Sadao Minami

Alternating current induced otoacoustic emissions in the guinea pig

Injection of alternating current (AC) into the scala media of the guinea pig cochlea induced otoacoustic emissions (OAEs) at the frequency of the AC fundamental, together with harmonic and intermodulation distortion products. Although the waveform of the injected ACs was distorted, probably due to nonlinear polarization of the metal electrodes, and was composed of the fundamental plus distortion products of every order, only a few of the lowest order distortion products were selectively emitted with the fundamental. AC injection at a basal site extended the high frequency limit of OAEs. Electrical stimulation of the crossed olivocochlear bundle inhibited the sideband emissions with little change in the fundamental. OAE was reduced reversibly by temporary impairment of the cochlea due to exposure to fatiguing sound, by intravenous application of furosemide and by temporary anoxia. Irreversible reduction resulted from intracochlear perfusion with excess K+ solution, acoustic trauma and cardiac arrest. These facts imply that AC-induced OAE is not an artifact generated electrically; rather, such emissions originate in the cochlea and normal metabolic activity in the cochlea is essential. A proposed mechanism of generation includes two components: 1) electromechanical transduction from AC to mechanical vibration in the cochlea and 2) a distortion-producing process; the contribution of each component to the receptor mechanism is discussed.

The acoustic middle ear muscle reflex in albino rats

The acoustic middle ear muscle reflex was studied in albino rats anesthetized with chloralose. The best frequency of the reflex and the threshold at this frequency were on average about 3 kHz and 57 dB SPL, respectively. The threshold increased as frequency increased above, and decreased below, the best frequency at a rate of about 20 dB/octave. Above about 12 kHz, the muscular response showed instability and habituation. Thresholds were similar between stapedius and tensor tympani reflexes and between ipsilateral and contralateral reflexes. The middle ear transmission loss due to the reflex was the greatest and nearly constant below about 1 kHz, where the loss was about 18 dB at the maximal stimulation. Above this frequency the loss decreased as frequency increased up to 20 kHz. Thus the reflex, unlike that in other animals, suppressed transmission over the whole range of reflex-eliciting frequencies. The transfer function of the reflex had a well damped low-pass characteristic with a cut-off frequency of about 20 Hz. From the above characteristics of the reflex, the role of the rats tympanic muscles in improving ultrasonic hearing under ambient noises was suggested.

Characteristics of electrically induced otoacoustic emission

Acoustic emissions were detected using the fast Fourier analysis of sounds in an ear canal during injection of alternating currents at different frequencies into the scala media of guinea pigs anaesthetized with urethane (1.2 g/kg i.p.). The amplitude of the distortion products relative to the fundamentals in the emitted sound was about ten times as large as that in the cochlear current, which was already slightly distorted, probably due to nonlinearity at the metal electrode surface. This discrepancy implies a larger additional nonlinearity in the cochlea. During COCB stimulation or after moderately intense sound exposure, the emitted distortion products were suppressed reversibly, in contrast to almost no change in the fundamental emissions. All the emissions were suppressed reversibly after temporary anoxia or furosemide administration, and they disappeared completely after severe acoustic trauma or death. These results suggest that the normal metabolic activity of the cochlea is required for generatio...