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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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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ItemThe effects of muscle relaxants on the auditory cortical evoked potentials in humans( 1972)There are a number of studies on sensory information processing based on an analysis of cortical evoked potentials. Few studies however, provide direct evidence that the cortical evoked potentials are produced by neural generators in the brain. In fact, some research work has shown that most of the cortical evoked activity may be muscular in origin. (Bickford et al, 1964; Prichard et al, 1965). Consequently the present study was necessary as a preliminary to further research on sensory information processing in the auditory system.
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ItemField potentials in cat auditory nuclei in response to frequency and amplitude modulated sound( 1972)This study was undertaken to determine how the field potentials recorded from the cochlear nucleus, inferior colliculus, and primary and secondary auditory cortices varied for different modes of frequency and amplitude modulated sound. The sound was presented to anaesthetized animals as a burst of five consecutive modulation envelopes. It had modulation rates which varied from 10- to 80- Hz, and depths of 10%, 50% and 100%. The field potentials were summed over 63 stimulus presentations, and then a spectral analysis was carried out on the result. The evoked responses produced in the various nuclei and by different stimulus parameters were compared using a coherence function.
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ItemPre-operative hearing assessment in young cochlear implant candidates( 2000)Early implantation of congenitally deaf candidates is now a goal of most cochlear implant clinics. This objective can only be achieved through the use of audiological tests that are able to identify and quantify hearing loss in infancy. Behavioural conditioning techniques provide the most accurate indication of hearing acuity in older subjects, but these procedures are unreliable in young (6 months) or developmentally delayed children. This paper investigates the degree of accuracy with which two electrophysiological test techniques (click-ABR & steady-state evoked potential [SSEPD can be used to assess auditory function in infants with significant hearing loss. ABR and SSEP findings from 108 children were compared with hearing levels obtained behaviourally.
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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 (SSEP) in infants: modulation frequency effects [Abstract](International Evoked Response Audiometry Study Group, 1997)The University of Melbourne experience shows that steady-state evoked-potentials are a sensitive indicator of hearing loss in children and adults (Rance et al., 1996). Cognizant of the effect of stimulus rate upon ABRs in early development (Lasky, 1987, Cone-Wesson et al., 1995), we investigated the effect of modulation frequency (MF) upon the detectability of the SSEP recorded from full-term (FT) neonates and those with a history of prematurity (PM). The influence of gender was also evaluated, as it has been shown to be a significant variable in ABR results (Sininger et al., 1995, Eldredge and Salamy, 1996). Ten male and 9 female FT infants were tested on the second day post partum (range 1-5 days).
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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.