Graeme Clark Collection
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ItemAuditory steady-state evoked potential in newborns( 1994)Steady-state evoked potential responses were recorded from 337 normal full-term sleeping newborns to combined amplitude and frequency modulated tones. Responses were automatically detected by statistical analysis of the response phase. Responses were most easily and consistently recorded at carrier frequencies of 500 Hz, 1500 Hz and 4000 Hz when the modulation frequency was between 60 Hz and 100 Hz. In this modulation frequency range, the response latencies were found to be between 11 ms and 15 ms, depending on carrier frequency, and the mean response thresholds for the three carrier frequencies were found to be 41.36 dB HL, 24.41 dB HL and 34.51 dB HL respectively. The results of this study suggest that steady-state evoked potentials at modulation rates in excess of 60 Hz may be useful for frequency specific, automated hearing screening in newborns.
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ItemEvaluation of a new Spectral Peak coding strategy for the Nucleus 22 channel cochlear implant system( 1994)Sixty-three postlinguistically deaf adults from four English-speaking countries participated in a 17-week field study of performance with a new speech coding strategy, Spectral Peak (SPEAK), and the most widely used strategy, Multipeak (MPEAK), both of which are implemented on wearable speech processors of the Nucleus 22 Channel Cochlear Implant System; MPEAK is a feature-extraction strategy, whereas SPEAK is a filterbank strategy. Subjects' performance was evaluated with an experimental design in which use of each strategy was reversed and replicated (ABAB). Average scores for speech tests presented sound-only at 70 dB SPL were higher with the SPEAK strategy than with the MPEAK strategy. For tests in quiet, mean scores for medial vowels were 74.8 percent versus 70.1 percent; for medial consonants, 68.6 percent versus 56.6 percent; for monosyllabic words, 33.8 percent versus 24.6 percent; and for sentences, 77.5 percent versus 67.4 percent. For tests in noise, mean scores for Four-Choice Spondees at +10 and +5 dB signal-to-noise ratio (S/N) were 88.5 percent versus 73.6 percent and 80.1 percent versus 62.3 percent, respectively; and for sentences at +15 dB, +10, and +5 dB S/N, 66.5 percent versus 43.4 percent, 61.5 percent versus 37.1 percent, and 60.4 percent versus 31.7 percent, respectively. Subjects showed marked improvement in recognition of sentences in noise with the new SPEAK filterbank strategy. These results agree closely with subjects' responses to a questionnaire on which approximately 80 percent reported they heard best with the SPEAK strategy for everyday listening situations.
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ItemCochlear pathology following chronic electrical stimulation of the auditory nerve: II Deafened kittens( 1994)The present study examines the effects of long-term electrical stimulation of the auditory nerve on cochlear histopathology and spiral ganglion cell survival in young sensorineural deafened cats. Eight kittens were deafened using kanamycin and ethacrynic acid, and implanted with bipolar or monopolar scala tympani electrodes. Following recovery from surgery the animals were unilaterally stimulated using charge balanced biphasic current pulses for 450-1730 hours over implant periods of up to four months. Charge densities varied from 0.6-0.9 µC.cm ^-2 geom. per phase for monopolar electrodes to 12-26 µC.cm ^-2 geom. per phase for the bipolar electrodes. Electrically-evoked auditory brainstem responses (EABRs) were periodically monitored during stimulation to confirm that the stimulus levels were above threshold, and to monitor any change in the response of the auditory nerve. Following completion of the stimulation program cochleae were prepared for histological examination. EABRs exhibited relatively stable thresholds for both stimulated and implanted, unstimulated control cochleae for the stimulus duration. While the growth in response amplitude as a function of stimulus current remained stable for the bipolar control and monopolar stimulated cochleae, the five cochleae chronically stimulated using bipolar electrodes exhibited a moderate to large increase in response amplitude. These increases were associated with a more widespread fibrous tissue response which may have altered the current distribution within these cochleae. Implanted control cochleae exhibited significantly less tissue response within the scala tympani. Importantly, we observed no statistically significant difference in the spiral ganglion cell density associated with chronic electrical stimulation when compared with unstimulated control cochleae. While the present study supports the safe application of cochlear implants in young profoundly deafened children, it does not corroborate previous studies that have reported electrical stimulation providing a trophic effect on degenerating auditory nerve fibres.
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ItemHearing, vocalization and the external ear of a marsupial, the northern quoll, Dasyurus hallucatus( 1994)As part of a continuing study of the development of the marsupial auditory system, auditory brainstem responses (ABR) were recorded and an ABR audiogram was constructed for five female Northern Quolls (Dasyurus hallucatus), which are nocturnal carnivores. The best frequency for hearing lies between 8 and 10 kHz, and at 50 dB SPL there is a range from about 0.5 to 40 kHz. Vocalizations of adult quolls and pouch-young were recorded with a digital audio tape recorder, and the power spectra of representative calls were compared with the ABR audiogram. The common adult vocalizations have most energy at the lower end of the hearing range, whereas frequencies that are dominant in the isolation calls of the pouch-young lie close to the best frequency of hearing. Samples of nocturnal sounds of the habitat of the quoll were also recorded and analyzed. Power spectra have peak energy at frequencies between 2 and 5 kHz, with a smaller contribution above 10 kHz. The spectrum contains relatively little power at the best frequency of hearing. Measurements of the sound pressure level at the external ear canal as a function of stimulus frequency and location in space suggest that the directional amplifying properties of the pinna will operate most effectively on sound frequencies at the upper end of the quoll's hearing range, a region that may be important in prey detection. Comparisons are made with other mammalian nocturnal carnivores and with other marsupials. We speculate that, for nocturnal carnivores, one role of the low-frequency part of the hearing range concerns the recognition of adult conspecifics, the mid-frequency range is important for the detection of pouch-young, and the upper range may be particularly concerned with prey/predator detection.
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ItemPitch ranking with nonsimultaneous dual-electrode electrical stimulation of the cochlea( 1994)Abstract not available due to copyright.
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ItemPreliminary evaluation of a formant enhancement algorithm on the perception of speech in noise for normally hearing listeners( 1994)The effects on speech perception in noise of dynamic filtering with bandpass filters centred at the first formant (f1) and second formant (f2) frequencies were evaluated with four normally hearing listeners. Multitalker babble was added to the speech signal with signal-to-noise ratios of-5 to -15 dB, chosen to reduce intelligibility to about 50%. The combined signal was then filtered with two-pole programmable bandpass filters centred at fl and f2 under the control of a real-time speech processor. The f1 and f2 frequencies were estimated from the speech signal before noise was added to avoid hardware processing errors. Closed set vowel and consonant tests (using 11/h/vowel/d/ and 12 /a/consonant/a/ stimuli), the Consonant-Nucleus-Consonant Monosyllabic Word Test and the Bamford-Kowal-Bench Sentence Test were carried out for three filter bandwidths (3/4, 1/3 and 1/6 octave) and for unprocessed speech in noise. The processing produced a small significant improvement for vowels in all three processed speech conditions and for monosyllables at the broadest filter setting compared to the unprocessed speech condition. There was no significant effect on consonants. A small negative effect was observed for sentences at the narrowest filter setting.
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ItemFormant-based processing for hearing aids( 1994)A body-worn hearing aid has been developed with the ability to estimate formant frequencies and amplitudes in real time. These parameters can be used to enhance the output signal by "sharpening" the formant peaks, by "mapping" the amplitudes of the formants onto the available dynamic range of hearing at each frequency, or by resynthesizing a speech signal that is suited to the listener�s hearing characteristics. Initial evaluations have indicated small improvements in speech perception for three groups of subjects: users of a combined cochlear implant and speech processing hearing aid, normally hearing listeners in background noise, and a hearing aid user with a severe hearing loss.
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ItemDepartment of Otolaryngology( 1994)The Department of Otolaryngology at The University of Melbourne commenced on the 1st of January 1970. It was established because otolaryngology had become a major specialty and one of importance to medical students: 15 per cent of referrals in family medicine can be attributed to conditions of the ear, nose and throat. The Chair was the first in Australasia and was also meant to have a significant role outside The University of Melbourne. Its specific aims were to foster teaching and research in the discipline.
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ItemCochlear implants for congenitally deaf adolescents: is open-set speech perception a realistic expectation?( 1994)The prognosis for benefit from use of cochlear implants in congenitally deaf adolescents, who have a long duration of profound deafness prior to implantation, has typically been low. Speech perception results for two congenitally deaf patients implanted as adolescents at the University of Melbourne/Royal Victorian Eye and Ear Hospital Clinic show that, after 12 months of experience, both patients had significant open-set speech discrimination scores without lipreading. These results suggest that although benefits may in general be low for congenitally deaf adolescents, individuals may attain significant benefits to speech perception after a short period of experience. Prospective patients from this group should therefore be considered on an individual basis with regard to prognosis for benefit from cochlear implantation.
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ItemNeural processes in the dorsal cochlear nucleus of the anaesthetised cat investigated from unit responses to electrical stimulation of the auditory nerve( 1994)Extracellular responses of dorsal cochlear nucleus single units were recorded in response to biphasic, bipolar electrical stimulation of spiral ganglion cells and their peripheral processes using a banded electrode array in the scala tympani of the barbiturate anaesthetised cat. The DCN responses to this stimulus were the result of excitatory and suppressive (including inhibitory) processes. The excitatory responses from DCN units were usually within a range of 1.8-2.8 ms and these responses were probably the result of monosynaptic input from the auditory nerve. Latencies > 2.8 ms were most likely due to activation of di- and poly-synaptic pathways from auditory nerve fibres, except that latencies between 3.5-4.75 in hearing animals could have arisen from electrophonic mechanisms. Suppression of spontaneous activity was usually long acting, lasting > 70 ms following each pulse of the pulse train, but short acting suppression with a latency of 3.5-4.75 ms and a duration of < 10 ms was occasionally observed. These suppressive responses probably resulted from synaptic inhibitory input, but neural membrane properties may have contributed. In hearing animals, excitatory latencies within the range 1.8-5.2 ms were similar for units with different response area types or different PSTH patterns in response to acoustic CF tones or noise.