Evaluation of mental workload by using probe-tasks
Effects of prolonged workload are frequently explored in fatigue studies. Discussing workload and stress effects, Hockey (1993) identified four general patterns of degradation in task effectiveness and efficiency: 1) primary decrement, which refers to the impairment of primary task parameters; 2) compensatory costs, which are observed as increments in sympathetic dominance (activation) and/or negative affect during the task; 3) strategic adjustments, which appear as changes towards the use of low-effort strategies or reduced use of working memory during the task; 4) fatigue after-effects, which can be derived from an observed preference for low-effort
strategies on a probe-test following sustained work. The choice of the pattern depends on the goals of the individual, the availability of resources and strategies, and the controllability of the situation. Earlier studies of fatigue after-effects on probe-tasks administered after work failed to find any clear detoriation in performance even after extended work periods. Thorndike (1900), and after him many others (Broadbent, 1971; Hockey, 1993), pointed out that performance rarely deteriorates as a consequence of fatigue, induced by sustained work or sleep deprivation, as long as individuals are willing to compensate by spending more effort. Empirical evidence for this compensatory effort was found by Wilkinson (1962). He pointed out that the effects of 32-56 hours of sleep deprivation were less marked in subjects who showed greater increments in forearm EMG during a vigilance task. Therefore, Hockey (1993), Meijman (1997), and many others, pleaded for the use of an efficiency paradigm, wherein performance is related to the effort required for the task.