Cardiac arrhythmia and secondary tasks as measures of mental load

Five methods of scoring cardiac arrhythmia (mean heart rate, standard deviation of instantaneous heart rate, Kalsbeek's S/N score, a cumulative tolerance score, and a cumulative difference score) as well as self-paced and adaptive versions of the secondary-task method were compared as measures of mental load. Each was used to differentiate among easy, medium, and difficult versions of a mental arithmetic task. In this standardized task situation where task difficulty was a function of complexity (load stress), only the secondary-task measures reflected changes in mental loading which corresponded to the increasing difficulty of the arithmetic task. Consistent with previous research, secondary-task performance decreased as loading task difficulty increased. The self-paced and adaptive secondary task versions were quite similar as indices of the mental load imposed. Interference of the self-paced secondary task with the primary loading task was not a problem. None of the cardiac arrhythmia measures consistently nor accurately quantified levels of task difficulty. These indices of heart rate variability were not reliable measures of load stress and were judged not comparable to the secondary tasks as measures of mental load.