There is a demand for the objective evaluation of emotion. Thus, ρ
min has previously been suggested as a psychophysiological excitement index to meet this demand in various applications. ρ
min is the minimum value of the cross-correlation coefficient between the respiratory sinus arrhythmia components of heart rate variability (HRV) and the respiratory wave, considering the lag time. In this study, the feasibility of using this index was verified in an experiment in which the ρ
min values of the physiological responses of individuals playing games, which were classified according to the level of difficulty, were analyzed. Three major results were obtained in the analyses. First, the ρ
min values of the responses changed significantly while playing the game. Second, the ρ
min values of the responses changed linearly with the game's level of difficulty, and the rank correlation was found to be significant. Finally, the variation in the ρ
min value corresponded with the subjects' self-reported excitement scores. The HR and indexes of the autonomic nerve activities were investigated simultaneously by means of the HRV spectrum; however, no change or correlation with the game's difficulty level was found. The feasibility of using ρ
min values as the psychophysiological index of excitement was therefore verified.
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