Single Resource Models

Described in outline, single resource theory (SRT) assumes that cognitive resource allocation works simply in terms of supply and demand. Performance on one or more tasks is assumed to suffer whenever resource demands exceed available supply (see Figure 9). In some resource models the total supply of mental resources is assumed to be fixed (e.g., Navon and Gopher, 1979). Most SRT models, however, have assumed the amount of total resource to be variable in increasing or diminishing amounts - contingent on the presence or absence of psychological stressors and other Performance Influencing Factors (e.g., Kahneman, 1973).

Figure 9 SRT predicted relation between resources, demands and task

performance

One early influential SRT model with implications for the analysis of mental workload was the capacity theory of attention and effort proposed by Daniel Kahneman (1973). Kahneman argued that the cognitive system has a single pool of limited capacity generally available to resource task performance at any one time. Difficult tasks were assumed to require large amounts of resources leading to possible problems when these tasks were paired with other concurrent tasks requiring mental capacity. Conversely, easy, well practiced and highly automated tasks were likely to be time-shared with relatively few task performance implications. In Kahneman’s view, the total pool of resources available at any one time is variable, dependant upon a number of influencing factors. Factors believed likely to affect the availability of mental resources included the enduring dispositions and momentary intentions of the actor, as well as the person’s general levels of psychological arousal and, the complexity of the task (see Figure 10). When faced with a difficult, demanding task, arousal levels can be increased in the cognitive system, providing additional resources to cover the extra demand. It was suggested that this adaptive reaction to mental workload could be monitored via the collection of physiological data indicative of increased autonomic nervous system activity1.

1

The relationship between levels of psychological arousal and increased autonomic nervous system activity is well documented and will be discussed later in the section dealing with workload assessment techniques.

Figure 10 Kahneman's Capacity Model of Attention

(Source: DiDimenico, 2003: p.13)

The SRT approach to workload analysis was further developed by Norman and Bobrow (1975). They distinguished between the effects of resource- limited processes and data-limited processes in human cognition. Resource- limited processes are those where task performance improves as long as sufficient resources are allocated to meet task demand. For these tasks, the subject always has the opportunity to improve performance by adopting strategies that lead to the freeing up of mental capacity. Data limited processes, on the other hand, are those where the performance limitation is embedded within the structure of the task, as occurs, for example, in gambling situations and tasks involving judgments of uncertainty. For these situations, further allocation of mental resources is unlikely to improve task performance and other, data driven methods, must be adopted to improve the quality of human performance. It should be noted, however, that in some situations apparent data-limited processes may simply be reflecting ceiling effects of the task. Figure 11 shows a Performance Resource Function curve plotted to exemplify the effects of resource and data limited processes on task performance (cf: Norman and Bobrow, 1975).

Figure 11 Performance Resource Function (PRF) showing resource-limited and

data-limited regions

In many respects single resource theory provides a much more coherent account of the relation between human cognition and task performance than was the case for the single channel hypothesis. In particular, it has proved particularly suited to accounting for the observation that people are able to perform concurrent tasks under certain circumstances and the approach seemed to provide the basis for a strong scientific foundation for workload assessment through physiological measurement techniques. Some investigators (Beatty, 1982; Just and Carpenter, 1993) took this further by proposing that spare capacity available in the resource pool could be measured by giving subjects secondary tasks to perform, leading to the prospect of developing an index of the capacity required to perform a primary task. In this way, the capacity demands of a variety of tasks can be measured and used to predict dual task performance. Despite the promise of the approach, however, SRT approaches to workload assessment have proved problematic. Contrary to the predictions of the model, task interference and performance decrements are still frequently observed even for multitasking scenarios involving simple tasks. Indeed, the amount of interference seems more related to task similarity than task difficulty. Thus, concurrent visual processing tasks interfere more readily than a visual and auditory one even thought the estimated extrinsic tasks demands were viewed comparable. Kahneman explained this finding by suggesting that a degree of structural interference between simultaneous tasks is to be expected when both tasks require use of a non-sharable mechanism or resource.

As a final comment on SRT, it is worth noting that the Kahneman’s model of attention and effort proposed, perhaps for the first time, that people can exercise a degree of volition over how mental resources can be allocated to tasks and thus they can influence their reaction to task loading. The strategies that people can adopt to help them cope with high task demands are very important in workload research and will be considered in more detail below.