Geary A. McCandless

Differential susceptibility to noise-induced permanent threshold shift between albino and pigmented guinea pigs

Evidence that reduced levels of cochlear melanin are associated with increased auditory sensitivity, increased levels of auditory fatigue and an increased susceptibility to noise-induced hearing loss led us to investigate the effects of noise exposure on the cochlear microphonic (CM) in albino and pigmented English shorthair guinea pigs. CMs were recorded from the round window prior to and at 90 min and 7 days after exposure to 45 min of 126 dB noise. Thresholds for the first detectable elicitation of the CM for four pure tones were determined and the output voltage of each cochlea was measured in 10 dB steps through intensity levels which produced a maximum voltage amplitude in the CM and voltage rollover. This analysis demonstrated that: albino guinea pigs displayed significantly lower auditory thresholds than did pigmented animals before exposure to noise; thresholds were elevated to comparable levels in both groups 90 min after noise exposure; pigmented guinea pigs showed a reliable recovery in CM thresholds 7 days after exposure to noise while thresholds in the albinos remained elevated to the same degree at both 90 min and 7 days after noise; 90 min after noise exposure, the maximum voltage output of albino cochleas was significantly less than that recorded from the cochleas of the pigmented guinea pigs. These results demonstrate that albino guinea pigs are more susceptible to the ototoxic effects of high intensity noise than pigmented guinea pigs. Converging evidence indicates that some aspects of cochlear function involve melanin pigment and that its absence may produce auditory abnormalities. Reduced melanin pigmentation may also contribute to such phenomena as noise-induced threshold shifts and individual differences in noise-induced hearing loss.

Word recognition by 50 patients fitted with the Symbion multichannel cochlear implant.

We describe the word recognition ability of 50 patients who use the multichannel cochlear implant manufactured by Symbion, Inc. The median scores for open set tests involving auditory stimulation alone were: 14% correct (range 0 to 60) for monosyllabic words, 44% correct (range 0 to 100) for spondees, and 45% correct (range 0 to 100) for words in the Everyday CID Sentences. In the visual stimulation condition of the Everyday CID Sentences, the median score was 64% correct (range 0 to 100). In the visual plus auditory stimulation condition, the median score was 99% correct (range 46 to 100). These results demonstrate that relatively good speech understanding can be achieved using a cochlear implant which employs only a few channels of stimulation and which simultaneously activates monopolar electrodes.

Aging and the Auditory Brain Stem Response

30 elderly subjects, 30 young subjects with comparable sensorineural hearing losses and 30 normally hearing subjects were evaluated to examine the interaction of advanced age and high-frequency hearing loss upon the auditory brain stem response. Hearing loss plus age accounts for greater deterioration of auditory brain stem response waveforms than hearing loss alone. Comparison of interpeak latencies revealed no consistent pattern of lengthened latency due to aging.

Effects of aging on normal hearing loss and noise-induced threshold shift in albino and pigmented guinea pigs

In a previous investigation into noise-induced hearing loss by comparing 2-month-old albino with pigmented guinea pigs, albinos displayed significantly greater shifts in cochlear microphonic (CM) threshold and less recovery than the pigmented animals 7 days after noise exposure. The present study compared the responses of 14-month-old albino and pigmented guinea pigs to the same noise parameters used previously. Thresholds for the first detectable elicitation of CM for three pure tones were recorded prior to, at 90 min and at 7 days after a 45-min exposure to 126 dB broadband noise. Before exposure to noise, thresholds for pigmented guinea pigs were 24 dB higher than those in the albinos. Following noise exposure, the pigmented animals showed less than half the amount of threshold shift displayed by the albinos. This change ws attributed to the higher pre-exposure thresholds in the pigmented guinea pigs. Converging lines of evidence suggest that cochlear pigmentation may have both protective and toxic influences on the inner ear.

Pitch scaling and speech understanding by patients who use the ineraid cochlear implant

Pitch scaling was assessed for 10 normal-hearing listeners and 8 patients who use the Ineraid multichannel cochlear implant. For two patients who were excellent users of the prosthesis, pitch increased over a wide range of frequencies (100 Hz to 2333-3000 Hz). For three patients who were above average users of the prosthesis, pitch increased with frequency over a smaller range (100 Hz to 1200-2300 Hz). For three patients who demonstrated poor word recognition ability, pitch increased with frequency over a very small range (100 Hz to 600-1000 Hz). These results suggest that differences in speech understanding among patients who use the Ineraid may be accounted for, in part, by the range of pitch available through the implant.

Longitudinal changes in word recognition by patients who use the Ineraid cochlear implant

The time course for reacquisition of spondee word recognition was examined for 27 patients who use the Ineraid cochlear implant. At 1 month postfitting, test scores ranged from 0 to 84% correct. The median score was 10% correct. At 22 to 28 months postfitting, the scores ranged from 0 to 100% correct. The median score was 56% correct. The rate of improvement in spondee recognition varied greatly within the sample. Most patients continued to extract new information from the electrically evoked representation of the speech signal over a period of a year or more.

Vowel and consonant recognition with the aid of a multichannel cochlear implant

In this report we review the vowel and consonant recognition ability of patients who use a multichannel cochlear implant and who achieve relatively good word identification scores. The results suggest that vowel recognition is accomplished by good resolution of the frequency of the first formant (F1) combined with poor resolution of the frequency of the second formant (F2). The results also suggest that consonant recognition is accomplished (1) by using information from the amplitude envelope, including periodicity/aperiodicity, as cues to manner and voicing, (2) by using F1 as an aid to the identification of manner and voicing, and (3) by using information from cochlear place of stimulation to provide a very crude indication of the shape of the frequency spectrum above 1kHz.

Auditory/phonetic categorization with the Symbion multichannel cochlear implant

The phonetic identification ability of an individual (SS) who exhibits the best, or equal to the best, speech understanding of patients using the Symbion four-channel cochlear implant is described. It has been found that SS: (1) can use aspects of signal duration to form categories that are isomorphic with the phonetic categories established by listeners with normal auditory function; (2) can combine temporal and spectral cues in a normal fashion to form categories; (3) can use aspects of fricative noises to form categories that correspond to normal phonetic categories; (4) uses information from both F1 and higher formants in vowel identification; and (5) appears to identify stop consonant place of articulation on the basis of information provided by the center frequency of the burst and by the abruptness of frequency change following signal onset. SS has difficulty identifying stop consonants from the information provided by formant transitions and cannot differentially identify signals that have identical F1s and relatively low-frequency F2s. SSs performance suggests that simple speech processing strategies (filtering of the signal into four bands) and monopolar electrode design are viable options in the design of cochlear prostheses.

Aging and auditory site of lesion.

The purpose of this study was to examine and quantify the functional auditory problems of presbycusis through a battery of recently developed diagnostic tests and to evaluate the usefulness of these tests with an elderly population. Diagnostic measures used were impedance measures, speech discrimination tests, synthetic sentence identification, compressed speech, two measures of tone decay, the short increment sensitivity index, a digit span test, and auditory brain stem response audiometry. Significant differences were found between scores for elderly subjects and those of young subjects who had similar audiograms. Use of the Metz test as an objective measure of recruitment yielded results suggesting a higher incidence of recruitment than evidenced by previous studies using loudness balancing procedures. The Olsen-Noffsinger procedure of quantifying tone decay revealed a greater difference between age groups than did the Suprathreshold Adaptation Test. Synthetic sentence identification revealed the most consistent age effect among the tests of central auditory function. Auditory brain stem response audiometry revealed several examples of abnormally long interpeak latencies. It is concluded that there is both behavioral and electrophysiological evidence of central and peripheral auditory disorder frequently accompanying senescence.

Hearing aid performance relative to site of lesion.

This study investigated hearing aid acceptance and use as functions of the probable site of a lesion: conductive, cochlear, neural, and central as determined by an audiometric test battery. Results indicate significantly better acceptance of aids in conductive and cochlear disorders than in more central neural and central disorders.