External and internal attentional environments the utilization of cardiac deceleratory and acceleratory response data for evaluating differences in mental work load between machine paced and self paced work

Previous evidence has suggested that self-paced (SP) task performance may constitute a higher mental workload than machine-paced (MP) performance. These differences in mental workload were thought to be due to the presence, when working SP, of an internal pacing mechanism serving to maintain the worker's rhythm. In MP tasks, this function would be maintained externally by the machine. The present investigation attempted to directly test this hypothesis. For this purpose, Lacey's psychophysiological model relating changes in heart rate (HR) to attentional demands was employed. Differences in cardiac deceleratory and acceleratory activity between MP and SP performance were evaluated for each of two tasks. In one task, the emphasis was predominantly on visual detection. Based on the suspected direction of attentional demands, this task was characterized as external. The other task required mental solution to arithmetic problems and was categorized accordingly as internal. Psychophysiological findings were consistent with Lacey's basic model and offered no support for the existence of an internal pacing mechanism under SP conditions. Instead, they suggested the presence of uncertainty factors reflecting higher mental workload during the MP performance of both tasks. Performance data, however, did not support the causal interpretation given by Lacey for his psychophysiological model, and were explained in terms of a complex interplay between HR level and HR change.