Graeme Clark Collection
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ItemThe relationship between speech perception and electrode discrimination in cochlear implantees( 2000)Abstract not available due to copyright.
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ItemA frequency importance function for a new monosyllabic word test( 1998)A frequency importance function, characterising the relative contribution of different frequency bands to speech intelligibility, was determined for a CNC monosyllabic word test designed for Australian usage at the University of Melbourne. The importance function was derived from the phoneme scores of 12 normally-hearing listeners who were tested under various conditions of low-and high-pass filtering presented at signal-to-noise ratios of -8 to +6 dB, using noise which was shaped across frequency 10 match the speech spectrum. The importance function showed a dominant peak at approximately 2000 Hz, which is consistent with previously published word test importance functions. The word test, along with the importance function, will be useful in advanced hearing-aid fitting procedures and research aimed at improving speech perception.
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ItemThe perceptual dimensions of single-electrode and nonsimultaneous dual-electrode stimuli in cochlear implantees( 1996)Abstract not available due to copyright.
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ItemThe importance of different frequency bands to the speech perception of cochlear implantees [Abstract]( 1996)It is well known that cochlear implantees exhibit a wide range of speech perception ability. Understanding the reason for this variability may lead to improved speech processors. This study investigates whether implantees rely on different areas of the speech spectrum for speech cues, compared to normally hearing listeners, and whether poor performers rely on different spectral areas than better performers. Six subjects with the Mini System 22 implant and using the SPEAK strategy participated in this experiment. Scores for monosyllabic words were obtained using the full speech spectrum and with selected frequency bands removed from the subjects’ speech processor maps. The Articulation Index (AI) is a measure of the proportion of speech information available to a listener, and the relative contribution to AI from different frequency bands is termed the Importance Function. The five frequency bands studied in this experiment were determined to be of equal importance to normally hearing listeners for the speech material used. The scores for each implantee were transformed into AI values, and hence the relative importance of the bands was determined. This relative importance was compared between the implantee group and normally hearing listeners to determine the way in which speech perception by electrical stimulation varies from that by acoustical stimulation. Comparisons were also made between individual implantees to determine whether correlations exist between their speech perception ability and their use of cues in different parts of the spectrum. Further research will determine whether the differences among implantees are correlated with their ability to perceive changes in stimulation place or temporal characteristics.
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ItemLoudness summation for two channels of stimulation in cochlear implants: effects of spatial and temporal separation( 1995)An experiment with four implantees with the Mini System 22 device was undertaken to measure the loudness summation across two channels of stimulation, with stimuli in which the current pulses were delivered alternately to each channel. The effects of varying spatial separation, temporal separation, and extent of stimulation were investigated. It was found that the absolute amount of summation varied among subjects, and was in general independent of electrode separation, except for a reduction at zero separation. Widening of the spatial extent of the stimulation did not have a consistent effect. There was a reduction in summation for all subjects at zero electrode separation when the time between the two pulses was increased from less than I millisecond to 2 milliseconds. In conclusion, loudness summation did not appear to be highly dependent on parameters that affect the spatial current spread in the cochlea. Further study of the effect of temporal parameters on loudness may help to quantify interaction between stimulation channels.
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ItemPitch matching of amplitude-modulated current pulse trains by cochlear implantees: the effect of modulation depth( 1995)Abstract not available due to copyright.
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ItemThe spectral maxima sound processor: recent findings in speech perception and psychophysics(Wien, 1994)The Spectral Maxima Sound Processor (SMSP) was developed at the University of Melbourne for use with the Mini System 22 implant manufactured by Cochlear Pty Ltd. The SMSP has been shown in recent studies to provide improved speech perception to implantees when compared to the currently commercially available processor for this implant (the MSP (MULTIPEAK) processor). In the first of three experiments, the effect on speech perception of increasing the rate of stimulation of the SMSP and of increasing the number of electrodes activated in each stimulation cycle was studied. It was found that these parameter changes made little difference to speech perception in quiet but both changes were advantageous for some subjects when listening in noise. The second and third experiments investigated psychophysically the effects of two aspects of the SMSP strategy which differ from previous processors for this implant. In the second experiment, it was found that concurrent stimulation of two adjacent or nearby electrodes evoked a pitch which was intermediate to that of either electrode. This may explain, in part, the better discrimination of vowel formants by users of the SMSP. In the third experiment, it was found that a pitch related to the modulation frequency was evoked by amplitude-modulating a constant rate stimulus, provided that the rate of stimulation was sufficiently high (four times the modulation frequency) or a multiple of the modulation frequency. This result may explain the equal ability of SMSP and MSP users to perceive speaker differences and intonation patterns, even though the rate of stimulation is constant In the SMSP.
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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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ItemPitch ranking with nonsimultaneous dual-electrode electrical stimulation of the cochlea( 1994)Abstract not available due to copyright.
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ItemSpeech processing for multichannel cochlear implants: variations of the Spectral Maxima Sound Processor strategy( 1994)The Spectral Maxima Sound Processor (SMSP) incorporates a bank of 16 band-pass filters which are assigned to 16 intracochlear electrodes. In each stimulation period 6 electrodes are activated, based on the outputs of the filters with the largest amplitudes. The SMSP has previously been compared with the present MSP (MULTIPEAK) processor and found to improve speech comprehension results. The SMSP speech processing scheme has recently been implemented successfully in a new speech processor, also developed at the University of Melbourne, which utilises digital signal processing techniques. The programming flexibility of this processor has facilitated the investigation of variations of the SMSP strategy which might provide further enhancement of speech perception. Three variations have been investigated: firstly, increasing the constant pulse rate from the usual 250 Hz to 400 Hz; secondly, changing the number of electrodes selected in each stimulation period from 6 to numbers between 4 and 8; thirdly, sharpening the spectral peaks prior to selection of the active electrodes. The results of these studies showed that all three variations had minimal effect on speech perception in quiet, but that increasing the number of electrodes selected for stimulation to 8, or increasing the rate of stimulation, was advantageous for some subjects when listening in background noise.