Basilar membrane motion was measured at the 16-19 kHz place of the guinea pig cochlea using the Mössbauer technique. The threshold of the gross cochlear action potential (CAP) evoked by pure-tone bursts was used as an indication of neural threshold. CAP threshold deteriorated progressively after the cochlea was opened and the Mössbauer source placed on the basilar membrane. A close relationship was found between the amplitude of basilar membrane motion at the source place frequency and CAP threshold. Basilar membrane velocity at CAP threshold SPL was about 0.04 mm/s over a 60-dB range of CAP threshold. Intensity functions for basilar membrane motion were linear for frequencies more than an octave below the source place frequency but demonstrated progressive saturation for frequencies greater than an octave below the CF. This nonlinear behavior was eliminated as the CAP threshold became less sensitive and was absent post mortem. Isovelocity curves at the 0.04 mm/s criterion were remarkably similar to frequency threshold curves from primary afferent fibers innervating a similar place on the basilar membrane. The isovelocity curve was a better fit than the isoamplitude curve suggesting that inner hair cells respond to basilar membrane velocity. As the CAP threshold deteriorated, the isovelocit curves lost sensitivity around the best frequency, whereas sensitivity to frequencies below 10 kHz remained constant even after the animal was killed. We suggested that most of the frequency response and nonlinear behavior of inner hair cells and afferent fibers may be found in basilar motion.

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This paper describes the international education and practice of audiology with the broader aim of proposing possible cost-effective and sustainable education models to address the current situation. Major audiology organizations worldwide were surveyed from February 2005 to May 2007, and organizations from 62 countries (78% of the world population) returned a completed survey. Overall, the results suggested a wide range of professionals providing hearing health care, and 86% of the respondents reported a need for more audiologists. There was also considerable variation in the scope of practice among the different hearing health care professionals, and the minimum education levels of audiologists with similar scopes of practice. The countries surveyed fell into four broad categories in terms of professional resources, and the results highlighted the urgent need for forward planning at both national and international levels. The study highlights options for addressing some of the challenges in educating audiologists and the provision of hearing health care services globally.

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SUMMARY1. Potassium concentration was measured with double-barrelled K+-selective microelectrodes within the organ of Corti in the first turn of the guinea-pig cochlea.2. Penetration of the electrode from scala tympani through the basilar membrane was accompanied by an increase in K+ resting level from 3 0 mmol/l in perilymph to 3-4 mmol/l in cortilymph (n = 8). K+ resting level was not significantly different in various extracellular regions of the organ of Corti. On penetration of the cuticular plate, the K+ level reached 140 mmol/l simultaneously with the occurrence of a + 80 mV endocochlear potential. Impalement of hair cells and supporting cells was accompanied by an increase in K+ level, but intracellular K+ level was not systematically measured.3. Stimulation with pure tones over the frequency range 500 Hz to 25 kHz produced changes in the K+ level in the organ of Corti. The magnitude of these changes was dependent on stimulus frequency and intensity. At high sound intensities the K+ level in the tunnel of Corti could increase by typically 1 mmol/l, while a maximum increase of 3 mmol/l with respect to the resting level was observed immediately adjacent to inner hair cells.4. During brief exposures to moderate intensity, pure tone acoustic stimulation (10 s, < 80 dB SPL (sound pressure level)) of frequency 4 kHz or greater the K+ level in the extracellular fluid of the organ of Corti rose monotonically to a steady peak level. On cessation of the stimulus the K+ level fell monotonically with a time constant of about 2 s to a level close to the pre-stimulus level. In some cases this level was slightly above the pre-stimulus level.5. For brief exposures to moderate intensity sound (10 s, < 80 dB SPL) the extracellular potential in the organ of Corti became more positive. The amplitude of this sound-evoked change adapted during stimulation to a level approximately onefifth of its initial value. Upon cessation of the stimulus the potential fell transiently below its pre-stimulus level, before recovering to that level. The time constant of these changes was between 2 and 3 s. B. M. JOHNSTONE AND OTHERS6. Iso-response tuning curves for the sound-evoked elevation in K+ level in the organ of Corti in animals in good condition were similar to iso-rate tuning curves for primary afferent fibres reported previously. High-frequency slopes of tuning curves ranged from 130 to 260 dB/octave, while low-frequency slopes ranged from 40 to 60 dB/octave. Maximum Qlo dB values of 10 were measured.7. Input-output functions relating sound intensity to the amplitude of soundevoked changes in K+ level and the extracellular potential in the tunnel of Corti were linear for frequencies below the most sensitive frequency and non-linear at and above that frequency.8. Stimulation with loud sound (12 kHz, 105 dB SPL, 30 s), resulted in an initial 1 mmol/l increase in the K+ level in the extracellular space of the organ of Corti. During stimulation this elevated K+ concentration decreased, and after stimulation an 'undershoot' in resting l...