Graeme Clark Collection
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ItemSynchronization of the neural response to noisy periodic synaptic input( 1999)The relationship between the timing of the synaptic inputs and the output spikes of leaky integrate and fire neurons with noisy periodic synaptic input is addressed using the recently developed integrated-input technique. The conditional output spike density in response to noisy periodic input is evaluated as a function of the initial phase of the inputs. This enables the phase transition matrix to be calculated, which relates the phase at which the output spike is generated to the initial phase of the inputs. The interspike interval histogram and the period histogram for the neural response to ongoing periodic input are then evaluated by using the leading eigenvector of this phase transition matrix. The dependence of the synchronization index of the neural response upon the number and amplitude of synaptic inputs, the membrane time constant, the average rate of inputs and their frequency of modulation is examined.
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ItemAdults with a severe-to-profound hearing impairment: investigating the effects of linguistic context on speech perception( 1998)Linguistic context is known to influence speech perception abilities in adults with normal hearing. Recent reports question the importance of context for adults with a severe-to-profound hearing impairment. The severe reduction and distortion in acoustic input may result in the listener perceiving insufficient acoustic-phonetic cues to allow access to higher level linguistic processing. To investigate this further, a detailed study of the speech recognition of adults with a severe-to-profound hearing impairment (N=34) was undertaken. A series of aided speech recognition tasks, sequentially examined the different levels of processing in the speech perception chain. The investigation concluded that the effects of severe-to-profound hearing impairment did not reduce the listener's ability to take advantage of contextual cues. There was, however, wide variability between participants in the utilisation of contextual processing. This indicates that to estimate "real-life" speech perception skills, an evaluation of contextual processing ability is required.
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ItemApplication of MEMS to cochlear implants( 1998)A cochlear implant restores some hearing by electrically stimulating residual auditory nerve fibers in the cochlea. The cochlear implant represents a major scientific and technological breakthrough and is now providing hearing for thousands of profoundly and totally deaf people around the world. In this paper, we review the present multiple-channel cochlear implant technology and explores potential applications of micro-electro-mechanical system (MEMS) technology. A new generation of electrode arrays based on the silicon micromachining technology is presented. Approaches in the use of MEMS technology for a middle ear acoustic sensor in a totally implantable prosthesis is also discussed, with key issues for its development highlighted.
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ItemModelling the response of neurons to auditory stimuli: differences between acoustical and electrical stimulation( 1999)There are significant differences in the responses of auditory nerves when they are stimulated acoustically (normal hearing situation) or electrically (with a cochlear implant). This paper addresses the underlying causes of these differences by studying the interspike interval histogram, the synchronization index, and the entrainment (degree of response to successive cycles of the stimulus). The new integrated-input technique is used to analyze the response to periodic synaptic input of integrate-and-fire neurons, in which the randomly arriving synaptic inputs are summed and an action potential is generated when the postsynaptic potential reaches threshold. The synaptic inputs in the model are a sinusoidally modulated inhomogeneous Poisson process, and each input generates a postsynaptic response that subsequently decays according to the membrane decay constant. The results provide a quantitative understanding of both the decrease of the synchronization index with increasing frequency of acoustical stimulation in the auditory pathway and the previously observed enhancement of synchronization in globular bushy cells of the cochlear nucleus. The differences in the responses of neurons in higher stages of the auditory pathway for acoustical and electrical stimulation may be accounted for by the differences in the degree of entrainment that they induce.
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ItemSpeech perception and spoken language in children with impaired hearing( 1998)Fifty seven children with impaired hearing aged 4-12 years were evaluated with speech perception and language measures as the first stage of a longitudinal study. The Clinical Evaluation of Language Fundamentals (CELF) and Peabody Picture Vocabulary Test (PPVT) were used to evaluate the children's spoken language. Regression analyses indicated that scores on both tests were significantly correlated with chronological age, but delayed relative to children with normal hearing. Performance increased at 45% of the rate expected for children with normal hearing for the CELF, and 62% for the PPVT. Perception scores were not significantly correlated with chronological age, but were highly correlated with results on the PPVT and CELF. The data suggest a complex relationship whereby hearing impairment reduces speech perception, which slows language development, which has a further adverse effect on speech perception.
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ItemCochlear implants in the second and third millennia( 1998)Much has been achieved in the Second Millennium in the development of cochlear implants for profoundly deaf people, but further advances in the Third Millennium should result in most severely to profoundly deaf people being able to communicate effectively in a hearing community.
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ItemNew method for anlayzing the synchronization of synaptic input and spike output in neural systems( 1998)We present a new technique for analyzing the probability distribution of output spikes for the integrate and fire model. Using this method we investigate models with arbitrary synaptic response functions and the results, which are compared with numerical simulations, are exact in the limit of a large number of small amplitude inputs. We apply this method to the synchronization problem, in which the relationship between the spread in arrival times of the inputs (the temporal jitter of the synaptic input) and the resultant spread in the times at which the output spikes are generated (output jitter) is analyzed. The results indicate that the ratio of the output jitter to the input jitter is consistently less than one and that it decreases for increasing numbers of inputs, in agreement with earlier studies. We identify the variation in the spike generating thresholds of the neurons and the variation in the number of active inputs as being important factors that determine the timing jitter in layered networks, in addition to those identified previously.
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ItemDirect current measurements in cochlear implants: an in vivo and in vitro study( 1998)Direct current (DC) was measured both in vivo and in vitro in cochlear implant electrodes with stimulation at moderate to high pulse rates in monopolar and bipolar modes. In vivo DC was approximately 2-3 times higher than that measured in vitro. In vivo DC levels were <100 nA even at very high rates, although DC levels increased as a function of stimulus rate and charge intensity. DC levels were lower: in the monopolar than in the bipolar stimulation condition. Stimulation with a monopolar capacitively coupled extracochlear electrode showed even lower DC levels in the intracochlear .electrodes. Our results indicated that the Nucleus electrode shorting system is able to maintain a low level of DC during very high rate stimulation for both monopolar and bipolar modes.
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ItemThe development of a tympanic membrane sensor for a totally implantable cochlear implant or hearing aid(Monduzzi Editore, 1997)We present the design and development of a tympanic membrane sensor for a totally implantable cochlear implant or hearing aid system. The sensor employs a fiber-optic lever which is hermetically sealed in a biocompatible cartridge and implanted in the middle ear cavity. The sensor prototype has been designed, constructed and tested in cats. In addition, the implantation procedure of the device has also been studied using human temporal bones.
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ItemPreliminary results on spectral shape perception and discrimination of musical sounds by normal hearing subjects and cochlear implantees( 1997)This paper presents an overview of an ongoing research project investigating the perception of musical timbre by people with normal hearing, impaired hearing, and cochlear implants. The investigation of musical timbre has been limited to the perception of steady-state frequency spectra from 10 different sources, including sampled acoustic instruments, sung vowels, and synthetic waveforms. Subjects were tested in three different tasks: I) the discrimination of spectra when presented in all possible pairs; 2) the measurement of the internally-perceived frequency spectra using a forward-masking paradigm; and 3) the identification of the spectra by name with the restricted set of sound sources from which they were sampled. Preliminary results from the normally hearing subjects show the spectra to be 99.8% distinguishable, and that significant detail is evident in the internal spectral envelopes from different sounds. There was around 50%-correct identification of stimuli by name with the original sound sources from which they were sampled. The experimental work with hearing impaired and cochlear implant subjects has commenced.