Graeme Clark Collection
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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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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.
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ItemEffects of rate and pulse manipulations of the spectral maxima speech processor upon speech production( 1993)Postlingually deafened adults experience many changes in their speech as a consequence of late-onset deafness. Previous studies have indicated changes in fundamental frequency, duration, intensity, and vowel formant frequencies in postlingually deafened adults. Postlingually deafened adults who receive multichannel cochlear implants demonstrate improved control of fundamental frequency and intensity. Shifts in formant frequencies to values similar to normal hearing. speakers also appears in some adult implant users. In order to examine how adult implant users adjust their vowel production in response to map changes, we examined vowel production following manipulations to the processing strategy of the Spectral Maxima Speech Processor. This processor uses a speech processing scheme in which six spectral maxima from the outputss of 16 bandpass filters stimulate the cochlea on a place basis at a constant rate. The rate of sampling of the filterbank output is 250 Hz, so six biphasic pulses are presented every 4 msec and there is no attempt to extract fundamental frequency or to find the formant peaks in the speech signal. Two manipulations to the scheme were examined. In the first condition, the rate of sampling remained at 250Hz but eight biphasic pulses were presented rather than six. In the second condition, six biphasic pulses were presented but the rate of sampling of the filterbank output was increased to 400 Hz. Speech samples also were acquired using the standard spectral maxima processor .and with no auditory feedback when the processor was turned off. Speech samples from three �subjects were acquired immediately after receiving the manipulated speech processors and after two weeks experience with the various processors. Preliminary data indicate one subject experienced increased fundamental frequencies while using the 400 Hz high rate strategy. No significant changes were observed in fundamental frequency between the normal SMSP processor and the eight pulse variation. Elimination of feedback resulted in significantly lower fundamental frequencies. The high rate and eight pulse variations resulted in significantly higher first formant frequencies. Second formant frequencies also appear to be influenced by the processing strategies. Comparisons of values produced immediately after receiving a new strategy versus those produced after two weeks use show shifts in primarily first formant values. Data will be presented for all three subjects and discussed in regard to sensitivity to variations in speech processing schemes and the influence of experience with manipulated schemes. (Work supported by the N1H-NIDCD).