Graeme Clark Collection
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ItemEvoked responses in humans to tones which are sinusoidally amplitude modulated [Abstract]( 1980)Evoked responses have been recorded from the scalp of normal human subjects to continuous sinusoidally-modulated Amplitude-modulated tones. The responses were periodic in nature, and a Fourier transform was used to quantify the amplitude and the phase of the constituent fundamental frequency and harmonic components. The fundamental frequency of the response equalled the frequency of the modulation envelope and the amplitude of the harmonic components was usually less than the fundamental. The responses were also found to be invariant with time for periods exceeding 30 minutes.
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ItemClinical findings for a group of infants and young children with auditory neuropathy( 1999)Objective: To examine the prevalence of auditory neuropathy in a group of infants at risk for hearing impairment and to present an overview of the clinical findings for affected children. Design: Results for 20 subjects who showed repeatable cochlear microphonic potentials in the absence of click-evoked auditory brain stem responses are included in this study. Behavioral and steady state evoked potential thresholds were established in each case. Where possible, otoacoustic emission and speech perception results (unaided and aided) also were obtained. Results: One in 433 (0.23%) of the children in our series had evidence of auditory neuropathy. The audiometric findings for these subjects varied significantly, with behavioural thresholds ranging from normal to profound levels. Discrimination skills were also variable. Approximately half of the subjects showed little understanding, or even awareness, of speech inputs in both the unaided and aided conditions. There were, however, a number of children who could score at significant levels on speech discrimination tasks and who benefited from the provision of amplification. Conclusion: The results suggest that auditory neuropathy is more common in the infant population than previously suspected. The effects of neuropathy on auditory function appear to be idiosyncratic, producing significant variations in both the detection and discrimination of auditory signals. As such, the management of children with this disorder must allow for individual differences.
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ItemSteady-state evoked potential and behavioural hearing thresholds in a group of children with absent click-evoked auditory brain stem response( 1998)Objective: 1) To examine the distribution of behavioural hearing thresholds in a group of children who had shown no click-evoked auditory brain stem response (ABR) at maximum presentation levels. 2) To describe the relationship between the 90 Hz steady-state evoked potential (SSEP) and behavioural thresholds in these subjects. Design: A retrospective study based on clinical findings obtained from 108 infants and young children. Each of these children had shown no recordable ABR to clicks presented at maximum levels (100 dB nHL). SSEP audiograms were obtained using AM/FM tones at the octave frequencies 250 to 4000 Hz. The results of these evoked potential assessments were compared with hearing thresholds established behaviourally. Results: Click-ABR assessment could not differentiate between the subjects in our sample with total hearing losses and those with useful residual hearing. Although some of the ears were anacusic, more than a quarter showed residual hearing ate each of the audiometric frequencies. Furthermore, at least 10% of the behavioural thresholds at each frequency fell within the moderate/severe hearing loss range. A far closer relationship was observed between SSEP and hearing thresholds. On occasions where the SSEP was absent at maximum levels, 99.5% of the ears showed either a total loss or a behavioural threshold within 10 dB of that level. When an SSEP was obtained, the hearing threshold was typically within 5 dB of the SSEP threshold. Conclusion: The results suggested that in our group of selected subjects, the SSEP technique was able to assess ears with only minimal amounts of residual hearing. Where the brevity of the acoustic click limits both its frequency specificity and its presentation level, the modulated tones used for SSEP testing allow accurate, frequency-specific assessment at high presentation levels.
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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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ItemThe automated prediction of hearing thresholds in sleeping subjects using auditory steady-state evoked potentials( 1995)Objective: To examine the relationship between auditory steady-state evoked potentials (SSEPs) and behavioral thresholds in sleeping subjects. Design: 60 adults and children with hearing thresholds ranging from normal to profound were selected on the basis of appropriate audiograms. Behavioral audiograms were determined at the octave frequencies 250-4000 Hz. These behavioral thresholds were then compared with the SSEP thresholds obtained during natural sleep for adults, or natural or sedated sleep for children. Results: A strong relationship between behavioural and SSEP thresholds was observed. The strength of the relationship increased with increasing frequency and increasing degree of the loss. On the basis of these data, the prediction of behavioural thresholds from SSEP levels was determined. It was found that the standard deviation of the error in this prediction decreased with increasing frequency and increasing degree of the loss. There was no significant age effect in the results obtained at any of the frequencies. Conclusion: The results suggest that the SSEP technique can be used as a predictor of behavioural thresholds in adults and children at the frequencies 250-4000 Hz.
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ItemAccuracy of behavioural threshold prediction using steady-state evoked potentials [Abstract]( 1994)This paper examines the confidence with which predictions of hearing level can be made using the steady-state evoked potential (SSEP) technique. Steady-state evoked potentials are scalp potentials that can be elicited in response to sinusoidally amplitude and/or frequency modulated tones in sleeping or awake subjects. SSEP thresholds were obtained using frequency specific stimuli at octave frequencies between 250Hz and 4000Hz in 25 children and 35 adults with varying degrees of sensori-neural hearing loss. These levels, determined automatically by a computerised detection system, were then compared with thresholds obtained behaviourally. Linear regression analyses of this data have shown that the SSEP procedure allows objective estimates of hearing level to be made for a range of carrier frequencies to within 10dB accuracy on 96% of occasions.
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ItemThe diagnostic assessment of infants using steady-state evoked potentials [Abstract]( 1994)This paper examines the relationship between the steady-state potential and behavioural thresholds obtained in two groups of young children. The first group consists of 10 babies referred to the Victorian Children's Hearing Centre following abnormal findings on screening ABR assessments. The second group contains 20 difficult to test infants who had unconfirmed hearing losses at the time of the SSEP assessment. The hearing levels predicted by the regression lines (described in the companion paper presented at this conference) were in close agreement with the behavioural levels obtained subsequently from children in both groups.
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ItemCochlear implants in children, adolescents, and prelinguistically deafened adults: speech perception( 1992)A group of 10 children, adolescents, and prelinguistically deafened adults were implanted with the 22-electrode cochlear implant (Cochlear Pty Ltd) at the University of Melbourne Cochlear Implant Clinic and have used the prosthesis for periods from 12 to 65 months. Postoperative performance on the majority of closed-set speech perception tests was significantly greater than chance, and significantly better than preoperative performance for all of the patients. Five of the children have achieved substantial scores on open-set speech tests using hearing without lipreading. Phoneme scores in monosyllabic words ranged from 30% to 72%; word scores in sentences ranged from 26% to 74%. Four of these 5 children were implanted during preadolescence (aged 5:5 to 10:2 years) and the fifth, who had a progressive loss, was implanted during adolescence (aged 14:8 years). The duration of profound deafness before implantation varied from 2 to 8 years. Improvements were also noted over postoperative data collection times for the younger children. The remaining 5 patients who did not demonstrate open-set recognition were implanted after a longer duration of profound deafness (aged 13:11 to 20:1 years). The results are discussed with reference to variables that may affect implant performance, such as age at onset of loss, duration of profound loss, age at implantation, and duration of implantation. They are compared with results for similar groups of children using hearing aids and cochlear implants.
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ItemSteady-state evoked potentials to amplitude modulated tones in the monkey( 1992)A frequency-specific, objective assessment of hearing thresholds is required for use in subjects unable to perform behavioural audiometry. One such method using steady-state evoked potentials (SSEPs) in response to amplitude-modulated tones was evaluated in an experimental animal, the macaque monkey. An amplitude-modulation frequency of 165 Hz was found to produce optimum response detection in the anaesthetised animal. Auditory thresholds determined by a computerised automatic response detection system accurately reflected behavioural thresholds previously described in this species.
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ItemSpeech perception, production and language results in a group of children using the 22-electrode cochlear implant( 1992)Five children out of a group of nine (aged 5.5 to 19.9 years) implanted with the 22-electrode cochlear implant (Cochlear Ply. Ltd.) have achieved substantial scores on open-set speech tests using hearing without lipreading. Phoneme scores for monosyllabic words ranged from 40% to 72%. Word scores in sentences ranged from 26% to 74%. Four of these five children were implanted during preadolescence. The fifth child, who had a progressive loss and was implanted during adolescence after a short period of very profound deafness, scored highest on all speech perception tests. The remaining four children who did not demonstrate open-set recognition were implanted during adolescence after a long duration of profound deafness. Post-operative performance on closed-set speech perception tests was better than pre-operative performance for all children. Improvements in speech and language assessments were also noted. These improvements tended to be greater for the younger children. The results are discussed with reference to variables which may contribute to successful implant use: such as age at onset, duration of profound hearing loss, age at implantation, aetiology, educational program, and the type of training provided.