1. Physiological bases of mental and physical work.

2. What Is Mental Workload?
Mental workload is the portion of operator’s limited mental capacities actually required to perform a particular task.
Mental reserves are the difference between capacity required and capacity available.
Mental effort is the voluntary matching of mental capacities with that needed for task success.
Increase in Mental Workload often precedes Performance Failure.
The first definition is from O’Donnell and Eggemeier (1986), the second from Gopher and Donchin (1986)

3. Measuring the Load of Mental Work
There are FOUR Basic Methodologies:
Primary Task Performance.
Subjective Response.
Physiological Assessment (Central and Peripheral).
Secondary Task Techniques.
Meshkati, N., Hancock, P.A., Rahimi, M., & Dawes, S.M. (1995). Techniques of mental workload assessment. In: J. Wilson and
E.N. Corlett, (Eds.). Evaluation of human work: A practical ergonomics methodology. London: Taylor & Francis.
Available at:

4. Measurement of the physiological cost of work
The classical method of determining energy expenditure at work involves the measurement of oxygen uptake using the Douglas bag
Source:

5. How does a Douglas bag work?
50 litres of exhaled air will be collected in a bag.
From the person doing some task,
Air in the bag is then analyzed,
To find the amount of oxygen metabolized
Rate of energy expenditure can be calculated

6. Indirect measures of energy expenditure
Heart rate increases as a function of workload and oxygen uptake.
Because it is more easily measured than oxygen uptake, heart rate is often used as an indirect measurement of energy expenditure.

7. Heart rate can be likened to a signal that integrates the total stress on the body.
Heart rate measurement can therefore be used as an index of the physiological workload.

8. Energy Expenditure Rates for Various Activities
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9. Applied Physiology at the Workplace
Rate for energy expenditure of a work is linearly related to the amount of oxygen consumed by the body and heart rate.
Therefore, oxygen consumption rate and heart rate are often used to quantify the workload of physical work.

10. Applied Physiology at the Workplace
There is a linear relationship between oxygen consumption and energy expenditure.
For every liter of oxygen consumed, an average of about 4.8 kcal of energy is released. Thus, the amount of aerobic metabolism or energy expenditure of work can be determined by multiplying the oxygen consumption (liter/min) by 4.8 kcal/liter.

11. A Representation of Stress Effects
Experience
Health
Stressors
Direct (e.g.,vibration)
Direct (e.g., lighting, noise)
Physiological arousal
Indirect
Lighting and vibration directly affect visual capabilities, while noise affects auditory, time stress may limit amount of info that can be perceived.
Indirect effects such as anxiety or fear affect the efficiency of info processing
Information
Processing
Performance
Input

12. Possible Effects of Stress
A psychological experience (e.g., frustration)
A change in physiology (e.g., increased heart rate)
Reduced efficiency of information processing
Long-term negative consequences for health (e.g., heart disease, G-I problems)

13. Environmental Stressors
Motion – vibration, G forces & motion sickness
Whole body vibration:
.3-1 Hz – motion sickness, vomiting
1-4 Hz – blurred vision, difficulty breathing, impaired psychomotor
4-10 Hz – chest pain, rattling jaw
8-12 Hz – backache
10-20 Hz – headache, eyestrain, speech disturbance, G-I problems
Limb vibration:
Hz – pain in arm/wrist, arthritis, bone atrophy, VWF
Thermal stress – body temperature, air movement, amount of physical work
Heat – drowsiness, fatigue, heatstroke, dehydration, sweating, vomiting
Cold – restlessness, lower alertness, numbness, shivering, hypothermia
Air quality – anoxia (lack of O2)
Vibration: on limbs can cause long term numbness (Vibrating White Finger) e.g., use of power sander
Whole body vibration (riding in helicopter) can cause muscle tension, headache Hz, blurred vision
between 2.5 to 5 Hz resonate in neck/spine, 4 to 6 Hz in trunk, shoulder/neck, 20 to 30 in head/shoulder
Motion sickness can cause dizziness and inability to concentrate, vomiting, etc.
Thermal stress: optimal temp for performance 73 to 79 deg in summer, 68 to 75 in winter
Excessive heat interferes with info processing, can cause heat stroke, exhaustion, sweating, dehydration, etc
Heat stress affected by 3 variables: body temp (moderated by clothing), Air movement (fans), physical work (metabolic activity).
Cold stress: frostbite & hypothermia. Performance degraded by shivering, distraction, numbness, or the thick protective clothing worn to prevent those things.
Wet bulb temperatures (thermometer with soaked rag on end, slung around) takes humidity into account.
Air quality – pollution and anoxia (lack of oxygen, such as in hi altitudes).

14. Psychological Stressors
Resulting from the perceived threat of harm or loss of esteem, something valued, or of bodily function through injury or death.
Cognitive appraisal – person’s understanding / interpretation of the situation
Level of arousal – heart rate, pupil diameter, hormonal chemistry
Performance changes with overarousal – e.g., tunneling
Remediation of psychological stress – simplifiers in emergency situations
Cognitive appraisal is determined by one’s experience, expertise, confidence, risk acceptance, etc.
Level of arousal – physiological correlates of stress
Remediation – simplify elements of situation in emergency (displays, controls, etc). Rely less on capacity of working memory, and make procedures compatible with well-learned behaviors. Train to reduce arousal to optimal level (breathing, relaxation, techniques)

15. Effects of Psychological Stressors on Information Processing
Narrowing of attention
may be positive or negative
Diverted attention
Working Memory Loss
Disrupts articulatory loop (subvocal speech)
Perseveration
Revert to what people know best – implications for overlearning of emergency behaviors
In times of stress people tend to narrow their attention (tunnel), like the pilots who CFIT while concentrated on landing gear light or 3-mile island where operators focused on alarm and ignored water level or the investigator in the Chutes and Ladder story who nearly electrocuted himself oblivious to the powerlines.
Cognitive tunneling – focusing on one hypothesis.
Overarousal affects capacity of working memory and ability to access long-term memory (e.g., forgetting what you’re about to say while nervously giving a speech), revert to things one knows best (even if inappropriate).
demonstrates the need to overlearn emergency procedures (turning toward skid direction on ice).
may wish to have operators in emergency situation take a few sec to think before they act

16. Life Stress Causes lack of attention, distraction or diversion
e.g. Deaths in the family, financial problems
Related to different aspects of attention
Could lead to airplane accidents, industrial accidents, lower motivation and performance

17. Adapting to Stress How do people adapt to stress?
Use more resources - Try harder
Work faster, speed/accuracy tradeoff, avoid Type A behaviors
Remove stressor – leave environment
Earplugs, coping strategies (relaxation techniques)
Change task goal – use simpler, stress-resistant strategy
Rely on pattern recognition skills, heuristics
Do nothing – continue until stress takes its toll

18. Moderating Variables of Stress
Interacting effects of multiple stressors
Noise & sleep loss both decrease performance, but effects not additive
Noise increases arousal, sleep loss decreases arousal
Personality (individual differences)
Differences in locus of control, Type A behavior, etc.
Training
Experience may reduces negative effects of stress by:
Reducing anxiety
Increasing repertoire of responses
Increasing knowledge of situation and ability to create solutions

19. Workload TIME Task Now Future Task Overload Normal Underload Overload
Fatigue
Sleepiness
Sleep Loss
Circadian Rhythm

20. Work Overload Time-line Model
“So much work to do, so little time”
Time-line model
Workload percentage = Time required/Time available
Can have over 100% workload and handle it okay or less than 100% and not
Moderators of time requirement estimations:
Individual differences
Spare capacity
Level of automaticity
Shared vs. separate resources
TR estimates should take into account 1) indiv diffs, such as expertise, 2) leave room for spare capacity (pilots begin to err at 80% capacity, so leave 20% as ROT), 3) level of automaticity,if performing task at the skilled-level then can take on other tasks (riding a bike, no longer worry about balance), and 4) resources that are shared, easier to do 2 things if they use diff resources.
Time stress effects

21. Work Overload Time-stress Effects
Under time stress, people tend to:
restrict tasks to those believed to be more important
restrict available info sources to those believed to be more important
Problem occurs when subjective evaluation of importance is wrong
e.g., trying to understand one difficult concept for a test, and not studying rest of material

22. Remediation Eliminating Stressors at Work
Engineering solutions
Sound absorbing materials, temperature regulation, glare shields, earplugs, vibration dampening
System design solutions
S-R compatibility, automation, increased cue saliency, use of command displays (over status), redundancies
Training
Train task management skills – prioritizing tasks
Train important procedures to automaticity
Stress exposure or inoculation training

23. Effort and Workload
Effort – changes in workload related to demands other than time
Precision
Force
Discriminability
KSA requirements
Working memory requirements

24. Work Overload Prediction
When two or more tasks are carried out concurrently
Predictions must account for differences in task automaticity & multiple resource competition
both of which will influence performance
Figure shows comparison of predicted to subjective and empirically tested workload

25. Mental Workload Measurement
Primary Task Measures
measures of system performance on the task of interest
Secondary Task Methods
measures reserve capacity by looking at performance on a secondary or concurrent task
Physiological Measures
e.g., measuring heart rate variability for mental workload & measuring mean heart rate to look at physical workload and stress
Subjective Measures
done by asking the operator to rate workload on a subjective scale (e.g., NASA TLX)
if assessment is done after the system has been built it can provide a contribution to usability analysis
Primary task measures – speed and accuracy, not a good measure of workload, can’t tell whether user is working at max capacity or if he has room to spare
Secondary task measures – perform primary task as well as possible, using leftover resources to complete the secondary task. The more resources available to do 2nd task, the lower the primary task workload requirement. 2nd tasks typically include math, memory, tracking, or RT tasks. Must consider whether 2nd task uses the same resources as primary task (e.g., may be easy to perform RT task while reading, but more difficult to do a memory task)
Physio measures – HRV- HR varies less at higher wklds. Other measures include eye tracking (eye fixations), skin conductance (arousal), EEG wave activity, pupil diameter, EMG, etc.
Subjective measures – NASA TLX, measures are biased or just not accurate depictions of truth
Use of multiple measures are recommended
Dissociation occurs when workload measures are found to increase at the same time that primary task performance is found to improve
Subjective and physiological workload measures are good assessments of how much cognitive effort the user invests in a task

26. Fatigue Due to effects of high or even moderate workload
Can be mental or physical
e.g., observed during a military combat mission
Symptoms - Feelings of weariness, faintness, sluggish thinking, reduced alertness, poor and slow perception, unwillingness to work, decline in physical and mental performance
Measures
EEG – increased alpha & theta waves, decreased beta
Flicker-fusion frequency – lowered with fatigue
EEG – electro encephalogram – alpha Hz, awake but relaxed
theta – 4-7 Hz, sleep
delta – less than 4 Hz, deeper sleep
Beta – Hz, Alert
Flicker-fusion frequency measured with a flickering lamp (strobe). When alert you can tell that the light is flickering at higher Hz, but with fatigue you have to lower the freq in order to see flicker.
FFF has been shown to decrease by as much as 6 Hz following mental stress

27. Vigilance and Underarousal
Vigilance – Sustained attention to low stimulus-changing environment
Low-arousal environments can be just as fatiguing as high workload environments.
e.g., seen in low-workload shifts for air traffic controller’s and on repetitive assembly line jobs

28. Vigilance Decrement Causes
Time – longer duration of vigilance, increases chance of misses
Event salience – subtle events increase chances of error
e.g., typesetting error
Signal rate – when signal events occur at low rates, likelihood of detection will be reduced
Arousal level – problems occur when there is little intrinsic task-related activity

29. Vigilance Remediations
Short work shifts – with frequent breaks
Salient signals
e.g., by using signal enhancement
Use payoffs when miss rates are high or change the signal expectancy
e.g., can introduce false signals
Increase/sustain level of arousal
e.g., use of caffeine, music, noise, conversation

30. Boredom Boredom – the affective reaction to monotony
Boredom proneness associated with: sensation seeking, job dissatisfaction, poor vigilance, ADHD, Type A behavior (Vodanovich & Kass)
Boredom proneness greatest for:
People in state of fatigue
Not-adapted night workers
People with low motivation
Highly educated, knowledgeable people
Challenge seekers
Boredom proneness least for:
Alert or fresh people
People who are still learning
People whose jobs suit their abilities (Grandjean)

31. Sleep Loss
Sleepiness blamed for over 200,000 auto accidents per year
Caused by:
Purposely staying awake (all-nighter, night shift)
Trying to sleep during the day (against circadian rhythm)
Stimulants (caffeine)
Stress
Aspects of performance that are most susceptible:
tasks requiring visual input, tasks involving judgment, learning, or storing new material

32. Remediation to Sleep Disruption
Get more sleep!!! – even if it is only 3-4 hours per night
Build up sleep credits
e.g., gain extra sleep prior to deprivation
Napping helps
make sure you allow time for full mental recovery
Sleep management
e.g., easier with more controlled jobs – the military
Rebound effect – loss of REM sleep is made up after person does have chance to sleep (even if actual sleep time is normal)
REM sleep is what refreshes.
NASA study on fatigue countermeasures showed that pilots who can take a 15 minute nap during cross-continental flights performed better. Study conducted after a pilots in a commercial passenger jet found themselves over the Pacific Ocean when the fell asleep in the cockpit.

33. Desynchronization
Desynchronization - Occurs when the circadian rhythms are out of synchrony with the level of activity that one is trying to maintain
Shiftwork –strategies to deal with the disruption of circadian rhythms
e.g., Assignment to permanently different shifts, continuous rotation, alter shift periods
Jet Lag – analogous to shift changes (east-bound more difficult than west-bound)
Remediation – bring the body into the local cycle rapidly