2.1. 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.
2.2. 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 18000 Hz, while the right ear received a frequency of 18010 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.
2.3. 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 second, 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.
2.4. Cerebral hemodynamics measurement & 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 (760nm, 850nm) 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.