On the investigation of the neurophysiological correlates of knowledge worker mental fatigue using the EEG signal

Applied Ergonomics 199425(6) 355-365

On the investigation of the neurophysiological correlates of knowledge worker mental fatigue using the EEG signal O. Geoffrey Okogbaa* Department of Industrial and Management Systems, College of Engineering, University of South Florida, 4202 East Faulen Avenue, Tampa, FL 33620--5350, USA

Richard L. Shell Department of Mechanical, Industrial and Nuclear Engineering, University of Cincinnati, Cincinnati, OH, USA

Davorka Filipusic Department of Industrial and Management Systems, College of Engineering, University of South Florida, Tampa, USA/University of Zagreb Technological trends and advances in automation have underscored the importance of task performance of certain jobs requiring mental functions such as information processing and decision analyses. Most experts agree that such work environments produce increased mental activities, with profound implications for mental fatigue and stress. Consequently, productivity measurement and improvement for white collar or 'knowledge worker' occupations remains a major challenge and concern. This investigation defines an experimental approach that examines the neurophysiological correlates of white collar worker mental fatigue using the EEG signal. A 6 h laboratory experiment was conducted to simulate work output. The methods of assessing fatigue employed were mental tests and physiological measurements. The experiment involved reading of standardized texts, finding solutions to arithmetic--logical problems and a combination of both task types. Two primary performance measures were obtained, work output and brain waves. Fast Fourier transform and correlation analyses are used to quantify the relationship between certain brain waves and mental fatigue. This research is a major step towards the development of a model that explores the relationship between mental fatigue and factors associated with output performance, optimal recuperation periods and related variables. Such a model would be useful in human reliability prediction based on task parameters and worker profiles. Keywords: mental fatigue, stress, neurophysiologicalcorrelates, knowledgeworkers, Fourier transform The need to improve quality of life through reduction and easing of work task loads has always been a major challenge and concern to engineers and scientists interested in the design of work systems. The industrial revolution marked a significant reduction in manual effort through mechanization, while the advent of the digital computer led to the reduction in the effort required for information processing tasks. During the past decade there has been a growing trend towards more knowledge/information-based occupations. According to Drueker (1982), the term 'knowledge worker' refers to a worker who 'by virtue of position or knowledge, is responsible for a contribution that materially affects the capacity of the organization to perform and obtain results'. This clssification includes *Corresponding author

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executives, managers, and professionals such as design engineers, accountants, marketing managers and industrial engineers (Brisley and Fielder, 1983). Today, the white collar or knowledge worker accounts for about 50% of the US workforce (USBL, 1992). Current U S B L projections are that by the year 2000, the number of workers in knowledge/informationbased occupations will increase to 70 million from about 40 million in 1990. Additionally, USBL has projected that the percentage of managers and administrators will grow about 22% through the 1980s and 1990s. At the same time, professional and technical workers are projected to increase from 14.2 million in 1978 to 16.9 million in the early 1990s for an increase of 19%. On the other hand, decline in the blue collar workforce is expected to contribute to a decrease of about 33.5% of the total workforce in the 1990s.

0003-6870/941060355-11 ~) 1994 Butterworth-Heinemann Ltd

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Neurophysiological correlates of knowledge worker mental fatigue." O.G. Okogbaa et al

The increase in the number of white collar workers makes it important to investigate the activities of these workers. Additionally, it is important to attempt to improve their output. A major problem in this is the phenomenon of mental fatigue. As Cameron (1971) succinctly suggests, the complex phenomenon of mental fatigue is on the increase while simple physical fatigue is on the decrease, owing to the increased knowledge/ information content of most jobs. While several studies suggest that repetitive white collar worker related tasks (such as engineering design, medical diagnostics, process monitoring, vigilance, or military command and control) performed for sustained periods of time could increase fatigue, reduce productivity, and alter cardiovascular and neurophysiological functioning (Weber et al, 1980; Smith, 1981; Okogbaa and Shell, 1986; Schleifer and Okogbaa, 1990), research concerning the temporal pattern of these effects has been sparse. Moreover, there has been little attempt to examine the inherent variability in task performance and neurophysiological activity, or the relationship between task performance and cardiovascular activity, over continuous intervals of time (e.g. minute by minute). As a result, an understanding of the temporal and functional relationships between performance variability and these psychophysiological measures is lacking. The absence of knowledge about such relationships limits efforts to define meaningful correlates of job/system characteristics that are prerequisites for proper job design. The establishment of such temporal ordering and precedence relationships will provide insight into the nature of the functional relationship between job characteristics that determine task performance and their physiological correlates. Current technological advances have enhanced the use of neurophysiological techniques for the investigation of the brain and neural process during mental activities. Bipolar recordings with electrodes implanted on different parts of the scalp (Cooper et al, 1980; Gogolitsin and Kropotov, 1981) and on the chest have been instrumental in the acquisition of data on the neurophysiology of human mental activities. Contemporary approaches include recording and processing different physiological parameters such as ECG, EEG and brain impedance (Okogbaa, 1983; Okogbaa and Shell, 1986). Weber et al (1980) showed that alpha activity is elevated during repetitive tasks. Gardner (1975) suggests that the alpha rhythm is characteristically responsive to arousal and mental activities. The relationship between alpha rhythms and mental activity is also supported by Bechtereva (1981), Haider et al (1981) and Gogolitsin and Kropotov (1991). Most authorities agree that a fatigue state should manifest itself generally as decrement in performance. Of particular interest is the deterioration in working performance as a result of having worked for a considerable length of time (hours). Mental fatigue is a gradual and cumulative process. Consequently, multiple measures taken on a number of sequential occasions in any work period should provide an indication of the presence of fatigue. Unfortunately, most of the studies utilizing direct objective measures of subjects engaged in mental tasks have shown conflicting results. In some of these there were no dramatic and reliable decrements

356

in performance while others clearly demonstrated some deterioration. The objective of this investigation was to identify an experimental approach suitable for determining the onset of mental fatigue through neurophysiological correlates. In this paper we first look at the history and definition of mental fatigue, and related issues of causal factors, mitigation regimens, measurement approaches, and classification methods. Then the experimental design used and some of the assumptions made 'in the research are presented, followed by data collection, data handling and reduction, including Fourier transform analyses of the E E G signal to provide the power spectra and the corresponding time and frequency components of the waveform. The next two sections deal with data analyses, including correlation and nonparametric analyses to determine possible assignable cause relationships, and results and discussions. The last section provides some conclusions about the study, some of the limitations and possible extensions.

Mental fatigue 'Fatigue' is a state that is familiar to all of us in everyday life. The term usually denotes a loss of efficiency and a disinclination for any kind of effort, but it is not a single, definite state. Fatigue is a diverse problem and the literature on it spans not only numerous disciplines ranging from engineering to medicine but eras as well. The subject of fatigue can be divided into two parts, physical and mental fatigue. Physical fatigue deals with the reduction of performance of the muscular system. Mental fatigue occurs with a sense of weariness, reduced alertness, and reduced mental performance (Grandjean, 1981). An important area of research interest for industrial engineers is mental fatigue and the examination of the various methodologies employed in its measurement as well as their applicability to the problem of determining the effect of long periods of continuous work on white collar worker performance. Interest in fatigue has been demonstrated a long time ago and actual documentation dates as far back as World War I. Research in fatigue since that time has passed through different eras of interest, three of which are relevant to us here. The first era of interest in the phenomena of fatigue was in England during World War I, and the research at this time was conducted by the Industrial Research Fatigue Board. The Board's studies dealt with environmental factors, hours of work, workplace design, and plant layout. The studies maintained that workers' output was retarded by fatigue and that alleviation of fatigue could maintain high productivity (Cameron, 1973). The second segment of research investigated mental fatigue among aviation workers, particularly pilots. These studies commenced during the 1940s and were spawned by the increased use of aviation in warfare. The studies were generally concerned with the quality of performance of the aviation crewmen, with pilot errors as the major point of interest. Prominent researchers include Drew (1940), Davis (1948) and Bartlett (1953). It should be noted that studies continue to this day investigating the relationship between

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Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al mental fatigue and pilot error, and there has been an increased interest in other aviation workers, such as air traffic controllers. The third segment of research also involves the operation of a transportation vehicle. This time the emphasis centred upon mental fatigue and driving. The research efforts for driving closely parallel those in aviation. The main concern of the studies was usually driver errors. Other concerns of many of these studies involved the relationship of sleep deprivation, driver performance and mental fatigue. Two researchers that have completed important studies in this area are Crawford (1961) and Brown (1967). Research in this area of driving and mental fatigue continues to this day. It is interesting to note that the major emphasis in the area of mental fatigue has been on the operation of vehicles. However, there has been a paucity of studies investigating the mental performance of the white collar worker. Mental fatigue is described by Cameron (1971, 1973) as a generalized response to stress over a period of time. Cameron interjects the notion of recovery time, defined as the period of time necessary to recover from mental fatigue. Cameron also delineates mental fatigue into two types: acute and chronic. Acute mental fatigue is defined as an aspect from which total recovery can occur under normal resting processes. Chronic mental fatigue represents the condition from which total recovery fails to transpire through normal resting. A variety of external factors can affect mental task performance. Some of these factors are the cardiac rhythm (Corlett and Mahadeva, 1970; Luczak and Laurig, 1973; Mulder and van der Meulen, 1973; Amaria, 1974, in Beith, 1981; Hitchen and Harness, 1980), nutrition, physical health (Wisner, 1981), environment, physical activity during the task (McGlyn et al, 1979; Sjoberg, 1980), and recuperation periods and their duration. The subject of recuperation periods has been investigated fairly thoroughly for physical fatigue but not as thoroughly for mental fatigue. However, some of the principles derived for physical fatigue may be valid for mental fatigue. For example, it has been found that the recovery value of a rest break is not directly related to time duration (Cakir et al, 1980). It appears that overall performance may be more improved by frequent, short-duration breaks than by longer, less frequent breaks (Sharit and Salvendy, 1982; Conn, 1984). According to these authors, besides the time factor, attention must be paid to the activity engaged in during a recuperation period. In Okogbaa's (1983) experiment the results showed a significant difference between output for rest versus no rest task conditions. Huston (1985) found in his experiment that subject differences were extremely significant, and rest break duration had only a weak significance. He also found that greatest overall efficiency occurred with the shortest rest breaks. However, while some research has been completed on each external factor, it still remains an area where further research needs to be done.

Measuring fatigue The science of ergonomics is just as interested in the quantitative measurement of mental fatigue as is

Applied Ergonomics 1994 Volume 25 Number 6

industry itself. Methods currently used fall into four groups: primary task measurement; secondary task measurement; subjective rating measures; physiological measures.

Primary task measures These are probably the most obvious method of mental workload assessment. For example, if we want to know how driving is affected by differing task demands (such as traffic conditions, fatigue or lane width) we should be able to utilize the driving performance itself as a criterion (Hicks and Wierwille, 1979). Secondary task measures A secondary task is one that the operator is asked to do in addition to his or her primary task. If he or she is able to perform well on both the primary and the secondary task this is taken to indicate that the primary task is relatively easy; if however, he or she is unable to perform the secondary task and at the same time maintain a reasonable level of performance on the primary task, then this indicates that the primary task is more demanding (Knowles, 1963; Brown and Poulton, 1978). The difference between the performance obtained under the two conditions, with and without inclusion of the primary taks, is then taken as a measure, or index, of the workload imposed by the primary task. Subjective rating measures These include direct or indirect queries to the individual for their opinion of the workload involved in a task. The easiest way to estimate the mental workload of a person who performs a certain task is to ask him or her what he or she feels about the mental load level of the task. Physiological Individuals who are subjected to some degree of mental workload commonly exhibit changes in a variety of physiological functions. As a result, several researchers have advocated the measurement of these changes to provide an estimate of the level of workload experienced. Among the most promising of the physiological measurements in use today is the electroencephalogram (EEG), a measure of the electrical activity present in the brain. The electrical activities are classified according to rhythms, and the rhythms are in turn defined in terms of frequency bands. The rhythmicity of E E G signals provides a means of quantitatively describing E E G records. Once the signal has been acquired, it is then broken down into its sine and cosine wave components and analysed for contents. The frequency bands of an E E G rhythm are typically grouped into the following classifications: alpha, theta, beta and delta bands. It is important, in attempting to study the mental fatigue of white collar workers, to distinguish the type of work activity in which the typical worker engages from other types of activities. While it may be difficult to dichotomize a task as strictly mental or physical, there is some agreement about the types of task that are reasonable indicators of mental or physical activities. Schouten et al (1962), Zwaga (1973),

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Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al

Laplat (1978), Bechtereva (1981) and Sjoberg (1980) all agree that tasks involving arithmetical-logical relationships are good representations of the basic mental operations that are fast becoming the major constituents of many industrial activities. A good proportion of white collar worker job requirements consist of cerebral activities. A typical work assignment may involve cognitive tasks, analytical tasks, or decisionmaking tasks. For this reason, mental tests have been selected as a means of measuring the effect of mental fatigue. In the literature, the test types are frequently mathematics-oriented and reading-oriented. Reading tasks are quite difficult to analyse and quantify. A reason for this difficulty is that reading is composed of a number of varying and interrelated factors involving both the material and the reader. Some of these factors are text length, text difficulty, reading rate, text interest or appeal, and text comprehension. Regarding the relationship between reading rate and difficulty, the studies of Pitcher (1953), Klare (1963), Miller and Coleman (1972) and Carver (1975) have shown that if reading rate is measured in terms of letters (or characters) per minute or standard words, then the reading rate is constant over varying material difficulty. However, in these experiments, the level of comprehension was not measured. The relationship between reading rate and comprehension has also been examined by several researchers. Goldstein (1940), Jester and Travers (1966), and Carver (1975) all maintain that there is a linear loss in comprehension as the reading rate is increased. Text length or duration is an element that is often overlooked by researchers in the area of reading. Usually, the researchers' experiments are of very short time duration. For example, in Carver (1982) the subjects read and were tested on a passage 330 words in length. The short time required is all too often ignored and researchers may assume, without justification, that their findings apply to reading over extended periods. In order to obtain a valid assessment of the effects of fatigue on cognitive tasks, the methods of measurement employed in the research reported here take into account the essential elements of all of the variables referred to above, such as text difficulty and length. Mathematically oriented tests represent another form of the mental tests that are used to simulate analytical tasks. They often involve the addition, multiplication and division of digits as well as problems of logic. In several studies (e.g. Bennett, 1976; Sjoberg, 1980; Okogbaa, 1983; Huston, 1985) the researchers used addition, multiplication and division of digits as well as textual, logical and trigonometric problems to represent the analytical or the problem-solving aspect of the knowledge worker task. The level of difficulty of both the Okogbaa and Huston studies was the equivalent of that of the Graduate Records Examination (GRE), the Law School Entrance Examination (LSAT) and the Graduate Management Examination (GMAT). In the investigation reported by Zwaga (1973) a task period of 45 min was used, and this author also reported the work by Kalsbeek and Ettema (1963), Ettema and Zielhius (1971) and Kalsbeek (1964) in which subjects performed at one-third of their personal maximum for four hours. In another investigation by

358

Bnuma and Diesfeldt (in Zwaga, 1993) a task period of 20 min was used during which subjects worked at 80% of their personal maximum. In a separate experiment, Zwaga (1973) used a task duration of 5 min, followed by 6 rain rest and then another 5 rain of work. Bennett (1976) used a task duration time of 3 h for an arithmetic task. A great majority of the studies examined used task times that were 1 h or less. Another form of mental test that is frequently used is the concentration test. This test requires the individual to pay close attention to a flow of input data and then make a response. A concentration test designed by Brown and Poulton (1978) has been popular with other researchers. The test requires the identification of a sequence of odd-even digits from a continuous stream of digits presented to a subject. The reaction-time mental test is a test that requires a quick response to a stimulus. Generally, reaction time tests are simply a binary choice test. A strong advocate of the binary choice task is Kalsbeek (Kalsbeek and Ettema, 1963; Kalsbeek, 1964). Experimental design and task description Assumptions and constraints

The average age of the subjects was 21.8 years with a standard deviation of 1.2 years. One argument against this sample population is that the subjects do not represent the wide range of professions that denote the white collar worker. The age of the subjects was also a concern as, although the age of white collar workers ranges from early adulthood to old age, the majority are middle aged. While these arguments are valid, there is a case to be made for using this particular sample population. By using a uniform sample population, the experimenter may be able to dispel any notion that the experimental results are due merely to the sample population differences rather than to the effects of the treatments. The homogeneity of the subjects also helps to justify comparisons between different experiments (both task types and task conditions) within the same sample population. Another constraint of the experiment was the modelling of the white collar workers' mental task. The spectrum of mental tasks performed by white collar workers makes them difficult to model, and the use of mental tests seems the best means of monitoring mental fatigue amongst white collar workers, It may be debated though that not all white collar workers solve mathematical problems or engage in prolonged reading. Another problem with using the mental tests is that for some white collar workers these mental tasks change frequently or involve tasks that are not easily quantifiable; for instance, the white collar worker may spend a large portion of his or her time in meetings. Still, most white collar workers at some point in time will engage in cognitive tasks, analytical tasks or decision tasks - hence the use of mental tests. In addition, several researchers agree that arithmetic problem-solving (Bennett, 1976; Sjoberg, 1980) and reading (Thorndike, 1971; Carver, 1975) constitute the bulk of most mental activities, and therefore solving analytical problems and reading (including comprehension) are representative of most mental effort and thus could be used to stimulate mental activities.

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Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al Description of tasks and conditions Reading. The two treatment groups engaged in the reading task were required to read, over a 5 h period, standardized texts made up of materials from the SAT and ACT reading examinations. For this research, the general requirements for the reading text were as follows. (1) The text should have constant difficulty level across the duration of the experiment so that any change in performance (reading rate and reading comprehension) would be attributable to factors other than material difficulty. Reading tests administered over different time periods were utilized, and while there was no specific assessment to determine the difficulty level, the assumption was that the difficulty level of similar tests over a fairly short period of time would be similar or fairly constant over the time period of interest. (2) The text should also be such that the quality of reading (i.e. comprehension) and the rate of reading by the subject could be obtained or estimated as a function of time. Subjects were instructed to read as fast as possible while maintaining maximum comprehension. The first group, which was designated the control group, was required to work for 5 h, the only restriction being occasional pauses for scheduled rest. The length of each rest period was 10 rain for every 50 min of continuous work. In all, there were four 50 min work periods each followed by 10 min of rest. Various rest duration periods ranging from 5 to 15 min have been suggested and used by various authors (Zwaga, 1973; Bennett, 1976; Sjoberg, 1980). NIOSH (National Institute of Occupational Safety and Health) recommends 15 min for 2 h of continuous VDT work for moderate work load and 15 min for 1 h of continuous work under high load. A timing device (pulse count, alarm clock) was used to indicate the end of each work session. The experimental group was required to perform the same task as the control group but with no scheduled rest. To control for 'skimming', a test of comprehension was scheduled during each work period.

Arithmetic problem solving. A standardized arithmetic test package was developed from materials taken from the SAT and ACT examinations analytical section. The requirement for the analytical problems was that the difficulty level should remain constant throughout the experiment. An additional requirement was that the difficulty level should not exceed the capabilities of the experimental subjects consisting of engineering college sophomores and juniors. The type of task involved was one that induces a high information load. Standardized tests were chosen and the criterion was that the problems be such that they should be completed by over 95% of the population in the skill level chosen. The problems were also challenging enough to require some thought before a response. The control group had a scheduled rest: 10 min for every 50 min of continuous work. The experimental group had no scheduled rest. Two treatment groups engaged in this task.

Applied Ergonomics 1994 Volume 25 Number 6

Combined reading and problem solving. Two groups were involved in both reading and problem solving. The control group performed the reading task for one half of the 5 h period and the problem solving task for the other half of the work period, with the rest scheduled as before. The experimental group performed the combined tests but with no rest. Procedure Standard experimental procedure was adhered to in the study. Fourteen male subjects were randomly selected from a group of male undergraduate engineering students registered in a junior-level engineering human factors/ergonomics course at the time of the experiment. Participation in the experiment was one way of partially fulfilling the requirements of the course. All the subjects were unpaid volunteers. Of the 14 subjects, two were used for the pilot study that preceded the actual experiments (to validate the test materials and debug the experimental procedures) and the remainder (12 in all) were the actual experimental subjects. A sample size of 12 was chosen because this enabled adequate control of the experiment and it also provided a schedule that was flexible enough to accommodate the needs of the subjects within a ten-week academic quarter. In this investigation, the experimental tasks consisted of the reading of standardized text materials and obtaining solutions to arithmetic-logical problems. For each subject, the experiment was run in one allday session of about 6 h (5 h for actual data acquisition and 1 h for preparation). The experimental design was implemented via a modified incomplete 23 design (Table 1). In addition to the mental tests, the subjects were monitored for changes in EKG and EEG output. A subjective rating test was also administered before and at the conclusion of each session. The task duration and the fact that subjects were all unpaid volunteers precluded scheduling the subjects for more than 5 h for the entire experiment. In addition, the actual number of hours taken up by the experiment, including preparation time, subjective rating assessment and so on was slightly over 6 h. Consequently only one treatment per block (subject) without replication was

Table 1 Experimental design layout Treatment*

Block

(subject) 1 2 3 4 5 6 7 8 9 10 11 12

~

~

ATNR A ~ S

~

R~

X X X X X X X X X X X X

aRDNR, reading without rest; RDRS, reading with rest; ATNR, arithmetic without rest; ATRS, arithmetic with rest; RAN'R, reading/ arithmetic without rest; RARS, reading/arithmetic with rest

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Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al

eventually run, yielding 12 data points excluding the pilot data. Two treatment groups, with each group consisting of two subjects, were used for each of the two tasks with different task conditions (reading, rest versus no rest, and arithmetic-logical problems, rest versus no rest) with randomization of treatments within the group (Hicks, 1982). Two additional treatment groups, with two subjects per group, performed the combined task treatment (combined reading and arithmetic-logical problems with rest breaks; combined reading and arithmetic-logical problems with no rest breaks), with treatment randomized within the group. (The tasks for the two subjects who participated in the pilot study were randomly assigned within each group of rest versus no rest. As it turned out, the first subject performed the reading test without rest, while for the second subject the task was combined reading/ arithmetic with rest.) Table 2 is a summary of the data collection structure with subject, task and condition for all 12 main subjects. The level of difficulty of the reading materials and the arithmetic-logical problems was constant throughout the duration of the experiment. The length for the reading material was commensurate with the task duration, as per Educational Testing Service Standard Tests. The same was true of the arithmetic-logical problems. Subjects were habituated to the experimental setting by participating in a pre-test experiment lasting 20 rain. In summary, the experiment conducted consisted of three task types (reading, arithmetic, and combined reading and arithmetic). For each task type there were two conditions, rest and no rest. Two subjects participated in each task and a given experimental condition (i.e. rest versus no rest) for a total of 12 subjects, two subjects being used previously in the pilot stage.

Special procedure for EEG signal. One of the major assumptions for this experiment was that it would be possible to extract information from the E E G signal that would corroborate changes in increases of output performance due to mental fatigue. Using the experimental design described above, the E E G signal that was acquired turned out to be corrupted with a high level of noise, and several attempts to remove the noise were not successful. Hence a new experimental design was specially implemented so as to extract a noise-free E E G signal. The original experiment was repeated, but with only four rather than six task types, using the same subjects. The combined task (of reading and arithmetic) was not performed because the relevant subjects were not available during this phase of the project.

Table 2 Data collection s t r u c t u r e for aH 12 main subjects

Subject size

Task

Condition (rest vs no rest)

2 2 2 2 2 2

Read Read Arithmetic Arithmetic Combined Combined

Rest No rest Rest N o rest Rest No rest

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However, the output performance measures for the new experiment were combined with those from the original design for a repeated-measures analysis. The E E G output for the original design was never used in the ensuing analyses.

Data collection E E G electrodes were attached to subjects on each temple on both sides of the head. Prior to attachment, the anatomical areas were first sterilized, contact between the electrode and the skin was interfaced with electrode jelly, and the electrode setup was secured to the skin by electrode collars. Pressure on the skin while securing the electrode was minimal in order to prevent local occlusion of blood flow, which could result in severe attenuation of the signal. During each trial about 2-5 min of the E E G signal from the subjects was recorded at the beginning and at the end of each session to establish baseline response values. The signal was appropriately amplified and routed to a recorder for storage. Later the signal was put through a fast Fourier analyser for processing and analysis to provide the power spectra and the corresponding time and frequency components of the wave form.

Performance measures The data collection for the reading/problem-solving tasks included the following: (1) Obtaining a count of the number of words read in each segment of the experiment. From this the rate of reading (words/rain) was computed under the assumption that the difficulty level was constant over the duration of the reading task. (2) Obtaining a count of the number of correct responses to questions and the number of questions attempted in the reading and problem solving efforts. This analysis yielded the accuracy of performance for each segment and for the entire experiment.

Signal analysis The rhythmicity of E E G signals provided a means of quantitatively describing E E G records. The signal was analysed and broken down into its sine and cosine wave components with different amplitudes and frequencies, classified in terms of rhythms and in turn defined in terms of frequency bands. The frequency bands of an E E G rhythm are usually grouped into the following classifications (where f0 --'- frequency in Hz): • • • •

f0 ~< 4 4~
delta 8theta 13 alpha beta

Brown (1967) and Grandjean (1981) described or identified the different E E G signal frequency components as follows.

• Delta rhythm. Delta (less than 4 Hz) components, like the theta components, are slow waves and are present only during sleep. • Theta components. Theta (4--8 Hz) rhythms indicate

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Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et ai a state of drowsiness. They replace the alpha components at the onset of sleep. • Alpha components. The alpha rhythms include an electrical activity with frequencies of 9-13 Hz. According to Grandjean (1981), alpha rhythms are present when an individual is in an alert relaxed state. • Beta components. Beta components (14-30 Hz) are associated with states of excitement or arousal. The presence of high components of beta rhythms is manifested in the form of increased alertness. Hence, with the EEG, one is ableto establish whether or not a human subject is alert (Wierwille, 1979). In this research, attention was focused only on the theta, alpha and beta rhythms. Particular attention was devoted to the alpha components, as they are known to be markedly present when the eyes are shut or closed, and are highly attenuated during attention. An amplified E E G output signal consists of noise coupled with the actual signal, with frequencies below and above the desired bands. Electronic filtration was used to facilitate the separation of the frequency components of the compound E E G waves into desired bands. Low-pass filters were used to eliminate noise from the trace (noise increases with increasing bandwidth), while high-pass filters were used for eliminating the bandwidth below 4 Hz (the range for delta). There are currently several methods of quantifying E E G signals, including amplitude analysis, power spectrum analysis, and wave indices analysis (Bechtereva, 1981; Gogolitsin et al, 1981; Grandjean, 1981; Ivanitsky, 1981; Anthony, 1984). Selection between these methods depends on the specific application or use; the power spectral density approach was utilized in this investigation. Amplitude analysis Amplitude averages or histograms can be computed using pulse-height analysers. The analyser uses the 'window principle' wherein a preset window is the threshold at which pulses are counted. The pulses at each threshold are then tallied by electronic scalers. These are plotted and displayed on video screen or other useful medium. It is also possible to store the signal on magnetic tapes or diskettes for future use. A major disadvantage of amplitude analysis is that amplitude is not a function of time; consequently such analysis would invariably ignore time relationships. In amplitude analysis, it is also possible for waves of different frequency contents to have identical amplitudes. In addition, artefacts also show up as 'glitches' or oversized waves. Thus the use of amplitude as a measure of the wave content in this case would reveal very little about the signal, as the actual signal would be masked by the presence of the artefact. Wave indices Amplitude measurement alone does not take into account the nature of waveforms and their frequencies unless the measurement is applied to a particular frequency component. One of the most promising efforts to quantify E E G signals involves the decomposition of the waveform into delta, theta, alpha and beta frequencies and the assessment of the percentage of

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time occupied by an activity in each of these bands. This percentage becomes the wave index. The beta index, for example, is the proportion of time for which the beta waves were present in a given record. A composite plot of the index for each frequency component would indicate which of these frequencies dominated during a given mental activity. Results

and discussion

The power spectral density values for each frequency band were plotted in time domain and examined for trends. Theoretically, as time progresses, and with the onset of fatigue, the alpha frequencies should dominate the spectrum. This means that the spectral density of the alpha waves would show higher values. However, a close examination of the plot revealed a random or sinusoidal trend with no clear indication as to the frequency band that was dominant. A plot of the thetaJ alpha ratio, as suggested by Matousek (1973) did not indicate a trend. Because the signal was first recorded before the spectral analysis was carried out, it was thought that the problem might be equipment-related. The tapes were re-edited for noise and other possible interference. The noise signal was easily identifiable, as it appeared as 'glitches' and had much higher amplitudes (in the present case) than the regular signal. The idea was to eliminate noise by physically removing it from the signal. This was accomplished by running the tapes until a noise signal was encountered; then the feed to the spectral analyser stopped while the tape continued running until there was no appearance of noise. This process was extremely tedious because the noise showed up at random and hence it was impossible to tell when it was going to appear. The results after the editing did not improve. A different approach was used in an attempt to improve the results. The method used was to set a cutoff voltage corresponding to the maximum signal voltage. As this was much lower than the noise signal's voltage, it had the effect of cutting off all amplitudes over and above those corresponding to the maximum voltage amplitude. This produced little improvement in the results. Because of the foregoing problems, a decision was made to rerun the experiment and, while doing so, conduct the spectral analysis in real time and on line. In this last experiment the power spectral densities were recorded for the following task types: • R D N R - reading without rest; • R D R S - reading with rest; • A T N R - arithmetic without rest; • ATRS - arithmetic with rest. Because the results of the E E G of the original experiment was not usable, they were discarded. Hence for the rest of the paper the data and results refer only to the modified (smaller) experiment, with eight subjects and four task types. The results for the frequency bands theta, alpha, beta and the theta/aipha ratio were plotted against time on the same graph for each task type identified above (Figure I). The dimension or measurement unit is voltage2/hertz.

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.

.

.

.

.

t

o t6-

!

I

].2 . . . . . . . . . . . . . . . . . 0.8 0

50

100

150

200

250

300

350

400

0

I 50

I 100

TIME (MINUTe)

a

--

THETA

.... ~. 150 TIME

" 4 - - ALPHA

~

b

BETA

--

I 200

* 250

300

350

(MIm~ES)

RATIO: T H E T A / A L P H A

-~200 3.55 180 ......................................................................................................

1

--

-

3.05 2.55

120

2,05 1,55

40

1.05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

..............

0.55 0.0~

0

100

C

--

200 TIM~ (MIMUT]~)

THETA

- 4 - - ALPHA

300

400

~

~

J

100

200

300

400

TtME (NINUTL~) ~

d

BETA

~" 240

--

RATIO: T H E T A / A L P H A

~8

-2

200 ....................................................................................................

i

100

........................................................................................

120

.....................................................................................................

80

..........................................................

................................................................................................

40 0

0 0

50

100

150

200

250

300

0

350

50

100

. r i ~ (MINb'r~$)

e

--

THETA

-+--ALPHA

--

"-J-- BE TA

150 200 T.'[t,I;~ (I,II NUT,T,S )

250

300

350

RATIO: T H E T A / A L P H A

180

2 .............................................................................................................................................................................................. 8120

d ~

1.5 80

1

I 40

~0,5 O--

0

100

200

300

0

400

I

~

I

100

200

300

400

TI~E (MXN~Z$)

TI~E (Mlslrr F.S) i

a

--

THETA

--4-- ALPHA

+

BETA

RATIO: T H E T A / A L P H A

h

Fi&mre 1 (a) Power spectral density: reading without rest. (b) Theta/aipha ratio: reading without rest. (c) Power spectral density: reading with rest. (d) Theta/aipha ratio: reading with rest. (e) Power spectral density: arithmetic without rest. (f) Theta/alpha ratio: arithmetic without rest. (g) Power spectral density: arithmetic with rest. (h) Theta/alpha ratio: arithmetic with rest To investigate further the possible relationship between brain waves and mental fatigue, a correlation analysis was performed on the spectral density data. A run analysis (randomness test) was also implemented in order to determine whether the data were random or whether there was a trend in the sequence of values obtained for the power density.

362

Correlation analysis The correlation analysis was carried out using the correlation procedure C O R R in SAS (SAS, 1985). The correlation procedure computes point estimates and correlation coefficients between variables using the Pearson product-moment approach, weighted product-

A p p l i e d E r g o n o m i c s 1994 V o l u m e 25 N u m b e r 6

Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al

moment approach and the Spearman rank order approach. The output consists of the mean, the medium, the minimum and maximum values and a significance probability of the correlation coefficient. The significance probability of a coefficient is the probability that a value of the coefficient larger than the one computed would have arisen by chance were the two random variables truly uncorrelated. In other words, if the magnitude of the correlation coefficient were large, then the significance probability would be small, etc. The tables that follow show one half of a symmetric correlation matrix for the tasks R D N R , R D R S , A T N R and ATRS. The first element in each entry represents the correlation coefficient while the second element represents the associated significant probability. All data refer to the second, smaller, noise-freeexperiment.

RDNR. Time was negatively correlated with all the other variables (theta, alpha, beta, and theta/alpha). The correlations between theta and alpha and between theta and beta were quite high. There was a weak correlation between beta and theta/alpha (r = 0.239) and time and theta/alpha (r ---- 0.135) (Table 3).

the theta/alpha ratio. The alpha versus beta value was also high (r --- 0.642, Table 5).

A T R S . Except for alpha versus beta (r -- 0.6), theta versus alpha (r = 0.5) and theta versus thetaJalpha (r -0.83), all other correlation coefficients were less than 0.5. The results of the observed patterns are as shown in Table 6. The correlation between alpha and beta waves for the no rest case was quite high. The same was true for the correlation between alpha and theta waves for the same conditions. This suggested that while alpha waves were present during both types of task (rest/no rest), they dominated during the task type with no rest breaks (no rest).

Test of randomness (runs test). The test statistic of the Run Analysis for a sequence of occurrence or phenomena which contains nl symbols of one kind and n2 symbol of another (where nl >i 10, n2 >I 10) and U total runs can be approximated by the normal distribution with the mean and standard deviation equal to 2nln2

Ixv =

2

+ 1,

2n ln2 (2nln2 --nl --n2)

02

( h i + n 2 ) 2 ( n l + n 2 --1)

A TNR. While the magnitude of the coefficients for this task type were greater than zero for time versus all the other variables, none of the values was as high as 50%. For theta, however, the correlations versus all other variables were considerably higher (r >I 0.78 for all cases). The alpha versus beta value was also quite high (r = 0.78) (Table 4).

RDRS. The only values of significance for this task type were the values for theta versus alpha, beta and Table 3 Symmetric correlation matrix for reading without rest

(RDNR)

Time

Time

Them

Alpha

Beta

1.000 0.000

-0.404 0.022 1.000 0.000

-0.566 0.001 0.918 0.000 1.000 0.000

-0.511 -0.135 0.003 0.459 0.589 0.798 0.001 0.000 0.767 0.544 0.000 0.001 1.000 0.239 0.000 0.188 1.000 0.000

Theta Alpha Beta Theta/alpha

Theta/alpho

The test statistic is given by U-

P'u

OU

The test of hypothesis for randomness is given by: H0: Sequences are random H i : Sequences are not random with the critical region: [ Z I > Z for a two-sided test Given a significance level of 0.05 (a = 0.025) corresponding to a critical region of 1.96 (Zo.o25), only three of the entries were significant. It is noteworthy that all three entries belong to the task types with no Table 5 Symmetric correlation matrix for reading with rest (RDRS)

Time

Time

Them

Alpha

Beta

Theta/alpho

1.000 0.000

-0.225 0.207 1.000 0.000

-0.329 0.061 0.756 0.000 1.000 0.000

-0.680 0.000 0.413 0.017 0.642 0.000 1.000 0.000

0.128 0.479 0.656 0.000 0.092 0.610 -0.131 0.469 1.000 0.000

Theta Alpha

Table 4 Symmetric correlation matrix for arithmetic without rest

(ATNR)

Beta Theta/alpha

Time Theta Alpha Beta

Time

Theta

Alpha

Beta

Theta/alpha

1.000 0.000

0.120 0.466 1.000 0.000

0.225 0.117 0.788 0.000 1.000 0.000

0.153 0.354 0.779 0.000 0.775 0.000 1.000 0.000

-0.006 0.973 0.844 0.000 0.424 0.007 0.648 0.188 1.000 0.000

Theta/alpha

Applied Ergonomics 1994 Volume 25 Number 6

Table 6 Correlation coefficient summary

Correlation ~ Signal frequency

RDNR

Alpha versus beta 0.78 Alpha versus theta 0.92

t

ATNR

RDRS

ATRS

0.78 0.80

0.64 0.80

0.60 0.50

363

Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al

scheduled rest break. If the significance level was reduced to 0.1 (ix = 0.05), the critical region would decrease to 1.645. In this case all the entries for the no rest tasks would become significant. T h e theta/alpha entry for R D R S (reading rest) and the alpha entry for A T R S (arithmetic with rest) were also significant at this new level. Conclusions

and recommendations

Several studies suggest that when repetitive knowledge worker related tasks, such as in engineering design, medical diagnostics, process monitoring, vigilance, military c o m m a n d and control are p e r f o r m e d for sustained periods of time, there is a strong possibility of increased fatigue effects, reduction in productivity, and alterations of cardiovascular and neurophysiological functioning. T o understand this p h e n o m e n o n requires an understanding of the temporal patterns of these effects as well as the functional relationships between performance variability and psychophysiological measures. Such an understanding is vital in order to define meaningful correlates of job/system characteristics that are prerequisites for p r o p e r job design. The establishment of such temporal ordering and precedence relationships will provide insight into the nature of the functional relationships between job characteristics that determine task performance and their physiological correlates. Such analyses when p e r f o r m e d over continuous intervals of time (e.g. minute by minute) would help to determine when to implement necessary countermeasures such as rest breaks or o t h e r types of fatigue allowance, so as to mitigate the effects of stress due to fatigue, and to maximize productivity. H u m a n performance, in and o f itself, is of very little use without considering the related issue of human reliability. H u m a n reliability is particularly important in maintenance, inspection and monitoring tasks where failures could have grave implications for both worker and system safety. Research has shown (Okogbaa, 1983) that the human performance deterioration function (or the human hazard function) appears to follow an exponential probability distribution. The corresponding reliability function is quite complex and has not yet been fully explored. In addition, the h u m a n renewal or recovery process and the resulting performance function, in the presence of mental fatigue, is not well understood, particularly with regard to whether the renewal renders the human system 'as good as new', 'as bad as old', or 'better than old'. Thus it is apparent that characterizing the reliability function and the failure rate/hazard function (or performance deterioration function) as well as the system renewal capabilities will represent a m a j o r step towards the understanding of the human system. T h e issue of the nature of the reliability and renewal functions is of singular importance if countermeasures and mitigation regimens for mental fatigue are to be effective. This research has allowed a major step towards the development of a model that explores the relationship between mental fatigue and factors associated with output performance, optimal recuperation periods and related variables. Such a model would be useful in human reliability prediction, in terms of the ability to achieve a reliability objective based on task parameters

364

as well as w o r k e r profiles, and a m a j o r step in the d e v e l o p m e n t of feature-based task classification schemes that will encompass the entire spectrum of knowledge workers. This would ultimately result in a better marriage of workers to tasks, leading to a reduction in stress and fatigue. The investigation has identified an experimental approach that is useful to determine the onset of mental fatigue through neurophysiological correlates. Preliminary results are promising and support the need for additional research with a different classification of knowledge workers, increased age of subjects, and changes in work task and/or workplace. T h e result of future research could provide the basis for optimizing white collar w o r k e r rest breaks, task assignments for specific individuals, workplace evaluation and improvement, safety and health (stress) conditions, and overall h u m a n productivity.

References

Anthony, G. 1984 'Designing productivity standards for the white collar employee' Ind Eng 16 (I), 76-81 Bartlett, F. 1953 'Psychological criteria of fatigue' in Floyd, W. and Welford, A. (eds) Symposium on Faa'gue H. K. Lewis, London, pp 284-290 Bechtertwa, N.P. 1981 'Neurophysioiogical correlates of mental processes in man' in Bechtereva, N.P. (ed) Psychophysiology Today and Tomorrow Pergamon Press, New York, pp 11-21 Beith, B. 1981 'Work repetition and pacing as a source of occupational stress' in Salvendy, G., and Smith, M. (eds) Machine Pacing and Occupational Stress, Taylor & Francis, London, pp 197-200 Bennett, C. 1976 'Counteracting psychological fatigue effects by stimulus change' in Proceedings of the 18th Annual Human Factors Society Meeting Huntsville, Alabama, pp 180-189 Brisley, C. L. and Heider, W. 1983 'Measuring the "unmeasurable": knowledge worker productivity' lnd Eng 15 (7), 42-47 Brown, I. D. 1967 'Car driving and fatigue' Triangle 8, 131-137 Brown, I. n. and Ponlton, E. C. 1978 'Measuring the spare mental capacity of car drivers by a subsidiary task' Ergonomics 4 (3), 103-208 Caidr, A., Hart, D. and Stewart, T. 1980 Visual Display Terminals John Wiley, New York Cameron, C. 1971 'Fatigue problems in modem industry' Ergonomics 14 (6), 713-720 Cameron, C. 1973 'A theory of fatigue' Ergonomics 16 (5), 633-648 Carver, R.P. 1975 'Comparing the reading-storage test to the paraphrase test as measures of the primary effect of reading' J Educ Psychol 67, 274-284 Carver, R. P. 1982 'Optimal rate of reading prose' Reading Res Q 18 (1), 56-88 Corm, H. P. 1984 'Improving the output of the knowledge worker' lnd Eng 16 (7), 70-77 Cooper, R., Ossdton, J. W. and Shaw, J. I. 1980 EEG Technology Butterworth, New York Corlett, E. and Mahadeva, K. 1970 'A relationship between freely chosen working pace and energy consumptioncurves' Ergonomics, 13 (4), 517-524 Crawford, A. 1961 'Fatigue and driving' Ergonomics 4, 143-154 Davis, D. R. 1948 'The disorder of skills responsible for accidents' Q J Exp Psychol 1, 136-142 Drew, D. 1940An Experimental Study of Mental Fatigue Air Ministry Hying Personal Research Committee Report No. 227, London Drocker, P. 1982 The Changing Role of the Executive Times Books, New York Ettema, J. H. and Ztelhhts, R. L. 1971, 'Physiologicalparameters of mental load' Ergonomics 14 (1), 137-144 Gardner, E. 1975 Fundamentals of Neurology W. B. Saunders Co., Philadelphia GogoUtsin, Y.L. and Kropotev, Y.D. 1981 'Application of the stimulus-time histogram to the analysis of discharge post patterns

Applied Ergonomics 1994 Volume 25 Number 6

Neurophysiological correlates of knowledge worker mental fatigue: O.G. Okogbaa et al in neuronal populations of the human brain during intellectualmnestic activity' in Beehtereva, N. P. (eli) Psychophysiology Today and Tomorrow Pergamon Press, New York, pp 23-31 Goldstein, H. 1940 'Reading and listening comprehension at various controlled rates' Unpublished Doctoral Dissertation, Teachers College, Columbia University Grandjean, E. 1981 Fitting the Task to the Man Taylor & Francis, London Elaider, M., KoUer, M., Grnll-Knapp, E., Cervinka, R. and Kundi, M. 1981 'Psychophysiological studies on stress and machine paced work' in Solvemly, G. and Smith, M. (eds) Machine Pacing and Occupational Stress Taylor & Francis, London, pp. 303-309 Hicks, C. R. 1982 Fundamental Concepts in the Design of Experiments Holt, Rinehardt & Winston, New York Hicks, T. and Wierwille, W. 1979 'Comparison of five mental workload assessment procedures in a moving-base simulator' Hum Factors 21 (6), 753-761 Httcben, D. and Harnem, J. 1980 'Cardiac responses to demanding mental load' Ergonomics 23, 379-385 Huston, T. R. 1985 'An investigation into knowledge worker mental fatigue and rest break duration' Unpublished PhD Dissertation, University of Cincinnati Ivanitsky, A. M. 1980 'The physiologyof brain signal evaluation and the psychology of perception' in Bechtereva, N. P. (ed) Psychophysiology Today and Tomorrow Pergamon Press, New York, pp 191-200 Jester, R.E. and Travers, R.M.V. 1966 'Comprehension of connected meaningful discourse as a function of rate and mode of presentation' J Educ Res 59, 302-309 Kalsbeek, J. H. W. 1964 'On the measurement of deterioration in performance caused by distraction stress' Ergonomics 7 (2), 187-195 Kalsbeek, J. H. W. and Ettema, J. F. 1963 'Scored regularity of the heart pattern and the measurement of perceptual and mental load'

Eronomics 6, 306-307 Kiare, G.P. 1963 The Measurement of Readability Iowa State University Press, Ames, Iowa Knowles, W. 1963 'Operator loading tasks' Hum Factors $, 155-161 Leplat, J. 1978 'Factors determining work-load' Ergonomics 21 (3), 143-149 Luczak, H. and Laurig, W. 1973 'An analysis of heart rate variability' Ergonomics 16 (1), 85-97 Matousek, R. 1973 'Frequency analysis: processing of output data' in Redmond, A. (ed) Handbook of Electroencephalography and

Applied Ergonomics 1994 Volume 25 Number 6

Clinical Neurophysiology Vol 5, Elsevier Publishers, Amsterdam, part A, pp 61-67 McGlyn, G. H., Laughlin, N. T. and Rowe, V. 1979 'The effect of increasing levels of exercise on mental performance' Ergonomics 22 (4), 406-414 Miller, G. R. and Coleman, E. B. 1972 'The measurement of reading speed and the obligation to generalize to a population of reading material' J Reading Behav 4 (3), 48-56 Mulder, G. and Meulen, M. van tier 1973 'Mental load and the measurement of heart rate variability' Ergonomics 16 69--83 Okogban, G. 1983 'An empirical model for mental work output and fatigue' Unpublished PhD Dissertation, University of Cincinnati Okogban, G. and Shell, R. 1986 'The measurement of knowledge worker fatigue' lie Trans. 18 (4), 335-342 Pitcher, R. 1953 'An experimental investigation of the validity of the Fiesch readability formula as related to adult material' Unpublished Doctoral Dissertation, University of Michigan SAS 1985 Statistical Analysis Stystem, Version 5 Schleifer, L. and Okoglma, O. G. 1990 'System response time and method of pay: cardiovascular stress effects in computer-based tasks' Ergonomics 33 (12), 1495-1509 Schouten, J., Kalsbeek, J. and Leopold, F. 1962 'On the evaluation of perceptual and mental load' Ergonomics 5, (1), 251-260 Sharit, J. and Solvendy, G. 1982 'Occupational stress: review and reappraisal' Hum Factors 24 (2), 129-162 Sjoberg, H. 1980'Physical fitness and mental performance during and after mental work' Ergonomics 23 (10), 977--985 Smith, M. 1981 'Occupational stress: an overview of psychosociai factors' in Solvendy, G. and Smith, M. (eds) Machine Pacing and Occupational Stress Taylor & Francis, London, pp 13-19 Thorndike, E. L. 1971 'Reading as reasoning: a study of mistakes in paragraph reading' Reading Res Q 6 (4), 425-434 USBL 1992 Department of Labor, US Bureau of Labor Statistics Weber, A., Fussier, J., O'Hanlan, J., Grierer, R. and Grandjean, E. 1980 'Psychophysiologicai effects of repetitive tasks' Ergonomics 23 (10), 1033-1046 Wierwille, W.W. 1979 'Physiological measure of aircrew mental workload' Hum Factors 21 (5), 575-593 Wisner, A. 1981 'Organizational stress, cognitive load, and mental suffering' in Salvendy, G. and Smith, M. (eds) Machine Pacingand Occupational Stress Taylor & Francis, London, pp 37--44 Zwaga, H. 1973 'Psychophysiologicai relations to mental tasks: effort or stress' Ergonomics 16 (1), 61-67

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