Graeme Clark Collection
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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.
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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.
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ItemElectrical transmission line properties of the cat cochlea( 1977)Some properties of cochlear microphonic and summating potential distributions recorded from the cochlear scalae may be accounted for by a consideration of the distributed resistive and reactive elements which load the cochlear generators (Haas, 1973). These distributed elements result in the cochlea behaving as an electrical transmission line. One property of the transmission line which characterises its longitudinal properties is the length constant and this has been measured by Bekesy (1951) and Johnstone et al. (1966) who have demonstrated in the guinea pig a value of 2 mm between scala media and scala tympani and 1.4 mm between scala vestibuli and scala tympani in the basal turn.
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ItemSpeech analysis and synthesis [Abstract]( 1976)Speech sounds are digitized and stored in mass storage units of a Hewlett Packard computer system. A short-time frequency analysis of the speech sounds is then performed, and sound spectrographs are displayed on a Tektronix 4014 terminal. The speech sounds are also processed using a digital computer model of hasilar membrane motion, and envelope details of the cochlear analysis are displayed using the same technique as the sound spectrograph. Speech sounds are synthesized by rule using a hardward terminal analog synthesizer. A digital computer is used to translate a sequence of phonemes which comprise the speech sound into the control signals required by the synthesizer.
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ItemBehavioural responses in the cat to simple patterns of electrical stimulation of the terminal auditory nerve fibres( 1974)Behavioural studies on electrical stimulation of the terminal auditory nerve fibres have shown that there are limitations to the perception of rates of electrical stimulation above 200 Hz (Clark et al., 1972), and that the difference limens for electrical stimulation are only similar to those of sound for frequencies below 200 Hz (Clark et al., 1973). The present study has been undertaken to extend this research, and help determine whether the cat can not only detect a change in the rate of electrical stimulation, but also judge whether it is higher of lower than a reference frequency.
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ItemAuditory evoked potentials and auditory sequential memory [Abstract]( 1974)The evaluation of intelligence in infants and young children is important in assessing their prognosis and institutional placement, and is difficult with present clinical methods. Therefore, the recent studies which show a correlation between visual evoked responses and intelligence are of value. It is also of importance to determine if auditory evoked potentials may be used in assessing children with communication disorders, as their defect frequently involves the auditory system, and they will often require evoked response audiometry to exclude loss of hearing from the diagnosis.
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ItemAuditory evoked responses to frequency and amplitude modulated sound( 1973)Auditory evoked responses to pure tone bursts have been described in a number of studies and have been characterized by the P1, N1 and P2 components. The presence of later components, namely N2, P3 and the Contingent Negative Variation, depends largely on the cerebral processing of the stimulus. These evoked responses have been recorded using tone bursts. However, neurophysiological studies have shown that the cortex codes complex sounds. Therefore, we performed a set of acute experiments on the cat, using frequency and amplitude modulated sounds. This was reported in a previous study (Richards and Clark, 1972) which showed that similar onset and later waves could be recorded from the cortex of the cat. In some areas of the cortex the later waves were in synchrony with the modulation envelope.
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ItemSurface negative potential shifts in the auditory cortex( 1973)A number of investigations have shown that cortical surface negative potential shifts may occur in response to peripheral and central stimulation, and accompany various behavioural states. The present study is an attempt to indicate some of the important variables in producing a surface negative shift in the auditory cortex. The purpose of this was to help determine whether this phenomenon might play some role in sensory memory.
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ItemResponse thresholds to frequency modulated sound and electrical stimulation of the auditory nerve in cats( 1973)A previous study (Clark et al., 1972) has shown the effects of rate, current and site of electrical stimulation of the cochlea and central auditory pathways on cats' behavioural thresholds. It was observed that the thresholds were lower for low rates of electrical stimulation of the basal or high frequency end rather than the apical or low frequency end of the cochlea.
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ItemThe design of an ear bar system for auditory neruophysiological research( 1972)Absolute sound intensity measurements at the ear drum are important in auditory research. Previous attempts to measure sound intensity using a small bore probe tube coupled to a microphone have proved unsatisfactory. In the present investigation an alternative system employing two condenser microphones coupled to the input end of the ear bar was used to estimate the sound intensity at the ear drum. Consideration was also given to minimizing harmonic distortion and resonance in the system.