Shore S E, Nuttall A L
J Acoust Soc Am. 1985 Oct;78(4):1286-95. doi: 10.1121/1.392898.
The auditory compound action potential (CAP) represents synchronous VIIIth nerve activity. Clicks or impulses have been used in the past to produce this synchrony under the assumption that the wide spectral spread inherent in transient signals will activate a large portion of the cochlear partition. However, the observation that only auditory nerve units tuned above 3 kHz contribute to synchronous activity in the N1P1 complex of the CAP [Dolan et al., J. Acoust. Soc. Am. 73, 580-591 (1983)] suggests that temporal delays imposed by the traveling wave result in an asynchronous pattern of VIIIth nerve activation. In order to determine if units tuned below 3 kHz could be recruited into the CAP response, the present study uses tone bursts of exponentially rising frequency to hypothetically activate synchronous discharges of VIIIth nerve fibers along the length of the cochlear partition. The equations defining the frequency sweeps are calculated to be the inverse of the delay-line characteristics of the guinea pig cochlear partition. The resultant sweeps theoretically cause a constant phase displacement of a large portion of the cochlear partition at one time. Compound action potentials recorded in response to the rising frequency sweeps were compared to CAPs evoked by corresponding falling frequency sweeps and clicks. Analysis of the CAP waveforms showed narrower N1 widths and larger N1 and P1 amplitudes for rising sweeps when compared to falling sweeps. This is consistent with the hypothesis of increased synchrony. A further test of the hypothesis was made by using high-pass masking noise to evaluate the contributions of discrete cochlear locations to the CAP ("derived" CAP). Latency functions of the derived CAPs for clicks and falling frequency sweeps showed progressive increases in latency as the cutoff frequency of the high-pass filter was lowered. The latency of the derived CAP for these stimulus conditions reflects traveling wave delays [Aran and Cazals, "Electrocochleography: Animal studies," in Evoked Electrical Activity in The Auditory Nervous System (Academic, New York, 1978)]. In contrast, derived CAPs obtained from rising sweeps showed no change in latency for any cutoff frequencies, indicating a constant delay of response for fibers with different characteristic frequencies (CFs). These results support the theoretical premise underlying the derivation of the rising sweep: Spectral energy with the appropriate temporal organization, dictated by basilar membrane traveling wave properties, will increase CAP synchrony.
听觉复合动作电位（CAP）代表同步的第八对脑神经活动。过去曾使用 clicks 或脉冲来产生这种同步性，其假设是瞬态信号固有的宽频谱扩展将激活大部分耳蜗隔板。然而，观察发现只有调谐频率高于 3kHz 的听觉神经单元对 CAP 的 N1P1 复合波中的同步活动有贡献[Dolan 等人，《美国声学学会杂志》73，580 - 591（1983）]，这表明行波所施加的时间延迟导致了第八对脑神经激活的异步模式。为了确定调谐频率低于 3kHz 的单元是否能被纳入 CAP 反应，本研究使用指数上升频率的短纯音脉冲，假设其能激活沿耳蜗隔板长度的第八对脑神经纤维的同步放电。定义频率扫描的方程被计算为豚鼠耳蜗隔板延迟线特性的倒数。由此产生的扫描理论上会使耳蜗隔板的大部分在某一时刻产生恒定的相位位移。将响应上升频率扫描记录的复合动作电位与由相应下降频率扫描和 clicks 诱发的 CAP 进行比较。对 CAP 波形的分析表明，与下降扫描相比，上升扫描的 N1 宽度更窄，N1 和 P1 幅度更大。这与同步性增加的假设一致。通过使用高通掩蔽噪声来评估耳蜗离散位置对 CAP（“衍生”CAP）的贡献，对该假设进行了进一步测试。对于 clicks 和下降频率扫描，衍生 CAP 的潜伏期函数显示，随着高通滤波器截止频率的降低，潜伏期逐渐增加。这些刺激条件下衍生 CAP 的潜伏期反映了行波延迟[Aran 和 Cazals，“电耳蜗图：动物研究”，载于《听觉神经系统中的诱发电活动》（学术出版社，纽约，1978）]。相比之下，从上升扫描获得的衍生 CAP 在任何截止频率下潜伏期都没有变化，这表明具有不同特征频率（CF）的纤维的反应延迟是恒定的。这些结果支持了上升扫描推导所基于的理论前提：由基底膜行波特性决定的具有适当时间组织的频谱能量将增加 CAP 的同步性。
