Verschooten Eric, Strickland Elizabeth A, Verhaert Nicolas, Joris Philip X
Laboratory of Auditory Neurophysiology, Department of Neurosciences, University of LeuvenLeuven, Belgium.
Department of Speech, Language, and Hearing Sciences, Purdue UniversityWest Lafayette, IN, United States.
Front Neurosci. 2017 Jun 8;11:331. doi: 10.3389/fnins.2017.00331. eCollection 2017.
Development of electrophysiological means to assess the medial olivocochlear (MOC) system in humans is important to further our understanding of the function of that system and for the refinement and validation of psychoacoustical and otoacoustic emission methods which are thought to probe the MOC. Based on measurements in anesthetized animals it has been hypothesized that the MOC-reflex (MOCR) can enhance the response to signals in noise, and several lines of evidence support such a role in humans. A difficulty in these studies is the isolation of efferent effects. Efferent activation can be triggered by acoustic stimulation of the contralateral or ipsilateral ear, but ipsilateral stimulation is thought to be more effective. However, ipsilateral stimulation complicates interpretation of effects since these sounds can affect the perception of other ipsilateral sounds by mechanisms not involving olivocochlear efferents. We assessed the ipsilaterally evoked MOCR in human using a transtympanic procedure to record mass-potentials from the cochlear promontory or the niche of the round window. Averaged compound action potential (CAP) responses to masked probe tones of 4 kHz with and without a precursor (designed to activate the MOCR but not the stapedius reflex) were extracted with a polarity alternating paradigm. The masker was either a simultaneous narrow band noise masker or a short (20-ms) tonal ON- or OFF-frequency forward masker. The subjects were screened for normal hearing (audiogram, tympanogram, threshold stapedius reflex) and psychoacoustically tested for the presence of a precursor effect. We observed a clear reduction of CAP amplitude by the precursor, for different masking conditions. Even without an MOCR, this is expected because the precursor will affect the response to subsequent stimuli via neural adaptation. To determine whether the precursor also activated the efferent system, we measured the CAP over a range of masker levels, with or without precursor, and for different types of masker. The results show CAP reduction consistent with the type of gain reduction caused by the MOCR. These results generally support psychoacoustical paradigms designed to probe the efferent system as indeed activating the MOCR system, but not all observations are consistent with this mechanism.
开发评估人类内侧橄榄耳蜗（MOC）系统的电生理方法，对于增进我们对该系统功能的理解以及完善和验证被认为可探测MOC的心理声学和耳声发射方法非常重要。基于对麻醉动物的测量，有人提出假设，即MOC反射（MOCR）可以增强对噪声中信号的反应，并且有几条证据支持其在人类中的这种作用。这些研究中的一个困难是分离传出效应。传出激活可由对侧或同侧耳的声刺激触发，但同侧刺激被认为更有效。然而，同侧刺激使效应的解释变得复杂，因为这些声音可通过不涉及橄榄耳蜗传出神经的机制影响对其他同侧声音的感知。我们使用经鼓膜程序记录来自耳蜗岬或圆窗龛的群体电位，评估人类同侧诱发的MOCR。采用极性交替范式提取对4kHz带掩蔽探测音有无前导刺激（设计用于激活MOCR但不激活镫骨肌反射）的平均复合动作电位（CAP）反应。掩蔽音要么是同时的窄带噪声掩蔽音，要么是短（20毫秒）的音调开或关频率前向掩蔽音。对受试者进行正常听力筛查（听力图、鼓室图、镫骨肌反射阈值），并进行心理声学测试以检测前导刺激效应的存在。我们观察到，在不同的掩蔽条件下，前导刺激使CAP幅度明显降低。即使没有MOCR，这也是预期的，因为前导刺激会通过神经适应影响对后续刺激的反应。为了确定前导刺激是否也激活了传出系统，我们在有或无前导刺激以及不同类型掩蔽音的情况下，测量了一系列掩蔽音水平下的CAP。结果显示CAP降低与MOCR引起的增益降低类型一致。这些结果总体上支持旨在探测传出系统的心理声学范式确实激活了MOCR系统，但并非所有观察结果都与该机制一致。
