More generally, the present study also demonstrated the potential of the autonomic-space approach to increase sensitivity and diagnosticity over using heart period alone for mental workload assessment. Other than the cold pressor, significant change in autonomic space was observed for each task condition for which there was a significant heart period change. The potential for greater sensitivity was evident in the velocity and low-disturbance conditions of the tracking task. In these task conditions, heart period was shorter than baseline (i.e., faster heart rate), but not statistically significant, whereas the change in autonomic space was significantly different from the baseline. The potential for greater diagnosticity was evident in the exercise and tracking tasks, where a coupled reciprocal mode of control was observed during exercise compared to an uncoupled parasympathetic withdrawal mode of control observed during tracking.
The PCA components examined in the present study were proposed as a method for obtaining cardiac autonomic information in the field. Impedance cardiography and pharmacological blockades can be used to obtain cardiac autonomic information in the laboratory, but these methods are not practical for most field studies. Although ambulatory impedance cardiographs have recently been developed, the technique is difficult to use in the field and is more susceptible to artifact than is the electrocardiogram. One advantage of the PCA method is that it does not require the collection of any more data than are needed to obtain heart period. Another advantage of the PCA method is that it can be applied post hoc to heart period data that have been previously collected (e.g., Backs, Lenneman, & Sicard, 1999; Backs, Wilson, & Hankins, 1995), which could potentially disambiguate instances where other mental workload assessment measures were sensitive but heart period was not. Thus, the PCA method presented in this chapter appears to be a valid and logical choice for obtaining cardiac autonomic information in the field.
Backs R. W. (in press). An autonomic space approach to the psychophysiological assessment of mental workload. In P. A. Hancock & P. A. Desmond (Eds.), Stress, workload, and fatigue. Mahwah, NJ: Lawrence Erlbaum Associates.
Backs R. W. ( 1998). "A comparison of factor analytic methods of obtaining cardiovascular autonomic components for the assessment of mental workload". Ergonomics, 41, 733-745.
Backs R. W. Lenneman J. K., & Sicard J. L. ( 1999). "The use of autonomic components to improve cardiovascular assessment of mental workload in flight simulation". The International Journal of Aviation Psychology, 9, 33-47.