Hearing and hearing loss in 5-year-old children. Pure-tone thresholds and the effect of acute otitis media

Air and bone conduction pure-tone thresholds of 420 unselected urban children were measured with standard clinical audiometry. The mean of air conduction pure-tone averages (average threshold at 0.5, 1 and 2 kHz) was 8.6 dB HL in girls and 8.3 dB HL in boys. In only 5 ears (0.6%), was this average greater than or equal to 35 dB HL. The threshold greater than or equal to 35 dB HL at 4 kHz was found in 1.4% of the ears and at 8 kHz in 4.1%. The bone conduction threshold greater than 20 dB HL at any of the frequencies from 0.5 to 4 kHz was very rare, and only once, at 4 kHz, was it greater than 35 dB HL. Earlier attacks of acute otitis media seemed to have only a marginal long-term effect on air conduction hearing, and an almost negligible effect on bone conduction hearing.     

Attenuation of Neomycin Ototoxicity by Iron Chelation

Increasing evidence suggests that aminoglycoside ototoxicity is mediated by the formation of an aminoglycoside-iron complex and that the creation of this complex is a preliminary step in generation of free radical species and subsequent hair cell death. In this study we have assessed the ability of the iron chelator deferoxamine to attenuate the hearing loss induced by an ototoxic dose of the aminoglycoside neomycin (100 mg/kg per day for 14 days). Experiments were carried out on pigmented guinea pigs weighing 250 to 300 g. Changes in auditory sensitivity were characterized by monitoring shifts in compound action potential(CAP) thresholds, recorded through indwelling electrodes implanted at the round window, vertex, and contralateral mastoid. Results show that animals receiving neomycin alone suffered a mean threshold shift exceeding 35 dB at all test frequencies (2.0, 4.0, and 8.0 kHz) 30 days after initiation of treatment. In comparison, all animals receiving cotherapy of neomycin and deferoxamine (150 mg/kg twice daily for 14 days) maintained their CAP threshold, suggesting significant protection from neomycin ototoxicity. A statistical comparison of treatment groups showed that in the animals receiving cotherapy with neomycin and deferoxamine, deferoxamine produced a significant protective effect against neomycin-induced ototoxicity (P < 0.001). These results provide further evidence of the intrinsic role of iron in aminoglycoside ototoxicity and suggest that deferoxamine may have a therapeutic role in attenuating the cytotoxic action of aminoglycoside antibiotics.     

An individualized, sensitive frequency range for early detection of ototoxicity

OBJECTIVE: The aim of this study was to identify auditory frequencies at which serial threshold testing would provide the greatest sensitivity for early detection of ototoxicity. The overall objective is to develop a more time-efficient ototoxicity monitoring protocol. DESIGN: Threshold data were analyzed from 370 hospitalized patients treated with aminoglycoside antibiotics (AMGs) or cisplatin (CDDP) who received serial auditory monitoring before, during, and after treatment at conventional (0.25 to 8 kHz) and high (9 to 20 kHz) frequencies. RESULTS: For patients showing hearing changes due to ototoxicity, a frequency range was identified for its apparent high sensitivity to initial ototoxicity. This sensitive range is identified according to an individual's hearing threshold configuration, and is, therefore, unique for each patient. The range consists of five frequencies, generally separated by 1/6 octave, e.g., 8, 9, 10, 11.2, and 12.5 kHz. To determine frequencies and combinations of frequencies that were most often involved in ototoxicity detection, threshold data in the sensitive range were analyzed in detail. This analysis suggests that patients receiving treatment with AMG or CDDP can be monitored for hearing thresholds at only five frequencies, resulting in an 84% detection rate for AMG and 94% for CDDP compared with monitoring at all conventional and high frequencies. CONCLUSIONS: This comprehensive analysis supports earlier observations that a sensitive, limited frequency range exists in which serial threshold monitoring will provide early warning of ototoxicity before effects in the speech frequency range. This finding is now being evaluated in a prospective investigation.     

Distortion-product otoacoustic emissions in humans with high frequency sensorineural hearing loss

Distortion-product otoacoustic emissions (DPOAEs) and pure-tone behavioral thresholds were compared in 20 ears with normal hearing and in 20 ears with high-frequency sensorineural hearing loss. The purpose was to determine if DPOAE amplitude is associated with pure-tone behavioral threshold. Comparison of results from the two groups of ears indicated that DPOAEs were reduced in amplitude or were absent in ears with high-frequency hearing loss. The differences occurred at frequencies above 1,500 Hz. Comparing results from 750 to 8,000 Hz within the same ear revealed a frequency-related correspondence of elevated behavioral threshold to reduced DPOAE amplitude. When behavioral thresholds were better than 20 dB HL, DPOAE amplitude was within the range (+/- 2 SDs) determined for the ears with normal hearing. When pure-tone threshold was greater than 50 dB HL, DPOAEs were absent or were significantly attenuated in 16/17 subjects (94%). The association of emission level with behavioral threshold level was variable when threshold was between these two extremes. Results imply that the measurement of DPOAEs has clinical potential as a means of detecting hearing loss by frequency.     

Differential susceptibility to gentamicin ototoxicity between albino and pigmented guinea pigs

The known chemical affinity of melanin pigment for aminoglycoside antibiotics has led to the suggestion that higher concentrations of these drugs will bind to the pigmented inner ear and produce greater ototoxicity compared to the nonpigmented albino cochlea. Although this has provided a compelling hypothesis, results from the few investigations to address this question have been equivocal. In the present study, cochlear microphonic (CM) thresholds were recorded from albino and pigmented guinea pigs both before and two weeks after exposure for 14 consecutive days to 100 mg/Kg gentamicin. Cochleae were dissected and half-turn segments prepared for surface examination of the organ of Corti. After gentamicin exposure, threshold shifts averaged a statistically reliable 33 dB in albinos and 19 dB for the pigmented animals. Anatomical studies revealed a significant 44% mean outer hair cell loss in albinos compared to a 21% loss in the pigmented inner ears. The results showed that albinos display greater ototoxicity from gentamicin than do pigmented guinea pigs. Aminoglycosides are known to exert toxicity through interaction with polyphosphoinositides found in high concentrations in the inner ear. Cochleae in both albino and pigmented animals appear to possess significant phospholipid concentrations and bind toxic levels of these drugs independent of inner ear pigment content. However, evidence showing that melanin can inhibit aminoglycoside activity in vitro suggests that, once these drugs bind to pigmented tissue, they may undergo inactivation in a manner unavailable to the nonpigmented albino cochlea. The present results are consistent with the possibility that cochlear melanin may inhibit gentamicin activity in vivo and decrease the severity of aminoglycoside ototoxicity in the pigmented inner ear.     

Interpretation of pure-tone thresholds in sensorineural hearing loss (SNHL): a review of measurement variability and age-specific references

When assessing a patient with a sensorineural hearing impairment, the most simple and most widely available technical investigation is pure-tone audiometry. Although it is a subjective measure, the test is very reliable if the patient is cooperative. In this paper we review standards and test-retest reliability for pure-tone audiometry. A pure-tone threshold measurement at a single frequency has a chance of 90% to be repeated between -10 dB and +10 dB compared to the first measurement, assuming no real change in hearing thresholds has occurred. It is also of great importance to use correct gender- and age-specific reference values when interpreting pure-tone threshold measurements. Several large-scale epidemiological studies have been conducted during recent years, and have provided us with reliable gender- and age-specific references. A method to take into account the age-related deterioration is presented. In this method, Z-score audiograms represent traditional thresholds in an age- and gender-independent way. At each frequency, the Z-score value is the number of standard deviation units that the threshold differs from the median value for the otologically normal population (ISO 7029).     

Audiologic testing and molecular analysis of 12S rRNA in patients receiving aminoglycosides

BACKGROUND: Pathogenic mutations in the mitochondrial genome are associated with a wide variety of maternally inherited human diseases including sensorineural hearing loss (HL). A specific mutation, m.1555A>G in the mitochondrial 12S rRNA gene, is associated with predisposition to aminoglycoside ototoxicity and HL. Mutation screening in this gene has been recommended before use of aminoglycosides as a preventative strategy to reduce the risk of ototoxicity. OBJECTIVE: To study the incidence of mutations in the 12S rRNA gene in patients being treated with aminoglycosides and its correlation with ototoxicity. METHODS: Patients undergoing treatment with aminoglycosides were prospectively enrolled in this study (n = 27). Total dosage administered and therapeutic levels of the antibiotic were noted. All patients underwent high-frequency pure-tone audiometry pre- and posttherapy and sequencing of the 12S rRNA gene. In addition, 12S rRNA gene was also sequenced in 50 controls to characterize population specific polymorphisms. RESULTS: Five of 27 patients suffered from HL involving the high frequencies: four mild and one moderate. Only one of the five patients with ototoxicity harbored two sequence alterations in 12S rRNA of uncertain pathogenicity. The m.1555A>G and m.961delTInsCn mutations were not detected. CONCLUSIONS: High-frequency pure-tone audiometry is critical for detection of aminoglycoside-induced HL. In the Swiss population, screening for mutations in the 12S rRNA gene, before the initiation of aminoglycoside therapy, is not supported by this limited study. A larger multicenter and multicultural study is warranted to more definitively address this critical clinical issue.     

Incidence of amikacin ototoxicity: a sigmoid function of total drug exposure independent of plasma levels

A sigmoid curve was found to closely describe the relationship between the incidence of amikacin ototoxicity (greater than or equal to 15 dB hearing loss at a given frequency) and either (1) total dose, or (2) the area under the curve (AUC) describing plasma drug concentration v time over the total period of amikacin administration (total AUC) in continuously infused guinea pigs. Total dose or total AUC estimates of the drug exposure required to produce ototoxicity in 50% of the animals (ED50s) were not significantly different over an eight-fold range of dosing rates or plasma concentrations. A theoretical explanation for this result is that ototoxicity occurs only when a critical amount of drug is accumulated at the ototoxic site by an essentially unidirectional process with a rate that is slow and linearly related to the extracellular drug concentration. The sigmoid relationships for pooled data were parallel in slope for all hearing frequencies from 2 to 32 kHz, and the ED50s showed a strong negative linear relationship to the log of the hearing frequency over this range. The magnitude of ototoxicity expressed as the number of octaves (frequency ratios of 2) for which hearing loss damage was continuous from 32 kHz downward, was correlated to both total dose (r = .605) and total AUC (r = 0.703). No relationship between ototoxicity and plasma level or dosing rate was found. The extreme steepness of the dose-effect curve for the incidence of ototoxicity greatly amplified the variability between individuals and offers an explanation for the unpredictability of aminoglycoside ototoxicity in human patients. The results indicate that either total dose or total AUC (in cases of highly unpredictable blood levels), and not peak or trough serum levels, should be used as an index of ototoxic risk and that the safety limits of drug exposure should be set conservatively.     

Reproducibility of hearing threshold measurements. Supplementary data on bone-conduction and speech audiometry

The reproducibility of bone-conduction pure-tone audiometry and speech recognition thresholds has been tested in groups of normal-hearing subjects. Each person was tested twice during the same day, and the test-retest difference was calculated. The reproducibility is presented as the standard deviation of this difference. Bone-conduction threshold measurements have a high degree of test-retest precision, whereas air-bone gaps show a large range of distribution in these normal-hearing subjects. This makes the interpretation of such gaps spurious when values are below 20-30 dB. Speech recognition threshold has the highest degree of test-retest precision of all audiometric tests, and this is probably due to the steep slope of the psychometric function at 50% intelligibility. A more detailed graphic presentation of the 50% point of intersection will bring the reproducibility down to less than 2.5 dB.     

Rate of false-positive results for threshold shifts during initial treatment with aminoglycosides

Specific recommendations from the American Speech-Language Hearing Association (1994) exist for the audiometric surveillance of patients receiving aminoglycoside therapy. However, these recommendations are not based primarily on test-retest repeatability in often seriously ill patients. The probability of ototoxicity is very low during the first three days of aminoglycoside therapy, and significant threshold shifts can be assumed to represent a false-positive result. Baseline thresholds were measured in 28 patients before or not later than 24 h after the beginning of aminoglycoside therapy. These measurements were repeated in 22 patients during the first three days of the drug administration. Three out of 22 patients fulfilled the American Speech-Language-Hearing Association's (1994) criteria for cochleotoxicity making up a false-positive rate of 13%. In actual patients, the false-positive rate of audiometric surveillance during ototoxic drug administration may be substantially higher than in healthy subjects when these criteria are used.     

Changes in Pure-Tone Thresholds in Individuals Aged 70-81: Results from a Longitudinal Study

The results of audiometric evaluation of 376 randomly selected men and women, 70 years old and born in 1901, are reported. The investigation is part of a large study on a gerontological population in which the original participants were tested again with pure-tone and speech audiometry at ages 75, 79 and 81. We also report audiometric results obtained at ages 70 and 75 from a second group, consisting of 297 men and women born in 1906. Hearing loss was most pronounced at higher frequencies for both sexes, and men had an average of 10 dB greater hearing loss at 8 kHz than women. The decrease in hearing threshold in men between the ages of 70 and 81 was more pronounced at 2 kHz (27 dB) than at 4 and 8 kHz (15 and 20 dB, respectively). The average hearing loss in women increased at a constant rate between the ages of 70 and 79 (15 dB), while betweeen the ages of 79 and 81 the change in pure-tone threshold was minimal. There were no significant differences in pure-tone thresholds for women born in 1901 when compared to those born in 1906 at the ages of 70 and 75. However, men born in 1906 had a more pronounced hearing loss at the age of 75 than those born in 1901.     

Acoustic-reflex adaptation: morphology and half-life data for subjects with normal hearing

Acoustic-reflex adaptation was studied in 35 subjects with normal hearing using nine pure-tone activators (250-6000 Hz) and a broadband-noise activator. The individual subject data generated by the 31-s activators presented 10 dB above the reflex threshold were digitized, corrected for baseline drift and ear canal volume, and analyzed in terms of the acoustic-admittance change in acoustic mmhos and in terms of the percentage of maximum-reflex magnitude. Reflex adaptation increased as a function of frequency. The adaptation functions for the lower frequencies (less than or equal to 1500 Hz) were essentially linear over the 31-s activator period, whereas the functions for the higher frequencies (greater than or equal to 2000 Hz) were curvilinear over the activator period. The experimental half-life data are compared with a theoretical half-life function that was generated to estimate normal acoustic-reflex adaptation as a function of activator frequency. Finally, the means and standard deviations are reported and discussed for (a) the onset time of adaptation, (b) the half-life time, and (c) the percentage of maximum reflex magnitude at 5-s intervals from 5 to 30 s.     

Aminoglycoside ototoxicity

PURPOSE OF REVIEW: To summarize mechanisms of ototoxicity associated with aminoglycoside antibiotics and discuss possible protective strategies. RECENT FINDINGS: Studies in the past 15 years have demonstrated that aminoglycoside ototoxicity is mediated by an apoptotic form of cell death which employs caspase-dependent pathways. Reactive oxygen species have been demonstrated in the sensory epithelium after aminoglycoside administration and methods of blocking reactive oxygen species in the cochlea have been attempted, but not found to be uniformly effective in protecting against cell loss or threshold shift. Aspirin has recently been studied in a human chemoprevention trial in China, and while data suggest the possibility of protection, there was a significant increase in gastrointestinal bleeds associated with aspirin use. SUMMARY: There are currently no recommendations for pretreatment or posttreatment therapies to attenuate ototoxicity associated with aminoglycoside antibiotics. Our understanding of the mechanisms of ototoxicity has improved and apoptotic pathways are clearly responsible for hair cell demise. Further studies are necessary before significant improvement in hearing outcome can be expected after use of ototoxic antibiotics.     

Distortion product otoacoustic emissions in normal and hearing-impaired ears

Otoacoustic emissions of distortion products (DPOAE's) were recorded in normal and hearing-impaired human ears using relatively straightforward methods. Two pure-tone stimuli at fixed frequency levels of 73 dB HL for f1 and of 67 dB HL for f2 were used. The frequencies of the two primaries were chosen so that their geometric mean represented standard audiometric frequencies. Measurements of the emission amplitudes at 2f1-f2 and the adjacent noise floor were achieved by spectral averaging. A total of 101 subjects (199 ears) were tested. Seventy-seven ears in 46 subjects had normal hearing (hearing levels less than or equal to 20 dB at standard audiometric frequencies; average hearing levels, less than or equal to 10 dB). Thirty-six ears in 25 subjects had near-normal hearing (no hearing complaints, hearing levels less than or equal to 40 dB; average hearing levels, less than or equal to 20 dB). No significant differences in mean DPOAE values were apparent between these two groups of ears. All but two of these 113 ears (98%) showed emissions at three or more of the six frequencies tested between 1 and 6 kHz. Emissions were detected in more than 75% at each frequency between 1 and 6 kHz and in more than 85% between 1 and 4 kHz. A further 86 ears in 44 subjects exhibited varying degrees of sensorineural hearing loss caused by different pathologies. In general, emission amplitudes approximated the shapes of the audiograms, and a highly significant correlation between hearing thresholds and emission amplitudes was demonstrated in the frequency range of 1 to 4 kHz.(ABSTRACT TRUNCATED AT 250 WORDS)     

Distortion-product otoacoustic emission: early detection in deferoxamine induced ototoxicity

OBJECTIVE: Deferoxamine therapy in lifelong transfusion-dependent anaemias, as beta-thalassemia major, is associated with an increased risk of ototoxic changes. With increasing survival rates, prevention and/or early detection of ototoxicity are important for providing management options. The predictive value of pure-tone audiometry in early detection of ototoxicity has been questioned, particularly in the higher frequencies. Otoacoustic emissions appear to be more sensitive to cochlear insult than the conventional pure-tone audiometry. The aim of our study was to compare the efficacy of otoacoustic emissions (distortion-product otoacoustic emissions) with that of pure-tone audiometry as method of audiological monitoring. METHODS: Baseline audiometric (0.25-8kHz) and otoacoustic emission testing (distortion-product otoacoustic emissions) was conducted in a group of patients with beta-thalassemia major, 60 of whom met the criteria for inclusion in the study. Comparisons were performed between baseline measurements and those recorded after 20 months. Distortion-product otoacoustic emissions were obtained as DP-grams. The DP-gram amplitude was determined for each child. RESULTS: Threshold changes from baseline were found to be statistically significant from 4 to 8kHz in 68.4% of the subjects (P<0.01). Distortion-product otoacoustic emissions demonstrated a significant threshold shift and a decreased amplitude in the frequencies >3kHz (P<0.05). Furthermore, DP-gram amplitude also reduced significantly at 3kHz (P<0.05) without any similar change in pure-tone audiometry. CONCLUSIONS: As ototoxicity screening tool DP-gram was extremely sensitive and superior to pure-tone audiometry. Their use is recommended for regular monitoring of cochlear function, aiming in prevention of permanent damage.     

Evidence of subtle auditory deficit in a group of patients recovered from bacterial meningitis.

OBJECTIVES: Sensorineural hearing loss of greater than 30 dB hearing loss occurs in up to 30% of patients after acute bacterial meningitis. This study investigated whether postbacterial meningitic patients with no apparent clinical sensorineural hearing loss had any evidence of more subtle subclinical cochlear deficit. DESIGN: Prospective case-controlled clinical trial. SETTING: Departments of Otolaryngology in Leicester and Nottingham, England, UK. PATIENTS: Fifty-eight controls and 20 postbacterial meningitic patients aged between 18 and 38 years were screened by a questionnaire and tympanometry to exclude hearing loss attributable to other causes. All participants fell below the 90th percentile pure-tone audiometry threshold of the Lutman and Davis UK data sets. MAIN OUTCOME MEASURES: In both ears, standard (0.25-8 kHz) pure-tone audiometry, high-frequency pure-tone audiometry (10-16 kHz), and distortion product otoacoustic emissions at 2, 4, and 6 kHz were measured. RESULTS: Mean thresholds over the range of standard pure-tone audiometry (analyzed independently) for the postbacterial meningitic patients were significantly elevated at most frequencies (p < 0.05-p < 0.001) between 4 and 7 dB in both ears above control group values. There was no evidence of significant high-frequency threshold elevation (10-16 kHz). The mean iso-distortion product values at 2, 4, and 6 kHz were elevated in both ears in the meningitis group; significantly so (p < 0.05-p < 0.01) at all three frequencies in the right ear and at 4 kHz in the left. CONCLUSIONS: Postbacterial meningitic patients with hearing below the 90th percentile range had a slight but significant subclinical threshold elevation over the standard pure-tone audiometry. This may reflect a real effect of the infection at the level of the cochlea, or it may be attributable to a mild residual cognitive defect. The moderate increases in iso-distortion product values are more likely to be real and reflect an effect on outer hair cell function in response to lower stimulus intensities.     

Middle-latency responses to assess objective thresholds in patients with noise-induced hearing losses and Ménière's disease

In this study 23 patients with noise-induced hearing loss (NIHL) referred for medico-legal evaluation and a group of 18 cases with Ménière's disease were evaluated audiologically using the middle-latency response (MLR). Cross-correlation functions were used to assess precisely MLR thresholds in low and middle frequencies. Cross-correlation data obtained from suprathreshold levels to below threshold levels were compared with the normal limits of parameters found at threshold levels, allowing us to determine the true MLR threshold. Our results revealed that this MLR threshold and visual detection thresholds were different in 18% of both the NIHL and Ménìere's disease groups. In this population the true MLR threshold was greater by 5 dB. These findings demonstrated that cross-correlation functions can enhance the sensitivity of the definition of the MLR threshold. True MLR thresholds were compared with subjective pure-tone audiometric (PTA) thresholds at the same frequencies (0.5, 1 and 2 kHz). The true MLR threshold and PTA threshold were in agreement within 10 dB in 91% of the NIHL group and all of the Ménière's disease group. The PTA threshold was greater by 15 dB or more in the remaining NIHL group. If a criterion of 15 dB discrepancy indicates non-organic hearing loss, it can be inferred that 9% of an NIHL population referred for medico-legal evaluation is exaggerating subjective audiometric thresholds.     

Auditory characteristics of children with autism

OBJECTIVES: The objectives of this study were (1) to describe the auditory characteristics of children with autism relative to those of typically developing children and (2) to describe the test-retest reliability of behavioral auditory test measures with this population of children with autism. DESIGN: Audiometric data were obtained from 22 children diagnosed with autism and 22 of their typically developing peers. The audiologic test battery consisted of behavioral measures (i.e., visual reinforcement audiometry, tangible reinforcement operant conditioning audiometry, and conditioned play audiometry) and physiological measures (auditory brain stem response audiometry, distortion product otoacoustic emissions, and acoustic reflexes). RESULTS: Children with autism had physiologic test results equivalent to their typically developing counterparts. That is, no differences in auditory brain stem response audiometry, distortion product otoacoustic emissions, or acoustic reflex results were noted between the children with autism and typically developing children. However, behavioral measures revealed that about half of the children diagnosed with autism presented pure-tone averages outside of normal limits (i.e., >20 dB HL), although their response thresholds to speech were within normal limits. All behavioral test results were within normal limits (i.e., 20 dB HL) despite having normal to near-normal hearing sensitivity as determined by other audiometric measures.     

Sensorineural loss in chronic otitis media. Is it clinically significant?

Charts of 161 patients with unilateral chronic otitis media were reviewed for evidence of sensorineural hearing loss, defined as the difference in preoperative bone conduction thresholds between diseased and normal contralateral ears. Mean bone conduction differences were small but statistically significant, ranging from 5.6 to 12.8 dB across the frequencies. Approximately 45% of the subjects had differences greater than 10 dB for high frequencies, but less than 12% had a difference greater than 20 dB for the pure-tone average. Significant relationships were found between sensorineural hearing loss and the presence of acquired cholesteatoma in the middle ear, diseased mucosa of the promontory and hypotympanum, and diseased ossicles. These findings suggest that more severe middle ear disease may result in sensorineural hearing loss. However, for the majority of subjects, the amount of sensorineural hearing loss was judged not to be clinically significant.     

Hearing thresholds with outer and inner hair cell loss

Hearing impairment and related cochlear histopathologic changes were evaluated in experimental animals after treatment with aminoglycoside antibiotics or exposure to intense sound. In the course of treatment with kanamycin, neomycin, or dihydrostreptomycin, permanent hearing loss in monkeys and guinea pigs occurred first at the high frequencies and progressed toward the lows. Exposure to different octave bands of noise at 120 dB SPL in monkeys and chinchillas produced permanent hearing loss at frequencies related to the spectral characteristics of the octave band. In most instances loss of outer hair cells was substantially greater than that of inner hair cells. In fact, the pattern and location of missing outer hair cells on the basilar membrane were most often correlated with threshold shifts of 50 dB or less. Generally inner hair cell loss was observed when the threshold shift was greater than 50 dB. Our data support the place principle and the inference that the outer hair cells are essential for hearing from threshold to about 50 dB SL. The inner hair cells, if functioning normally, apparently take over above that level. Although there is little doubt that such a generalization will, in the long term, be found to have been greatly oversimplified, there is every reason to believe that a combination of behavioral and morphologic procedures, as used in this study, will play an important part in elucidating the differences in functional significance of the two types of hair cells.