numerous fairly independent processors or modules). Evidence for modularity comes from the study of language in brain-damaged patients (see Chapters 12 and 13). If the processing system consists of specific processing mechanisms, then it is clear why the degree of similarity between two tasks is so important: similar tasks compete for the same specific processing mechanisms or modules, and thus produce interference, whereas dissimilar tasks involve different modules, and so do not interfere.

Allport (1989) and others have argued that dual-task performance can be accounted for in terms of modules or specific processing resources. However, there are significant problems with this theoretical approach. First, it does not provide an adequate explanation of findings on the psychological refractory period effect. Second, there is no consensus regarding the nature or number of these processing modules. Third, most modular theories cannot be falsified. Whatever the findings, it is always possible to account for them by assuming the existence of appropriate specific modules. Fourth, if there were several modules operating in parallel, there would be substantial problems in terms of co-ordinating their outputs to produce coherent behaviour

Synthesis theories

Some theorists (e.g., Baddeley, 1986; Eysenck, 1982) favour an approach based on a synthesis of the central capacity and modular notions. According to them, there is a hierarchical structure. The central processor or central executive is at the top of the hierarchy, and is involved in the co-ordination and control of behaviour. Below this level are specific processing mechanisms operating relatively independently of each other.

One of the problems with the notion that there are several specific processing mechanisms and one general processing mechanism is that there does not appear to be a unitary attentional system. As we saw in the earlier discussion of cognitive neuropsychological findings, it seems that somewhat separate mechanisms are involved in disengaging, shifting, and engaging attention. If there is no general processing mechanism, then it may be unrealistic to assume that the processing system possesses a hierarchical structure.

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