Effects of 90 dB pure tone exposure on auditory and cardio-cerebral system functions in macaque monkeys

Excessive noise exposure can pose significant health risks to humans. In addition to causing direct damage to the auditory system, noise exposure is also associated with pathological changes or injuries in the cardiovascular and central nervous systems. Uncovering the physiological effects of noise exposure on the body contributes to a better understanding of the potential health risks associated with noise exposure, aiding in the establishment and updating of safety standards and limits for occupational noise exposure. This study focused on three male macaque monkeys as subjects. Low-intensity pure tone exposure (intensity: 90 dB, frequency: 500 Hz, repetition rate: 40 Hz, 10 min per day, continuously exposed for 5 days) was administered. Evaluations were conducted before noise exposure, immediately after exposure, and at 7, 14, and 28 days post-exposure. Auditory brainstem response (ABR) tests were conducted to assess the impact of noise exposure on the hearing of the macaque monkeys. Electrocardiograms (ECG) were performed to evaluate changes in the heart function of the monkeys after noise exposure. Electroencephalograms (EEG) were recorded to assess noise-induced changes in brain function. The results of the study revealed that, compared to pre-exposure levels, the average threshold for the V wave of the right ear of the Macaque monkeys increased by approximately 30 dB immediately after exposure (p < 0.01), and it returned to normal within 7 days. The ECG results indicated that one of the macaque monkeys exhibited an RS-type QRS wave, and inverted T waves from immediately after exposure to 14 days, which normalized at 28 days. The other two macaque monkeys showed no significant changes in their ECG parameters. Changes in EEG parameters demonstrated that main brain regions exhibited significant activation at 40 Hz during noise exposure. Noise stimulation with a 40 Hz repetition rate induced abnormal resting-state electroencephalogram power spectral density (PSD) across the δ (1–3 Hz), θ (4–8 Hz), α (8–12 Hz), β1 (12–20 Hz), β2 (20–30 Hz), and γ (30–48 Hz) frequency bands. After noise exposure, the PSD in main brain regions, particularly those represented by the temporal lobe, exhibited a decreasing trend across all frequency bands, with no clear recovery over time. In summary, exposure to 90 dB pure tone resulted in transient hearing threshold shifts, inverted T waves, activation of brain regions at 40 Hz, and disrupted PSD of resting-state EEG in macaque monkeys. While these effects were observed, they exhibited individual differences. This study investigated the physiological changes in auditory and cardio-cerebral systems in macaque monkeys following low-intensity noise exposure, which could be informative for noise protection and the establishment of safety standards for human. However, the exact mechanisms involved require further pathological and behavioral exploration.