Mental effort causes vigilance decrease due to resource depletion
Introduction
Performance on sustained attention tasks is known to decrease with time on task. One of the first theories that sought to explain this decrement was the arousal theory, a derivative of arousal theories on general human performance (see e.g. Eysenck, 1982). This approach argues that the decrement in performance is due to lack of stimulation, needed to maintain alertness at a required level (Ballard, 1996; Eysenck, 1982). More specifically, it assumes that a state of vigilance cannot be maintained under conditions of repetitive stimulation (Parasuraman, 1985), due to perceptual habituation (Mackworth, 1969; Stroh, 1971). Mackworth maintained that monotonous stimulus presentation causes habituation. This, in turn, decreases arousal and thus results in a performance decrement (Mackworth, 1969). The arousal theory predicts that a greater complexity of the stimulus situation results in a smaller decrement, which has indeed been found (see Stroh, 1971, for an overview).
Some findings do not corroborate with the arousal theory’s predictions. First, arousal primarily affects overall performance, rather than performance decrement (Jewett, Dijk, Kronauer, & Dinges, 1999; Parasuraman, 1998). However, decrement effects have been found in some studies (Smit, Eling, & Coenen, 2002). Second, participants report a substantial processing load during vigilance tasks (Hoffman, Sherrick, & Warm, 1998). This suggests that vigilance tasks are not merely boring, but can be mentally demanding.
Another explanation for vigilance decrements may be found in the resource or capacity view on performance. Resource models have been considered in the literature on general performance (Parasuraman, 1998; Wickens & Kessel, 1980), but can also be applied to sustained attention or vigilance research (see Matthews & Davies, 2001; Warm & Dember, 1998). The resource view asserts that energy or resources are needed during task performance and these resources are limited in their availability (Wickens, 1984). Especially controlled processes, in contrast to automatic processes, require resources (Parasuraman, 1985). Furthermore, as tasks become more difficult, they require more resources (Wickens, 1984). In dual-tasks, often used to test resource models, performance is indeed worse than in single tasks in some studies (Gorissen & Eling, 1998; Matthews, Davies, Westerman, & Stammers, 2000). However, interpretation is hindered by the fact that not only instructions are dissimilar between tasks, but usually also the amount and nature of stimuli. Direct comparison of dual and single tasks is problematic as long as these variables are not held constant.
Prolonged performance on tasks is thought to deplete the pool of resources (Parasuraman, 1985), but it is not exactly clear how this depletion works (Matthews et al., 2000). Unfortunately, cognitively demanding tasks have rarely been used in studies on sustained attention performance (Matthews & Davies, 2001). Task difficulty in this field of research is generally manipulated by decreasing discriminability of stimuli (Parasuraman & Mouloua, 1987). Cognitively demanding tasks have been used in the form of dual-tasks, but, as stated above, comparison with single tasks has been problematic due to differences between single and dual-tasks with respect to the nature and amount of presented stimuli. This experiment was conducted with the aim of directly testing resource view predictions with respect to long-term performance on a cognitively demanding task, which is directly compared with an equivalent, but low demanding task.
We examined whether additional irrelevant stimuli influence performance. A simple task was compared with a similar simple task that only differed with respect to occasional presentation of irrelevant stimuli. According to the resource view, additional stimuli should either have a small detrimental effect or no effect at all if they are properly ignored. The arousal theory predicts that these additional stimuli, since they decrease monotony in stimulus presentation, should lead to a smaller decrement in performance. We then directly tested the resource prediction that vigilance is declined most in a complex task that requires many processing resources. Stimulus presentation in the complex task was identical to the task with additional stimuli, in order to make direct comparison possible. Merely the instructions changed.
Section snippets
Participants
Seventeen students (three men and 14 women; mean age: 21 years) participated and signed an informed consent. They fulfilled a course requirement or received 20, -for partaking.
Design and procedure
There were four conditions of 50 min each: a control condition (in which participants conversed with the experimenter and read magazines); a simple, low demanding task (sLDT) condition; a composite, low demanding task (cLDT) and a high-demanding task (HDT) condition. Participants were subjected to all conditions.
Each
Statistical analyses
Data were analyzed with repeated measures ANOVAs with Bonferroni’s confidence interval adjustments and Pearson’s bivariate correlations. Task data were analyzed with a 3 × 2 (Condition × Part) ANOVA; there are only three conditions in this ANOVA, as the control condition did not include performance data. Subjective data were computed per dimension of the Thayer scale and analyzed with a 4 × 2 (Condition × Time: before and after task) ANOVA.
Task data
For performance data, see Table 1. For RTs, a main effect of
Discussion
Contrary to the arousal theory prediction, performance remained stable throughout the sLDT and cLDT: no decrement was found. Although it is often stated that vigilance tasks need to last long, decrements have been reported in tasks that have durations of less than 10 min (Matthews, Davies, & Holley, 1993; Robertson, Manly, Andrade, Baddeley, & Yiend, 1997). A 30-min duration is usually sufficient to yield decrement effects (Matthews et al., 2000). The fact that complexity of the stimulus
Acknowledgements
We thank Sylvia Claassen for assistance during the experiment and Hubert Voogd for programming the software of the mental tasks.
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