This phenomenon is called entrainment of neural oscillation ( Wahbeh et al., 2007). The binaural beat is then conducted to the auditory cortex in phase-locked fashion described by the response of the inferior colliculus – a part of the ascending auditory pathway – as a binaural beat ( Kuwada et al., 1979 McAlpine et al., 1996 Spitzer and Semple, 1998 Schwarz and Taylor, 2005 Karino et al., 2006).Īfter the primary auditory cortex receives the binaural beat signal, the signal is consequently sent to associated auditory areas and other associated areas inducing the brain to oscillate at the rate of binaural beat frequency, which can be measured by EEG rhythms.
The so-called binaural beat is spontaneously generated with a frequency equaling the difference of the frequencies of the two tones. However, at the brainstem, auditory signals from both sides are passed to the superior olivary complex ( Kuwada et al., 1979 Schwarz and Taylor, 2005), the first nucleus in the ascending auditory pathway receiving bilateral auditory signals. The auditory signal from each ear is conducted ipsilaterally along the ascending auditory pathway to the auditory cortex. Moreover, one study measured perception of the beat on different frequency carrier tones – the tone that is generated at each ear – and suggested that an intermediate frequency carrier tone of 440 Hz facilitated a wider range of beat perception than lower or higher frequency carrier tones ( Licklider et al., 1950). The binaural beat cannot be measured by measurement tools but can be perceived by humans because the origination of the beat is in the brain however, the difference of the two presented frequencies is greater than 35 Hz or two separate tones would not be perceived ( Oster, 1973). For example, when a sinusoidal 250-Hz pure tone is presented to the left ear, and a 253-Hz tone is simultaneously presented to the right ear, a fluctuation of amplitude with a frequency rate of 3 Hz is perceived by the brain (Figure 1). A binaural beat, which occurs when the two mentioned tones are simultaneously presented to different ears, a virtual fluctuation of a single tone is generated in the brain by the ascending auditory pathway. The fluctuation single tone is perceived as amplitude modulation with the frequency equaling the difference of the frequencies of the two interfering tones. Moreover, the N3 latency of the experimental group was shorter.īeat is a phenomenon that occurs by interference of two almost equivalent sinusoidal tones but with slightly different frequencies and presents as fluctuation of a single tone. The results showed that the N3 duration of the experimental group was longer than that of the control group, and the N2 duration of the experimental group was shorter than that of the control group. However, the participants were blinded to their stimulus group. For the control group, a silent sham stimulus was used. The stimulus was initiated when the first epoch of the N2 sleep stage was detected and stopped when the first epoch of the N3 sleep stage detected. Participants in both groups underwent the same procedures, but only the experimental group was exposed to the 3-Hz binaural beat on the experimental night. The experimental period was three consecutive nights consisting of an adaptation night, a baseline night, and an experimental night. Twenty-four participants were allocated to experimental and control groups. This study aimed to investigate the effect of a 3-Hz binaural beat on sleep stages, which is considered a behavioral state. The binaural beat can entrain neural activities to synchronize with the beat frequency and induce behavioral states related to the neural activities. In this phenomenon, the so-called binaural beat has a frequency equaling the difference of the frequencies of the two pure tones. The dichotic presentation of two almost equivalent pure tones with slightly different frequencies leads to virtual beat perception by the brain. Brain Computer Interface Laboratory, Department of Biomedical Engineering, Faculty of Engineering, Mahidol University, Salaya, Thailand.Nantawachara Jirakittayakorn * and Yodchanan Wongsawat *
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