Natural sounds produce higher EEG hemispheric coherences than artificial sounds.

[Objective] Present study examined the hemispheric coherences of electroencephalogram (EEG) to identify the differential effects of natural sounds and mechanical sound, as inspired by the research reporting the association between meditative state of consciousness and increased EEG hemispheric coherence (Kato & Kawano, 2017). [Procedure] Thirty-three healthy volunteers, including twenty women (24.41+10.42 yrs.) and thirteen men (25.00+10.04 yrs.), listened to six 1-minute-long audio files (helicopter, rowing-boat, water-sprinkler, ocean-wave, waterfall, rain) and their 16-channel EEGs were monitored based on International 10-20 Method. [Analyses] Coherences were computed for eight hemispherically-paired channel combinations of EEGs (Fp1-Fp2, F3-F4, F7-F8, C3-C4, P3-P4, T3-T4, T5-T6, & O1-O2) by using Coherence Similarities as well as Long Common Subsequence methods. Coherence Similarity is obtained by dividing the absolute value of cross power spectral density by square-root of power spectral densities of two waveforms. Long Common Subsequence is based on the length of common subsequence of two waveforms. Mixed Design ANOVAs were applied by using gender as between IV, measurement stages as within IV, and above-mentioned coherence values as DVs. [Results] Significant treatment effect of T5-T6 was found for Coherence Similarity, and post-hoc-t-test with Bonferroni correction showed that T5-T6 coherence during ocean-wave and waterfall sounds were significantly higher than during helicopter sound (P<.01). Further, there were significant treatment effects of F3-F4 and C3-C4 for Long Common Subsequence of alpha frequency component (8-13 Hz). Post-hoc-t-test with Bonferroni correction demonstrated significantly higher coherence during waterfall sounds than during rowing-boat sound for F3-F4, and significantly higher coherence during rain sound than during water-sprinkler sound for C3-C4 (P<.01). [Discussion] ANOVA results suggested that exposure to natural sounds produced significantly higher EEG hemispheric coherences than did the exposure to artificial sounds, and thus suggesting relaxing and meditative effects of natural sounds.