The human brainwave (EEG) signals are associated with different neural activities. Particularly, frequency bands exceeding approximately 30Hz are referred to as the gamma frequency band, and they are linked to various cognitive functions, sensory integration, short-term memory, and more.

Moreover, numerous reports have discussed the association between gamma waves and conditions such as Alzheimer's disease and schizophrenia. It is suggested that presenting stimuli with the power of gamma waves may regulate cognitive processes in the brain, potentially mitigating the severity of these disorders.

 

Integration of 40Hz stimuli into music: Exploring cognitive enhancement and comfort.

 

Moreover, the ASSR (Auditory Steady-State Response) induced by auditory stimulation at 40Hz holds potential as a biomarker for evaluating the extent of human cognitive function. When actual stimuli combining visual and auditory inputs at 40Hz are presented, there is evidence demonstrating a reduction in brain atrophy and loss of functional connectivity, leading to improved performance in associative memory tasks.

However, humans generally perceive monotonous auditory stimuli at 40Hz as unpleasant sounds. Therefore, methods have been proposed to integrate 40Hz stimuli with music to make them more tolerable. Nevertheless, incorporating this often perceived as rough and unpleasant stimulus into music while maintaining a comfortable listening experience is a challenging task.

 

A novel attempt at achieving comfortable music: the integration of 40Hz gamma stimuli.

 

In this endeavor, we synthesized tones resembling drums from 40Hz vibrations, adjusted the amplitude envelope of bass and keyboard sounds to the gamma frequency band, and evaluated the effectiveness of this Gamma Music. As a control condition, we used music stimuli (Control Music) comprising drums, bass, and keyboard sounds without strong power in the vicinity of 40Hz. Additionally, as a third condition, we included music with traditionally unpleasant gamma stimuli directly incorporated into the target music (Conv. Gamma Music). Subsequently, after listening to each piece of music, we conducted a survey assessing relaxation indicators and levels of arousal (Table 1).

The results of subjective evaluations for auditory stimuli are depicted in Figure 1. As a result, significant differences were evident in relaxation, emotional evaluation, preference, pleasantness, and oddness. However, no significant differences were observed in immersion and arousal.

In the multiple comparison test for relaxation, the Gamma Keyboard sound revealed significant differences in relaxation evaluations compared to Gamma Drum and Conv. Gamma Music. Furthermore, Conv. Gamma Music exhibited significantly higher ratings for discomfort and oddness compared to Gamma Music and Control Music. These results suggest that Conv. Gamma Music was perceived as the least preferable music, while the tone of Gamma Keyboard was the most preferred, invoking a sense of relaxation and comfort.

Figure 2-3 (A) represents the cumulative average of power and Phase Lag Index (PLI) for each frequency from the forehead to the central region. As depicted in the figure, clear peaks in both 40Hz power and PLI were observed from the forehead to the central region. This suggests that the 40Hz component present in auditory stimuli induces Auditory Steady-State Response (ASSR). Figure 2-3 (B) presents box charts of 40Hz power and PLI for each stimulus in the frontal and prefrontal to central regions. It reveals that Gamma Music, Gamma Instrument, and Conv. Gamma Music strongly induce ASSR. Particularly, Gamma Keyboard sound exhibits the highest values in both 40Hz power and PLI※.

*Phase Locking Index (PLI)* is a statistical method used in the field of neuroscience to measure the phase synchronization between trials of brainwave (EEG) data or other physiological signals. In this study, it is utilized as an indicator to assess the extent to which Gamma Music consistently induces the 40Hz oscillatory component in the brain.

Gamma Music: Potential for ASSR Induction without Discomfort

 

From these results, it has been revealed that Gamma Music has the potential to induce powerful ASSR while maintaining a pleasant sensation when listening to music. In recent years, there has been suggestive evidence of the regulatory impact of gamma stimulation on cognitive functions in the brain, increasing the significance of gamma stimulation. The Gamma Music we have created holds the potential for participants to use it for extended periods during ASSR and gamma interventions in clinical settings without experiencing discomfort.

In the future, our plan is not only to focus on 40Hz but also to identify individual gamma frequencies that elicit specific ASSR responses for each person. We aim to develop personalized Gamma Music based on these frequencies and validate its effectiveness.

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詳細（Preprint paper）:
Yokota, Y., Tanaka, K., Ming, C., Naruse, Y., Imamura, Y., & Fujii, S. (2023). Gamma Music: A New Acoustic Stimulus for Gamma-frequency Auditory Steady-State Response. bioRxiv, 2023-08.

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