Abstract
This paper examines the development and use of cardiac arrhythmia as an index of mental workload in terms of several issues. These include the relevance of autonomic response to the measurement of task difficulty and the implications of general principles of psychophysiology to the application of cardiac arrhythmia as an applied measure. In addition the complexity of the psychological factors which may influence task difficulty is discussed with particular reference to second-by-second changes in heart rate variability. In conclusion it is suggested that the usefulness of global concepts of task difficulty such as mental load may be questionable. Moreover that a greater understanding of what a task entails, with reference to increased psychophysiological knowledge, is necessary in order that global changes in cardiac arrhythmia may be interpreted in relation to task difficulty.

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