This paper proposes a concept of the adaptive asynchronous human machine system (Async-HMS) which maintains asynchronous periodic operations against user's actions for avoiding user intention. In order to find the dynamics of the operation period presented by Async-HMS, discrimination thresholds of the operation period were obtained by psychophysical experiment. According to the thresholds, three levels of fluctuation of the operation period were defined as ‘constant’, ‘unrecognizable’, and ‘recognizable’. Psychophysiological measurement was conducted on the performance of the synchronized key typing task by the three levels of the operation period. A synchronization error as performance index, the hemodynamic parameters as indices of autonomic nervous system's activity, and the amount of the senses as psychological indices were evaluated. As a result, monotonous feeling was eliminated without compromising the performance in case of `unrecognizable' fluctuations in the operation period presented by Async-HMS.
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