Quantitative evaluation of the effect of visually-induced motion sickness using causal coherence function between blood pressure and heart rate

To evaluate the effect of visually-induced motion sickness on the human, blood pressure variability (BP) and heart rate variability (HR) of 51 normal subjects watching a 15min-long video image taken by a vibrating handy camera were analyzed. Not only coherence function (K²) between BP and HR but also two causal coherence functions: K²_BP→HR from BP to HR and K²_HR→BP from HR to BP were introduced to divide causal linearity of the cardiovascular system regarded as a closed-loop system. K² represents total linearity of the system. K²_BP→HR and K²_HR→BP correspond to the baroreflex system and the mechanical hemodynamics, respectively. The results revealed that K²_BP→HR at the Mayer wave-band (around 0.1 Hz) of the subjects prone to motion sickness decreased gradually and was significantly lower than that of the subjects not prone to in later scenes. This result has never been obtained from conventional methods dealing with a cardiovascular system as an open-loop system.