Kể từ đó, bản ghi âm của sự thay đổi trở kháng âm thanh của tai đã được sử dụng bởi các nhà điều tra rất nhiều cho các nghiên cứu lâm sàng của viêm tai giữa âm thanh phản xạ [92, 296] và cho mục đích nghiên cứu [194]. Trong khi Metz [151] và Jepsen [92] được sử dụng cầu Schuster, | 184 Section II The Auditory Nervous System BOX 1 cont d middle-ear muscles. Since then recordings of the change of the ear s acoustic impedance have been used by numerous investigators for clinical studies of the acoustic middle-ear reflex 92 296 and for research purposes 194 . While Metz 151 and Jepsen 92 used the Schuster bridge the investigators who followed mainly used an electroacoustic method 33 182 194 296 and that is also the principle used in the equipment that is presently used clinically. Most commercially available equipment that is designed for clinical recording the response of the acoustic middle ear reflex and for tympanometry use test tones of approximately kHz but investigators of the function of the acoustic middle ear reflex have used a kHz probe tone 194 . Another non-invasive method makes use of recordings of the displacement of the tympanic membrane as an indicator of contractions of the middle ear muscles but this method does not provide a reliable measure of the contraction of the stapedius muscle see p. 38 . Recording electromyographic EMG potentials 19 229 from the exposed stapedius muscle or recording the change in the cochlear microphonic CM potentials 177 has also been used to study the function of the acoustic middle ear reflex. Recording of EMG potentials makes it possible to discriminate between the contractions of the two muscles which is not possible by recording of the ear s acoustic impedance. Recording CM makes it possible to measure the change in sound transmission through the middle ear that is caused by contractions of the middle-ear muscles 177 . Both the EMG and the CM methods are invasive and are not practical for use in humans except in special situations where the middle-ear cavity becomes exposed during a surgical operation 19 . . Responses to Stimulation with Tones The response amplitude of the acoustic middle ear reflex to sounds just above threshold of the reflex increases gradually after a brief latency
