Background: It is known that the activities of the mirror system are related to imitation and understanding of the intention of an action. It has been reported that the activity of the mirror system is higher for observations for imitating and understanding the intention of an action than for simple observations. However, observations that facilitate the mirror system's activities, if they are observations intending to imitate an action or observations for understanding the intention of an action, have not been clarified to date. Methods: The types of observations of actions that highly facilitate mirror system activities were investigated. Participants were right-handed university students (N = 23). They observed videos showing hand actions following three types of instructions: (1) to observe the videos intending to understand the intention of the action (action understanding, AU), to observe the videos intending to imitate the hand action (imaginarily imitation, II), and to observe the videos without any intention (observation, OB). Brain waves during observation were measured, and the suppression rate of 8-10 Hz (lower mu/α) and 10-12 Hz (upper mu/α) in the central and occipital regions of the brain was calculated. The rate of suppression was compared among the conditions using a repeated measures analysis of variance for each region. Results: There was a main effect of the condition in the central region in 10-12 Hz. The degree of suppression in the AU condition was significantly larger than SO condition (p < 0.05) and II condition (p < 0.1). However, there were no differences among conditions in 8-10 Hz, the occipital region, or in either frequency band. Conclusions: These results suggest that activities of the mirror system are enhanced when observing an action with the purpose of understanding the intention of the action. Differences in the mirror system activities according to the changes of inner states might be better reflected in high-frequency mu waves.
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