Otolaryngology - Theses
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ItemSpeech production skills, vocabulary development, and speech perception abilities in children with hearing loss: intervention and outcomes( 2007)Despite early diagnosis, early fitting of more advanced sensory aids, early intervention, and intensive educational management, many children with severe to profound hearing loss are delayed in their acquisition of spoken language compared with their peers with normal hearing. Some of the greatest challenges facing educators of children with hearing loss include determining where to focus intervention in order to maximise benefit, and establishing the most effective strategies for the development of age-appropriate language. The experimental research in this thesis, conducted across three studies, examined the relationship between hearing, speech production, and vocabulary knowledge, and investigated the contributions of these factors to the overall speech perception performance of children with hearing loss. This research also investigated the areas in which intervention would be most beneficial, and examined the effects of different types of intervention on the development of spoken language and speech perception skills in children with hearing loss. The first study collected and analysed data to validate a simple non-linear mathematical model that describes the effects of hearing, vocabulary knowledge, and speech production on the perception test scores for monosyllabic words by children with hearing loss. Thirty-three primary-school children with hearing loss, fitted with hearing aids and/or cochlear implants, were evaluated, using speech perception, reading-aloud, speech production, and language measures. Results from these measures were analysed using the mathematical model. It was found that performance on an open-set speech perception word test in the auditory-alone mode is strongly dependent on residual hearing levels, lexical knowledge, and speech production abilities. Further applications of the model provided an estimate of the effect of each component on the overall speech perception score for each child. The separation of these components made it possible to ascertain which children would benefit most from specific language intervention, and which children would benefit from more advanced sensory aids. However, further investigation of the effectiveness of different intervention strategies on the development of speech perception skills is required. In the second study, 12 primary school-aged children with hearing loss participated in two types of speech production intervention to determine which was the most effective in improving speech production skills. After an 8-week intensive program, speech production skills improved for all children, with greater improvements evident in the articulation of phonemes trained at a phonological level. Untrained vowels and consonants also improved after intervention. These findings suggest that intensive speech production intervention in the context of words, sentences, and discourse is effective not only in improving the production of those phonemes trained, but may also result in the generalisation of taught speech skills into other aspects of children's spoken language. The final study applied the mathematical model postulated in the first study to the speech perception scores of 21 primary school-aged children with hearing loss. The children participated in intensive speech production and vocabulary intervention programs. The speech production intervention program implemented the strategies that were found to be effective in the second study, while the vocabulary intervention involved learning the meanings of words. The speech production intervention produced a small but significant improvement in the production of consonants in words, while the vocabulary intervention improved knowledge of word meanings substantially. Both types of intervention significantly improved speech perception performance. These findings demonstrate that the relationships between speech perception, speech production, and vocabulary knowledge are causal rather than merely associative. The application of the model also assisted in identifying the most effective methods of improving receptive and expressive spoken language skills for individual children with hearing loss. In summary, the results from this research provided further evidence of the complex relationship between hearing, speech perception, vocabulary knowledge, and speech production. This research highlights the factors requiring consideration in the interpretation of speech perception scores. Separation of the contributions of hearing, lexical knowledge, and speech production to speech perception scores enabled a better understanding of factors contributing to children's performance levels, and facilitated the development of more appropriate intervention. Speech production and vocabulary intervention were shown to be valuable and beneficial in the individual education programs of many children with hearing loss who exhibit delays in spoken language skills. The evaluation, analysis, and intervention methods reported in this thesis provide an experimentally validated program for improving speech perception, speech production, and spoken language skills of school-aged children with hearing loss.
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ItemPitch perception with cochlear implants( 2008)Most cochlear implant recipients achieve good speech perception under good listening conditions. However, pitch perception is generally poor. The first aspect of cochlear implant pitch perception investigated was place of stimulation. Sequential stimulation of neighbouring channels can produce pitch percepts intermediate to those of individual channels. This was evident when recipients pitch-ranked pure tones processed by the ACE strategy. The existing centroid model of place pitch perception was extended by incorporating a loudness model to provide a better fit to the experimental results. The second aspect investigated was temporal cues on a single channel. A high-rate pulse train, modulated on and off at frequency F0, had a higher pitch than a train of pulses at the rate of F0. If amplitude modulation of high-rate pulse trains is to be used to convey pitch, then the shape of the modulating waveform is important: a half-wave shape is better than a square-wave (on-off) shape. The experimental results were consistent with a perceptual model that derived pitch from the times between successive auditory nerve firings (first-order inter-spike intervals). The results were not consistent with a model utilising auto-correlation or all-order inter-spike intervals. Recipients showed limited ability to combine place and temporal cues to pitch. In voiced speech, the harmonics are not resolved by the sound processor, and there are no useful place cues to pitch. Several experimental sound processing strategies that attempted to enhance temporal cues provided little benefit in laboratory testing. A strategy that used half-wave rectification (HWR) to provide temporal fine structure cues was implemented on the Freedom processor, but a take-home trial showed no significant difference in speech or pitch perception between HWR and the standard ACE strategy (which provided only envelope cues). Pitch can be defined as that attribute of sensation whose variation is associated with melody. The Modified Melodies test was developed to measure pitch perception according to this definition. Normal hearing listeners had higher scores with harmonic tones than pure tones, and musicians performed better than non-musicians. Cochlear implant recipients performed much worse than subjects with normal hearing, but a contralateral hearing aid provided a large benefit. Previous research had suggested that cochlear implant place pitch was more akin to brightness (an aspect of timbre) than to pitch. However, the Modified Melodies results supported the hypothesis that place pitch can support melody perception.
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ItemImproving high-frequency audibility for hearing-impaired listeners using a cochlear implant or frequency-compression aid( 2007-05)Listeners with severe-sloping losses often don’t perceive high-frequency sound cues. Conventional amplification fails to provide these cues due to loudness discomfort experienced by the listener, and/or acoustic feedback. Alternative signal-processing solutions include shifting higher frequencies down to lower frequencies, or providing electrical stimulation via a speech processor. Three experiments were carried out on adult hearing-impaired listeners to determine the best way of providing high-frequency information: conventional amplification, frequency compression or cochlear implantation.
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ItemElectrode-tissue interface: development and findings of an in vitro model( 2006)In the period immediately following the implantation of a cochlear implant electrode array within the cochlear environment, the power required to stimulate the auditory nerve at preset current levels increases. This rise is due to increases in electrode impedance which in turn is suggested to be a result of tissue growth around the electrode array. The foreign body response initiated by the immune system encapsulates the array in a matrix of fibrous tissue, separating the electrode array from the rest of the body. A second change in electrode impedance occurs with the onset of electrical stimulation. A transitory reduction in impedance has been recorded in animals and humans after stimulation of electrodes. Impedance returns to pre-stimulation levels following the cessation of stimulation. It was suggested that these changes in impedance with stimulation were also related to the tissue growth around the electrode array. A more thorough understanding of the interface was required to ascertain these concepts.
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ItemA trainable hearing aid( 2003-09)Digital hearing aids have an increasing number of adjustable amplification parameters, which offer the potential of greater customisation of their amplification settings to the needs and preferences of individual aid users. However, an increased number of adjustable amplification parameters, which may have counteracting effects, can increase the complexity of the fitting procedure, and the difficulty in achieving the combination of amplification parameter values that are optimal for the individual aid user. The latter is compounded by the fact that hearing aids are adjusted with a limited range of acoustic stimuli in a clinic environment, away from the real-life acoustic environments where the amplification provided by hearing aids may be less satisfactory. The customisation of hearing-aid amplification parameters for the individual aid user could be more efficiently and effectively performed with a trainable hearing aid. Such a hearing aid 'learns' the aid user's amplification preferences in everyday acoustic environments, and after sufficient training, automatically varies its amplification settings to the preference of the aid user for the current acoustic environment. However, to the knowledge of this author, the efficacy of trainable hearing aids has not been published in scientific journals. This thesis describes the design, development, and evaluation of a novel trainable hearing aid. In order to evaluate the novel trainable hearing aid, two laboratory experiments and three field trials were conducted to test three main hypotheses: and one sub-hypothesis. For one experiment, the amplification preferences of a hypothetical aid user were communicated to the developed aid under controlled conditions, and used by the aid to calculate a set of trained amplification parameters. The trained amplification parameters resulted in the application of amplification settings that were closer to the hypothetical preferred settings than were the original untrained settings. The results of this experiment supported the first hypothesis. For the other laboratory experiment, the speech-to-noise ratio estimation technique used by the aid was evaluated under controlled conditions (the developed aid periodically estimated the speech-to-noise ratio from a statistical analysis of the microphone signal, and could vary its gain with the speech-to-noise ratio estimates as determined by a trainable amplification parameter). It was found that when speech was present, the speech-to-noise ratio estimation technique used by the novel aid provided estimates that were linearly correlated with the reference estimates over a range of (mainly positive) speech-to- noise ratios, which partly supported sub-Hypothesis 2a. A training trial was conducted, where the developed aid was fitted to 18 hearing-impaired subjects, who trained the aid in the acoustic environments they encountered in everyday life. The data that was logged in the aids during this trial indicated that, for some compression channels, the relationship between the amplification preferences of the subjects and the speech-to-noise ratio relative to the average for all channels (as estimated by the aid) was significant, which partly supported the second hypothesis. Most subjects who had completed the training trial participated in the comparison trials, where a modified version of the developed aid allowed the subjects to blindly compare their trained and untrained settings in everyday acoustic environments, and log their preferred settings in the aid's non-volatile memory. The results of the comparison trials showed that, in everyday environments, most subjects preferred the trained settings to the untrained settings on a statistically-significant majority of occasions, which supported the third hypothesis. Furthermore, it was found that the proportion of occasions when the trained settings were preferred was positively correlated with the amount of time the subjects used the aid during the training trial. Therefore, the main findings of this research project were that under controlled acoustic conditions, a hearing aid can be trained to provide amplification settings that are closer to hypothetical preferred settings than were the initial untrained settings, and in everyday acoustic environments, hearing aid users can train an aid to provide amplification settings that they prefer to the untrained settings on a significant majority of occasions.
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ItemThe cochlear nucleus commissural pathway: an electrophysiological investigation( 2005-11)The cochlear nucleus (CN), as the first brain centre in the auditory system is responsible for sorting the neural signals received from the cochlea, into parallel processing streams for transmission to the assorted higher auditory nuclei. A commissural connection formed between cochlear nuclei through direct projections, thereby provides the first site in the central auditory system at which binaural information is able to influence the ascending auditory signal. This thesis investigates the nature of commissural projections and the impact of their input upon neurons of the ventral CN (VCN) through in vivo intracellular and extracellular electrophysiological recordings together with both acoustic and electrical stimulation of the contralateral CN.
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ItemSurvival and regeneration in the deaf ear: the potential of neurotrophic factors( 2004)Spiral ganglion neurons (SGNs) within the cochlea degenerate following the loss of the auditory sensory epithelium, the auditory hair cells. Since these neurons are the target cells of the cochlear implant, which bypasses damaged or lost hair cells to stimulate the SGNs directly, enhanced SGN number and integrity may provide enhanced outcomes for cochlear implant patients. Improved contact between the cochlear implant electrode array and the auditory nerve fibres is also likely to enhance the benefits received by cochlear implant patients. Therefore, the identification of molecules with the capacity to support SGN survival and stimulate axonal growth has significant clinical implications. Based on their roles in the development and maintenance of the auditory system, some neurotrophic factors are expected to play an important role in enhancing the survival of auditory elements following deafening. This thesis investigates various molecules for their capacity to stimulate and guide the growth of SGN axons, and also investigates the survival-promoting effects of specific neurotrophic factors on SGN survival in clinically relevant animal models of deafness. Two neurotrophic factors were identified specifically to stimulate axonal growth from SGNs in an in vitro model of deafness.
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ItemStochastic resonance in a neuron model with application to the auditory pathway( 2001-03)In this thesis, the transmission of spike trains in a neuron model is studied in order to obtain a better understanding of the role played by stochastic activity, i.e. uncorrelated spikes, in the auditory pathway. Fluctuations of the neuron membrane potential are given by a first-order stochastic differential equation, using a leaky integrate-and-fire model. In contrast to most previous studies the model has a finite number of synapses, and the usual diffusion approximation does not hold. The input signal is modeled by spike trains with spiking times described by inhomogenous Poisson processes. The membrane potential is a shot noise process for which statistical properties are derived with a Gaussian approximation. The statistics of the output spike train are obtained by using the property that a pool of a large number of output spike trains can be modeled by an inhomogeneous Poisson process. It is shown that, under certain conditions, the addition of uncorrelated input spikes, i.e. noise, can enhance the transmission of periodic temporal information. This phenomenon, called stochastic resonance, is demonstrated analytically and supported by computer simulations. Results are compared with those obtained from the traditional leaky integrate-and- fire neuron receiving a continuous waveform input. The shot-noise property of the membrane potential, which implies that its variance is de facto modulated by the input stimulus, is shown to enhance the phenomenon of stochastic resonance. Indeed, for a given average noise level, a modulated noise gives a higher output signal-to-noise ratio than an unmodulated noise with the same average amplitude. The derivation is then extended to certain polyperiodic stimuli mimicking vowel sounds. The fact that the addition of uncorrelated input spikes can enhance the transmission of information is discussed in the context of cochlear implants. The results provide supportive evidence to the postulate that a cochlear implant speech coding strategy that elicits stochastic firing neural activity might benefit the user.
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ItemAn advanced cochlear implant hearing prosthesis for profound to total deafness( 1988)This thesis describes the design and application of an advanced cochlear implant for a hearing prosthesis. When used with a suitable external speech processor, the device is intended to convey more auditory information to deaf implantees than existing prostheses. The implant comprises an electrode array, a capsule containing a receiver-stimulator integrated circuit, and a coil to which radio frequency (RF) signals are transmitted from the speech processor. The electrode array includes 20 platinum bands mounted on a flexible carrier. It is surgically inserted into the scala tympani of the cochlea (inner ear) to bring the electrodes into proximity with surviving auditory neurons. Each electrode is connected individually to the receiver-stimulator. It is the task of the receiver-stimulator to demodulate the RF signals picked up by the receiving coil and to deliver stimulating current pulses to the electrodes under control of the received data. The entire device is implanted beneath the skin, and derives an internal power supply by rectification of the RF signal transmitted from the speech processor. The receiver-stimulator chip, which contains approximately 8000 transistors in digital and analogue circuits, was custom designed. On receipt of each data sequence, it can generate three pulsatile stimuli controlled independently in onset time, duration and current level. Each stimulus can be delivered to any two electrodes selected from the array and configured as a bipolar pair. Bipolar stimulation is used to reduce the spread of neural excitation distributions so as to minimize undesirable interactions between stimuli. The stimuli can be presented sequentially or effectively simultaneously. In the simultaneous stimulation mode, a novel current waveform is generated which was developed to ensure that chronic stimulation would be biologically safe. The implant also contains a telemetry system which enables a selected electrode voltage waveform to be conveyed to an external receiver for display and analysis. The receiver-stimulator chip was fabricated externally using a low-power high-reliability process. Subsequently, the complete advanced cochlear implant was constructed and tested intensively. The device has been implanted in two patients so far. In initial audiological evaluations, both implantees are achieving good results.
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ItemThe cochlear prosthesis: safety investigations( 1986)The present research used both physiological and histological techniques to assess the effects of chronic intracochlear electrical stimulation on the residual auditory nerve population in cats. Stimuli consisted of charge balanced biphasic current pulses presented at 500 pps. Stimulus levels were in the range 0.5 - 0.9 mA, and 200 us per phase, and developed charge densities of 18 - 32 uC.cm^-2 geom. per phase. These stimulus levels are within the range used clinically. The animals were stimulated for periods of up to 2000 hours during which time electrically evoked auditory brainstem responses (EABRs) were periodically recorded. At the conclusion of the stimulus program spiral ganglion cell survival was assessed for both stimulated and control cochleas; comparison of the two groups showed no statistically significant difference in ganglion cell population. A number of cochleas exhibited various degrees of cochlear pathology in association with a general inflammation reaction. Severe inflammation, observed in four of the 20 cochleas examined, was attributed to the presence of infection and resulted in significant and widespread neural degeneration. The histopathological changes were correlated with changes in the EABR input-output functions and confirmed the physiological viability of these cells. The results of this study indicates that long-term intracochlear electrical stimulation, using carefully controlled biphasic pulses, does not adversely affect the auditory nerve population. However, widespread infection can result in severe loss of auditory nerve fibres and care must be taken in this regard during implant surgery. Finally, the correlation between cochlear histopathology and EABR recordings suggests that the EABR may be a useful physiological tool in determining auditory nerve survival in patients. The impedance of these scala tympani electrodes were monitored throughout the chronic stimulation program and were compared with impedance data from similar electrodes chronically stimulated in inorganic saline. The changes in impedance of the in vivo stimulated electrodes generally correlated with the degree of fibrous tissue reaction adjacent to the electrode surface. These scala tympani electrodes were examined for evidence of corrosion using a scanning electron microscope. The surface of these in vivo electrodes were compared with in vivo control electrodes and in vitro electrodes stimulated in inorganic saline using similar stimulus parameters. The in vitro stimulated electrodes showed evidence of platinum dissolution at high charge densities (36 uC.cm^-2 geom. per phase) and aggregate charge (270 C). Significantly, the in vivo stimulated electrodes showed no evidence of stimulus induced corrosion. Indeed, their surfaces were similar to the in vivo control electrodes. Previous in vitro electrochemical studies have demonstrated that proteins play a significant role in the inhibition of platinum dissolution. The present study has demonstrated an inhibitory effect in vivo which may be due to the presence of proteins. The results from these studies reflect the biocompatible nature of this neural stimulator. Finally, temporary and permanent reductions in the excitability of the auditory nerve were observed following acute stimulation at intensities and rates above the range used clinically. The extent of these changes correlated with the degree of stimulus evoked neural activity. Furthermore, these stimulus induced changes were metabolically active. These findings suggest that the changes in neural excitability were a result of long-term metabolic changes in the stimulated neural population. Moreover they indicate upper functional operating ranges for auditory prostheses using this form of stimulus regime.