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.     

The influence of cochlear hearing loss and probe tone level on compound action potential tuning curves in humans

The effect of cochlear hearing loss and of probe tone level on slopes and sharpness of compound action potential tuning curves was investigated. Thirty-one simultaneously masked isoreduction (50%) tuning curves were determined in 26 adults with cochlear hearing losses up to 60 dB. Probe tone frequency was 2 or 3 kHz. Probe tone level was chosen as close as possible to the action potential threshold, usually within 30 dB. In 5 cases a second tuning curve was determined at a 20–30 dB higher probe tone level in order to differentiate between effects of hearing loss and of probe tone level itself on decrease of selectivity. Tuning was analysed in terms of high- and low-frequency slopes of the tuning curves, both in the steepest parts near the tip and overall, and in terms of Q_10dB. Slopes and tuning quality diminished with increasing hearing loss up to 60 dB. Part of the decrease in Q₁₀ could be attributed to increased probe tone level, implying that frequency selectivity is also a level-dependent property. In the same group of subjects so called ‘narrow-band’ (or ‘derived response’) compound action potential latencies were determined at 90 dB per SPL and a derived frequency similar to the probe tone in the tuning curve experiments. Narrow band latencies did not change significantly out of the normal range (2 periods) with increasing hearing loss. This implies that narow band latencies are not related to hearing loss, but reflect only the probe-level dependent impulse response delay. Analysis shows that it is possible to derived Q_10dB from narrow band latencies with probe level as a parameter.     

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.     

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.     

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.     

Derived and enhanced compound action potentials at near-threshold levels: forward masking increases sensitivity of audiograms and tuning curves.

The amplitude of a cochlear nerve compound action potential (CAP) can be increased by forward maskers having levels close to the visual detection threshold of the CAP. This effect, termed enhancement, varies as a function of the frequency of the masker and probe stimulus, and is nonmonotonic with respect to the level of the masker. Other studies using the derived CAP have used a subtraction technique to evaluate the ability of simultaneous maskers having levels near the CAP visual detection threshold to influence the CAP produced by an above threshold tone. The present paper compares audiograms produced by the conventional nonmasked CAP visual detection threshold technique with audiograms produced by both forward masked derived CAPs and forward masked enhanced CAPs. In response to low and middle frequency stimuli, both masked CAP measures produce more sensitive audiograms than does the conventional nonmasking method. Forward masked amplitude tuning curves (TCs) were also produced, comparing the conventional 50% amplitude reduction and 20 microV amplitude reduction methods with TCs obtained with derived and enhanced CAPs. When the same criteria are used, both masked CAP measures result in sharply tuned amplitude TCs that are approximately 60 dB more sensitive than the conventional CAP technique. At near-threshold levels, the properties of forward masked enhanced and derived CAPs appear to be similar.     

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.     

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.     

Action potential suppresion, tuning curves and thresholds: comparison with single fiber data

The collection of compound action potential (AP) threshold curves and their use to define the sensitivity of individual animals are described. Forward masking AP tuning curves (APTC) have also been collected in the chinchilla. Characteristics of APTCs are compared with single fiber frequency-threshold curves (FTC) in the same group of animals. The two sets of data are quite similar when the probe frequency used to collect the APTC is equated with a fiber's characteristic frequency (CF). The major difference is that APTCs are usually broader than FTCs. A paradigm utilizing two maskers in a forward masking situation, developed to study psychophysical unmasking [19,37], has been modified for measuring AP suppression. AP suppression areas are described as similar to single fiber two-tone suppression areas when probe frequency and CF are above 3 kHz. Relationships among single fiber, AP and psychophysical thresholds, tuning curves and suppression areas are discussed.     

Relation between the waveform of the cochlear whole nerve action potential and its intensity function

The whole-nerve action potential (AP) was recorded by intracochlear electrodes in the guinea-pig. As is well known, in normal conditions, the AP elicited by tone bursts displays negative and positive deflections. Inactivation of the central end of the nerve at the internal auditory meatus (IAM), by introduction of a few drops of KC1 solution, or by mechanical pressure, produces a change in the wave shape of AP which is transformed into a single negative deflection. The relation of the amplitude of the monophasic AP to the intensity of the tone burst is monotonic, in contrast to the classical two-slope variation observed in normal conditions. These results are interpreted by the disappearance of a positive component of the response produced at the IAM. The convolution of the monophasic unit AP would explain the monotonic intensity function.     

Hydrogen clearance and cochlear microcirculation at different levels of blood pressure

To study the autoregulative capabilities of the cochlear vessels, hydrogen clearance (HC) measurements were carried out by means of various blood pressure levels in hemorrhagic hypotension. H2-sensitive platinum--needle electrode were applied and a polarographic microanalysis method was used. The measurements were performed in the scala media of anesthetized cats during measuring of blood pressure and blood gases. The statistical analysis of the experimental data shows that the relationship between the HC and aortic blood pressure represents a regression line. According to the regression line the half-life time of HC was delayed for 0.7 min, when the blood pressure was reduced by 10 mm Hg (40-150 mm Hg blood pressure range). This corresponds to a cochlea flow rate of 0.3 ml/100 g/min. By mean aortic blood pressure of 40 mm Hg, the speed of H2 exchange still remains about 50% of the initial clearance. This indicates that the cochlear vessels have autoregulative capability. This autoregulative capability differs from the saturation curves of most autoregulated blood flow in the brain, heart and kidney vessels. These findings are in accordance with the observations of other authors. As our experimental data show, and as long as an analogy can be drawn between man and cat, it is hardly possible to explain that the fall of the blood pressure alone could lead to a disturbance of the oxygen supply to the inner ear, as often discussed in certain inner-ear dysfunctions.     

Hydrogen clearance and cochlear microcirculation at different levels of blood pressure

Summary To study the autoregulative capabilities of the cochlear vessels, hydrogen clearance (HC) measurements were carried out by means of various blood pressure levels in hemorrhagic hypotension. H₂-sensitive platinum — needle electrodes were applied and a polarographic microanalysis method was used. The measurements were performed in the scala media of anesthetized cats during measuring of blood pressure and blood gases. The statistical analysis of the experimental data shows that the relationship between the HC and aortic blood pressure represents a regression line. According to the regression line the half-life time of HC was delayed for 0.7 min, when the blood pressure was reduced by 10 mm Hg (40–150 mm Hg blood pressure range). This corresponds to a cochlea flow rate of 0.3 ml/100 g/min. By mean aortic blood pressure of 40 mm Hg, the speed of H₂ exchange still remains about 50% of the initial clearance. This indicates that the cochlear vessels have autoregulative capability. This autoregulative capability differs from the saturation curves of most autoregulated blood flow in the brain, heart and kidney vessels. These findings are in accordance with the observations of other authors. As our experimental data show, and as long as an analogy can be drawn between man and cat, it is hardly possible to explain that the fall of the blood pressure alone could lead to a disturbance of the oxygen supply to the inner ear, as often discussed in certain inner-ear dysfunctions.Part of an M.D. thesis     

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.     

The relative importance of amplitude,temporal, and spectral cues for cochlear implant processor design

Speech understanding with cochlear implants has improved steadily over the last 25 years, and the success of implants has provided a powerful tool for understanding speech recognition in general. Comparing speech recognition in normal-hearing listeners and in cochlear-implant listeners has revealed many important lessons about the types of information necessary for good speech recognition--and some of the lessons are surprising. This paper presents a summary of speech perception research over the last 25 years with cochlear-implant and normal-hearing listeners. As long as the speech is audible, even the relatively severe amplitude distortion has only a mild effect on intelligibility. Temporal cues appear to be useful for speech intelligibility only up to about 20 Hz. Whereas temporal information above 20 Hz may contribute to improved quality, it contributes little to speech understanding. In contrast, the quantity and quality of spectral information appear to be critical for speech understanding. Only four spectral "channels" of information can produce good speech understanding, but more channels are required for difficult listening situations. Speech understanding is sensitive to the placement of spectral information along the cochlea. In prosthetic devices, in which the spectral information can be delivered to any cochlear location, it is critical to present spectral information to the normal acoustic tonotopic location for that information. If there is a shift or distortion of 2 to 3 mm between frequency and cochlear place, speech recognition is decreased dramatically.     

Effects of body mass index and adenotonsillar size on snoring sound intensity levels at highest power

Objectives

Snoring during sleep is a major clinical symptom of adenoid and tonsil hypertrophy in paediatric patients. The aim of this study was to determine the effects of adenoid and tonsil size on snoring sound frequency and intensity in children.

Methods

Twenty-seven patients with adenotonsillar hypertrophy were included the study. Adenoid size was graded from 1+ to 4+ by rigid endoscopy. Patients were staged (I–III) according to body mass index (BMI) and tonsil and adenoid size. Snoring was recorded and analysed. The analysis focused on the highest power frequency (Fmax) and snoring sound intensity levels (SSILs).

Results

SSIL and Fmax values for Stage III were significantly higher than those for Stages I and II. BMI for Stage III was higher than for Stages I and II, and that for Stage II was higher than for Stage I. The BMI, SSIL, and Fmax values increased at each stage and tonsil/adenoid grade.

Conclusions

SSIL seems to be related to Adenoid and Tonsils size and BMI. As stage increased, both Fmax and SSILs increased proportionally. Also, Fmax values shifted to higher frequencies. Physicians and parents should be aware of snoring, and be informed that a higher frequency and intensity may be related to obesity and/or adenotonsillar hypertrophy. Snoring analysis may be a useful tool for detecting cases of Adenoid and Tonsils hypertrophy and/or upper airway obstruction during sleep.     

Compound action potential offset and onset tuning curves generated by simultaneous masking in the mongolian gerbil. Effects of varying the intensity of the probe stimulus from 55 to 85 dB SPL

Action potential tuning curves (TCs) were produced by simultaneous masking of both the onset and offset of tone bursts. Offset TCs are much more sharply tuned than onset TCs when both responses are generated by stimuli at SPLs of 55 to 75 dB. The onset TC tip and the offset TC high frequency tip thresholds accurately reflect increases of the probe SPL, but the low frequency tip and the peak of the offset TC compress this change. With increasing probe SPL, the onset TC and low frequency TC tip (but not the high frequency tip and peak) of the offset TC become progressively detuned.     

Spatial spread of neural excitation: comparison of compound action potential and forward-masking data in cochlear implant recipients

The feasibility of using the electrically evoked compound action potential (ECAP), measured with the NRT system (Neural Response Telemetry) and the NRT software (version 2.04), to quantify the longitudinal spread of neural excitation was examined in four subjects fitted with the Nucleus C124M cochlear implant. The ECAP and psychophysical forward-masking profiles were measured using stimulation on each of three electrodes, in basal, middle and apical positions. Spatial spread profiles derived from the ECAP measure produced broader functions than those derived from the psychophysical measure. These results, together with investigation of the change of ECAP spread pattern with stimulation current, suggest that functions derived from this 'simple' method were more influenced by the spread of electric field from excited neuron to electrode array than by breadth of the neural excitation pattern. The recently released NRT version 3.0 permits the masker and probe pulses to be delivered to separate electrodes, thus removing a fundamental limitation of version 2.04. Results from a pilot study, in which this capability was exploited, suggest that the spatial functions from this 'advanced' method may provide a better correlation with results from psychophysical forward masking.     

Spatial spread of neural excitation: comparison of compound action potential and forward-masking data in cochlear implant recipients

The feasibility of using the electrically evoked compound action potential (ECAP), measured with the NRT system (Neural Response Telemetry) and the NRT software (version 2.04), to quantify the longitudinal spread of neural excitation was examined in four subjects fitted with the Nucleus CI24M cochlear implant. The ECAP and psychophysical forward-masking profiles were measured using stimulation on each of three electrodes, in basal, middle and apical positions. Spatial spread profiles derived from the ECAP measure produced broader functions than those derived from the psychophysical measure. These results, together with investigation of the change of ECAP spread pattern with stimulation current, suggest that functions derived from this ‘simple’ method were more influenced by the spread of electric field from excited neuron to electrode array than by breadth of the neural excitation pattern. The recently released NRT version 3.0 permits the masker and probe pulses to be delivered to separate electrodes, thus removing a fundamental limitation of version 2.04. Results from a pilot study, in which this capability was exploited, suggest that the spatial functions from this ‘advanced’ method may provide a better correlation with results from psychophysical forward masking.     

High levels of sound pressure: acoustic reflex thresholds and auditory complaints of workers with noise exposure

IntroductionThe clinical evaluation of subjects with occupational noise exposure has been difficult due to the discrepancy between auditory complaints and auditory test results. This study aimed to evaluate the contralateral acoustic reflex thresholds of workers exposed to high levels of noise, and to compare these results to the subjects’ auditory complaints.MethodsThis clinical retrospective study evaluated 364 workers between 1998 and 2005; their contralateral acoustic reflexes were compared to auditory complaints, age, and noise exposure time by chi-squared, Fisher's, and Spearman's tests.ResultsThe workers’ age ranged from 18 to 50 years (mean = 39.6), and noise exposure time from one to 38 years (mean = 17.3). We found that 15.1% (55) of the workers had bilateral hearing loss, 38.5% (140) had bilateral tinnitus, 52.8% (192) had abnormal sensitivity to loud sounds, and 47.2% (172) had speech recognition impairment. The variables hearing loss, speech recognition impairment, tinnitus, age group, and noise exposure time did not show relationship with acoustic reflex thresholds; however, all complaints demonstrated a statistically significant relationship with Metz recruitment at 3000 and 4000 Hz bilaterally.ConclusionThere was no significance relationship between auditory complaints and acoustic reflexes.