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Haddadzade Niri H, Rahbar N, Pourbakht A, Haghani H. Auditory Efferent System Influencing the Auditory Overshoot Phenomenon: An Auditory Brainstem Response Study in Guinea Pigs. BCN 2025; 16 (2) :403-416
URL: http://bcn.iums.ac.ir/article-1-1558-fa.html

Introduction: Detection of a brief tonal signal at the beginning of a longer masking noise is difficult, but it becomes easier when the onset of the signal is delayed. This phenomenon is known as overshoot or temporal effect. Our study investigates the impact of the auditory efferent nerves (AENs) function on the auditory electrophysiological overshoot, further introducing an objective tool examining one of the AENs performances. Therefore, the effect and the trend of changes induced by low- and high-frequency stimuli on electrophysiological overshoot were studied in different delay onset times (DOTs) between the signal and the noise before and after dissecting the AENs. 
Methods: Right internal auditory canals were exposed in 16 young male guinea pigs weighing 250 to 350 g. The inferior and posterior vestibular nerves known to carry AENs were transected in half of the subjects. Then, the ABR waveforms were recorded at 16 and 8 kHz tone burst stimuli at 0, 30, 60, and 100 ms DOTs relative to wideband noise. The latency values of auditory brainstem response (ABR) waves I and III were compared among the different DOTs and 5 signal-to-noise ratios (SNRs) before and after the surgery.
Results: By increasing DOTs, ABR waves I and III latency decreased in the control group before and after the surgery and in the case group before surgery at 16 kHz. However, the observed overshoot-like effect disappeared after the surgery in the case group. The wave’s latency I and III increased from 0 to 30 ms, remained approximately constant from 30 to 60 ms, and then decreased toward 100 ms DOTs. However, none of the measurements at 8 kHz before and after surgery in both groups showed an overshoot effect.
Conclusion: Using the ABR paradigm, the overshoot phenomenon disappeared after the transection of AENs. The results confirmed the role of the efferent system in auditory overshooting. Therefore, an objective tool to measure auditory efferent function is provided.