Physiological bases of mental and physical work.

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)

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: www.mit.ucf.edu

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: http://www.vacumed.com/images/Douglas%20Bag%20Components.gif

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

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.

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.

Energy Expenditure Rates for Various Activities 2/26/11

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.

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.

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

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)

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: 40-300 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 10-20 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).

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)

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

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

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

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

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

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

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

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

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

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

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

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- 8-12 Hz, awake but relaxed theta – 4-7 Hz, sleep delta – less than 4 Hz, deeper sleep Beta – 14-30 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

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

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

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

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)

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

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.

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