Avan P, Magnan P, Smurzynski J, Probst R, Dancer A
University of Auvergne, School of Medicine, Laboratory of Biophysics, Clermont-Ferrand, France.
Eur J Neurosci. 1998 May;10(5):1764-70. doi: 10.1046/j.1460-9568.1998.00188.x.
The fine tuning mechanisms involved in the normal processing of sound in the cochlea are non-linear, hence combination tones are generated inside the cochlea when a pair of low-level pure tones with neighbouring frequencies f1 and f2 is used as a stimulus. Their detection as sounds in the ear canal proves that they undergo backward propagation in the cochlea and through the middle ear, and the non-invasive measurement of the combination tone at 2f1-f2, called the cubic difference tone (CDT), has become a routine method of monitoring cochlear function. In order to gain information on the hypothetical places where CDTs are generated, on their intracochlear levels and propagation velocities, direct measurements of CDT pressure waves were carried out in scala vestibuli and tympani of the first and second turn of the guinea-pig cochlea. Cubic difference tones at 2f1-f2 varied from 0.75 to 9 kHz and were measured with a miniature piezoresistive transducer. Its high sensitivity allowed the detection of CDTs whenever their levels exceeded 5 dB SPL in the ear canal, i.e. 40 dB SPL (re: 20 microPa) inside the cochlea. The levels of CDTs were similar in scala vestibuli of the first and second turn. Phase comparisons between measurements at 2f1-f2 in the first and second turn allowed determination of the place where the CDT phase was minimum. It provided an estimation of the generation site of the CDT, which appeared to be close to the place tuned to f2 for stimulus levels lower than 70 dB SPL. Forward and backward travel times from one turn to the other were assessed at several frequencies, and both values were shorter than 0.2 ms. In contrast, the overall 'round-trip' delay of CDTs, measured in the ear canal, was about five times larger, suggesting that local filtering processes rather than propagation delays account for the overall CDT delay.
耳蜗中声音正常处理所涉及的微调机制是非线性的，因此，当使用一对频率相邻的低强度纯音f1和f2作为刺激时，耳蜗内会产生组合音。在耳道中检测到它们作为声音，证明它们在耳蜗中以及通过中耳进行逆向传播，并且对2f1 - f2处的组合音（称为三次差音，CDT）进行非侵入性测量已成为监测耳蜗功能的常规方法。为了获取有关CDT产生的假设位置、其在耳蜗内的水平以及传播速度的信息，在豚鼠耳蜗第一和第二圈的前庭阶和鼓阶中对CDT压力波进行了直接测量。2f1 - f2处的三次差音频率在0.75至9 kHz之间，使用微型压阻式换能器进行测量。其高灵敏度使得只要CDT在耳道中的水平超过5 dB SPL，即在耳蜗内超过40 dB SPL（相对于20 μPa），就能够检测到。第一和第二圈前庭阶中CDT的水平相似。通过比较第一和第二圈中2f1 - f2测量值之间的相位，确定了CDT相位最小的位置。这提供了对CDT产生部位的估计，对于低于70 dB SPL的刺激水平，该部位似乎接近调谐到f2的位置。在几个频率下评估了从一圈到另一圈的正向和反向传播时间，两个值均短于0.2 ms。相比之下，在耳道中测量的CDT的整体“往返”延迟大约大五倍，这表明整体CDT延迟是由局部滤波过程而非传播延迟造成的。
