在過去對於中高強度聲音刺激的聽覺反射已經有相當好的研究(例如驚嚇反射、頭部轉向、耳廓反射以及鬍鬚凍結反射)，但在低強度聲音刺激下的聽覺反射則尚有待探討。耳鳴是一種常見的聽力障礙，通常與低強度聽覺有關(在耳鳴中的幻聲強度約略為30分貝或更低)。在此我們推測如果存在低強度的聽覺反射將有助於在實驗性耳鳴模式中客觀的評估動物其聽力損失(或屏蔽)。耳鳴之現行理論肇因於聽力損失導致中樞神經增益效應以補償受損之聽力，而產生聽覺過敏和幻聲感知(耳鳴)等副作用。幻聽會對弱聲刺激產生屏蔽效應(或聽力損失)的感知而與耳鳴類似。基於此，實驗性耳鳴評估之行為模式，包含學習行為的條件反射(polydypsia avoidance)和前脈衝抑制驚嚇反射(prepulse inhibition)。但兩種試驗都具有缺點:(A)需要時間訓練動物以及(B)運用高強度聲音刺激(>90分貝,與耳鳴特性不相似)。因此需要發展出運用低強度聲音刺激來進行簡單與客觀的聽力評估方法。為此，我們開發了一套數位化影像行為試驗模式用來偵測清醒大鼠對於低強度聲音刺激所誘發之聽覺反射。我們以Matlab為基礎設計一套影像分析軟體用來量化大鼠細微的身體動作。我們發現在正常的大鼠中，對於弱聲刺激的突現會使規律的呼吸運動產生偶然的擾動。藉由觀察大鼠胸、腹部的呼吸運動能偵測到此擾動，我們稱為”聽覺對呼吸的干擾”(acoustic perturbation of breathing, APB)。我們還發現:並非所有用來測試的聲音種類都能誘導APB。例如以粗糙表面摩擦效果最好。於實驗開始前，實驗組的大鼠於產後第四周中每日夜間以中強度單頻音給予刺激(4千赫單頻音、65分貝、每日八小時)。在過去同實驗室的研究指出:對幼鼠給予早期聲音刺激會使位於大腦聽覺皮層、中腦以及其下行聽覺路徑的神經細胞增大，如同耳鳴所產生的現象。對比於其他耳鳴動物模式如過量水楊酸與高音量暴露，早期聲音刺激模式(除了其他神經可塑性的變化)較接近常見於人類因老化產生的耳鳴(人類終其一生常暴露於中強度聲音下)。我們發現受過早期聲音刺激的大鼠於低強度聲音刺激中其APB會下降而其他的聽覺反射則否，代表此耳鳴模式之動物對低音量聽覺的損失(或遮蔽)與耳鳴類似。我們於此首次報告:APB是一種可能有效的工具用於低強度聲刺激的反射偵測有助於評估因耳鳴所造成之低強度聽覺損失。

While acoustic reflexes to loud sounds are well studied (viz., startle, head orienting, pinna, and vibrissa freezing responses), those to soft sounds remain unknown. Tinnitus, a leading hearing disorder, typically affects low intensity hearing (i.e., phantom sound perception of <30 dB SPL). Here we speculate that low-intensity acoustic reflexes, if in existence, could reflect the phenomenon of masking (or hearing loss) that accompanies tinnitus. The prevailing theory of tinnitus is that an initial hearing loss leads to an increased central gain to compensate the impaired intensity coding, with the unwanted side-effect of a phantom sound percept (tinnitus) and hyperacusis. The phantom sound produces a masking effect (or hearing loss) of soft sounds perceived similar to the tinnitus percept. Based on such masking, experimental tinnitus is assessed with either the traditional polydypsia avoidance or the more recent prepulse inhibition of acoustic startle reflex. These two methods have shortcomings like (a) time-consuming behavioral training or (b) the use of loud sound stimuli, hence inappropriate for studying the low level hearing in tinnitus. Methods that are simple and objective to estimate low intensity hearing are therefore in demand. Here, we developed a behavioral setup equipped with a digital imaging system designed to detect in awake rats the acoustic reflexes induced by soft sounds. Custom-made software was written to quantify the subtle body movements by optimizing the region of interest in the rat images. We found that for a normal rat, following the sudden presentation of a soft sound, its regular breathing pattern showed occasional perturbations. The perturbation in breathing was identified as an arrest in the respiratory-driven regular displacement of chest or abdomen. We coined this response the ‘acoustic perturbation of breathing (APB)’. We also found that not all sounds we tested had the same effectiveness in inducing APB: e.g., the rubbing sound was the most effective among three tested stimuli. The rubbing sound was therefore adopted as the stimulus for this study. Before the experiment, rats in the exposed group were first exposed during their early life (postnatal week-4) to a moderate level tone (4 kHz, 65 dB SPL, 8 hrs/day). From the previous studies of this laboratory, the same sound-exposure has led to enlarged neurons in the auditory cortex, midbrain and the descending system, consistent with the presence of tinnitus. Compared with other tinnitus-models (e.g., loud sound exposure, salicylate overdose), this early sound-exposure mode, apart from other accompanying neural plasticity, is likely closer to the human case as most tinnitus occurs with senility (likely due to, among other factors, the life-long sound exposure at moderate levels). APB in early sound-exposed rats was depressed when compared with the control, while other acoustic reflexes were not. We interpret this finding in the exposed animals as a masking of low intensity hearing likely associated with tinnitus. We conclude that the APB (reported here for the first time) is useful for assessing low intensity hearing loss associated with tinnitus.

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