Age-related changes in the compound action potentials of the eighth nerve in guinea pigs

The eighth nerve compound action potential (CAP) in 95 guinea pigs was measured using click stimuli to investigate age-related changes in their neural auditory thresholds. The animals were separated into three groups: group A (n = 43, 86 ears; 2–4 months old); group B (n = 29; 58 ears, 13–15 months old); and group C (n = 23; 46 ears, 23–25 months old). With increasing age, a gradual elevation of CAP thresholds was clearly seen among the three groups. The negative peak (N1) latencies of the CAP were prolonged, and the N1 amplitudes of the CAP decreased. There were significant differences in N1 latencies among the three groups and in N1 amplitudes between groups A and B, and between groups A and C. However, the rate of decline of the thresholds as well as the input-output function curves of the CAP varied in some of the oldest animals, suggesting that there were some individual differences in degenerative aging processes of the auditory system.     

乙状窦后进路暴露兔小脑脑桥角及直接听神经复合动作电位的测定

目的:探讨乙状窦后进路暴露兔小脑脑桥角以及测定直接听神经复合动作电位(directcompoundactionpotentialoftheauditorynerve,DCAP)的方法,观察DCAPN1波潜伏期的稳定性。方法:选取15只耳廓反射正常的新西兰大白兔,均以左耳为实验耳,麻醉后无菌状态下暴露小脑脑桥角并在面听神经复合体上插入针状记录电极,在插入电极后即刻、2h及5h行DCAP的测定,采用配对样本t检验对15只兔DCAPN1波潜伏期进行统计分析。结果:8侧耳3次均引出清晰的DCAP曲线,各波分化良好,15侧耳3次测得的N1波潜伏期之间差异无统计学意义。结论:本实验方法能成功地暴露兔小脑脑桥角及进行DCAP的测定,正常状态下个体的DCAPN1波潜伏期较稳定,在暴露小脑脑桥角的手术中,DCAPN1波潜伏期是较好的监控听觉损伤的指标。     

Effects of sympathetic stimulation on the round window compound action potential in the rat

The effect on the ear of stimulating the sympathetic nervous system was studied in rats by recording the compound action potentials (N1N2) in response to 2 kHz tonebursts presented to anesthetized rats before, during, and after electrical stimulation of the superior cervical ganglion, and evaluating the changes in N1 latency which resulted. Stimulation of the superior cervical ganglion was found to cause an increase in the N1 latency which was more pronounced at low stimulus intensities (mean value 0.09 +/- 0.04 ms (S.E.) at 5 dB above threshold) than at moderate stimulation intensities (0.08 +/- 0.04 ms at 15 dB above threshold), with little change in latency occurring at the highest intensity tested (0.02 +/- 0.01 ms at approximately 25-30 dB above threshold). In addition, individual animals varied in their responses to stimulation of the superior cervical ganglion, with some animals evidencing a great change in the latency of the response (0.4 ms increase at 10 dB above threshold) and others showing very little change in latency. This variability could not be related to the condition of the animal at the time of observation of the response. In one of the twelve animals there was a slight decrease in latency as a result of sympathetic stimulation (0.07 ms at 5 dB above threshold), and although not studied systematically, low frequencies seemed to be affected more than high frequencies. Further, the change in the amplitude of N1 was not systematically related to sympathetic stimulation. After the administration of hexamethonium (which blocks transmission in autonomic ganglia) in three rats there was no effect on the latency of the N1 potential from sympathetic stimulation as recorded before sympathetic stimulation.     

Anesthesia and surgical traum: their influence on the guinea pig compound action potential

A chronic implant consisting of a fine thermocouple placed on the round window permitted measurement of temperature and of the compound action potential (CAP) of the cochlear nerve in guinea pigs. Thresholds and latencies of the CAP, in response to tone bursts (2–40 kHz), were measured when the animal was awake and unrestrained, and again after several hours of anesthesia. The CAP remained unchanged with a variety of common anesthetics when precise control of round window temperature was maintained. However, when anesthesia was accompanied by several hours of slight cochlear cooling, thresholds were elevated for CAPs evoked by frequencies above 24 kHz and latencies were increased for CAPs evoked by all frequencies tested.

The effects of surgery on CAP threshold and latency were also examined. Guinea pigs were tested while still anesthetized at the conclusion of the implantation procedure, and then again several days later while awake. Thresholds and latencies were unchanged. In two anesthetized guinea pigs already implanted with thermocouples, ventral and post-auricular surgery to expose the middle ear had no effect on the CAP, when low-speed drilling was used to open the auditory bulla. However, when small portions of the bulla were broken away with forceps, the CAP in response to high-frequency tone bursts immediately showed elevated thresholds. This alteration of the CAP was clearly different from that produced by lowered temperature, since the latencies at threshold were significantly decreased.     

Single auditory nerve fiber and action potential latencies in normal and noise-treated chinchillas

Latency-intensity functions for the auditory nerve action potential (AP) and single auditory nerve fibers were measured in a group of normal and a group of noise-treated chinchillas using click stimuli. The noise-treated group was exposed for 5 days to an octave band of noise centered at 0.5 kHz and having an SPL of 95 dB. The AP visual detection level (VDL) and the single neuron thresholds were approximately 40–50 dB higher in the noise-treated group than in normals. At the VDL, the latency for AP was shorter in the noise-treated group than in the normal group; however, for the same intensity the latency was the same for both groups. Similar results were obtained from single fibers of similar characteristics frequency (CF). At threshold, the fiber latencies were shorter for the noise-treated group than for normal animals, but at the same intensity, the latencies for the two groups were similar. The results indicate that AP and single fiber latencies are a function of absolute intensity rather than intensity relative to a ‘neural threshold’. A comparison of poststimulus time (PST) histograms made at the same intensity revealed fewer peaks in the units from noise-treated animals than in normal units; however, the width of the peaks and the time between peaks appeared normal.     

Nucleus24型人工耳蜗使用者电诱发听神经复合动作电位的测定

目的 :探讨电诱发听神经复合动作电位 (ECAP)的特点及在人工耳蜗临床中的应用价值和意义。方法 :应用NRT(neuralresponsetelemetry)软件 ,通过体外言语处理器和耳蜗内的植入电极系统 ,采用单极模式电极刺激和近场记录方法 ,对 37例NucleusCI2 4M装置使用者进行ECAP的记录。对其中 12例使用者做了行为测试。结果 :87.6 %的使用者记录到ECAP波形。分析了有行为阈值的 12例测试者的ECAP阈值和行为阈值的关系 ,二者之间存在显著的相关性。结论 :ECAP的检出率高且波形稳定可靠 ,在临床人工耳蜗装置的调试中可作为对行为测试的补充 ;对于年幼儿童和首次开机的患者的调试尤其重要     

Threshold sensitivity of the auditory pathway: Effects of worsening signal-to-noise ratio in the auditory nerve

Perfusions of scala tympani with high potassium solutions increase spontaneous activity of auditory nerve fibres without affecting their threshold sensitivity. In these circumstances, however, the signal-to-noise ratio of the auditory nerve response to a given stimulus is worsened. Recordings from cochlear nucleus neurones during such perfusions indicate that this worsening is critical for threshold sensitivity at higher levels in the auditory pathway.     

多巴胺对豚鼠听觉传入神经的抑制作用及其频率选择性

目的 观察多巴胺对听觉传入神经的抑制作用及其频率选择性,为进一步探讨多巴胺在内毛细胞下突触复合体中的调控作用奠定基础.方法 健康杂色豚鼠40只,随机分为4组,行全耳蜗灌流:①灌流人工外淋巴液组;②灌流10 mmol/L多巴胺组;③灌流30 mmol/L多巴胺组;④灌流50 mmol/L多巴胺组.灌流过程中分别在第0 h、1 h和2 h通过蜗窗记录不同频率刺激声(250、500、1000、2000、4000、8000及16 000 Hz)所引出的复合动作电位(CAP)阈值和4000 Hz刺激声所引出的耳蜗微音器电位(CM)的幅值.结果 灌流人工外淋巴液前后各频率CAP阈值差异均没有统计学意义(P值均>0.05);灌流多巴胺后大部分频率的CAP阈值提高,与灌流前相比差异具有统计学意义(P值均<0.05),而且随着多巴胺浓度的增加,其抑制作用也逐渐增强;灌流后各组内不同刺激声频率间的CAP阈移存在差异,灌流30 mmol/L多巴胺时最明显,该组中4000 Hz和8000 Hz处阈移最大.灌流人工外淋巴液和多巴胺对CM的非线性没有明显影响,灌流前后CM的相对幅值差异没有统计学意义(P值均>0.05).结论 多巴胺对豚鼠听觉传入神经具有抑制性作用,而对外毛细胞无影响;这种抑制作用具有频率选择性,对高频纤维的抑制作用较强,而对低频的抑制作用较弱.     

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.     

Eighth-nerve action potentials evoked by tone bursts in cats before and after inducement of an acute noise trauma

Properties of eighth-nerve action potentials (AP) evoked by single-frequency tone-bursts (test tone) were studied in cats. Curves representing AP threshold as a function of test-tone frequency have a shape similar to behavioral and single-fiber threshold curves. The absolute level of AP thresholds is higher than that of behavioral and single-fiber thresholds. Cats were exposed to broad-band noise (equal intensities per octave) halfway into the experiments. This exposure resulted in a long-term temporary threshold shift (TTS) which remained fairly steady during the measurements. APs were measured before and during an acute noise trauma in the same animal. After inducement of the trauma the greatest threshold shift is found between 2 and 6 kHz. Curves representing AP amplitude as a function of stimulus SPL are displaced to higher stimulus SPLs. Sometimes the slope of the curve is steeper after the noise exposure than before. AP latency at threshold did not change due to the excessive noise exposure. AP-latency values compared at equal sound pressure levels before and after inducement of the trauma showed higher values during the trauma than before.     

Latencies of eighth nerve fibre responses with respect to their relative contribution to the compound action potential in the guinea pig

Summary We present a study of the latencies of click-evoked post-stimulus time histograms (PSTHs) in the guinea pig in the context of the auditory nerve fibre's contribution to the compound action potential (CAP) recorded at the round window. The latencies of the dominant PSTH peak were studied as a function of relevant physiological fibre properties, in particular the characteristic frequency (CF) and the spontaneous discharge rate (SR). We found that high-SR fibres have shorter latencies than low-SR fibres. These findings are discussed in the context of correlation between synaptic morphology and SR as described in the literature. The PSTH latency as a function of CF is described separately for low- and high-CF fibres for each of the two SR sub-groups. Finally, we discuss to what extent the various subgroups of fibres contribute to the N₁ peak of the CAP, the most commonly studied component.Presented at the 25th Workshop on Inner Ear Biology in London, England, 4–7 September 1988     

Electrically evoked compound action potential (ECAP) in cochlear implant children: Changes in auditory nerve response in first year of cochlear implant use

AimRecording of the electrically evoked compound action potential (ECAP) of the auditory nerve in cochlear implant (CI) patients represents an option to assess changes in auditory nerve responses and the interaction between the electrode bundle and the neural tissue over time. The aim of the present work is to assess ECAP changes during the first year of cochlear implant for the purpose of predicting thresholds and adjustment of the patients’ programs over time.MethodData were collected from 25 children using Cochlear Nucleus 24 implants. ECAP thresholds were examined at the time of surgery, at initial stimulation, and 3, 6 and 12 months post-stimulation. Five electrodes located at basal, middle, and apical positions in the cochlea were tested at each time interval and ECAP thresholds were analyzed and compared.ResultsThere was a significant decrease in ECAP thresholds between the intraoperative measure and fitting time at all electrode sites. Mean ECAP thresholds measured at 3, 6 and 12 months post-stimulation remained similar to initial stimulation levels. Although there was no significant difference in ECAP thresholds recorded at fitting time and 12 months follow up session, there was significant increase in behavioral T and C levels from initial stimulation to the 12 months’ time point.ConclusionMost electrodes undergo non-significant change in ECAP thresholds over time, and therefore thresholds obtained on the day of initial stimulation can be used to estimate the patients’ map levels at any time. On the other hand, intraoperative thresholds demonstrated significant change relative to postoperative recording times, limiting the ability to use intraoperatively recorded ECAP thresholds to predict postoperative measurements.     

Age-dependent changes of the compound action potential in the guinea pig

Summary As a measure of age-related changes in the most peripheral neural part of the auditory pathway, the compound action potential of the guinea pig was analyzed. In addition to a marked threshold elevation, there was a significantly lower potential amplitude in old animals. By contrast, the latency of the compound action potential was unchanged. In view of the fact that the relative amplitude increase in the intensity range tested was the same in old as in young animals, the implication is that the auditory-nerve neurons that are still excited do not exhibit functional deterioration with aging.With the support of the Deutsche Forschungsgemeinschaft (DFG)     

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.     

Effects of temporal properties on compound action potentials in response to amplitude-modulated electric pulse trains in guinea pigs

The electrically evoked compound action potential (ECAP) of the auditory nerve in response to amplitude-modulated pulse trains varies over time, but the response amplitudes are not linearly proportional to the level of stimulus pulses. At least two mechanisms could contribute to the deviations of the ECAP response pattern from that of the stimulus envelope. The first mechanism is time-invariant or stationary that reflects the non-linear growth of response amplitude with changes in stimulus level that is evident in the response to single pulses. This can be considered a time-invariant or stationary effect. The second mechanism is time-variant or non-stationary and reflects neural refractoriness and adaptation. The purpose of this study was to characterize the auditory nerve responses to amplitude-modulated pulse trains and also to evaluate the extent to which the stationary and non-stationary effects may contribute to those responses. ECAP amplitudes were predicted from single-pulse growth functions of the auditory nerve to account for time-invariant effects. Linear regression was performed on the measured vs. predicted ECAP amplitudes to quantify the discrepancies between the two datasets, thereby separating the influence of non-linear growth from time-varying effects on ECAP amplitudes. The results demonstrated a bandpass function of the modulated response amplitudes, with a low-cutoff modulation frequency at 300 Hz and a high-cutoff modulation frequency at 800 Hz, depending on the carrier pulse rate. The relative contribution of the temporal effects on ECAP amplitudes is greatest at low stimulus levels and low modulation depths.     

Frequency threshold curves and simultaneous masking functions in single fibres of the guinea pig auditory nerve

Tuning curves for simultaneous masking were measured electrophysiologically, in single fibres of the guinea pig auditory nerve. The masking paradigm used was an analogy of that used in the determination of psychophysical tuning curves in man. The resulting tuning curves ran nearly parallel to the corresponding neural frequency threshold curve, over all except the high-frequency portion of the tuning curve. There, the masking functions had a shallower slope than the excitatory frequency threshold curve. The frequency at which the slope became shallower had a close and consistent dependence on fibre characteristic frequency, reaching a value of 1.2 times fibre characteristic frequency in high-frequency fibres. Analysis of firing rates during the threshold determination gave information about the mechanism of the masking, and the results were supported by theoretical analysis. The results give information useful for the interpretation of psychophysical tuning curves, determined by simultaneous masking, in man.     

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.     

Electrophysiological study of the ototoxicity of kanamycin during development in guinea pigs

In order to test the ototoxicity of antibiotics during development, pregnant guinea pigs were intoxicated with kanamycin at different stages of gestation. Eighty newborn guinea pigs were tested electrophysiologically by recording the cochlear microphonic potential and the compound action potential from the round window in response to tone bursts and filtered clicks of various frequencies. Thirty-one animals presented antibiotic injury and showed electrophysiological changes similar to those previously described in adult mammals after kanamycin intoxication.The results suggest a relationship between the ototoxicity of kanamycin and the onset of the auditory function: cochlear potentials were mostly affected when the intoxication was performed during the last 15 days of gestation.     

Frequency threshold curves and simultaneous masking functions in high-threshold, broadly-tuned, fibres of the guinea pig auditory nerve

Tuning curves for simultaneous masking were measured electrophysiologically, in single fibres of the auditory nerve. The recordings were made in guinea pigs with cochlear hearing losses. The masking paradigm was an analogy of that used in the determination of psychophysical tuning curves in man. Below the probe frequency, the slope of the masking function was similar to that of the frequency-threshold curve. Here, changes in the slope of the frequency-threshold curve with hearing loss were closely mirrored by changes in the slope of the masking function. Above the probe frequency, however, the masking function for low-threshold fibres had a shallower slope than the frequency-threshold curve. When the high-frequency slope of the frequency-threshold curve became shallower with hearing loss, the changes were not mirrored in the masking function, until the threshold was raised to 70-80 dB SPL. The results are discussed in terms of the influence of cochlear nonlinearity on frequency resolution, and the vulnerability of the nonlinearity.