Graeme Clark Collection
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ItemSteady state evoked potentials (SSEPs): An objective measure of residual hearing in young cochlear imlpant candidates( 1993)The use of objective measures of residual hearing in young cochlear implant candidates has become more important as the minimum age of implantation has reduced. This paper examines the use of the steady-state evoked potential (SSEP) technique in the assessment of these children. SSEP thresholds were obtained using frequency specific stimuli at octave frequencies between 250llz and 4000Hz in ZS children with moderate to severe hearing losses. These levels, determined automatically by a computerized detection system were then compared with thresholds obtained behaviourally. Data was also collected from a group of 35 adult subjects with varying degrees of sensorineural hearing loss. Results indicate that the steady-state evoked potential procedure can provide accurate, frequency specific estimates of hearing thresholds in ears with even profound or total hearing losses.
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ItemSpeech processing for cochlear implants: variations of the spectral maxima sound processor( 1993)The Spectral Maxima Sound Processor (SMSP) incorporates a bank of sixteen band-pass filters which are assigned to sixteen intracochlear electrodes. In each stimulation period six electrodes are activated, based on the outputs of the filters with the largest amplitudes. The SMSP has been compared with the present MSP(MULTIPEAK) processor and found to improve speech comprehension results. The SMSP speech processing scheme has recently been implemented successfully in a new speech processor, also developed at the University of Melbourne, which utilises digital signal processing techniques. The programming flexibility of the new processor has facilitated the investigation of variations of the SMSP strategy which might provide further enhancement of speech perception. Three variations have been investigated: firstly, increasing the constant pulse rate from the usual 250Hz to 400Hz; secondly, changing the number of electrodes selected in each stimulation period from six to numbers between four and eight; thirdly, sharpening the spectral peaks prior to selection of the active electrodes. The results of these studies showed that all three variations had minimal effect on speech perception in quiet, but that increasing the number of electrodes selected for stimulation to eight, or increasing the rate of stimulation, may have advantages when listening in background noise.
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ItemFricative perception by cochlear implant users( 1992)Three implant users were tested with 45 syllables consisting of[v, f, �, ?, z, s, ?, ?, d?, t ?, ?, , d, n, 1] before the vowels [I ,a,u] with three wearable speech processors. The WSP3 processor coded first and second formant frequencies and amplitudes. The MSP1 processor used a similar scheme with improved measurement and coding of the formants. The MSP2 processor added amplitude information from three higher frequency bands. Average scores were 42% for WSP3, 54% for MSP1, and 57% for MSP2. Perception of voicing, manner, and place of articulation of the consonants was significantly greater for the MSP processors than the WSP3 processor. Place perception was slightly higher for MSP2 than MSP1. The listeners used three perceptual dimensions which were highly correlated with the frequencies and amplitudes of peaks in the low frequency region of the frication spectrum, amplitudes of high frequency peaks, and duration of the frication noise.
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ItemA digital computer model of electrical stimulation in the human cochlea for auditory prosthesis research( 1990)A three-dimensional model of electrical stimulation in the human cochlea has been developed and implemented on a digital computer. The model was used to estimate the distributions of electric potential and current density in the human cochlea in response to electrical stimulation using scala tympani electrodes. The computed distributions were used to investigate the relative merits of two scala tympani electrode designs. The results showed that the electrode design consisting of a medial electrode pair in the scala tympani is a more viable alternative than a lateral electrode pair for patients suffering from profound-to-total hearing impairment.
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ItemPreliminary results with a miniature speech processor for the 22-electrode Melbourne/Cochlear hearing prosthesis(Kugler & Ghedini, 1990)The 22-electrode cochlear prosthesis developed by the University of Melbourne in conjunction with Cochlear Pty Ltd has been used successfully by profoundly deaf patients since 1982 and is now a part of everyday life for some 2000 people in many countries around the world. The implanted part of the prosthesis has remained relatively unchanged in this time except for the alteration of the design in 1986 to incorporate an implanted magnet and reduce the overall thickness of the device. The implanted magnet eliminated the need for wire headsets which were difficult to fit and in some cases did not maintain the position of the external transmitter coil adequately. This was felt to be essential before the prosthesis could be used in young children.
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ItemSpeech processing strategies in an electrotactile aid for hearing-impaired adults and children(Australian Speech Science and Technology Association, 1990)An electrotactile speech processor (Tickle Talker) for hearing-impaired children and adults has been developed and tested. Estimates of second format frequency, fundamental frequency and speech amplitude are extracted from the speech input, electrically encoded and presented to the user through eight electrodes located over the digital nerve bundles on the fingers of the non-dominant hand. Clinical results with children and adults confirm that tactually-encoded speech features can be recognized, and combined with input from vision or residual audition to improve recognition of words in isolation or in sentences. Psychophysical testing suggests that alternative encoding strategies using multiple-electrode stimuli are feasible. Preliminary results comparing encoding of consonant voiced/voiceless contrasts with new encoding schemes are discussed.
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ItemElectrical stimulation of the auditory nerve: the influence of electrode position on neural excitation(Kugler & Ghendini, 1990)Improved speech recognition among cochlear implant patients would appear to be dependent on a number of factors including improved speech processing strategies and an improvement in the effectiveness of electrically stimulating residual auditory nerve fibers (i.e. lower thresholds, wider dynamic ranges and more localized current spread). Previous human temporal bone studies have shown that free fit scala tympani electrode arrays generally lay along the outer wall of the scala tympani. Therefore, there is a relatively large distance between the electrode array and the residual neural elements within Rosenthal's canal. In the present study, we systematically varied the location of the electrode within the scala tympani to examine the influence of electrode position on neural excitation.