Graeme Clark Collection
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ItemSignal processing for multichannel cochlear implants: past, present and future [Abstract]( 1994)Since the late 1970's, many groups have worked on developing effective signal processing for multichannel cochlear implants. The main aim of such schemes has been to provide the best possible speech perception for those using the device. Secondary aims of providing awareness and discrimination of environmental sounds and appreciation of music have also been considered. Early designs included some that attempted to simulate the normal cochlea. The application of such complex processing schemes was limited by the technology of the times. In some cases, researchers reverted to the use of single channel systems which could be controlled reliably with the existing technology. In other cases, as with the Australian implant, a simple multichannel processing scheme was devised that allowed a reliable implementation with available electronics. Over the next 15 years, largely due to the improvements in integrated circuit technology, the signal processors have slowly become more complex. Further psychophysical research has shown how additional information can be transferred effectively to implant users via electrical stimulation of the cochlea. This has lead to rapid improvement in the speech perception abilities of adults using cochlear implants. Some of the main developments in signal processing over the last 15 years will be discussed along with the latest speech perception results obtained with the new SPEAK processing scheme for the Australian 22-channel cochlear implant. Initial results for SPEAK show mean scores of 70% (equivalent to 85-90% phoneme scores) for open set monosyllabic word testing for experienced adult users. Although there remains a large range of performance for all users of cochlear implants, average speech perception scores for all implanted adults have also improved significantly with the developments in signal processing. It appears likely that multichannel cochlear implants will be a viable alternative for the treatment of severe hearing loss in the future.
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ItemPsychophysics of electrical stimulation of the auditory nerve: implications for coding of sound and speech processing for cochlear implants [Keynote address]( 1994)Psychophysical studies on electrical stimulation of the auditory nerve have contributed to our understanding of the coding of sound and speech signals. Those studies have also helped establish speech processing strategies for multiple-electrode cochlear implant patients. The first studies were on temporal coding of frequency and pitch perception to help determine whether a single or multiple electrode implant would be preferable for the coding of speech frequencies. Temporal frequency coding was initially studied in the experimental animal by measuring difference limens for frequency of stimulus rate. The results showed that rate coding occurs for low frequencies up to 200 or even 600 pulses per second. It was concluded that higher speech frequencies cannot be conveyed by variations in stimulus rate but require multiple-electrode stimulation. These studies in experimental animals were essentially confirmed in the human.
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ItemLoudness growth characteristics of cochlear implantees using the Spectral Maxima Sound Processor [Abstract]( 1994)The study of perceptual characteristics of subjects with cochlear implants can lead to improvements in the design of speech processors. One important aspect of speech processing which has received little attention in the past is the conversion acoustic signal amplitudes into appropriate levels of electrical stimulation. The optimum conversion would provide implantees with loudness growth characteristics that mimic those of normal hearing. To investigate how implantees using the Spectral Maxima Sound Processor (SMSP) perceive changes in loudness, an experiment involving production of fixed loudness ratios was conducted. Ten subjects participated: five users of the Mini System 22 cochlear implant, and five normally-hearing subjects. In the experiment, the subjects were required to adjust the loudness of two stimuli (white noise and speech-weighted noise) to equal half or twice that of a reference. The reference was presented at various levels over a range of 25 to 75 dBA. The results for three of the implantees were similar to those of all the normally-hearing subjects, who produced an average level change of 10.8 dB for the task. The remaining subjects, who had the largest electrical dynamic ranges, produced larger level changes (up to 20 dB) which were constrained by the limited electrical dynamic range of the processor (46 dB). The SMSP utilises an amplitude conversion function by which the stimulus level (in dB) is directly proportional to the input sound level (in dB). The experimental results suggest that the shape of this function is satisfactory, though not necessarily optimum, for these implantees.
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ItemValidation of a technique for establishing maximum comfortable levels for children using cochlear implants [Abstract]( 2002)The aim of fitting a cochlear implant is to establish electrical stimulation parameters that will provide the wearer with comfortable and useful auditory sensations. One parameter that is fundamental to achieving this aim is the Maximum Comfortable Level (C-level). A C-level is the amount of electrical current that produces a loud, but comfortable sound. C-levels need to be established for all channels that a person will use in their speech processor Map. Determining C-levels can be complicated as the person is required to make a judgment about the loudness of a sound. While most adults and older children have the ability to make such a judgment and provide feedback to the clinician, this is rarely the case for young children. Generally, the only way a clinician will be aware a sound could be too loud for a young child is when they observe the child giving an aversive reaction or an involuntary blink. A current level that produces such a reaction is called the Loudness Discomfort Level (LOL). This study examines the relationship between LDLs and C-levels. Testing was performed with a group of adults, using stimulation rates and stimulation modes that are commonly used by children. The LDL/C-level relationship established in this study provides a procedure for selling C-Levels for young children when only loudness discomfort responses can be obtained.