Introduction

Brain waves consist of rhythmic patterns of neuronal activity or synchronized electrochemical pulses from groups of neurons in the central nervous system (CNS)¹. There are several well-established brainwave range patterns: gamma (30-70 Hz), beta (13-30 Hz), alpha (8-13 Hz), theta (4-8 Hz), and delta (1-4 Hz)². Each one of these frequency bands has been correlated with different states of consciousness, such as awake, relaxed, rapid eye movement (REM) sleep, as well as non-REM sleep stages³.

According to Gruzelier⁴, prolonged audio stimuli in repetitive and synchronized manner may induce changes in brain waves patterns and, consequently, may modulate neurophysiological and behavioral responses. More specifically, repetitive external or environmental stimuli may temporarily affect the predominance of specific brain wave frequencies, a phenomenon namely brainwave entrainment (BWE)⁵-⁷. Therefore, BWE can be defined as rhythmic synchronization of brainwave oscillation with an external repetitive stimulus.

BWE is a recurrent phenomenon in nature and biologically present in living beings⁸. The principle of entrainment or harmonization was discovered around 1665, by the Dutch scientist Christian Huygens⁹. The synchronization obtained through the entrainment principle is the result of the harmonization principle, a physical phenomenon that occurs systematically in nature, and that is dependent on environmental stimuli, for example, visual, auditory, or tactile. These stimuli may be used to elicit synchronized brainwave patterns to match that of different mental states and/or levels of consciousness, as seen with data acquisition techniques, such as the electroencephalography (EEG). In this context, Oster¹⁰ stated the possibility to improve, amplify or modulate brain wave patterns to conditioned events in the cerebral cortex³,¹¹. The proposed therapeutic benefits have a wide scope, including the improvement of cerebral blood flow, neuroplasticity stimulation, and neurophysiological compensations between the cerebral hemispheres³.

In this review, we focused on the most common forms of auditory BWE, that is, binaural beats and isochronic tones. Acoustic waves are characterized in Hertz (number of cycles per second). The audible sound spectrum for humans comprises frequencies between 20 Hz and 20,000 Hz, regardless of its complexity, and as long as it has an amplitude greater than 0 dB (decibel)¹²,¹³.

Isochronic tones consist of distinct and repetitive regular beats of a single tone. The number of peaks per second in the signal amplitude is the isochronic frequency heard at regular and standardized time intervals. On the other hand, binaural beats represent the auditory experience that occurs when two sounds of close frequencies are presented separately to each ear with headphones or stereo speakers. The brain integrates the two signals, producing a third “phantom sound” representing the difference between the two auditory stimuli. For example, if a frequency of 114 Hz is presented to the right ear, and another of 124 Hz to left ear, a binaural beat of 10 Hz is created by the brain as a result of these stimuli. In this case, brain waves tend to match the binaural beat frequency, in this example 10 Hz, which corresponds to alpha brainwave pattern. Binaural beats are created in the superior olivary nucleus of the brain stem, the local of contralateral integration of auditory input¹⁴ (Fig. 1).

Figure 1 Binaural beats generated by the brain. 

The beat is neurologically transported to the reticular formation, which uses neurotransmitters to trigger changes in brain wave activity¹⁰ that synchronizes with that of the stimulus generated. The mental features associated with each brain wave pattern can be elicited based on the scientific principle of harmonization, also known as “brain wave entrainment.”

Most of studies with binaural beats and isochronic tones provides positive outcomes, indicating that audio brain entrainment may yield different benefits, both physically and mentally¹⁵,¹⁶. More specifically, brain entrainment can be applied to induce mental states and as adjunctive treatment to several brain disorders in a safe and non-invasive manner, such as for the treatment of depression and anxiety disorders¹⁷. For instance, it was demonstrated that a group of individuals exposed to 6 Hz sounds for 10 min presented a significant increase on theta wave (4-8 Hz) cortical activity in comparison to control group that did not receive the stimulus. These findings indicate a facilitatory effect on induction of a meditative state and altered states of consciousness¹⁸. In this context, the aim of this study was to review the scientific evidence on the therapeutic use of binaural beats and isochronic tones for the modulation of brain wave patterns and mood states.

Development

Results

The search yielded a total of 674 studies (PUBMED: 74; MEDLINE: 396; LILLACS: 138; and SCIENCE DIRECT: 66). As illustrated in figure 2, from the total of studies, 49 studies were in duplicate, and 592 studies addressed other themes beyond the scope of this review, and, therefore, were excluded from the study.

Figure 2 Randomization algorithm and study selection. 

The selected studies were analyzed according to the Cochrane Handbook for Systematic Reviews¹⁹, resulting in 33 randomized, controlled clinical trials that were then evaluated by the Jadad scale²⁰ (Fig. 3). Only 17 studies obtained a score of three points or more on the Jadad scale²⁰, and them were fully read and critically analyzed; studies are summarized in Table 1. Binaural beats were used in 15 studies (88.25%) whereas isochronic tones were used in only two studies (11.76%) (Fig. 3). In 82.35% of these publications monaural and/or binaural audio stimulation were more effective in comparison to control group.

Figure 3 Summary of the selected studies - handbook for systematic reviews. 

After the evaluation made by the Jadad scale, the 17 selected studies were classified according to the methodological criteria adopted in each experiment (Randomized Study (R); Double Blind Study (BD), Controlled Study with Placebo (PC), type of sample and number of individuals in the study and results obtained by each researcher. The summary of therapeutic targets and forms of intervention and approaches of these selected studies are described in Table 2.

Main findings of the studies addressing the effects of brain waves on mental states

In this section is presented a description of the different brainwaves, their effects, and the discussion of the main findings obtained in the studies reviewed here. The study performed by Washington and collaborators²¹ indicated that each brain wave frequency produces particular neurophysiological and cognitive effects, been associated with a specific state of consciousness.

Delta waves (< 4 Hertz): the slowest waves are associated with the deepest state of sleep and unconscious. These waves represent the ideal for sleep, physical and mental recovery, and deep meditation. Delta waves lead to a state of mental happiness and empathy where the person feels more connected with her/himself. This state improves intuition and memory. Delta waves are associated with the release of growth hormone²², which is beneficial for cell regeneration, as well as the production of endogenous opioids²³. None of the selected studies review here used delta waves.

Theta waves (4-8 Hz): this frequency pattern is related to the processes of creativity, enhanced intuition, more intense emotional connections that elevate sensitivity, and a sense of tranquility and reduced anxiety. Theta waves also contribute to the improvement of problem-solving skills and retention of much larger amounts of information in shorter period of time. This pattern is associated with decreased levels of serum cortisol and with the modulation of serotonin and melatonin. Theta waves generate a relaxed state of consciousness²⁴,²⁵.

Eight of the studies reviewed here were conducted with Theta waves, with binaural beats as well as isochronic tones interventions. Major targets were cognitive and/or pathological states. Increased long-term memory performance was observed in patients with epilepsy who underwent 6-min 5 Hz brain entrainment sessions, once a week for 6 weeks with binaural and isochronic tones²⁶. On the other hand, in non-epileptic adult subjects, 15-min 5 Hz brain entrainment sessions did not induce significant effects²⁷. Visuospatial working memory and cortical connectivity were not altered following a single intervention of 5 min with 5 Hz therapy²⁸. Another study using binaural beats also describes absence of significant effects in the working memory of healthy young adults²⁹. Interestingly, 20-min 6 Hz binaural beat entrainment twice a week for 14 days effectively reduced the perception of pain severity¹⁶.

In addition, in patients with temporal lobe epilepsy who underwent brain implants with EEG signal control, acute therapy of temporolateral (5 Hz for 5 min) increased memory acuity with EEG synchronization¹⁷. Moreover, in young adult’s post-physical training, binaural beat entrainment (4-7 Hz for 20 min) increased parasympathetic activation and self-reported relaxation³⁰. Finally, a single intervention with isochronic tones at 6, 10 and 40 Hz during 5 min reduced anxiety and improved well-being reports of healthy individuals³¹.

Alpha waves (8-13 Hz): this frequency pattern is related to mental relaxation³², visualization, and creative processes; therapeutically uses include memory optimization, and modulation of pain perception threshold. The use of this wave pattern in the elderly population has been shown an excellent therapeutic potential to treat memory disorders³³.

Alpha waves were used with a therapeutic and/or neurocognitive focus in six studies reviewed here. Vernon et al.³⁴ reported that 10 Hz alpha pattern for 1 or 5 min did not elicit significant EEG alteration. However, isochronic stimulation with 7 Hz for 9 min with 3 min intervals induced temporal and parietal lobe activation with the potential to alter brain networks in adult’s healthy young individuals³⁵. Alpha binaural beats for 3 min in healthy young adults did not affect attentional blink (AB) control with the EBR method (predictor of mood states associated with dopamine levels)³⁶. Moreover, patients with temporal lobe epilepsy who underwent brain implants, the acute exposure to 10 Hz isochronic tones for 5 min increased significantly medio temporal synchronization¹⁷,³⁰.

Beta waves (13-39 Hz): this frequency pattern is related to attention, focus, concentration, and cognition. Beta BWE was shown to be effective in improving fatigue and some symptoms of attention deficit hyperactive disorder (ADHD), including learning and attention deficits. Additional studies showed improvement in visual acuity, coordination, potential for the dyslexia treatment and low of concentration, as well as, to promote IQ gain in the range of 8-10 points³⁷. Beta wave entrainment influenced self-confidence and socialization and makes people more optimistic and energetic³⁸-⁴¹. Finally, it helps learning as well as sports-related abilities³,⁴².

Five independent studies reviewed here used beta waves. Beauchene et al.²⁸ demonstrated that 15 Hz beta pattern for 5 min increased short-term visuospatial working memory and cortical connectivity in healthy young subjects. A single 15-min session of 20 Hz significantly increased long-term memory, improving the codification of new information without previous memories²⁴. Interestingly, Vernon et al.³⁴ demonstrated that two daily sessions (14 Hz, during 10 min) increased motor and non-motor symptoms in Parkinson’s disease (PD) patients. In addition, Gálvez et al.⁴³ showed a decrease of functional connectivity and optimization of working memory, with no changes in gait or anxiety levels in PD patients in comparison to control group. Kennel et al.⁴⁴ submitted a group of children and adolescents with diagnosis of ADHD to brain entrainment sessions (duration of 10 min, 3 days per week for 3 weeks). Results were not totally conclusive, although parents reported improvement in the performance of homework tasks after the interventions.

Gamma Waves (> 40 Hz): this frequency pattern is involved in blinking and processing of information from all parts of the brain. High gamma wave activity in the brain is associated with intelligence, compassion, self-control, and feelings of happiness⁴⁵-⁴⁸. In addition, gamma brainwaves have been associated with improved memory and a greater ability of reality perception⁴⁹,⁵⁰. Gamma brainwave activity has been shown to be increased in monks during meditation⁵¹.

Six studies were conducted with gamma BWE. It was demonstrated that 40 Hz gamma stimulation for 20 min improved working memory performance and mnemonic function in healthy subjects⁵². AB control, using the EBR method (predictor of mood states associated with dopamine levels), was significantly affected by 3-min binaural gamma stimulation in healthy young adults, before and during a global-local task³⁶. These benefits were observed only in individuals with low rates of spontaneous blinking, which indicates low levels of dopamine in the striatum. Colzato et al.⁵³ reported that 3-min 40 Hz binaural beats stimulation improved focus and attention in healthy young adults. Moreover, a study using 10-min 40 Hz stimulation in healthy adults⁵⁴ demonstrated that BWE acts by modeling specific brain oscillations, in cognitive processes sustained in the gamma wave frequency range, such as mental processes related to intelligence, self-control, and well-being. In epileptic patients subjected to brain implants, acute isochronic tones and binaural beats exposure, in the range of 40-80 Hz for 5 min significantly decreased synchronization in medio-temporal sites, demonstrating their potential as a non-invasive therapy for modulating intracranial flow in synchronization of the EEG signals¹⁷. A single intervention with isochronic tones 6, 10, and 40 Hz over 5 min reduced anxiety and, consequently, increased the subjects’ well-being, however, without marked effects on cognition⁵⁵.

As illustrated in figure 3, in 82.35% of the reviewed studies, monaural, and/or binaural audio stimulation were more effective in comparison to control group. As previously mentioned, binaural beats were used as therapeutic modality in 15 studies (88.25%), and isochronic tones were only used in two studies (11.76%).

Discussion

The data reviewed in the present study indicates that binaural beats are more commonly used than isochronic tones, at least in research (as seen in Fig. 3). Unfortunately, in our view, there is no standard in the choice of therapy parameters, such as brainwave range (in Hz), treatment duration, frequency, and regimen, neither for the treatment of specific conditions or disorders, nor for simple increase of executive brain functions and stimulation of inherent mental processes, and neurocognitive performance. A relevant observation is the absence of properly controlled double-blinded studies, which compromises the validity of the results available in the literature. Nevertheless, the results reviewed here suggest that auditory BWE, although still scarcely explored in behavioral and neurophysiological therapy, may represent an effective and inexpensive therapeutic approach, with minimal side effects. It is well known that all brain activity occurs through the bioelectric activity of neural networks and that the brain wave phenomenon is produced as a result of the sum of bioelectric interactions of the billions of neurons and their trillions of synaptic connections⁵⁶. Changes in states of consciousness (concentration, excitement, relaxation, sleep, dreams, etc.) are closed related to changes in the frequency of the vibratory pattern of brain waves, which varies according to the intensity of these bioelectric activities.

According to this system and considering that our state of consciousness is influenced by brain wave patterns, it is plausible to conclude that BWE can modulate many aspects of behavior, from states of consciousness to perception, learning, and cognition⁵⁷.

The adaptation to daily activities requires the brain ability to modulate brain wave activity, in consonance to external stimuli and signals, as well as when faced with challenges and/or problems to be solved. Each type of brain wave can modulate different neurotransmitter systems, inducing particular synaptic and neurochemical readjustments¹²,⁵⁶.

Conclusions

Based on the data reviewed in this study, binaural beats and isochronic tones BWE may effectively modulate mood states, improving attention, and memory processes. Promising results were also obtained in subjects suffering from different CNS disorders, including ADHD, PD, epilepsy, chronic pain, and anxiety disorders.

Despite the audio BWE effectiveness described in many studies reviewed here, it is important to emphasize that some positive outcomes may require further investigation, with more refined and appropriate evaluation tools, better suited for each specific type of intervention, and/or therapeutic target. Considering these limitations, the performance of additional studies with more adequate experimental design and data analysis is recommended, particularly focusing on the neurophysiological and behavioral effects of brain wave entrainment on mental states.