Development of cochlear frequency selectivity tested by compound action potential tuning curves

Gross cochlear action potential tuning curves were recorded in kittens and rat pups to evaluate the development of cochlear frequency selectivity. In both species, cochlear tuning properties matured the same way and the appearance of adult-like sharp tuning curves coincided with the last histological stages of receptor development at the outer hair cell level.     

Relation between changes in compound action potential tuning curves and the pathology of cochlear hair cells stereocilia]

Relations of the changes of compound action potential tuning curve (AP-TC) and AP(N1) threshold to the pathology of the cochlear hair cells stereocilia were examined in acoustic-traumatized guinea pigs. The results showed that all animals with hair cell stereocilia damaged showed deteriorations of AP-TC. It supports the suggestion that mechanical properties of hair cell stereocilia contribute to the frequency selectivity of cochlea. After exposure, the characteristic frequency (CF) shift of AP-TC was observed. The mechanism of CF shift may be related to the lesion of hair cell stereocilia which may change normal tono-topic on cochlear level. Correlation between the damages of inner/outer hair cell stereocilia and changes of AP-TC and AP threshold suggested that the threshold and tuning properties of the cochlea are dependent upon the integrity of the inner and outer hair cells and their inter-relationship.     

Correlation of changes in compound action potential (CAP) tuning curves and cochlear lesion in guinea pigs after explosion

The purpose of the present study was to investigate the sensitivity of compound action potential (CAP) tuning curves to changes of the cochlear status in guinea pigs after explosion and their ability to reflect specific histological variations. The results were as follows: 1. The CAP tuning curves were abnormally broad and the Q 10 dB values were reduced by a factor of 1 after explosion, indicating wider tuning. 2. The degree of broadening of the CAP tuning curves seemed to increase as the hair cell loss increased. 3. After explosion, the tip of the tuning curve shifted to frequencies significantly higher or lower than that of the signal, it might be related to the location of hair cell loss in the cochlea. 4. In animals for which damage was restricted to only three rows of outer hair cells, changes of the CAP tuning curves were observed. It provides further evidence that the tuning properties of cochlear nerve fibers are dependent upon the integrity of the outer hair cells even though the great majority of fibers innervate inner hair cells only.     

Latency and amplitude tuning curves of the N1 and N2 components of the cochlear nerve compound action potential

Compound action potential tuning curves (CAP TCs) generated by masking the N1 component of the CAP provide a means of assessing the ability of the cochlea to selectively tune to certain stimuli. This paper examines some of the factors which can influence this TC when a moderately intense (i.e. 40-80 dB SPL) probe stimulus is used. At these levels, each of the four corners of the trapezoidal stimulus envelope is capable of generating a CAP. Also, short stimulus rise times can merge the CAPs produced by the first two corners, but this does not appear to have a major effect on the CAP TC. It was shown that the N2 component of the CAP for the first corner of the stimulus is equally capable of producing a well-tuned TC. Another study has shown that, in addition to amplitude decrements, one can use latency increases as a criterion for CAP TCs. We have demonstrated that latency TCs are more finely tuned than amplitude TCs at high levels, especially when the stimulus rise time is short.     

Effects of submarine engine room steady noise on the compound action potential tuning curves and its relation to cochlear pathology in guinea pigs

Compound Action Potential Tuning Curves (CAP-TC) for tone pip of 2k, 4 kHz were examined in 8 guinea pigs before and after exposure to noise with main energy centered in the range of 0.25-4.0 kHz. CAP-TC was measured with the pure tone simultaneous masking profiles. AP was evoked by tone pip with an intensity of 10 dB above threshold. Masker level producing 40% reduction in AP amplitude was used. Relations between changes in CAP-TC and AP threshold shifts and the pathology of the stereocilia of hair cells were evaluated by surface preparation and SEM observation in 13 ears. After noise exposure, animals with damaged stereocilia showed AP threshold shift of 20-50 dB, deteriorations of CAP-TC, decrease of Q10 dB value, threshold shift of characteristic frequencies (CF) and displacement of CF towards higher frequencies. It showed that stereocilia damage may affect the susceptibility and frequency selectivity of the cochlea. We consider the CAP-TC may be an useful and sensitive index for detecting physiological and pathological conditions of the cochlea.     

Latency and amplitude compound action potential tuning curves for tonal stimuli with nontraditional envelopes

To evaluate the influence of the acoustic context on the latency and amplitude tuning curves (TCs) of cochlear nerve compound action potentials (CAPs), tonal stimuli were generated with a variety of amplitude modulation envelopes. CAPs were produced by intensity increases (onsets) and decreases (offsets) from the low ambient sound level and by intensity changes from a preexisting tonal level. Onset CAPs from ambient levels generated V-shaped TCs. However, when simultaneous masking was used with onset CAPs which were produced by a 5- to 12-dB increase from preexisting levels of approximately 65 dB SPL, TCs were W-shaped and similar in appearance to those produced by simultaneous masking of offset CAPs. The forward masking of this same CAP resulted in a very sharp V-shaped TC. These data suggest that the preadaptation to 10 ms of a moderate level of a tonal stimulus can increase the tuning of ensembles of cochlear neurons to subsequent transient amplitude changes.     

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.     

The influence of hearing loss on directional hearing

Fifty patients with cochlear hearing loss were tested with phase audiometry. Thirty-three of the patients had unilateral hearing loss at 500 Hz and 17 bilateral, symmetrical hearing loss at 500 Hz. Those patients who had slight hearing loss (less than 40 dB HL) had as a rule normal directional hearing. Those who had hearing loss exceeding 40 dB HL at 500 Hz usually had abnormal directional hearing. The sound lateralization ability was independent of whether the hearing loss was unilateral or bilateral.     

Frequency-specific enhancement of the cochlear compound action potential: influence of the forward masker.

Under restricted frequency and intensity conditions, forward masking can result in the amplitude of the CAP being increased above its unmasked value (Henry, 1991). The present study provides a quantitative analysis of this enhancement effect. In response to forward maskers having the same frequency as the probe stimulus, central frequency (CF) enhancement varies as a function of the level of the forward masker: the lowest masker level at which it can reliably be detected is often well below the visual detection threshold of the CAP generated by the unmasked probe stimulus; the highest masker level at which it can reliably be detected corresponds to approximately 10 dB above the probe stimulus CAP threshold. A second low frequency (LF) enhancement region also exists, encompassing a narrow range of more intense maskers. CF enhancement can double the amplitude of the CAP, whereas LF enhancement is less pronounced. The magnitude of CF enhancement varies as a function of the duration of the forward masker, with longer durations generally increasing the magnitude of the effect. This duration effect, however, interacts with the level of the stimulus. Decreasing the interval between the end of the forward masker and the beginning of the probe increases the magnitude of CF enhancement.     

Enhancement of the cochlear nerve compound action potential: sharply defined frequency-intensity domains bordering the tuning curve.

Forward masking can either decrease or increase the response to a subsequent stimulus. In the gerbil, frequency-intensity domains of the maskers that decrease the amplitude of the compound action potential (CAP) can be plotted as the sharply defined CAP tuning curve (TC). Regions were also found over which masking increases (enhances) the amplitude of the CAP. Center-frequency (CF) enhancement domains were found in approximately 2/3 of the animals tested, in response to maskers having frequencies very near that of the probe stimulus, at levels ranging from below the CAP detection threshold to just below the tip threshold of the TC. Approximately 2/3 of the animals showing CF enhancement also displayed low-frequency (LF) enhancement, in response to a domain which borders the low-frequency tail of the TC.     

Psychoacoustic tuning curves and averaged electroencephalic responses in a patient with low-frequency sensory-neural hearing loss

A 10-year-old girl whose bilateral, low-frequency, sensory-neural hearing loss had been noted three years earlier showed a drop in speech discrimination in her left ear with no corresponding decrease in pure-tone sensitivity. Psychoacoustic tuning curves and middle-component averaged electroencephalic responses to tone-pips suggested that damage to the left ear had become greater than indicated by the pure-tone audiogram, accounting for the drop in speech discrimination. This case suggests caution in inferring the magnitude of damage to the peripheral auditory system from tonal thresholds.     

The influence of moderate-intensity noise on the compound action potential evoked by tone bursts in the guinea pig, Cavia porcellus

Noise-induced changes in the compound action potential (CAP) evoked by tone bursts in the frequency range 0.5-24 kHz were studied in 15 pigmented guinea pigs by means of chronically implanted electrodes positioned near the round window. The animals were exposed for 120 h to continuous pink noise at the intensities 80, 90 and 100 dB SPL. During the exposure period, all the animals exhibited an exponential rise in CAP threshold, leveling out after 24-72 h (asymptotic threshold shift, ATS). The largest threshold shifts were recorded during exposure to 100 dB SPL, for frequencies in the range 8-12 kHz. In the recovery phase, after the end of noise exposure, the threshold to tones at all frequencies tested fell exponentially, reaching the original level in about 72 h in all cases.     

Corresponding effects of hypoxia on the cochlear microphonic and the compound action potential

Mild hypoxia has an unexpected influence on the compound action potential (CAP) compared to its effect on the cochlear microphonic (CM). While the CM decreases in amplitude near threshold, the low-level CAP increases in amplitude by as much as 400% and decreases in latency and width. The magnitude of latency decrease is dependent on the center frequency of $\text{1}\text{3}$ octave band-filtered clicks used as stimuli. Below 13500 Hz the latency is increasingly shortened at a rate of 0.073 ms/octave. At high intensities moderate hypoxia has no effect on either the CM or the CAP. However, with more severe hypoxia the high-level CM also begins to deteriorate. Correspondingly both the high- and low-intensity CAP decrease in magnitude and increase in latency relative to their control values. Assuming that the low-level CM is generated by outer hair cells, these results suggest that one relationship between outer hair cell function and auditory nerve function near threshold is that of inhibition. The fact that N₁ latency is shortened suggests that this inhibition occurs in the basalward direction.     

人工耳蜗植入后对侧耳是否联合使用助听器声调识别的对比研究

目的 探讨人工耳蜗植入后对侧耳联合使用助听器对声调识别的影响;同时探讨人工耳蜗开机时间、康复训练时间、听障儿童的学习能力等因素对声调识别的影响。 方法 选择3~7岁的人工耳蜗植入儿童27例,采用《言语听觉反应评估》(EARS)中的“封闭式声调测试”作为测试材料,分别单独使用人工耳蜗(CI)和在人工耳蜗对侧耳联合使用助听器(CIHA)在安静和噪声两种环境下进行声调识别的测试,使用SPSS 17.0对测试结果进行统计学分析。 结果 无论在安静环境还是噪声环境下,CIHA的声调测试成绩均优于CI,此优势在噪声环境下更明显;人工耳蜗开机时间、康复训练时间较长者声调测试成绩均优于较短者;听障儿童的希-内学习能力测试结果学习能力DIQ≥100与DIQ<100者声调测试成绩差异无统计学意义。 结论 建议听障儿童人工耳蜗植入后对侧耳联合使用助听器,但助听器需要优化和调整。     

Corresponding effects of acoustic fatigue on the cochlear microphonic and the compound action potential

Moderate levels of acoustic fatigue, usually induced by a 100 dB SPL pure tone at a frequency appropriate to the location of intracochlear differential electrodes, have a surprising and paradoxical influence on the cochlear microphonic and the compound action potential of the auditory nerve. While the low-level microphonic becomes smaller, the low-level action potential becomes considerably larger and exhibits a shortened latency. The high-level microphonic and the high-level action potential are left virtually unchanged at these levels of fatigue. Nonetheless, if the duration or intensity of exposure is increased, both the high-level and low-level action potentials decrease as well. Assuming that the low-level microphonics are generated by outer hair cells, these data suggest that one relationship between outer hair cell function and neural function is inhibition at low intensities. Assuming that the high-level microphonic tends to be generated by inner hair cells, the decrease in the action potential at all intensities whenever the high-level microphonic potential is impaired suggests that the functional relationship between inner hair cells and auditory nerve function is excitatory.     

The behaviour of wave V latency in cochlear hearing loss

The audiological approach to the early diagnosis of cerebellopontine angle tumours (APC) is based mainly on ABR. In present work, wave V latency has been studied in two groups of patients: 308 cochlear cases and 74 retrocochlear cases (APC surgically confirmed tumours), in order to increase the sensitivity and specificity of the diagnostic indexes I-V, IT5 and Delta V. Wave V latencies have been evaluated in relation to hearing loss at 2-4 kHz and audiometric profile. Both these factors show a highly significant positive correlation with the latency, which consequently increases proportionally to hearing loss and high-frequency audiometric loss. A multiple regression analysis was therefore used to analyse the effects from the two variables, and a correction factor was calculated to compensate the latency values for hearing loss and threshold configuration. The effects of such a correction on the clinical results consist mainly of a reduction in the rate of cochlear false-negative results, which corresponds to improving the ABR specificity.     

极重度感音神经性聋患者耳蜗微音电位测试研究

目的:报告极重度感音神经性聋患者人工耳蜗植入术中圆窗耳蜗微音电位(CM)测试结果.方法:40例患者全身麻醉下人工耳蜗植入手术过程中,行术中圆窗CM测试,测出CM与术前畸变耳声发射(DPOAE)结果进行比较.结果:40例患者术中圆窗CM 0.5、1.0、2.0、4.0 kHz引出率分别为90.0%、97.5%、100.0%、92.5%;术前DPOAE 0.5、1.0、2.0、4.0 kHz引出率分别为2.5%、2.5%、17.5%、2.5%.结论:同一极重度感音神经性聋患者CM引出率明显高于DPOAE引出率,术中圆窗CM测试可以较准确客观地评估极重度感音神经性聋患者的部分耳蜗功能.