The objective of this study is to develop a quantified method for determining the mental workload imposed on train control officers (TCOs) and to express this mental workload by means of an index that is objective and can stand up to the tests of validity and reliability. The method addresses an existing operational shortcoming in Spoornet train control operations and could be used as a tool for predicting the mental workload imposed on operators at particular train control centres. The method could be applied to manage and improve operational safety in the rail transport environment. A participative systems approach was followed in the development of the measuring methodology. A work group comprising expert users of the specific train control system was involved in identifying task factors and assigning weights for task and moderating factors. The newly developed Mental Workload Index (MWLI) consists of three task factors and eleven moderating factors, each with a different weight in terms of its contribution to overall mental workload. The work group performed several iterations to reach final consensus on the following task factors and their respective contributions to the MWLI: the number of data transactions, the number of authorisations, and the number of communications via telephone and radio. The systems approach used in the development process is discussed, and the final index with the task and moderating factors is presented. In conclusion, the value and possible application of the MWLI are discussed. The MWLI is shown to provide an objective method for the assessment and prediction of mental workload in the train control environment. / Thesis (PhD)--University of Pretoria, 2008. / Electrical, Electronic and Computer Engineering / unrestricted

Workers experience combined physical and mental demands in their daily jobs, yet the contribution of these concurrent demands in the development of work-related musculoskeletal disorders (WMSDs) is not clearly understood. There is a need to understand how concurrent demands interact with different work parameters, such as force levels, muscles employed, and types of exertion, to influence physiological responses. Furthermore, whether muscle capacity is altered with these concurrent demands remains unclear. The current research was conducted to address these needs through three experimental studies that evaluated changes in physiological, performance, and subjective measures. The first study investigated muscle-specific responses to concurrent physical and mental demands during intermittent static work. Mental demands adversely affected physiological responses with increasing physical demand. Furthermore, greater motor and mental performance impairment was observed at either end of the physical demand spectrum. Finally, these interactions were muscle-dependent, with postural (shoulder and torso) muscles indicating a greater propensity to interference due to concurrent demands than executive (wrist) muscles. The aim of the second study was to evaluate differential effects of exertion type (static and dynamic) during concurrent physical and mental work. Concurrent physical and mental demands adversely affected physiological responses during static exertions compared to dynamic exertions. Furthermore, static exertions were more susceptible to decrements in muscle output and mental task performance than dynamic exertions, specifically at higher force levels. The last study quantified the effects of concurrent physical and mental demands on muscle capacity (endurance, fatigue, and recovery) during intermittent static work. Additional mental processing was associated with shorter endurance times, greater strength decline, increased fatigability, and slower cardiovascular recovery. Concurrent demand conditions were also associated with higher levels of perceived fatigue, and rapid increases in rates of perceived exertion, time pressure, mental load, and stress. Overall, the current research provides a comprehensive understanding of the interactive effects of physical and mental demands on physiological responses and task performance. These findings may facilitate the development of task design strategies to help reduce the risk of workplace injuries and to increase worker performance. Finally, outcomes from this research can contribute towards the revision of current ergonomic guidelines to incorporate concurrent assessment of physical and mental demands. / Ph. D.

New sensor and display technologies are expected to enhance the performance of soldiers by providing them more information about the battlefield. However, there is concern that greater quantities of information and increases in mental workload might cause distraction, reduce attention to dangers in the immediate environment, and threaten soldier survival. The purpose of this laboratory investigation was to quantify the effects of increases in mental workload on one of the soldier's most basic tasks --- avoiding ground hazards while walking. The participants were 12 U.S. Army infantry soldiers. The study was conducted on a treadmill that was modified to provide the participants a view of impending ground hazards up to 5 meters forward of their walking position. The study was a 2X3 fixed factor design with two levels of terrain difficulty (No Hazards and Hazards) and three levels of mental workload (No Load, Moderate load, and High load), all as within-subject effects. Mental workload was increased from the "No Load" to a "Moderate" level by requiring the participants to perform a mental arithmetic task while walking. Mental workload was increased from the "Moderate" to the "High" level of load by increasing the difficulty of arithmetic problems. The dependent variables included time and error in the performance of the mental arithmetic task, the mean and standard deviation in step length and step rate, the number of ground hazards contacted, and subjective ratings of workload. The participants" scores on the Armed Forces Qualification Test (AFQT) and subtests of the Armed Services Vocational Aptitude Battery (ASVAB) related to arithmetic skills were also obtained. The results of the investigation indicated that when the participants were required to avoid hazards, step length decreased and step rate increased, as was expected. Both measures of gait increased in variability. Subjective ratings of physical demand and effort obtained across the three levels of mental workload increased significantly, along with perceptions of workload associated with a perceived decline in performance. Subjective ratings obtained across the two levels of terrain difficulty indicated that ratings of mental demand and effort increased with each increase in level of mental workload. When the participants were confronted with the more difficult arithmetic problems at the "High" level of mental workload, time and error in performing the mental arithmetic task increased as did ratings of temporal demand, frustration, and workload attributable to a perceived decline in performance; however, subjective ratings of physical demand decreased. Interactions found between terrain difficulty and mental workload indicated that differences in ratings of performance and overall workload scores between the two levels of terrain difficulty decreased significantly between the "No Load" and the "Moderate" level of mental workload, and converged at the "High" level of mental load. Although relationships were found between perceived workload, gait, and performance of the mental arithmetic and hazard avoidance tasks, the analysis did not reveal a significant effect of mental workload on the number of hazards contacted. Some participants tended to contact more hazards at the "High" level of mental workload than at the "No Load" or the "Moderate" levels, as expected. However, other participants tended to contact more hazards at the "Moderate" level of mental load than at either of the two extremes. Still other participants tended to contact more hazards at the "No Load" level of mental workload than at the "Moderate" or the "High" levels. Correlations were found between subjective ratings of workload, mental arithmetic performance, and scores on the AFQT and subtests of the ASVAB related to arithmetic skills, but no relationships were found between test scores and performance of the hazard avoidance task. However, when test scores were used as covariates in the analysis of mental arithmetic performance, the findings revealed that the number of correct responses to the arithmetic problems decreased when the participants were required to avoid hazards. The results of the study may support the belief that the allocation of limited resources will vary based on past experience and other individual differences, and that the amount of resources allocated to a task may be influenced by the difficulty of the task, criteria for performance, and the motivation of the individual. / Master of Science

Multiple devices such as desktops, laptops, and cell phones are often used to manage users' personal information, such as files, calendars, contacts, emails, and bookmarks. This dissertation presents the results of two studies that examined users' mental workload in this context, especially when transitioning tasks from one device to another. In a survey of 220 knowledge workers, users reported high frustration with current devices' support for task migration, e.g. making files available on multiple machines. To investigate further, I conducted a controlled experiment with 18 participants. While they performed PIM tasks, I measured their mental workload using subjective measures and physiological measures. Some systems provide support for transitioning users' work between devices, or for using multiple devices together; I explored the impact of such support on mental workload and task performance. Participants performed three tasks (Files, Calendar, Contacts) with two treatment conditions each (lower and higher support for migrating tasks between devices.) This dissertation discusses my findings: workload measures obtained using the subjective NASA TLX scale were able to discriminate between tasks, but not between the two conditions in each task. Task-Evoked Pupillary Response, a continuous measure, was sensitive to changes within each task. For the Files task, a significant increase in workload was noted in the steps before and after task migration. Participants entered events faster into paper calendars than into an electronic calendar, though there was no observable difference in workload. For the Contacts task, task performance was equal, but mental workload was higher when no synchronization support was available between their cell phone and their laptop. Little to no correlation was observed between task performance and both workload measures, except in isolated instances. This suggests that neither task performance metrics nor workload assessments alone offer a complete picture of device usability in multi-device personal information ecosystems. Traditional usability metrics that focus on efficiency and effectiveness are necessary, but not sufficient, to evaluate such designs. Given participants' varying subjective perceptions of these systems and differences in task-evoked pupillary response, aspects of hot cognition such as emotion, pleasure, and likability show promise as important parameters in system evaluation. / Ph. D.

This thesis measured the mental workload associated with operating a voice activated software application run on wearable computer under five different communication styles (buttons, command line, icon buttons, icon text menus, and text menus). The goal of this thesis was to determine which communication style would be best allow wearable computer users to simultaneously perform other non-computer tasks. Thirty subjects were randomly assigned to using one of five software versions (n = 6), each of which utilized a unique communication style. The mental workload associated with operating each version was assessed by monitoring the performance of secondary tasks. Secondary tasks consisted of completing a block assembly, digit subtraction, and walking along a marked pathway. Each secondary task was performed twice by itself and once while operating one of the software versions, creating a total of nine trials per subject. Block assembly task performance measures included average assembly time, percentage correct blocks, and percentage correct blocks attempted. Digit subtraction measures included percentage of correct digits. And path walking measures included average walking speed. Subjective estimates of mental workload were also collected for those trials in which subjects operated the wearable computer and performed physical tasks using the NASA Task Load Index (TLX). Finally, usability information was collected for each software version via a questionnaire form. Each of the five versions of the experimental software application was operationally identical to the others, but utilized a separate communication style. The button version displayed available functions via sets of labeled buttons in the control screen. The icon button version replaced the appearance of these buttons with labeled icons. The text menu version displayed available functions textually via a pull down main menu. The icon text version displayed appended icons to the left of each main menu item. Finally, the command line version displayed no labels, buttons, menus, or icons for any functions. The experimental software was designed as a day planner/scheduling application used to set reminder dates on a calendar, edit task lists, and edit phone listings. Under the multiple resource view of mental workload, it was hypothesized that the different versions and secondary tasks would demand distinct types of mental resource and, consequently, that mental workload would be observed as lowest when the version and secondary task demanded different types of mental resources. In contrast, it was also hypothesized that mental workload would be observed as highest when the version and secondary task demanded the same type of mental resources. Although separate one way ANOVAs performed on all secondary task measures failed to indicate statistically significant differences in mental workload across the versions, secondary task performance was consistently observed as best for subjects using the icon button version. Analysis of NASA TLX subscale data indicated that the block assembly task was rated as requiring less effort and the digit task rated as requiring less mental demand when the icon button version was used. These results generally support using an icon button communication style for wearable computer software applications. Results of this study are applicable to the design of the user interface of wearable computers. These results not only report subjective and objective measures for assessing the amount of mental effort associated with operating a wearable computer and performing various physical tasks simultaneously, but also provide estimates for determining the amount of physical task performance decrement to expect when wearable computer are also operated. Such data may be used to determine human factors guidelines for matching wearable computer interfaces to physical tasks so that interference between the two is minimal. / Master of Science

Mental workload is commonly defined as the proportion of a person's total mental capacity in use at a given moment. A measure of mental workload would have utility in a number of rehabilitation medicine applications, but no method has been adequately examined for these purposes. A candidate measure is the intersaccadic interval (ISI), which is the duration between two successive saccades. Previous studies indicate that ISI length may be linked to mental workload, but this link is poorly understood for tasks that are not primarily visual. Therefore, the current study was an investigation of ISI and workload intensity in three non-visual tasks: mental arithmetic, verbal fluency, and audio perception. Workload was manipulated through changes in task difficulty as well as study participant motivation level. An analysis of eye movements and other experimental workload measures indicated a significant association between audio perceptual workload and ISI length.

Mental workload is commonly defined as the proportion of a person's total mental capacity in use at a given moment. A measure of mental workload would have utility in a number of rehabilitation medicine applications, but no method has been adequately examined for these purposes. A candidate measure is the intersaccadic interval (ISI), which is the duration between two successive saccades. Previous studies indicate that ISI length may be linked to mental workload, but this link is poorly understood for tasks that are not primarily visual. Therefore, the current study was an investigation of ISI and workload intensity in three non-visual tasks: mental arithmetic, verbal fluency, and audio perception. Workload was manipulated through changes in task difficulty as well as study participant motivation level. An analysis of eye movements and other experimental workload measures indicated a significant association between audio perceptual workload and ISI length.

As handheld computers are becoming powerful, portable and wireless, they have recently received considerable attention in education because their multi-functionality and mobility can be used for supporting learning activities in various manner. In spite of the potential in education, handheld computers have several limitations. One of the critical limitations is the small screen size. In an attempt to find factors affecting text readability on the small screen of handheld computers, this study, first, reviewed basic reading process and reading model, previous studies examining potential factors affecting text readability on a small handheld screen. This study investigated the effect of screen orientation and margin on reading performance (reading speed and comprehension) and subjective impressions (ease of reading, mental workload and satisfaction) with a handheld computer. A 2x2 within-subject factorial design was conducted with 16 participants reading texts from a PDA (Compaq iPAQ 3850). The results of this study indicated that neither screen orientation nor margin had a significant influence on objective reading performance and subjective impression. It was found that there were no prevailing preference of screen orientation and margin in reading texts from a handheld computer screen. It was also found that participants had a favorable reading experience with the handheld computers. Based on results of experiments and post-experiment questionnaire, a set of recommendations was developed to improve reading from small screens of handheld computers. / Master of Science

This research evaluated mental schema accuracy, user experience, and training methods of computer based tasks using educational software. Aims were to investigate the impact of mental schemas on individuals’ usability of technology and analyze the impact of training and user experience in terms of mental schemas and performance. Study one investigated schema accuracy as a predictor of perceived usability and mental workload; by analyzing the accuracy of users’ mental schema through task correctness. Task was found to be a significant predictor of the measures of usability, along with various demographic variables. When considering the effect of tasks, schema accuracy was a significant predictor of perceived usability and mental workload for task two (online quiz). Perceived usability showed lower values indicating higher perceptions of usability for task two and mental workload had lower values indicating reduced mental workload for task two. Significant, positive correlations were found between perceived usability and mental workload. Findings show schema accuracy as a preliminary measure of users’ subjective usability of non-problem solving tasks, based on the type of task tested and demographic data of students. Study two examined experience level (experienced and un-experienced) effects mental schema accuracy, robustness, completion time, and errors using three computer-based tasks. Experienced participants showed lower values for number of errors and robustness than un-experienced users. Significant, positive correlations were found between schema accuracy and completion time, and errors and completion time. The findings support the use of experience to analyze the impact of mental schemas and performance measures. Study three explored the change in training methods (no-training, paper, or video) on user changes in mental schema accuracy, robustness, completion times and errors. Training improved robustness, specifically paper-based training. Performance results showed that students who spent small periods of time using the software more times a week had made fewer errors and had more robust schemas. Demographic experience categories found that participants who spent more time using the software had fewer errors. Significant, positive correlations were found between schema accuracy and robustness. These results show that training improves mental schema robustness and reduces the number of errors while completing computer based tasks.

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