Evaluation of mental workload by fluctuation analysis of pupil area

It is generally known that the pupil is regulated by the autonomic nervous system. In this study, an attempt was made to assess mental workload on the basis of fluctuation rhythm of pupil area. Controlling the respiration interval, the pupil area during mental tasking was measured for 1 min. The respiration curve was measured simultaneously to monitor the respiration interval. Two mental tasks were used. One was a division task, the work level of which was set to two, three, four and five number of digits of the dividend. The other was a Sternberg memory search task, which had four work levels defined by the number of memory set. In the Sternberg memory search, the number of memory sets changed from five to eight. In such a way, the mental workload induced by mental loading was changed. As a result of calculating an AR power spectrum, we could observe two peaks which corresponded to the blood pressure variation and respiratory sinus arrhythmia under a low workload condition. With increasing workload, the spectral peak related to the respiratory sinus arrhythmia disappeared. The ratio of the power at the low frequency band (from 0.05-0.15 Hz) to the power at the respiration frequency band (from 0.35-0.4 Hz) increased with work level. In conclusion, the fluctuation of the pupil area can be used as a promising means to evaluate mental workload or autonomic function to supplement well-used heart rate variability measures.