Motor area activity for action-related and nonaction-related sounds in a three-dimensional sound field reproduction system.

The motor cortical area is often activated to auditory stimuli in the human brain. In this study, we examined whether the motor area shows differential activation for action-related and nonaction-related sounds and whether it is susceptible to the quality of the sounds. A three-dimensional sound field recording and reproduction system based on the boundary surface control principle (BoSC system) was used for this purpose. We measured brain activity during hearing action-related or nonaction-related sounds with electroencephalography using mu rhythm suppression (mu-suppression) as an index of motor cortical activation. The results showed that mu-suppression was observed when the participant heard action-related sounds, but it was not evident when hearing nonaction-related sounds. Moreover, this suppression was significantly larger in the 3D sound field (62-ch loudspeaker condition), which generates a more realistic sound field, than in the 1-ch loudspeaker condition. Our results indicate that the motor area was indeed activated for action-related sounds and that its activation was enhanced with a 3D realistic sound field. We discuss our findings in relation to the mirror neuron system and the possibility of using its activity as an objective measure that reflects the subjective sense of reality in various virtual reality settings when interacting with others.