Graeme Clark Collection
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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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ItemObjective hearing level estimation in infants with severe to profound impairment: role of Steady State Evoked Potential threshold testing [Abstract]( 1998)Accurate determination of hearing levels in potential cochlear implant candidates is an important part of pre-operative investigation. This is particularly the case for children younger than two years of age in whom speech perception testing is not possible. We have found the Steady-State Evoked Potential (SSEP) technique to be a reliable and accurate objective measure of residual hearing. Threshold estimation using the click-evoked auditory brainstem response cannot quantify hearing loss in the severe to profound range because of the presentation level restriction (? 100 dBnHL) and the broadband stimulus. The SSEP technique employs modulated tones at levels as high as 120 dBHL and can be used to estimate the audiograms of subjects with little or no residual hearing and absent click-auditory brainstem responses. We have demonstrated a strong relationship between behavioural and SSEP thresholds and believe that the procedure provides an important safeguard in the paediatric selection process. The possible exception is in children with retrocochlear abnormalities such as �Auditory Neuropathy�. In these subjects we have found poorer correlation between SSEP and behavioural thresholds.
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ItemSteady state evoked potentials: an objective measure of residual hearing in young cochlear implant candidates [Abstract]( 1993)Precise determination of hearing thresholds in prospective cochlear implant candidates is essential. As the minimum age of implantation for youl1g children has been reduced, the use of objective measures of hearing has become an important part of their pre-operative evaluation. Steady-state evoked potentials are scalp potentials elicited in response to sinusoidally amplitude and/or frequency modulated tones. A system has been developed at The University of Melbourne which allows the presence: of such a response to be detected automatically thus permitting an objective. frequency specific estimate of hearing to be made in sleeping or awake subjects. This paper investigates the use of SSEPs in determining hearing thresholds in young profoundly deaf children. Responses in such patients are compared with those: obtained in neonates, and a group of sleeping adult subjects with varying degrees of sensori-neural hearing loss. Results indicate that the SSE? procedure is well suited as a measure assessing residual hearing in profoundly deaf children suitable for a cochlear implant in that it can provide an accurate estimate of auditory thresholds using frequency specific stimuli presented at high levels.
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ItemSteady-state evoked potentials (SSEPS): an accurate, automated, frequency specific method for objectively assessing hearing thresholds [Abstract]( 1992)The determination of hearing thresholds in young children and some adults is impossible using conventional behavioural audiometry. The development of objective methods of assessment for use in such patients is highly desirable.
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ItemSteady-state evoked potentials (SSEPS): a new tool for the accurate assessment of hearing in cochlear implant candidates [Abstract]( 1992)Determining hearing thresholds is crucial in assessing cochlear implant candidates; never more so than in young children unable to undergo behavioural audiometry. In these children an objective measure of hearing is desirable. Steady-state evoked potentials (SSEPs) are potentials recorded from the scalp and simultaneously analysed. Stimuli ore amplitude and frequency modulated pure tones. A system has been developed which allows the presence of such a response to be automatically detected and the technique thus permits an objective, frequent specific assessment of hearing threshold to be mode in sleeping or awake subjects (1). This paper investigates the use of SSEPs in determining hearing thresholds in young profoundly deaf children who are candidates for cochlear implants. Responses in such patients are compared with those obtained in normal adults, neonates and experimental animals. Results indicate that the SSE? can provide a consistent and reliable measure of threshold and the technique appears to have a number of advantages over the auditory brainstem response: automated response detection removes the subjective element of threshold determination, higher levels of stimulus presentation are possible, low frequency threshold determination is more accurate and the testing procedure IS quicker.
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ItemResults for the Nucleus multiple-electrode cochlear implant in two children [Abstract]( 1988)Two males, 9 years 10 months (CHILD 1) and 5 years 5 months (CHILD 2) at time of surgery, were implanted with the Nucleus multiple-electrode cochlear implant. Both patients were deafened as a result of meningitis in their third year. Assessments of speech perception, speech production and language skills were undertaken at regular intervals, pre and post operatively. For both patients in the audition alone condition, some speech perception post operative scores were significantly higher than pre operative scores and progressive improvements in scores over successive post operative data collection times were seen. Significant differences between the visual alone and auditory-visual condition scores were also observed for CHILD 1 post operatively. Speech production post operative scores were significantly higher than pre operative scores for both patients. The receptive vocabulary scores for both patients improved at a higher rate than that of age-matched normal children. The acquisition of expressive and receptive language skills for CHILD 2 was at a higher rate than that of age-matched children. Differences in the results between the two patients were seen, and this may be related to age and duration of deafness.
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ItemSteady state evoked potentials to amplitude modulated tones [Abstract]( 1982)Evoked responses have been recorded from the scalp of normal human subjects to continuous sinusoidally-modulated amplitude-modulated tones. Phase locking the computer to the modulation envelope enabled an averaging technique to be used to improve the signal to noise ratio. The responses were found to be periodic with the same fundamental frequency as the modulation envelope. Fourier analysis was used to quantify the amplitude and phase of the first and second harmonic components of the responses. The variation of the response amplitude with SPL depended on the modulation and carrier frequency. At modulation frequencies of less than 20Hz and carrier frequencies of less than 1KHz, the amplitude of a response increases up to 50dBSPL but remains invariant at higher levels. At higher modulation frequencies, the response amplitude is uniform to 60-80dBSPL and then increases very rapidly, sometimes by as much as a factor of 20 for a 20dB increase in SPL. This is particularly true of high carrier frequencies. These amplitude growth functions can be explained in part by neural tuning curves. Phase locked responses ran be recorded down to 30dBSPL at most modulation rates and carrier frequencies. Estimates of latencies of these potentials were made by measuring the phase change of the first and second harmonic components whilst changing the modulation frequency. The latencies varied with modulation frequency, carrier frequency and SPL. Latencies of both first and second harmonic components fall into 14 discrete groups from 3msec up to 104msec, with the majority of responses having latencies of 9msec to 33msec. The origin of some of these responses is likely to be the auditory cortex.