Nachtigall Paul E, Supin Alexander Y
Marine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii, PO Box 1106, Kailua, HI 96734, USA.
J Exp Biol. 2008 Jun;211(Pt 11):1714-8. doi: 10.1242/jeb.013862.
The use of auditory evoked potential (AEP) measurements has added considerably to knowledge of the hearing mechanisms of marine mammals. We have recently measured the hearing of a stranded infant Risso's dolphin, the audiograms of white-beaked dolphins temporarily caught and released, and the hearing of anaesthetized polar bears. Most small toothed whales echolocate and hear very high frequency sounds underwater. While much has previously been learned about the echolocation performance and characteristics of the outgoing signals of echolocating dolphins and small whales, the hearing processes occurring while these animals actively echolocate have not previously been examined. Working with a well-trained echolocating false killer whale (Pseudorca crassidens) wearing latex surface suction cup electrodes, we have measured echolocation hearing AEPs in response to outgoing echolocation clicks, returning echoes, and comparable simulated whale clicks and echoes in a variety of situations. We have found that: (1) the whale may hear her loud outgoing clicks and much quieter returning echoes at comparable levels, (2) the whale has protective mechanisms that dampen the intensity of her outgoing signals - she hears her outgoing signals at a level about 40 dB lower than similar signals presented directly in front of her, (3) when echo return levels are lowered either by making the targets smaller or by placing the targets farther away - without changing the levels of her outgoing signals - the hearing of these echoes remains at almost the same level, (4) if targets are made much smaller and harder to echolocate, the animal will modify what she hears of her outgoing signal - as if to heighten overall hearing sensitivity to keep the echo level hearable, (5) the animal has an active 'automatic gain control' mechanism in her hearing based on both forward masking that balances outgoing pulse intensity and time between pulse and echo, and active hearing control. Overall, hearing during echolocation appears to be a very active process.
听觉诱发电位（AEP）测量的应用极大地增进了我们对海洋哺乳动物听力机制的了解。我们最近测量了一头搁浅的里氏海豚幼崽的听力、暂时捕获并放生的白喙海豚的听力图，以及麻醉状态下北极熊的听力。大多数小型齿鲸通过回声定位来感知水下的高频声音。虽然此前已经对回声定位的海豚和小型鲸类发出信号的回声定位性能和特征有了很多了解，但这些动物在积极进行回声定位时的听力过程此前尚未得到研究。我们与一头训练有素、能进行回声定位的伪虎鲸（Pseudorca crassidens）合作，它身上佩戴着乳胶表面吸盘电极，我们在各种情况下测量了其对发出的回声定位咔哒声、返回的回声以及类似的模拟鲸类咔哒声和回声的回声定位听力AEP。我们发现：（1）这头鲸能以相当的水平听到自己响亮的发出咔哒声和安静得多的返回回声；（2）这头鲸有保护机制来减弱其发出信号的强度——它听到自己发出信号的水平比直接在其前方呈现的类似信号低约40分贝；（3）当通过减小目标尺寸或使目标距离更远来降低回声返回水平时——在不改变其发出信号水平的情况下——对这些回声的听力仍保持在几乎相同的水平；（4）如果目标变得小得多且更难进行回声定位，这头动物会改变它对自己发出信号的听觉——似乎是为了提高整体听力灵敏度以保持回声水平可听；（5）这头动物在听力方面有一个基于前向掩蔽的主动“自动增益控制”机制，该机制既能平衡发出脉冲强度和脉冲与回声之间的时间，又能进行主动听力控制。总体而言，回声定位期间的听力似乎是一个非常活跃的过程。
