Pentatonic sequences and monaural beats to facilitate relaxation: an EEG study

Abstract

Introduction: In two studies we investigated if specific acoustic stimulations could be more effective to induce a relaxation response in comparison to silence. Acoustic stimulations included monaural beats and musical sequences based on a pentatonic scale.

Methods: In the first study, 47 participants evaluated monaural beats and pentatonic sequences presented through loudspeakers and varying along three frequencies (0.2, 2, 4 Hz). In the second study, 31 participants relaxed with their eyes closed for 10 min during a passive listening of monaural beats and a pentatonic sequence presented through loudspeakers. A silence condition was introduced as control. All auditory stimuli were designed with a temporal modulation of 0.2 Hz. Concomitant EEG was recorded with a 64-channel system and spectral analysis was performed on delta, theta, alpha, beta, and gamma oscillations to test if each of the three auditory stimulations had a significant effect on EEG spectral power in comparison to silence.

Results: In the first study, pentatonic sequences were evaluated as more pleasant and more relaxing than monaural beats. Pleasantness and relaxation were inversely related to frequency. Visual imagery and emotion induction had higher frequency and were rated with a more positive valence in pentatonic sequences than in monaural beats. In the second study monaural beats in comparison to silence strongly decreased beta and gamma oscillations in the first three minutes and strongly increased theta oscillations in the last three minutes. Pentatonic sequences increased delta, theta, and alpha oscillations in the last three minutes while decreasing beta, and gamma oscillations for the whole auditory stimulation.

Discussion: The results show that auditory signals with a very low temporal modulation (0.2 Hz) could be more effective than silence in inducing a relaxation response. Although 0.2 Hz monaural beats were effective in inducing a relaxation response, they tended to be perceived as unpleasant. Pentatonic sequences could be considered as a better alternative to promote relaxation by auditory stimulation.

Keywords: EEG; beats; pentatonic sequences; relaxation; silence.

Figure 1

Energy-time graphs of a selection of the auditory stimuli used in the first study, with a temporal modulation of 0.2 Hz: (A) monaural beats, (B) pentatonic sequence.

Figure 2

Pleasantness ratings (from unpleasant to pleasant) for each sound stimulus by frequency.

Figure 3

Relaxation ratings (from relaxing to arousing) for each sound stimulus by frequency.

Figure 4

Positive valence of visual imagery ratings (from negative to positive) for each sound stimulus by frequency.

Figure 5

Positive valence in emotion experience (from negative to positive) for each sound stimulus by frequency.

Figure 6

Pleasantness mean rating as a function of condition.

Figure 7

Positive valence of visual imagery mean rating as a function of condition.

Figure 8

Positive valence in emotion experience as a function of condition.

Figure 9

Relaxation mean ratings as a function of condition.

Figure 10

EEG power spectrum change (dB) in the beats condition with reference to the silence condition. Magenta channel marks highlight the channel with a significant difference in power spectrum between the two conditions.

Figure 11

EEG power spectrum change (dB) in the pentatonic condition with reference to the silence condition. Magenta channel marks highlight the channel with a significant difference in power spectrum between the two conditions.