Mental fatigue has long been recognized as having a central explanatory role in human performance, work psychology, health psychology, and other cognate areas. Research on fatigue has a long history, rooted firmly in practical problems such as impairment associated with the duration of the school or working day but also reflecting an enduring concern for theoretical and measurement development of the fatigue construct
Despite this early activity and the sustained interest over the last century remarkably little has been established about the origins of mental fatigue or the brain mechanisms underlying the state (in marked contrast to the related states of physical fatigue and fatigue associated with sleep disturbances, which have clearly identifiable causes). In its effects on performance, fatigue has usually been identified closely with a reduction in work effectiveness with extended or demanding mental work. Yet, marked impairments are not commonly observed, and many investigators have noted that changes in subjective fatigue and work output are only weakly related (Hockey, Wastell & Sauer, 1998).
Instead, effects may be quite subtle, such as an increase in the frequency of occasionally longer reaction times in sequential responding, a loss of priorities on complex tasks, or a reduced tendency to use effort on subsequent tasks.
It is also clear that task performance can appear remarkably stable across a wide range of other work conditions, such as stress or high workload so that mental fatigue may be considered a generalized response to any attempt to maintain performance under strain. More recent approaches have linked fatigue specifically with the costs of directed attention and executive control activity. However, the conditions under which a heavy task load or sustained effort lead to the development of fatigue have not been adequately explored.
A Compensatory Control Model of Mental Fatigue
The present research is guided by the compensatory control theory, developed by Meijman, (1997, 31-38) to account for the stability of performance under the threat of stress and demanding work conditions. The main elements of the model are illustrated as a highly simplified control diagram in Figure 1. In common with other two-level models the lower loop refers to the routine execution of familiar, highly skilled tasks, and the upper loop refers to the intervention of executive (or supervisory) control.
Figure 1. Main elements of Hockey's (1993, 1997 ) compensatory control model
As with all negative feedback systems, overt performance is stabilized by comparing goal targets against feedback (the action monitor in Figure 1) and modifying the output until discrepancies are minimized. However, as Kahneman (1970, 118-131) has argued, formal control models can say very little about the wide variety of reactions people have to perceived negative discrepancies; they may respond with increased or decreased task motivation, and performance may or may not be impaired. In Figure 1, effort regulation, implemented through activity in a second, upper loop, has the function of mediating between the experience of discrepancy and its effect on task ...