Introduction

Today, people utilize Auditory Beat Stimulation (ABS) technology for psychological relaxation and cognitive enhancement. ABS can be broadly classified into Monaural Beats (MB) and Binaural Beats (BB). MB involves presenting sine waves of two distinct frequencies simultaneously to both ears, while BB presents separate frequencies to each ear1. For instance, in MB, two different frequencies (e.g., 110 Hz and 120 Hz) are merged within a single speaker and simultaneously presented to both ears2. Conversely, BB delivers a stimulus of a base frequency of 100 Hz to one ear and a base frequency plus a difference frequency of 110 Hz to the other ear, inducing brainwave synchronization at the difference frequency of 10 Hz3.

In the case of MB, the perception of the stimulus arises from the direct interference of sound waves, which is recognized in the cochlea4. However, BB result from the combined neural activity induced by two separate but synchronized sound waves along the auditory pathway5. Consequently, the superior olivary complex (SOC) in the brainstem, responsible for integrating sound signals, has been identified as a key neuroanatomical structure associated with stimulus perception6,7. As a result, while MB perception depends on the intensity of sound, BB perception is independent of volume and relies solely on phase differences8.

Numerous studies have provided evidence that inducing brainwaves through BB can lead to positive effects on cognitive aspects9,10,11,12,13,14. Research has shown increased activation of attention and concentration indices when presenting alpha and beta frequency BB15. Additionally, studies have demonstrated improved cognitive abilities in groups exposed to alpha BB during short-term memory tasks, as well as reduced reaction times in groups exposed to alpha and gamma BB16.

However, there have been studies indicating that BB stimulation may not have an effect on cognitive function enhancement17,18,19. For instance, research has shown that while 15 Hz BB during working memory tasks increases response accuracy, both 5 Hz and 10 Hz BB result in decreased accuracy20. Additionally, there are studies suggesting that BB inducing beta waves does not affect cognitive functions such as attention21. These conflicting findings regarding the effects of BB are predicted to stem from various reasons. One possible explanation is that previous studies utilized audible range BB stimulation, which might have influenced cognitive processing due to the presence of sound within the audible range22,23,24.

In our study, we observed the brainwave induction effects of Binaural Beats (BB) after eliminating the influence of audible sounds in the audible range, as demonstrated in previous research. Specifically, we validated the ability to induce alpha waves using inaudible BB with a base frequency of 18,000 Hz and a difference frequency of 10 Hz25. Furthermore, by presenting inaudible BB stimuli while performing tasks to evaluate spatial memory, we found an improvement in task performance compared to conditions without stimulation. This clarifies the positive impact of pure BB stimulation, excluding sounds in the audible range, on cognitive processing26.

However, all prior research, including our team’s preliminary studies on BB, has solely observed changes in brainwave rhythms based on neural activity. Direct observation of brain activity has not been conducted. Thus, studies directly observing brain neuroactivity or changes in cerebral blood flow, which are directly associated with cognitive processing changes, are lacking regarding the effects of BB. Additional research, such as direct observation of brain neuroactivity, is necessary to validate the scientific effects of BB.

Therefore, in this study, we aim to re-examine the effects of pure BB stimulation, excluding the influence of sound, using inaudible BB stimulation with a base frequency of 18,000 Hz and a difference frequency of 10 Hz. Additionally, we seek to observe brain activation responses. We have selected spatial memory, a representative cognitive task, for examination. To measure changes in brain activation associated with cognitive processing, we will utilize functional Near Infrared Spectroscopy (fNIRS), which provides a convenient method for measuring brain hemodynamic responses.

Methods

Subjects

We recruited a total of 17 healthy individuals in their 20s (12 males with a mean age of 23.2 ± 1.7 and 5 females with a mean age of 21.0 ± 0.7) through participant recruitment announcements. Participants were required to have no hearing impairments or relevant medical history of brain disorders. Prior to the experiment, participants were advised to ensure sufficient sleep the night before, and they were instructed to refrain from behaviors such as smoking and drinking on the day of the experiment that could potentially affect fNIRS measurements.

This study was approved by the Konkuk University Institutional Review Board (7001355-202105-HR-439). All methods were performed in accordance with the relevant guidelines and regulations by including a statement, and informed consent was obtained from all subjects.

Binaural beats (BB)

To induce a 10 Hz Binaural Beats (BB) stimulation corresponding to the alpha wave, an auditory stimulator (Brand G, Model Q) was utilized. The left ear was presented with a frequency of 18,000 Hz, while the right ear received a frequency of 18,010 Hz. The experiment comprised three phases: Rest phase (5 min), Task phase (5 min), and another Rest phase (5 min). The Task phase included two conditions: “Task only,” where participants performed the task without BB stimulation, and “Task + BB,” where BB stimulation was presented concurrently with task performance (Fig. 1). All participants engaged in both task conditions, and the order was counterbalanced.

Fig. 1
figure 1

The experimental design.

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3-back Task

In this study, we measured visuospatial memory using the 3-back task. Participants were instructed to press the space bar as quickly as possible if the currently presented image matched the one presented three times earlier. The images consisted of a white display in one of the four quadrants of a divided rectangle (Fig. 2). Each image was presented for 1 s, followed by a black screen with a white cross (“+”) at the center during the 1-second interval between images. A total of 150 images, randomly selected from four categories, were presented, with 50 correct answers. To ensure smooth experiment execution, participants underwent sufficient practice trials before participating in the actual experiment. After completing all trials, Accuracy (%) and Reaction Time (ms) were calculated. To analyze the differences between the two conditions (Task only and Task + BB), a Paired t-test (SPSS 25, IBM, USA) was conducted.

Fig. 2
figure 2

3-back task.

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Cerebral hemodynamics measurement and analysis

We measured hemodynamic responses using fNIRS measurement system (NIRSport2®; NIRx Medical Technology, Berlin, Germany) with a sampling rate of 5.08 Hz. The system consists of sources emitting two wavelengths (760 nm, 850 nm) and 16 detectors each. To observe brain regions associated with visuospatial memory, optodes were attached to the Frontal, Temporal, Parietal, and Occipital areas according to the 10–20 system (sources: Fp1, Fp2, AFz, F1, F2, F5, F6, TP7, TP8, P1, P2, P5, P6, POz, O1, O2, detectors: Fpz, Fz, F3, F4, T7, T8, Pz, P3, P4, P7, P8, PO3, PO4, PO7, PO8, Oz), resulting in a total of 36 source-detector channels (Fig. 3). The distance between source-detector channels was set at 3.0 cm.

The nirsLAB®software (v.2019.04; NIRx Medical Technology, Berlin, Germany) was utilized to transform optical data into hemodynamic data using the modified Beer-Lambert law. Initially, bandpass filtering from 0.01 to 0.2 Hz was applied to remove physiological noises (heartbeat, respiration, etc.). The data from phases other than the pre-task rest phase were normalized based on the rest phase data. The Differential Pathlength Factor (DPF) was set at 7.25 (Wavelength 1) and 6.38 (Wavelength 2), and oxyhemoglobin (HbO) concentrations were extracted.

The extracted HbO concentrations in a time-series format were analyzed using MATLAB R2021b (Mathworks, USA). Paired t-tests (SPSS 25(IBM, USA)) were conducted to compare the average HbO concentrations across channels during the Task phase (5 min) between the two conditions.

Fig. 3
figure 3

The locations of optodes.

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Results

Task performance

The results of the 3-back task are presented in Fig. 4. The accuracy for the Task only condition was 66.8 ± 16.8%, and for the Task + BB condition, it was 68.2 ± 20.4%. The reaction time for the Task only condition was 753.2 ± 152.9 ms, while for the Task + BB condition, it was 699.4 ± 121.9 ms. The results of the paired t-test showed no significant difference in Accuracy (p = 0.785) between conditions. However, there was a significant decrease in Reaction Time in the Task + BB condition compared to the Task-only condition (p = 0.046).

Fig. 4
figure 4

The 3-back task result according to the presence or absence of BB stimulation.

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Cerebral hemodynamic responses

The results of the HbO concentration comparison between the Task Only and Task + BB conditions are illustrated in Fig. 5(a). Significant differences were observed in three channels: F1-F3 (p = 0.010), F2-F4 (p = 0.043), and P2-P4 (p = 0.047). In these regions, the HbO concentration significantly increased in the Task + BB condition compared to the Task Only condition (Fig. 5(b)).

Fig. 5
figure 5

Cerebral hemodynamic responses. (a) The channels F1-F3, F2-F4, P2-P4 exhibited a significant difference. (b) The concentration of HbO for three channels based on the two conditions.

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Discussion

This study aimed to revisit the influence of inaudible Binaural Beats (BB) stimulation on visuospatial memory while exploring the direct impact of the induced brainwave changes on brain activation. Task performance metrics (Accuracy, Reaction time) and hemodynamic responses (HbO) were concurrently observed.

The task performance results showed a significant decrease in Reaction Time in the Task + BB condition compared to the Task-only condition, indicating that the presentation of BB led to improved reaction time during task execution. Analysis of hemodynamic responses revealed increased HbO concentrations in the F1-F3, F2-F4, and P2-P4 regions during the Task + BB condition. Thus, increased activation in specific brain regions during the Task + BB condition, compared to Task only, is anticipated to positively influence visuospatial memory.

There have been reports suggesting that audible 10 Hz BB stimulation during Visual Reaction Time (VRT) tasks leads to shorter reaction times27. Furthermore, positive effects of BB in the alpha frequency band on cognitive abilities have been documented28,29,30,31,32. This study observed the positive impact of inaudible 10 Hz BB stimulation on visuospatial memory, reaffirming the pure effects of BB on cognition.

Previous research on objectively elucidating the effects of Binaural Beats (BB) stimulation has predominantly focused on observing changes in brainwaves thought to be directly influenced by BB. However, studies employing methods such as functional Magnetic Resonance Imaging (fMRI) or fNIRS, allowing the observation of changes in brain activation associated with actual cognitive processing, are notably scarce. This study addresses this gap by utilizing fNIRS, which provides a convenient means to capture brain hemodynamic signals reflecting brain activation in a relatively unconstrained environment. The frontal lobe plays a crucial role in performing working memory tasks. Specifically, the Right Frontal lobe is reported to be activated during short-term memory tasks related to visuospatial information33, while the Right Parietal lobe is activated during working memory tasks associated with visuospatial stimuli34. Therefore, the observed increase in activation in the F1-F3, F2-F4, and P2-P4 areas in this study is presumed to be closely related to the task, and the increased brain activation in these areas is expected to have a positive impact on task performance.

Alpha waves, a type of brainwave emerging during states of rest before the onset of substantial brain activity, are considered absent when the mind is unable to relax, indicating tension or anxiety35. Moreover, alpha waves play a significant role in the working memory process36, with reports suggesting an increase in alpha wave activity during the maintenance of working memory37,38,39. Although simultaneous measurement of brainwaves was not conducted in this study, the anticipated increase in alpha waves induced by BB stimulation suggests a positive impact on brain neural activity during task performance.

The significance of this study lies in the validation, through both behavioral data and brain hemodynamic responses reflecting direct neural activation, that inaudible frequency range BB stimulation positively influences spatial memory. However, while we confirmed the positive impact, we did not elucidate the causality of this effect. Additionally, we did not directly compare brainwave signals with cerebral blood flow signals, and the study was limited to participants in their twenties. In this study, due to the small number of participants, paired t-tests were conducted for each channel. However, to ensure more rigorous testing, it is important to apply p-value correction methods (such as Bonferroni and FDR) to address multiple comparison problem. Nonetheless, the activated regions observed in this study (Right & Left Frontal, Right Parietal) are highly relevant to visuospatial task performance, suggesting that the findings are reliable. Future research should address these limitations by implementing more stringent experimental controls and diverse analyses. It is imperative to conduct further mechanistic studies on BB effects from various perspectives, aiming for improved understanding.