Graeme Clark Collection
Permanent URI for this collection
Search Results
1 - 3 of 3
-
ItemCritical bandwidth in the cat following the selective destruction of cochlear inner and outer hair cells [Abstract](Australian Physiological and Pharmacological Society, 1978)It is of interest to determine the significance of the inner and outer hair cells of the cochlea in critical bandwidth measurements, as there is evidence for a difference in their roles in frequency selectivity (Dallos et at., 1977) and in frequency discrimination (Nienhuys and Clark, 1978). The present study has been undertaken on-four monauralized cats which were trained to respond by a conditioned suppression technique. Behavioural auditory thresholds were measured by plotting suppression ratios for frequencies of 1kHz, 4kHz, 8kHz, 10kHz, 12kHz and 16kHz. Critical bands were measured at the same frequencies by recording the pure tone thresholds in the presence of a masking noise with six different bandwidths, a constant total power, and geometrically centred on the test tone (Greenwood, 1961). The cats were then given a series of intramuscular injections of kanamycin (200mg kg-l d- l ) for 10 days to selectively destroy the outer hair cells, and behavioural thresholds determined shortly afterwards. Critical bands were again measured at all the previous frequencies except 16kHz, where the auditory behavioural threshold was too high. At the completion of the experiments behavioural thresholds were re-determined, the animals were anaesthetized with pentobarbital sodium (40mg/kg, i.p.) and auditory nerve action potentials recorded.
-
ItemElectrical network properties and distribution of potentials in the cat cochlea [Abstract](Australian Physiological and Pharmacological Society, 1978)The-patterns of electrical resistance and capacitance in the cochlea formed by the anatomical organisation of the tissue structures and fluids are important in determining the distribution of electrical potentials which arise during normal acoustic stimulation (von Bekesy,1951). Cochlear potential distributions have in the past been measured by recording from the scalar fluids both the spread of cochlear microphonics and also potentials due to electrical stimulation. However, similar distributions in the hair cell-nerve ending region of the organ of Corti may not necessarily occur because of current shunting effects due to the electrical network patterns. To examine these current shunting effects, a three dimensional mathematical model of the electrical properties of the cat cochlea was constructed. This was formed from a two dimensional cochlear cross-section model similar to that proposed by Johnstone et al., (1966) for the guinea pig. Sixteen such sections were resistively coupled to form the three dimensional model. Results derived from this model predict that during electrical stimulation of the cochlea, the current in the organ of Corti region attenuates quite differently to the scalar voltage by a degree which depends on the stimulus electrode configuration.
-
ItemCochlear implant: perspectives( 1979)Volta, who discovered the electrolytic cell, was the first person to stimulate the auditory system electrically. He connected a battery to two metal rods which were inserted into his ears, and he received "une secousse dans 1a t�te". In spite of his experience, interest in stimulating the auditory system electrically as a cure for profound hearing loss has persisted, and a number of attempts have been made as electronic developments have occurred. Following the introduction of the thermionic valve more precise studies of the effects of rate and intensity were possible. Then direct stimulation of the auditory nerve at surgery, to determine parameters of electrical stimulation, were carried out. These were followed by a few long term patient implantations in which the wires were connected directly to the outside through the skin. As this carries a high risk of infection, long term single electrode studies were also performed in which the stimuli were transmitted by inductive coupling to a coil buried beneath the skin. During this period developments in electronics and computers also led to acute and behavioural studies on animals to learn more about the physiology of hearing and the effects of electrical stimulation. All the results indicate that a multiple-electrode system offers the best chance of providing speech for a patient. A multiple-electrode system requires the implantation of a much more complicated electronic device which can now be miniaturized to an appropriate size.