Auditory Efferent System Influences the Auditory Overshoot Phenomenon- An Auditory Brainstem Response Study in Guinea Pigs

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 signal is delayed. This phenomenon is known as overshoot or temporal effect. Aiming of our study was to investigate the effect 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 in different delay onset time (DOTs) between the signal and the noise before and after dissecting the AENs were studied. Methods: Right internal auditory canals were exposed in 16 young male guinea pigs weighing 250 to 350 g. The inferior and posterior vestibular nerves that are known to carry AENs were transected in half of the subjects. Then, the ABR waveforms were recorded at 16 & 8 kHz tone burst stimuli at 0, 30, 60, 100 ms DOTs relative to wide-band noise. The value of latency of ABR waves I and III were compared among the different DOTs and five signal-to-noise ratios (SNRs) before and after the surgery. Results: By increasing DOTs, the latency of ABR waves I& III decreased in the control group before and after the surgery and 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& III increased from 0 to 30 ms, remained approximately constant from 30 to 60ms, and then began to decrease toward 100 ms DOTs. However, none of the measurements at 8 kHz, before and after surgery at both groups showed an overshoot effect. Conclusion(s): By using the ABR paradigm, the overshoot phenomenon disappeared after the transection of AENs. The results confirmed the role of the efferent system on auditory overshoot. Therefore, an objective tool to measure auditory efferent function is provided.

Peri-intraventricular hemorrhage: Study of the inhibitory effect of auditory efferent pathway

Purpose: to determine the functioning of the efferent auditory system in premature newborns with intraventricular hemorrhage. Method: the sample consisted of 44 newborns, divided into two groups. The study group was composed of 22 premature newborns with intraventricular hemorrhage/and the control group was composed of 22 newborns without intraventricular hemorrhage, matched to the study group for gestational age, correct gestational age and sex. The groups were submitted to the evaluation of the inhibitory effect of auditory efferent in otoacoustic emissions (equipment ILOv6-Otodynamics Ltda®) and auditory evoked potential with and without contralateral noise (equipment SmartEP-Intelligent Hearing Systems®). Results: newborns with intraventricular hemorrhage exhibited a higher occurrence of central hearing alteration as well as a lesser occurrence of the inhibitory effect of auditory efferent in otoacoustic emissions and auditory evoked potential compared to the newborns without this condition. An association was found between central hearing alteration and a lower occurrence of inhibitory effect. Agreement was found between the inhibitory effect test on otoacoustic emissions and latency of the auditory evoked potential. Conclusion: premature newborns with intraventricular hemorrhage have a greater occurrence of functional abnormality of the afferent auditory system, which can be effectively identified through an evaluation of the inhibitory effect of auditory efferent in otoacoustic emissions evoked by a transient stimulus and latency parameter in the brainstem auditory evoked potential.