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Figure 1

Unitary resource (capacity) model of Kahneman (1973). Attention and Effort © Daniel Kahneman. Reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ.

or modular. Cognitive psychologists have addressed this question in some detail in research on human attention. Beginning with Daniel Kahneman's seminal work on attention and effort (1973), mental workload has been linked to information-processing theories of attention. These theories assume that tasks require the allocation of information-processing resources for their efficient execution (except those that can be performed automatically) and that mental workload reflects the overall demand for such resources. Kahneman's resource (or capacity) model is shown in Fig. 1. Every individual has a pool of available resources that is modulated by arousal level, with capacity being low during drowsiness and sleep and high during attentive wakefulness. The available capacity is allocated in parallel to various possible information-processing activities, according to the current behavioral goals of the person.

Some resource theories, like Kahneman's model, assume a single pool of resources that can be flexibly allocated to different processing activities. This is the unitary view. In the modular view, functionally separate resources are applied to different processing activities. Modules may be defined by such factors as sensory modality and stages of information processing, as in a model proposed by Christopher Wickens (see Fig. 2). An influential model of "working memory'' by Alan Baddeley represents a hybrid view. This model proposes that modular "slave" systems for verbal and visuospatial processing are controlled by a unitary "central executive'' system (see Fig. 3).

According to both the unitary and modular views, the processing resources that are applied to a task (or a component of a task) increase with the mental effort invested by the individual. Resources also increase in response to task demand. The human information-processing system, like an economic system, thus is able to adjust the supply of processing resources in response to demand. However, there is an upper bound to the amount of processing resources that can be applied: information-processing capacity is limited (see Fig. 4). Moreover, as task demand increases, the amount of "spare" capacity decreases, so that less can be allocated to other processing activities, whose efficiency may then suffer. This line of reasoning accounts for the performance decrement that can occur when two tasks are performed simultaneously and the demand of one task is increased.

Wickens Ressourcen
Figure 2 Multiple resource model from Wickens (1984).

Capacity limitations appear to be a fundamental property of both artificial and biological systems. Work in computer vision and robotics has shown that fully parallel processing of an entire complex visual scene is computationally inefficient in comparison to limited-capacity serial processing of parts of the scene. The large receptive fields of neurons in the higher perceptual processing areas of the primate brain are also consistent with such a computational limitation;

Visuo-spatial Scratch-pad

Visuo-spatial Scratch-pad

Articulatory Loop

Figure 3 Baddeley (1992) model of working memory.

because several objects in a visual scene fall within a neuron's receptive field, they compete for neuronal processing. Identification of an object among distrac-ters therefore is dependent on the allocation of this limited processing resource.

Competition for limited processing resources is also a major factor in the decrement in performance that occurs when two tasks must be carried out simultaneously. According to unitary resource theories, interference between concurrent tasks occurs when

Supply = Demand

Supply = Demand

Capacity demanded by primary task

Figure 4 Relation between capacity (resources) demanded by a task and capacity supplied. From Kahneman (1973). Attention and Effort r Daniel Kahneman. Reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ.

Capacity demanded by primary task

Figure 4 Relation between capacity (resources) demanded by a task and capacity supplied. From Kahneman (1973). Attention and Effort r Daniel Kahneman. Reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ.

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