Gamma-modulated human speech-originated sound evokes and entrains gamma wave in human brain

Abstract

Numerous studies focusing on the gamma-band brain response to sound stimuli have been performed to treat or clarify the mechanism of cognitive diseases such as dementia or Alzheimer’s disease. However, the sound stimuli used in most studies have been limited to those with simple structures. In this study, we investigated the evoked gamma waves in human brains using sound with speech information, which was amplitude-modulated at a frequency in the gamma range (40 Hz), as the stimulus. Psychological investigations, such as establishing the level of discomfort and assessing the perception of the unnaturalness associated with hearing specific aural stimuli, were performed to examine the acceptability of listening to the sound stimulus in daily life. As a result, both the power of the evoked gamma wave and the phase locking index at 40 Hz showed significant differences from that of the non-modulated sound. Additionally, significant differences were also observed with both stimuli with simple structures and the amplitude-modulated speech sounds. In addition, a positive correlation between the degree of discomfort, and the degree of the unnaturalness of speech sound, respectively, could be observed with the gamma power. However, as a certain variation in the relationship between them was observed, the result implies that the comfort and natural stimuli that evoke and entrain gamma wave can be clinically applied to the prevention or improvement of dementia.