Binaural Beats and Brain Activity: Exploring the Research
Binaural beats are an auditory phenomenon that occurs when two tones of slightly different frequencies are presented separately to each ear. The brain perceives a third tone oscillating at the difference frequency of the two tones. The fluctuation in frequency equals the difference in the two pure tones that are presented. For example, when a sinusoidal pure tone of 250 Hz is presented to the left ear and a 256 Hz is simultaneously presented to the right ear, amplitude modulation with a frequency rate of 6 Hz is perceived by the brain.
Binaural beats are created by presenting different tones to each ear, resulting in the perception of a beat frequency.
The beat in this phenomenon is generated within the brain and is referred to as a binaural beat. Binaural beats can be perceived in the frequency range of about 1-30 Hz, a range that coincides with the main human EEG frequency bands. The superior olivary complex is believed to be the first nucleus that receives auditory information from both sides of the ears, and binaurally activated phase-sensitive neurons are also found in the inferior colliculus.
The brainwave entrainment hypothesis, which assumes that external stimulation at a certain frequency leads to the brain’s electrocortical activity oscillating at the same frequency, provides the basis for research on the effects of binaural beat stimulation on cognitive and affective states. The binaural beat induces an interesting effect termed the frequency following effect. This effect can induce brain activity corresponding to the perceived beat.
The process of brain activity synchronization to the perceived beat is called entrainment. However, a classic study reported that the maximum difference in the two tones for which humans can perceive them as beat is 35 Hz; otherwise, 2 separate pure tones were perceived instead. In addition to the difference in the 2 tones, the carrier tone, which is the lower tone of the 2 tones, is also involved in the beat perception.
Historical Context and Recent Research
Research on binaural beats started as early as 1839, when the phenomenon was first described by H. W. Dove. However, given that it is a perceptual response to stimuli which are artificially generated and do not occur in natural settings, binaural beats were dismissed as a mere curiosity for more than a century.
Scientific interest in binaural beats has been rekindled only much later when the results from early empirical studies were systematically integrated by Oster. Not only did he describe the phenomenon in more detail, he also highlighted the potential relevance of binaural beat stimulation for practical use. It has been only for the last two decades that systematic research on binaural beats has gained traction again.
This was mainly due to a series of studies in the 2000’s that investigated ASSRs using EEG and MEG, which provided first reliable neuroscientific evidence for specific responses in human brain activity after binaural beat stimulation. More recently, research has mainly focused on the psychological effects of binaural beat stimulation. Within this field of study, effects on cognition, emotion, as well as certain concomitant physiological changes are investigated.
EEG Studies and Brain Responses
An EEG experiment to investigate the brain responses to a 40-Hz binaural beat with low and high carrier tones of 400 Hz and 3,200 Hz, respectively, for 1.2 s was conducted. The results showed that 40 Hz activity is evoked, and the lower carrier tone induced higher responses than the higher carrier tone at the fronto-central region. Other EEG studies have reported different results.
Presentation of 7- and 15-Hz binaural beats to participants for 15 min led delta power to increase at the left temporal region for the 7-Hz binaural beat condition while gamma power increased for the 15-Hz condition. Utilization of 3- and 6-Hz binaural beats on 250 and 1,000 Hz carrier tones for 2 s. ERP responses were evoked at the left temporal region.
The responses were higher for the 250 Hz condition than the 1,000 Hz condition and higher for 3-Hz than for 6-Hz. Recording of EEG signal at only T3 and T4 positions while delivering 10- and 20-Hz binaural beats on a mean 400 Hz carrier tone to participants for 1 min was conducted. No change in alpha activity was detected in the 10-Hz condition, while the left temporal region expressed a higher amplitude of beta activity than the right side in the 20-Hz condition; however, clear evidence of a frequency following effect did not appear due to the number of recording electrodes.
A magnetic field study noted that the right temporal region responded to a 40-Hz binaural beat on a 500 Hz carrier tone within 1 s of exposure. Other magnetoencephalography (MEG) studies also reported similar findings. A 26 Hz auditory steady-state response (ASSR) occurred at the right parietal and left middle frontal regions by exposure to a 26-Hz binaural beat on a 250 Hz carrier tone for 500 ms was reported.
One study showed symmetrical responses to 4 binaural beats corresponding to 4.00- and 6.66-Hz beats on 240 and 480 Hz carrier tones for 10 min. ASSRs were induced at the temporal, frontal, and parietal regions, but symmetry did not always occur. However, these finding also suggested that the cerebral cortex can be synchronized with binaural beats.
The brain responses to binaural beats by sweeping a beat for 16 s from 3- to 60-Hz on a carrier tone with a mean frequency of 500 Hz and found different polarities between the left and right auditory cortices. These researchers also suggested that different brain processes occur for different frequency activities.
Theta Activity and Meditation
Several studies in literature have investigated different brain activities but one activity interested in nowadays lifestyle is theta activity and it is found during meditation. Theta activity is a type of brain activity classified by the frequency range of 4-8 Hz. It is associated with the behavioral states of alertness, attention, orientation, and working memory including the enhancement of cognitive and perceptual performances.
Meditation is the mental activity associated with attaining a deeply restful but fully alert state and is believed to reduce stress, which commonly occurs during daily life. One study claimed that 30 min of meditation is enough for a beginner to reduce stress. Peacefulness and the reduction of stress are important for improving brain functions, especially cortical brain functions.
Theta activity has been utilized to investigate the meditative state by both general theta and frontal midline theta rhythms. Theta activity, during meditative state, is found at frontal and parietal-central regions but is not found at posterior region. This phenomenon is general theta activity. Theta activity must be found at frontal midline cortical position, specifically Fz position, which is considered as meditative state; and this is frontal midline theta rhythms.
However, modern lifestyles are often characterized as stressful and highly active, and thus, 30 min of meditation may not achieve a meditative state if one is concerned of their surroundings; in such cases, it may take longer than 30 min to achieve a deep meditative state.
Methodological Differences and Study Aims
With methodological differences in the primary stimuli parameters, i.e., beat frequency, carrier tone frequency, duration of exposure, and recording procedure, shown in literatures have varied across previous studies. This leads to difficultly in comparing the brain responses to binaural beats and hinders valid discussions. In addition, to reduce stress in nowadays lifestyle meditation becomes an interesting procedure; if binaural beat can induce similar activity of meditative state, it can be used as stimulus for meditation induction.
Therefore, one study aimed to investigate the responses of the brain to a 6-Hz binaural beat on a 250 Hz carrier tone using QEEG for 30 min of listening. As previous studies have lacked QEEG recordings when evaluating the responses to binaural beats, we sought to use QEEG as a recording procedure in this study. The reasons for applying 6-Hz binaural beat are as follows: 6-Hz is the middle of theta activity, 4-8 Hz, so it can represent theta activity, and indicate more precisely that occurred responses are appeared due to the stimulus in theta range not in the others.
Only theta activity was observed due to the frequency following effect of binaural beats, as a 6-Hz binaural beat is in the range of theta activity. It was hypothesized that the power of the theta activity would be enhanced after listening to a 6-Hz binaural beat. The aim of this study was to investigate the responses of the brain to a 6-Hz binaural beat on a 250 Hz carrier tone using QEEG with a 30-min listening period.
Experimental Setup and Procedure
Experimental and control groups were included. The experimental room was a sound-attenuated room with the temperature controlled to 25°C. The wall color of the room was white for neutral perceptions of emotion and mood, as reported by Sroykham et al. The experimental station was setup as follows: an armchair faced the white wall at a distance of 3 m, and the height of the armchair was adjusted to the comfort of each participant.
The stimulus used in this study was a binaural beat stimulus that was specifically created for the experiment by providing two similar tones at slightly different frequencies. The carrier tone of 250 Hz was presented to the left ear, and the offset tone of 256 Hz was presented to the right ear. Twenty-eight participants with an average age of 21.9 years and a standard deviation of 1.9 years were included in the study.
The participants were allocated to experimental and control groups. The purpose of the study and the experimental procedures were described to all participants before participation, but details of the stimulus were not revealed. The experimental group was composed of 17 participants, 5 females and 12 males, with average age of 22.2 years and a standard deviation of 2.1 years, while the control group was composed of 11 participants, 4 females and 6 males, with an average age of 21.4 years and a standard deviation of 1.5 years.
The aims and procedures of the experiment were told to the participants. All participants were asked to give written informed consent before participation in the study. Each participant was asked to sit in the armchair within the experimental room in an upright position and to lay their feet on the footrest and relax. The size of participant's head was measured for fitting a suitably sized electrode cap.
The electrode cap, composed of an elastic cap with mesh electrodes at the positions corresponding to the international 10/20 system, was worn on the participant's head. Each electrode was filled with conductive gel to reduce the impedance between each electrode and the scalp to improve the recordings and to minimize noise. The impedance was <5 kΩ. Two electrode cups with conductive paste were then clipped on both the right and left ear lobules as ground and reference, respectively. All electrodes were wired to a BrainMaster® system for recording EEG signals using BrainMaster Discovery software.
EEG recording setup using BrainMaster system.
Consequently, 40 min of the experimental period began. The experimental period included 5 min of baseline recordings, 30 min of stimulus period, and a final 5 min of post-stimulus recording. The stimulus was provided to each participant based on the participant's group. The stimulus of experimental group was a 6-Hz binaural beat while that of the control group was silence. However, all participants were told that the stimulus was already provided.
Along the experimental period, EEG signals were recorded, and participants were asked to continue to sit idly and to make as few movements as possible. However, due to long experimental period, fatigue occurred in some participants. If the participants reported feelings of fatigue, a short duration of movements was permitted to release the fatigue. The signals recorded during movement and eye blinking were not included in the analysis.
Data Analysis
The recorded EEG signals included 40 min for each participant. Each recorded EEG signal was separated as follows: 5 min of baseline recording, followed by 6 intervals of 5 min each along the stimulus period, and finally 5 min of post-stimulus recording. A total of 8 intervals of 5 min each were recorded.
The EEG signals of each 5-min interval were selected based on the following criteria: clear signals without noise, eye blinking, or artifacts, and split half and test retest reliabilities of at least 90%. Fast Fourier transformation (FFT) was performed on each selected signal to convert the signal from the time domain to the frequency domain. The absolute power of the theta activity was assessed for statistical analysis.
All data analyses were conducted by NeuroGuide software. Within the same group, paired t-tests were conducted to compare mean of absolute power of the theta activity between each interval and baseline, position-by-position. The comparisons were performed to indicate the time-point of increased FFT absolute power of theta activity upon listening to the stimulus for both the experimental and control groups. P < 0.05 were considered significant.
Between the experimental and control groups, independent t-tests were conducted to compare the mean differences (d̄ in paired t-test) between the theta activity of each interval and baseline between the two groups, position-by-position. The changes in theta activity were compared to determine whether the changes that occurred at each time-point were a result of the stimulus rather than idly sitting. P < 0.05 were considered significant.
Results and Findings
The stimuli were presented to participants for 30 min depending on participants' group. A 6-Hz binaural beat was presented to the experimental group, and silence was presented to the control group. The absolute power of the theta activity showed a maximum value at the fronto-central cortical position, specifically between the Fz and Cz positions.
The absolute power of the theta activity gradually declined from the maximum position in radial fashion and reached a minimum value at the temporal cortical positions on both sides, i.e., the T3 and T4 positions. After listening to the 6-Hz binaural beat stimulus, the absolute power of the theta activity was enhanced at nearly all cortical positions except the T4 position within 10 min of exposure.
Table 1 shows the significantly different cortical positions of the absolute power of theta activity according to paired t-tests for each interval after exposure to the stimulus compared to baseline. The significant differences suggested that within 10 min of exposure, all cortical positions were enhanced by the stimulus; at 15 and 25 min, fewer cortical positions were changed by the stimulus.
Topographic mapping of theta activity during the experiment.
Binaural Beats and Cognitive Functions
Memory functions are linked to fluctuations in brain electrical activity, specifically in theta, alpha, and gamma frequencies, which can impact different aspects of memory. Generally, when binaural beats are stimulated for 5-30 minutes, they attract brain waves to the frequency of the beats. Binaural beats have been found to enhance working memory, an essential function of the central nervous system that temporarily stores information during cognitive activities, such as reading, comprehension, and learning.
Neuromodulation research has focused on the potential of working memory to improve cognitive and behavioral outcomes. Improving working memory through the use of binaural beats can help in the transfer of information from active memory to long-term memory, ultimately leading to better long-term storage and retrieval. Research has shown that the effect of binaural beats on long-term memory can vary depending on the frequency used.
In general, the beta frequency is more effective in enhancing long-term memory compared to the theta frequency. Several studies contradict the previously mentioned results. Also, the literature has revealed significant variability in results, with some studies reporting no significant changes in memory task performance when comparing binaural beats to control conditions.
Various studies have investigated the effect of binaural beats on cognition. The study by Sharp et al. showed that binaural beats at 40 Hz significantly improved cognition, while at 25 and 100 Hz, the amount of improvement was less. Evidence shows that gamma-frequency binaural beats can enhance cognitive flexibility and improve divergent thinking.
Conversely, research indicates that binaural beats may have no significant effect or even detrimental effects on cognition. A study involving 1,000 participants found that listening to binaural beats during cognitive tasks led to decreased performance scores, suggesting that rather than enhancing cognitive abilities, these auditory stimuli might impair them.
Binaural Beats for Mental Health
Major Depressive Disorder (MDD) was predicted to take the top rank by 2030 by the WHO, which placed it as the third most common cause of disease burden globally in 2008. In a randomized controlled trial study on patients with mild to moderate acute phase of depression, it was found that listening to binaural beats with an alpha frequency of 10 Hz for 30 minutes every day for 5 days significantly reduced depression scores.
Binaural beats can cause changes in the electrical activity of the brain cortex. By increasing alpha power in the occipital region and increasing gamma in the prefrontal region, they can have therapeutic effects on cortical activity and depression improvement. There are not many studies on the effect of binaural beats on depression, but those present vary in their conclusions.
A class of extremely common mental health issues known as anxiety disorders can have a crippling effect on everyday functioning and general well-being. Binaural beats can even be used to reduce anxiety in certain situations (such as preoperative). It has been shown in a study that binaural beats and 432 Hz music can both be effective in reducing preoperative anxiety. Several studies have shown the effectiveness of binaural beats listening in reducing anxiety levels and have also proven binaural beats to be more effective than monaural beats in reducing anxiety.
Binaural Beats and Sleep
Sleep is crucial for one's health and well-being. According to a preliminary study, binaural beats may improve sleep quality. In a study, binaural beats with a 3 Hz delta frequency were used to produce delta activity in the brain. Binaural beats lengthened stage three sleep as a result. For feeling fresh in the morning, stage three sleep must be deep.
The potential benefits of binaural beats in promoting improved sleep have been shown by other small research studies. In a study, soccer players who listened to binaural beats between 2 and 8 Hz reported better sleep quality, reduced sleepiness, and easier waking up. Additionally, binaural beats may help in lowering anxiety, in turn promoting better sleep.
Inconsistencies and Limitations
At first glance, however, the available literature on brainwave entrainment effects due to binaural beat stimulation appears to be inconclusive at best. The results corroborate the impression of an overall inconsistency of empirical outcomes, with some studies reporting results in line with the brainwave entrainment hypothesis, eight studies reporting contradictory, and one mixed results.
What is to be noticed is that the fourteen studies included in this review were very heterogeneous regarding the implementation of the binaural beats, the experimental designs, and the EEG parameters and analyses. The methodological heterogeneity in this field of study ultimately limits the comparability of research outcomes.
Conclusion
Binaural beats have been investigated for their potential to influence brain activity and cognitive functions. While some studies suggest positive effects on memory, cognition, anxiety, and sleep, there are inconsistencies and limitations in the research. Further studies with more standardized methodologies are needed to fully understand the effects of binaural beats on the brain.
The theoretical basis of psychological research on the effects of binaural beat stimulation is provided by the brainwave entrainment hypothesis which suggests that auditory or visual stimulation at a specific frequency will lead the brain’s electrocortical activity to oscillate at the external signal’s frequency or at its multiples.