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The authors analyzed facial video recordings and saccadic eye movements during 1-hour simulated driving in 10 subjects. Mean crosscorrelation coefficient between the visually determined facial sleepiness and the proposed index of saccade (i.e. PV/D) for 9 subjects was -0.56 and the maximum coefficient of inverse cross-correlation was 0.83. Mean crosscorrelation coefficient for 6 repetitive measurements for another subject was -0.72, and the maximum was 0.84. Variation in PV/D preceded that in facial sleepiness in 13 of 15 measurements and syncronized with it in other 2 measurements. From these results, we confirmed a fair potential of the PV/D to detect decline in vigilance levels earlier than facial sleepiness. We also revealed that narrow fluctuations throughout the measurement could lead to low inverse cross-correlation below 0.60 between the two indices. Therefore experimenter should pay attention to designing the experimental procedure to ensure broad fuctuations of the subject's vigilance levels in the measurement.

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