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Psychological Aspects

Monday, 14 March 2011 19:00

Mental Workload

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Mental Versus Physical Workload

The concept of mental workload (MWL) has become increasingly important since modern semi-automated and computerized technologies may impose severe requirements on human mental or information-processing capabilities within both manufacturing and administrative tasks. Thus, especially for the domains of job analysis, evaluation of job requirements and job design, the conceptualization of mental workload has become even more important than that of traditional physical workload.

Definitions of Mental Workload

There is no agreed-upon definition of mental workload. The main reason is that there are at least two theoretically well-based approaches and definitions: (1) MWL as viewed in terms of the task requirements as an independent, external variable with which the working subjects have to cope more or less efficiently, and (2) MWL as defined in terms of an interaction between task requirements and human capabilities or resources (Hancock and Chignell 1986; Welford 1986; Wieland-Eckelmann 1992).

Although arising from different contexts, both approaches offer necessary and well-founded contributions to different problems.

The requirements resources interaction approach was developed within the context of personality-environment fit/misfit theories which try to explain interindividually differing responses to identical physical and psychosocial conditions and requirements. Thus, this approach may explain individual differences in the patterns of subjective responses to loading requirements and conditions, for example, in terms of fatigue, monotony, affective aversion, burnout or diseases (Gopher and Donchin 1986; Hancock and Meshkati 1988).

The task requirements approach was developed within those parts of occupational psychology and ergonomics which are predominantly engaged in task design, especially in the design of new and untried future tasks, or so-called prospective task design. The background here is the stress-strain concept. Task requirements constitute the stress and the working subjects try to adapt to or to cope with the demands much as they would to other forms of stress (Hancock and Chignell 1986). This task requirements approach tries to answer the question of how to design tasks in advance in order to optimize their later impact on the—often still unknown—employees who will accomplish these future tasks.

There are at least a few common characteristics of both conceptualizations of MWL.

  1. MWL mainly describes the input aspects of tasks, that is to say, the requirements and demands made by the tasks on the employees, which might be used in forecasting the task outcome.
  2. The mental aspects of MWL are conceptualized in terms of information processing. Information processing includes cognitive as well as motivational/volitional and emotional aspects, since the persons always will evaluate the demands which they have to cope with and, thus, will self-regulate their effort for processing.
  3. Information-processing integrates mental processes, representations (for example, knowledge, or mental models of a machine) and states (for example, states of consciousness, degrees of activation and, less formally, mood).
  4. MWL is a multidimensional characteristic of task requirements, since every task varies in a couple of interrelated but nevertheless distinct dimensions which separately must be dealt with in task design.
  5. MWL will have a multidimensional impact which at least will determine (a) behaviour, for example, the strategies and the resulting performance, (b) perceived, subjective short-term well-being with consequences for health in the long run, and (c) psycho-physiological processes, for example, alterations of blood pressure at work, which may become long-term effects of a positive kind (promoting, say, fitness improvement) or of a negative kind (involving impairments or ill-health).
  6. From the point of view of task design, MWL should not be minimized—as would be necessary in the case of carcinogenic air pollution—but optimized. The reason is that demanding mental task requirements are inevitable for well-being, health promotion and qualification since they offer the necessary activating impulses, fitness prerequisites and learning/training options. Missing demands on the contrary may result in deactivation, loss of physical fitness, de-qualification and deterioration of so-called intrinsic (task content-dependent) motivation. Findings in this area led to the technique of health and personality promoting task design (Hacker 1986).
  7. MWL therefore, in any case, must be dealt with in task analysis, task requirement evaluation as well as in corrective and prospective task design.

 

Theoretical Approaches: Requirement-Resources Approaches

From the person-environment fit point of view, MWL and its consequences may be roughly categorized—as is shown in figure 1—into underload, properly fitting load, and overload. This categorization results from the relationships between task requirements and mental capabilities or resources. Task requirements may exceed, fit with or fail to be satisfied by the resources. Both types of misfit may result from quantitative or qualitative modes of misfit and will have qualitatively differing, but in any case negative, consequences (see figure 1).

Figure 1. Types and consequences of requirements-resources relationships

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Some theories attempt to define MWL starting from the resource or capacity side of the requirements, namely, resources relationships. These resource theories might be subdivided into resource volume and resource allocation theories (Wieland-Eckelmann 1992). The amount of available capacity may come from a single source (single resource theories) which determines processing. The availability of this resource varies with arousal (Kahneman 1973). Modern multiple resource theories suppose a set of relatively independent processing resources. Thus, performance will depend on the condition whether the same resource or different resources are required simultaneously and concurrently. Different resources are, for example, encoding, processing or responding resources (Gopher and Donchin 1986; Welford 1986). The most critical problem for these types of theories is the reliable identification of one or more well-defined capacities for qualitatively different processing operations.

Resource allocation theories suppose qualitatively changing processing as a function of varying strategies. Depending on the strategies, differing mental processes and representations may be applied for task accomplishment. Thus, not the volume of stable resources but flexible allocation strategies become the key point of interest. Again, however, essential questions—especially concerning the methods of diagnosis of the strategies—remain to be answered.

 

 

Assessment of MWL: using requirement-resource approaches

A strict measurement of MWL at present would be impossible since well-defined units of measurement are lacking. But, to be sure, the conceptualization and the instruments for an assessment should meet the general quality criteria of diagnostic approaches, which have objectivity, reliability, validity and usefulness. However, as of now, only a little is known about the overall quality of proposed techniques or instruments.

There are a sizeable number of reasons for the remaining difficulties with assessing MWL according to the requirement-resource approaches (O’Donnell and Eggemeier 1986). An attempt at MWL assessment has to cope with questions like the following: is the task self-intended, following self-set goals, or is it directed with reference to an externally defined order? Which type of capacities (conscious intellectual processing, application of tacit knowledge, etc.) are required, and are they called upon simultaneously or sequentially? Are there different strategies available and, if so, which ones? Which coping mechanisms of a working person might be required?

The most often discussed approaches try to assess MWL in terms of:

    1. required effort (effort assessment) approaches applying—in some versions psychophysiologically validated—scaling procedures such as those offered by Bartenwerfer (1970) or Eilers, Nachreiner and Hänicke (1986), or
    2. occupied or, vice versa, residual mental capacity (mental capacity assessment) approaches applying the traditional dual task techniques as, for example, discussed by O’Donnell and Eggemeier (1986).

       

      Both approaches are heavily dependent on the assumptions of single resource theories and consequently have to struggle with the above-mentioned questions.

      Effort assessment. Such effort assessment techniques as, for example, the scaling procedure applied to a perceived correlate of the general central activation, developed and validated by Bartenwerfer (1970), offer verbal scales which may be completed by graphic ones and which grade the unidimensionally varying part of the perceived required effort during task accomplishment. The subjects are requested to describe their perceived effort by means of one of the steps of the scale provided.

      The quality criteria mentioned above are met by this technique. Its limitations include the unidimensionality of the scale, covering an essential but questionable part of perceived effort; the limited or absent possibility of forecasting perceived personal task outcomes, for example, in terms of fatigue, boredom or anxiety; and especially the highly abstract or formal character of effort which will identify and explain nearly nothing of the content-dependent aspects of MWL as, for example, any possible useful applications of the qualification or the learning options.

      Mental capacity assessment. The mental capacity assessment consists of the dual task techniques and a related data interpretation procedure, called the performance operating characteristic (POC). Dual task techniques cover several procedures. Their common feature is that subjects are requested to perform two tasks simultaneously. The crucial hypothesis is: the less an additional or secondary task in the dual task situation will deteriorate in comparison with the base-line single task situation, the lower the mental capacity requirements of the primary task, and vice versa. The approach is now broadened and various versions of task interference under dual task conditions are investigated. For example, the subjects are instructed to perform two tasks concurrently with graded variations of the priorities of the tasks. The POC curve graphically illustrates the effects of possible dual-task combinations arising from sharing limited resources among the concurrently performed tasks.

      The critical assumptions of the approach mainly consist in the suggestions that every task will require a certain share of a stable, limited conscious (versus unconscious, automated, implicit or tacit) processing capacity, in the hypothetical additive relationship of the two capacity requirements, and in the restriction of the approach to performance data only. The latter might be misleading for several reasons. First of all there are substantial differences in the sensitivity of performance data and subjectively perceived data. Perceived load seems to be determined mainly by the amount of required resources, often operationalized in terms of working memory, whereas performance measures seem to be determined predominantly by the efficiency of the sharing of resources, depending on allocation strategies (this is dissociation theory; see Wickens and Yeh 1983). Moreover, individual differences in information processing abilities and personality traits strongly influence the indicators of MWL within the subjective (perceived), performance and psychophysiological areas.

      Theoretical Approaches: Task Requirement Approaches

      As has been shown, task requirements are multidimensional and, thus, may not be described sufficiently by means of only one dimension, whether it be the perceived effort or the residual conscious mental capacity. A more profound description might be a profile-like one, applying a theoretically selected pattern of graded dimensions of task characteristics. The central issue is thus the conceptualization of “task”, especially in terms of task content, and of “task accomplishment”, especially in terms of the structure and phases of goal-oriented actions. The role of the task is stressed by the fact that even the impact of contextual conditions (like temperature, noise or working hours) on the persons are task-dependent, since they are mediated by the task acting as a gate device (Fisher 1986). Various theoretical approaches sufficiently agree regarding those critical task dimensions, which offer a valid prediction of the task outcome. In any case, task outcome is twofold, since (1) the intended result must be achieved, meeting the performance-outcome criteria, and (2) a number of unintended personal short-term and cumulative long-term side effects will emerge, for example fatigue, boredom (monotony), occupational diseases or improved intrinsic motivation, knowledge or skills.

      Assessment of MWL. With task requirement approaches, action-oriented approaches like those of complete versus partialized actions or the motivation potential score (for an elaboration of both see Hacker 1986), propose as indispensable task characteristics for analysis and evaluation at least the following:

      • temporal and procedural autonomy regarding decisions on self-set goals and, consequently, transparency, predictability and control of the work situation
      • number and variety of subtasks (especially concerning preparation, organization and checking of the results of implementation) and of actions accomplishing these subtasks (i.e., whether such actions involve cyclical completeness versus fragmentation)
      • variety (“level”) of action-regulating mental processes and representations. These may be mentally automated or routinized ones, knowledge-based “if-then” ones or intellectual and problem-solving ones. (They may also be characterized by hierarchical completeness as opposed to fragmentation)
      • required cooperation
      • long-term learning requirements or options.

       

      The identification of these task characteristics requires the joint procedures of job/task analysis, including document analyses, observations, interviews and group discussions, which must be integrated in a quasi-experimental design (Rudolph, Schönfelder and Hacker 1987). Task analysis instruments which may guide and assist the analysis are available. Some of them assist only the analysis (for example, NASA-TLX Task Load Index, Hart and Staveland, 1988) while others are useful for evaluation and design or redesign. An example here is the TBS-GA (Tätigkeitsbewertungs System für geistige Arbeit [Task Diagnosis Survey—Mental Work]); see Rudolph, Schönfelder and Hacker (1987).

       

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      Monday, 14 March 2011 19:04

      Vigilance

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      The concept of vigilance refers to a human observer’s state of alertness in tasks that demand efficient registration and processing of signals. The main characteristics of vigilance tasks are relatively long durations and the requirement to detect infrequent and unpredictable target stimuli (signals) against a background of other stimulus events.

      Vigilance Tasks

      The prototypical task for vigilance research was that of radar operators. Historically, their apparently unsatisfactory performance during the Second World War has been a major impetus for the extensive study of vigilance. Another major task requiring vigilance is an industrial inspection. More generally, all kinds of monitoring tasks which require the detection of relatively infrequent signals embody the risk of failures to detect and to respond to these critical events.

      Vigilance tasks make up a heterogeneous set and vary on several dimensions, in spite of their common characteristics. An obviously important dimension is the overall stimulus rate as well as the rate of target stimuli. It is not always possible to define the stimulus rate unambiguously. This is the case in tasks that require the detection of target events against continuously presented background stimuli, as in detecting critical values on a set of dials in a monitoring task. A less obviously important distinction is that between successive-discrimination tasks and simultaneous-discrimination tasks. In simultaneous-discrimination tasks, both target stimuli and background stimuli are present at the same time, while in successive-discrimination tasks one is presented after the other so that some demands on memory are made. Although most vigilance tasks require the detection of visual stimuli, stimuli in other modalities have also been studied. Stimuli can be confined to a single spatial location, or there can be different sources for target stimuli. Target stimuli can differ from background stimuli by physical characteristics, but also by more conceptual ones (like a certain pattern of meter readings that can differ from other patterns). Of course, the conspicuousness of targets can vary: some can be detected easily, while others may be hard to discriminate from background stimuli. Target stimuli can be unique or there can be sets of target stimuli without well-defined boundaries to set them off from background stimuli, as is the case in many industrial inspection tasks. This list of dimensions on which vigilance tasks differ can be expanded, but even this length of the list suffices to emphasize the heterogeneity of vigilance tasks and thus the risks involved in generalizing certain observations across the full set.

      Performance Variations and the Vigilance Decrement

      The most frequently used performance measure in vigilance tasks is the proportion of target stimuli, for example, faulty products in industrial inspection, that have been detected; this is an estimate of the probability of so-called hits. Those target stimuli that remain unnoticed are called misses. Although the hit rate is a convenient measure, it is somewhat incomplete. There is a trivial strategy that allows one to achieve 100% hits: one only has to classify all stimuli as targets. However, the hit rate of 100% is then accompanied by a false-alarm rate of 100%, that is, not only the target stimuli are correctly detected, but the background stimuli are incorrectly “detected” as well. This line of reasoning makes it quite clear that whenever there are false alarms at all, it is important to know their proportion in addition to the hit rate. Another measure for performance in a vigilance task is the time needed to respond to target stimuli (response time).

      Performance in vigilance tasks exhibits two typical attributes. The first one is the low overall level of vigilance performance. It is low in comparison with an ideal situation for the same stimuli (short observation periods, high readiness of the observer for each discrimination, etc.). The second attribute is the so-called vigilance decrement, the decline of performance in the course of the watch which can start within the first few minutes. Both these observations refer to the proportion of hits, but they have also been reported for response times. Although the vigilance decrement is typical of vigilance tasks, it is not universal.

      In investigating the causes of poor overall performance and vigilance decrements, a distinction will be made among concepts that are related to the basic characteristics of the task and concepts that are related to organismic and task-unrelated situational factors. Among the task-related factors strategic and non-strategic ones can be distinguished.

      Strategic processes in vigilance tasks

      The detection of a signal like a faulty product is partly a matter of the observer’s strategy and partly a matter of the signal’s discriminability. This distinction is based on the theory of signal detection (TSD), and some basics of the theory need to be presented in order to highlight the distinction’s importance. Consider a hypothetical variable, defined as “evidence for the presence of a signal”. Whenever a signal is presented, this variable takes on some value, and whenever a background stimulus is presented, it takes on a value that is lower on the average. The value of the evidence variable is assumed to vary across repeated presentations of the signal. Thus it can be characterized by a so-called probability density function as is illustrated in figure 1. Another density function characterizes the values of the evidence variable upon presentation of a background stimulus. When the signals are similar to the background stimuli, the functions will overlap, so that a certain value of the evidence variable can originate either from a signal or from a background stimulus. The particular shape of the density functions of figure 1 is not essential for the argument.

      Figure 1. Thresholds and discriminability

      ERG130F1

      The detection response of the observer is based on the evidence variable. It is assumed that a threshold is set so that a detection response is given whenever the value of the evidence variable is above the threshold. As is illustrated in figure 1, the areas under the density functions to the right of the threshold correspond to the probabilities of hits and false alarms. In practice, estimates of the separation of the two functions and the location of the threshold can be derived. The separation of the two density functions characterizes the discriminability of the target stimuli from the background stimuli, while the location of the threshold characterizes the observer’s strategy. Variation of the threshold produces a joint variation of the proportions of hits and false alarms. With a high threshold, the proportions of hits and false alarms will be small, while with a low threshold the proportions will be large. Thus, the selection of a strategy (placement of the threshold) essentially is the selection of a certain combination of hit rate and false-alarm rate among the combinations that are possible for a certain discriminability.

      Two major factors that influence the location of the threshold are payoffs and signal frequency. The threshold will be set to lower values when there is much to gain from a hit and little to lose from a false alarm, and it will be set to higher values when false alarms are costly and the benefits from hits are small. A low threshold setting can also be induced by a high proportion of signals, while a low proportion of signals tends to induce higher threshold settings. The effect of signal frequency on threshold settings is a major factor for the low overall performance in terms of the proportion of hits in vigilance tasks and for the vigilance decrement.

      An account of the vigilance decrement in terms of strategic changes (threshold changes) requires that the reduction of the proportion of hits in the course of the watch is accompanied by a reduction of the proportion of false alarms. This is, in fact, the case in many studies, and it is likely that the overall poor performance in vigilance tasks (in comparison with the optimal situation) does also result, at least partly, from a threshold adjustment. In the course of a watch, the relative frequency of detection responses comes to match the relative frequency of targets, and this adjustment implies a high threshold with a relatively small proportion of hits and a relatively small proportion of false alarms as well. Nevertheless, there are vigilance decrements that result from changes in discriminability rather than from changes in threshold settings. These have been observed mainly in successive-discrimination tasks with a relatively high rate of stimulus events.

       

       

      Nonstrategic processes in vigilance tasks

      Although part of the overall poor performance in vigilance tasks and many instances of the vigilance decrement can be accounted for in terms of strategic adjustments of the detection threshold to low signal rates, such an account is not complete. There are changes in the observer during a watch that can reduce the discriminability of stimuli or result in apparent threshold shifts that cannot be considered as an adaptation to the task characteristics. In the more than 40 years of vigilance research, a number of nonstrategic factors that contribute to poor overall performance and to the vigilance decrement have been identified.

      A correct response to a target in a vigilance task requires a sufficiently precise sensory registration, an appropriate threshold location, and a link between the perceptual processes and the associated response-related processes. During the watch the observers have to maintain a certain task set, a certain readiness to respond to target stimuli in a certain way. This is a nontrivial requirement because without a particular task set no observer would respond to target stimuli in the way required. Two major sources of failures are thus inaccurate sensory registration and lapses in the readiness to respond to target stimuli. Major hypotheses to account for such failures will be briefly reviewed.

      Detection and identification of a stimulus are faster when there is no temporal or spatial uncertainty about its appearance. Temporal and/or spatial uncertainty is likely to reduce vigilance performance. This is the essential prediction of expectancy theory. Optimal preparedness of the observer requires temporal and spatial certainty; obviously vigilance tasks are less than optimal in this respect. Although the major focus of expectancy theory is on the overall low performance, it can also serve to account for parts of the vigilance decrement. With infrequent signals at random intervals, high levels of preparedness might initially exist at times when no signal is presented; in addition, signals will be presented at low levels of preparedness. This discourages occasional high levels of preparedness in general so that whatever benefits accrue from them will vanish in the course of a watch.

      Expectancy theory has a close relation to attentional theories. Variants of attentional theories of vigilance, of course, are related to dominant theories of attention in general. Consider a view of attention as “selection for processing” or “selection for action”. According to this view, stimuli are selected from the environment and processed with high efficiency whenever they serve the currently dominant action plan or task set. As already said, the selection will benefit from precise expectations about when and where such stimuli will occur. But stimuli will only be selected if the action plan—the task set—is active. (Drivers of cars, for example, respond to traffic lights, other traffic, etc.; passengers don’t do so normally, although both are in almost the same situation. The critical difference is that between the task sets of the two: only the driver’s task set requires responses to traffic lights.)

      The selection of stimuli for processing will suffer when the action plan is temporarily deactivated, that is when the task set is temporarily absent. Vigilance tasks embody a number of features that discourage continuous maintenance of the task set, like short cycle times for processing stimuli, lack of feedback and little motivational challenge by apparent task difficulty. So-called blockings can be observed in almost all simple cognitive tasks with short cycle times like simple mental arithmetic or rapid serial responses to simple signals. Similar blockings occur in the maintenance of the task set in a vigilance task as well. They are not immediately recognizable as delayed responses because responses are infrequent and targets that are presented during a period of absent task set may no longer be there when the absence is over so that a miss will be observed instead of a delayed response. Blockings become more frequent with time spent on the task. This can give rise to the vigilance decrement. There may be additional reasons for temporary lapses in the availability of the appropriate task set, for example, distraction.

      Certain stimuli are not selected in the service of the current action plan, but by virtue of their own characteristics. These are stimuli that are intense, novel, moving toward the observer, have an abrupt onset or for any other reason might require immediate action no matter what the current action plan of the observer is. There is little risk of not detecting such stimuli. They attract attention automatically, as is indicated, for example, by the orienting response, which includes a shift of the direction of the gaze toward the stimulus source. However, answering an alarm bell is not normally considered a vigilance task. In addition to stimuli that attract attention by their own characteristics, there are stimuli that are processed automatically as a consequence of the practice. They seem to “pop out” from the environment. This kind of automatic processing requires extended practice with a so-called consistent mapping, that is, a consistent assignment of responses to stimuli. The vigilance decrement is likely to be small or even absent once automatic processing of stimuli has been developed.

      Finally, vigilance performance suffers from a lack of arousal. This concept refers in a rather global manner to the intensity of neural activity, ranging from sleep through normal wakefulness to high excitement. One of the factors that is thought to affect arousal is external stimulation, and this is fairly low and uniform in most vigilance tasks. Thus, the intensity of central nervous system activity can decline overall over the course of a watch. An important aspect of arousal theory is that it links vigilance performance to various task-unrelated situational factors and factors related to the organism.

      The Influence of Situational and Organismic Factors

      Low arousal contributes to poor performance in vigilance tasks. Thus performance can be enhanced by situational factors that tend to enhance arousal, and it can be reduced by all measures that reduce the level of arousal. On balance, this generalization is mostly correct for the overall performance level in vigilance tasks, but the effects on the vigilance decrement are absent or less reliably observed across different kinds of manipulation of arousal.

      One way to raise the level of arousal is the introduction of additional noise. However, the vigilance decrement is generally unaffected, and with respect to overall performance the results are inconsistent: enhanced, unchanged and reduced performance levels have all been observed. Perhaps the complex nature of noise is relevant. For example, it can be affectively neutral or annoying; it cannot only be arousing, but also be distracting. More consistent are the effects of sleep deprivation, which is “de-arousing”. It generally reduces vigilance performance and has sometimes been seen to enhance the vigilance decrement. Appropriate changes of vigilance performance have also been observed with depressant drugs like benzodiazepines or alcohol and stimulant drugs like amphetamine, caffeine or nicotine.

      Individual differences are a conspicuous feature of performance in vigilance tasks. Although individual differences are not consistent across all sorts of vigilance tasks, they are fairly consistent across similar ones. There is only little or no effect of sex and general intelligence. With respect to age, vigilance performance increases during childhood and tends to decline beyond the age of sixty. In addition there is a good chance that introverts will show better performance than extroverts.

      The Enhancement of Vigilance Performance

      The existing theories and data suggest some means to enhance vigilance performance. Depending on the amount of specificity of the suggestions, it is not difficult to compile lists of various lengths. Some rather broad suggestions are given below that have to be fitted to specific task requirements. They are related to the ease of perceptual discriminations, the appropriate strategic adjustments, the reduction of uncertainty, the avoidance of the effects of attentional lapses and the maintenance of arousal.

      Vigilance tasks require discriminations under non-optimal conditions. Thus one is well advised in making the discriminations as easy as possible, or the signals as conspicuous as possible. Measures related to this general goal can be straightforward (like appropriate lighting or longer inspection times per product) or more sophisticated, including special devices to enhance the conspicuousness of targets. Simultaneous comparisons are easier than successive ones, so the availability of a reference standard can be helpful. By means of technical devices, it is sometimes possible to present the standard and the object to be examined in rapid alternation, so that differences will appear as motions in the display or other changes for which the visual system is particularly sensitive.

      To counteract the strategic changes of the threshold that lead to a relatively low proportion of correct detections of targets (and for making the task less boring in terms of the frequency of actions to be taken) the suggestion has been made to introduce fake targets. However, this seems not to be a good recommendation. Fake targets will increase the proportion of hits overall but at the cost of more frequent false alarms. In addition, the proportion of undetected targets among all stimuli that are not responded to (the outgoing faulty material in an industrial inspection task) will not necessarily be reduced. Better suited seems to be explicit knowledge about the relative importance of hits and false alarms and perhaps other measures to obtain an appropriate placement of the threshold for deciding between “good” and “bad”.

      Temporal and spatial uncertainty are important determinants of poor vigilance performance. For some tasks, spatial uncertainty can be reduced by way of defining a certain position of the object to be inspected. However, little can be done about temporal uncertainty: the observer would be unnecessary in a vigilance task if the occurrence of a target could be signaled in advance of its presentation. One thing that can be done in principle, however, is to mix objects to be inspected if faults tend to occur in bunches; this serves to avoid very long intervals without targets as well as very short intervals.

      There are some obvious suggestions for the reduction of attentional lapses or at least their impact on performance. By proper training, some kind of automatic processing of targets can perhaps be obtained provided that the background and target stimuli are not too variable. The requirement for sustained maintenance of the task set can be avoided by means of frequent short breaks, job rotation, job enlargement or job enrichment. Introduction of variety can be as simple as having the inspector himself or herself getting the material to be inspected from a box or other location. This also introduces self-pacing, which may help in avoiding signal presentations during temporary deactivations of the task set. Sustained maintenance of task set can be supported by means of feedback, indicated interest by supervisors and operator’s awareness of the importance of the task. Of course, accurate feedback of performance level is not possible in typical vigilance tasks; however, even inaccurate or incomplete feedback can be helpful as far as the observer’s motivation is concerned.

      There are some measures that can be taken to maintain a sufficient level of arousal. Continuous use of drugs may exist in practice but is never found among recommendations. Some background music can be useful, but can also have an opposite effect. Social isolation during vigilance tasks should mostly be avoided, and during times of day with low levels of arousal like the late hours of the night, supportive measures such as short watches are particularly important.

       

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      Monday, 14 March 2011 19:11

      Mental Fatigue

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      Mental strain is a normal consequence of the coping process with mental workload (MWL). Long-term load or a high intensity of job demands can result in short-term consequences of overload (fatigue) and underload (monotony, satiation) and in long-term consequences (e.g., stress symptoms and work-related diseases). The maintenance of the stable regulation of actions while under strain can be realized through changes in one’s action style (by variation of strategies of information-seeking and decision-making), in the lowering of the level of need for achievement (by redefinition of tasks and reduction of quality standards) and by means of a compensatory increase of psychophysiological effort and afterwards a decrease of effort during work time.

      This understanding of the process of mental strain can be conceptualized as a transactional process of action regulation during the imposition of loading factors which include not only the negative components of the strain process but also the positive aspects of learning such as accretion, tuning and restructuring and motivation (see figure 2).

      Figure 1. Components of the process of strain and its consequences

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      Mental fatigue can be defined as a process of time-reversible decrement of behavioural stability in performance, mood and activity after prolonged working time. This state is temporarily reversible by changing the work demands, the environmental influences or stimulation and is completely reversible by means of sleep.

      Mental fatigue is a consequence of performing tasks with a high level of difficulty that involve predominantly information processing and/or are of protracted duration. In contrast with monotony, the recovery of the decrements is time-consuming and does not occur suddenly after changing task conditions. Symptoms of fatigue are identified on several levels of behavioural regulation: dis-regulation in the biological homeostasis between environment and organism, dis-regulation in the cognitive processes of goal-directed actions and loss of stability in goal-oriented motivation and achievement level.

       

       

       

       

       

       

       

       

       

       

      Symptoms of mental fatigue can be identified in all subsystems of the human information processing system:

      • perception: reduced eye movements, reduced discrimination of signals, threshold deterioration
      • information processing: extension of decision time, action slips, decision uncertainty, blockings, “risky strategies” in action sequences, disturbances in sensorimotor coordination of movements
      • memory functions: prolongation of information in ultrashort-term storages, disturbances in the rehearsal processes in short-term memory, delay in information transmission in long-term memory and in memory searching processes.

      Differential Diagnostic of Mental Fatigue

      Sufficient criteria exist to differentiate amongst menta fatigue, monotony, mental satiation and stress (in a narrow sense) (table 1).

      Table 1. Differentiation among several negative consequences of mental strain

      Criteria

      Mental fatigue

      Monotony

      Satiation        

      Stress

      Key
      condition

      Poor fit in terms of overload
      preconditions

      Poor fit in terms
      of underload
      preconditions

      Loss of perceived sense of tasks

      Goals perceived
      as threatening

      Mood

      Tiredness without
      boredom exhaustion

      Tiredness with
      boredom

      Irritability

      Anxiety, threat
      aversion

      Emotional
      evaluation

      Neutral

      Neutral

      Increased affective aversion

      Increased anxiety

      Activation

      Continuously
      decreased

      Not continuously
      decreased

      Increased

      Increased

      Recovery

      Time-consuming

      Suddenly after task alternation

      ?

      Long-term
      disturbances in recovery

      Prevention

      Task design,
      training, short-break
      system

      Enrichment of job content

      Goal-setting
      programmes
      and job
      enrichment

      Job redesign,
      conflict and stress management

       

      Degrees of Mental Fatigue

      The well-described phenomenology of mental fatigue (Schmidtke 1965), many valid methods of assessment and the great amount of experimental and field results offer the possibility of an ordinal scaling of degrees of mental fatigue (Hacker and Richter 1994). The scaling is based on the individual’s capacity to cope with behavioural decrements:

      Level 1: Optimal and efficient performance: no symptoms of decrement in performance, mood and activation level.

      Level 2: Complete compensation characterized by increased peripheral psycho-physiological activation (e.g., as measured by electromyogram of finger muscles), perceived increase of mental effort, increased variability in performance criteria.

      Level 3: Labile compensation additional to that described in level 2: action slips, perceived fatigue, increasing (compensatory) psycho-physiological activity in central indicators, heart rate, blood pressure.

      Level 4: Reduced efficiency additional to that described in level 3: decrease of performance criteria.

      Level 5: Yet further functional disturbances: disturbances in social relationships and cooperation at workplace; symptoms of clinical fatigue like loss of sleep quality and vital exhaustion.

      Prevention of Mental Fatigue

      The design of task structures, environment, rest periods during working time and sufficient sleep are the ways to reduce symptoms of mental fatigue in order that no clinical consequences will occur:

      1. Changes in the structure of tasks. Designing of preconditions for adequate learning and task structuring is not only a means of furthering the development of efficient job structures, but is also essential for the prevention of a misfit in terms of mental overload or underload:

        • Information processing burdens can be relieved by developing efficient internal task representations and organization of information. The resulting enlargement of cognitive capacity will match information needs and resources more aptly.
        • Human-centred technologies with high compatibility between the order of information as it is presented and the required task (Norman 1993) will reduce the mental effort necessary for information recoding and, in consequence, relieve symptoms of fatigue and stress.
        • Well-balanced coordination of different levels of regulations (as they apply to skills, rules and knowledge) may reduce effort and, moreover, increase human reliability (Rasmussen 1983).
        • Training workers in goal-directed action sequences in advance of actual problems will lighten their sense of mental effort by making their jobs clearer, more predictable and more evidently under their control. Their psychophysiological activation level will be effectively reduced.

         

          2. Introduction of systems of short-term breaks during work. The positive effects of such breaks depend on the observance of some preconditions. More short breaks are more efficient than fewer long breaks; effects depend on a fixed and therefore anticipatable time schedule; and the content of the breaks should have a compensatory function to the physical and mental job demands.

          3. Sufficient relaxation and sleep. Special employee-assistant programmes and stress-management techniques may support the ability of relaxation and the prevention of the development of chronicle fatigue (Sethi, Caro and Schuler 1987).

           

          Back

          Contents

          Preface
          Part I. The Body
          Part II. Health Care
          Part III. Management & Policy
          Part IV. Tools and Approaches
          Biological Monitoring
          Epidemiology and Statistics
          Ergonomics
          Goals, Principles and Methods
          Physical and Physiological Aspects
          Organizational Aspects of Work
          Work Systems Design
          Designing for Everyone
          Diversity and Importance of Ergonomics
          Resources
          Occupational Hygiene
          Personal Protection
          Record Systems and Surveillance
          Toxicology
          Part V. Psychosocial and Organizational Factors
          Part VI. General Hazards
          Part VII. The Environment
          Part VIII. Accidents and Safety Management
          Part IX. Chemicals
          Part X. Industries Based on Biological Resources
          Part XI. Industries Based on Natural Resources
          Part XII. Chemical Industries
          Part XIII. Manufacturing Industries
          Part XIV. Textile and Apparel Industries
          Part XV. Transport Industries
          Part XVI. Construction
          Part XVII. Services and Trade
          Part XVIII. Guides

          Ergonomics Additional Resources

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