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PubMed1990

Cochlear threshold assessment using tone-derived action potentials.

An evoked-potential technique has been evaluated which detects whether the cochlea responds to a continuous, low level tone. The technique involves recording the cochlear action potential (AP) response to a suprathreshold probe tone, first in the absence and then in the presence of a continuous masking tone at the same frequency. Subtraction of the masked AP waveform from the unmasked AP yields a 'derived' potential, provided the continuous tone is above the threshold of cochlear sensitivity. Derived AP responses may be recorded with continuous masking tones over 10 dB below the threshold to the probe stimulus. In normally hearing guinea pigs, the mean best derived threshold using a 10-microV response criterion was 7.1 dB SPL, compared to 18.9 dB for conventional AP thresholds. The tone-derived response appears to provide a more sensitive and frequency-specific method for determining cochlear thresholds.

Animals↗

PubMed1989

Auditory brain stem responses in preterm infants: evidence of peripheral maturity.

This study explored further the relationship between peripheral and central auditory maturation on the basis of the auditory brain stem response. Auditory brain stem responses were recorded in preterm infants and adults to rarefaction and condensation click stimuli transduced through insert Tubephones. Infant recordings presented a triphasic waveform preceding wave I similar to that of the cochlear receptor potentials seen with adults during electrocochleography. Wave I latency and amplitude were found to be equivalent to those of adult subjects. Moreover, neither latency nor amplitude variability among infant wave I responses was found to be any greater than adults. Latencies of waves III and V, however, exhibited the expected differences relative to the adult comparison group. When the indirect evidence of cochlear receptor potentials in the infant are viewed adjacent to the observations that their ABR wave I latency, amplitude, and variability were entirely consistent with those of young adults, the data lend strong support for peripheral auditory electromaturity. These data are discussed relative to previously published reports of prolonged wave I latency in the infant which was attributed either to middle ear effects or immaturity of the cochlea and first order VIIIth nerve neurons.

Adult↗

PubMed1988

Modulation of cochlear tuning by low-frequency sound.

An intense, low-frequency tone (about 30 Hz) modulates the sensitivity of the inner ear to high-frequency stimulation. This modulation is correlated with the displacement of the basilar membrane. The findings suggest that the modulation may also affect cochlear tuning. We have investigated modulation of cochlear tuning by low-frequency sound in the guinea pig. Applying indirect methods of measurement (narrow-band analysis of compound action potentials and compound-action-potential tuning curves), the results suggest a shift of the excitation pattern along the basilar membrane towards higher-frequency areas. The shift occurred for both scala tympani and scala vestibuli displacement of the cochlear partition. Tuning curves, obtained from single units in the cochlear nerve, show sensitivity loss and a tip shift towards lower frequencies. This was also found for both scala tympani displacement and scala vestibuli displacement. The shift of the tip of the tuning curve towards lower frequencies corresponds to the inferred high-frequency shift of the excitation pattern. The relationship of these phenomena with the pathophysiology of Ménière's disease and with possible active mechanisms in cochlear transduction is discussed.

Animals↗

PubMed1989

The combined effect of cisplatin and furosemide on hearing function in guinea pigs.

The effect of the combined administration of cisplatin and furosemide on the electrophysiological hearing thresholds and endocochlear DC potential (EP) was studied in guinea pigs. A lack of interaction was found in animals given repeated intraperitoneal injections of a low dose of cisplatin with a pharmacological dose of furosemide. An ototoxic interaction occurred when a moderately high dose of cisplatin was administered intravenously at a time when the strial function was most affected by a very high dose of furosemide. The interaction was seen both as a decreased EP and a pronounced shift of auditory thresholds. It is concluded that the stria vascularis plays a role in the ototoxic mechanism of cisplatin.

Animals↗

PubMed1989

Thresholds of cat cochlear nucleus neurons to microwave pulses.

Action potentials of neurons in cat dorsal and posteroventral cochlear nuclei were recorded extracellularly with glass microelectrodes while the head of the cat was exposed to microwave pulses at 915 MHz using a diathermy applicator. Response thresholds to acoustic tones, acoustic clicks, and microwave pulses were determined for auditory units with characteristic frequencies (CFs) from 278 Hz to 39.2 kHz. Tests with pulsatile stimuli were performed for durations of 20-700 mus, principally 20, 70, and 200 mus. Brainstem midline specific absorption rate (SAR) threshold was as small as 11.1 mW/g per pulse, and specific absorption (SA) threshold was a small as 0.6 muJ/g per pulse. Microwave thresholds were generally lower for CF less than 9 kHz, as were most acoustic thresholds. However, microwave threshold was only weakly related to click threshold and CF-tone threshold of each unit.

Animals↗

PubMed1992

Rapid disruption of cochlear function and structure by trimethyltin in the guinea pig.

Trimethyltin (TMT) is a potent ototoxicant which acutely disrupts generation of the action potential evoked by a broad range of tone frequencies and subsequently produces selective high frequency impairment and outer hair cell (OHC) damage in the extreme basal turn of the cochlea. We investigated the development of TMT ototoxicity in the guinea pig 6-48 h following treatment using the compound action potential (CAP), cochlear microphonic (CM), endocochlear potential (EP) and light and electron microscopic examinations. At all time intervals studied, TMT reduced CAP sensitivity and CM amplitude. The effect was relatively broad across test frequencies at 6 h and subsequently became restricted to higher frequencies. No disruption of the EP was observed between 6 and 24 h following TMT. OHC pathology in the basal turn of the cochlea 12 h following TMT consisted of vacuolization in the supranuclear region and disruption of the cuticular plate; some mitochondria exhibited dark inclusions. Type 1 spiral ganglion cells appeared swollen at 24 h with separation of myelin from the cell bodies. No pathological changes were observed in the inner hair cells (IHC). The present data identify the OHC as targets responsible for the loss of CM sensitivity after TMT as the EP was unaffected. These data suggest that CAP and CM recovery at low and middle frequencies following acute TMT administration is accompanied by recovery of neurotransmission at the IHC or Type 1 SGC level and OHC recovery at apical regions of the cochlea.

Action Potentials↗

PubMed1990

Electrically evoked whole-nerve action potentials: data from human cochlear implant users.

This study describes a method for recording the electrically evoked, whole-nerve action potential (EAP) in users of the Ineraid cochlear implant. The method is an adaptation of one originally used by Charlet de Sauvage et al. [J. Acoust. Soc. Am. 73, 615-627 (1983)] in guinea pigs. The response, recorded from 11 subjects, consists of a single negative peak that occurs with a latency of approximately 0.4 ms. EAP input/output functions are steeply sloping and monotonic. Response amplitudes ranging up to 160 micro V have been recorded. Slope of the EAP input/output function correlates modestly (approximately 0.6-0.69) with results of tests measuring word recognition skills. The refractory properties of the auditory nerve were also assessed. Differences across subjects were found in the rate of recovery from the refractory state. These findings imply that there may be difference across subjects in the accuracy with which rapid temporal cues can be coded at the level of the auditory nerve. Reasonably strong correlations (approximately 0.74-0.85) have been found between the magnitude of the slope of these recovery curves and performance on tests of word recognition.

Cochlear Implants↗

PubMed1979

Origins of the scalp recorded frequency-following response in the cat.

The frequency-following response (FFR) is a short-latency scalp-recorded evoked potential elicited by the presentation of low-frequency acoustic stimuli. It is thought to be the result of the synchronous electrical activity in brain stemauditory nuclei to each wave in the acoustic signal. The present investigation constitutes an attempt to determine the generators of the FFR in the cat by analysis of the response and by section of brain stem auditory nuclei and tracts. Among the results were the following: (1) the cochlear nuclei contribute approximately 50% of the amplitude of the scalp-recordedFFR in the cat. (2) The cochlea also makes a significant contribution, accounting for an average of nearly 25% of the response amplitude. (3) The superior olivary nuclei (and/or the nuclei of the lateral lemnisci) account for about 20% of the response amplitude. (4) The contributions from the inferior colliculi (contrary to earlier studies) were found to be relatively insignificant. (5) As a consequence of the existence of multiple generators, the FFR ample area of hair cell excitation for each stimulus frequency involved in the mediation of the FFR, suggestion that scalp-recorded FFRs could be used to ascertain low-frequency hearingsensitivity in uncooperative human subjects.

Animals↗

PubMed1990

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.

Animals↗

PubMed1990

Compound action potential input/output functions in young and quiet-aged gerbils.

Auditory-nerve compound action potentials (CAP) and cochlear microphonic (CM) potentials were measured with round window electrodes in two sets of quiet-reared gerbils: young (N = 9 ears, 4-7 months) and aged (N = 11 ears, 35-37 months). CAP thresholds, measured at probe frequencies from 0.5 to 25.6 kHz, are plotted as audibility curves. Input/output (I/O) functions were derived from CAP and CM amplitude measurements at six frequencies. When compared to young controls, CAP audibility curves from aged animals all show some degree of threshold shift, ranging from minimal to severe, as well as increased variability. Our data suggest that some of the variability in the aged-animal audibility curves can be attributed to variations in individual genetic factors. Maximum CAP amplitudes for the aged animals average significantly less than those of the young controls at all frequencies tested. Young control I/O functions are generally steeper than those of the aged gerbils. Differences in the CM amplitudes of the young and aged gerbils are not as clear cut as the differences in the CAP. Possible mechanisms explaining the decrease in amplitudes and slopes of the CAP I/O functions in aged animals include changes in numbers or thresholds of primary ganglion cells, or a decrease in synchrony in discharges of auditory-nerve fibers.

Action Potentials↗

PubMed1988

Offset AP masker tuning curve and the FFT of the stimulus.

In previous experiments, it was noted that a cochlear compound action potential (CAP) can be produced by the offset of a tone, provided that the amplitude of the tone is modulated by a trapezoid with slopes that are typically much steeper than required to produce onset responses. Subsequently, such trapezoidal tone bursts with steep slopes were used as probe stimuli in simultaneous and forward masking experiments that were designed to evaluate the tuning characteristics of these offset CAPs. Masker tuning curves (MTCs) were generated by plotting the masker frequency necessary to reduce the amplitude of the offset CAP by 50%. Simultaneous masking of the offset CAP generated a W-shaped MTC, with two sharply tuned tips and one sharply tuned peak. Forward masking generated a sharply tuned V-shaped offset MTC. By contrast, for onset CAPs, both simultaneous and forward masking generated V-shaped MTCs. The very steep stimulus slopes required to produce an offset CAP are likely to generate much more acoustic splatter than the more gradual slopes required to produce an onset CAP, and this may be related to the different shapes of the onset and offset simultaneous MTCs. To explore this possibility, the relationship of the spectral characteristics (determined by fast Fourier transform, or FFT) to the shape of the MTC was studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Cochlear action potential tuning curves recorded with a derived response technique.

Previous action potential (AP) tuning curve methods have used a reduction in amplitude of the probe-elicited AP as an indication of tone-induced masking. The reduction criterion used in different studies has varied from 25% to 100%. For low level probe stimuli, which elicit a low-amplitude AP, this is a sensitive indicator. In contrast, for high-amplitude AP responses elicited by high-level stimuli, the required reduction in absolute terms is large, making it an insensitive indicator. AP tuning curves have been recorded using a sensitive method for detecting masker/probe interaction with a fixed criterion, unrelated to the unmasked AP amplitude. For each masking condition, a derived response was obtained by digitally subtracting the tone-masked AP waveform from the unmasked response. Derived responses are generated if there are ANY changes in the AP waveform induced by the masker, including amplitude changes, latency changes, or even changes in AP morphology not necessarily associated with the major peaks. A fixed criterion (10 microV) of tone-derived (TD) response was used as an indication of interaction of the responses to the masker and probe. Tuning curves generated by this method were compared with those generated by conventional amplitude reduction (AR) methods. TD tuning curves show different characteristics, especially with respect to increasing probe levels. They appear to give a good representation of the array of afferent fibers responding to a probe stimulus. In addition, frequency regions making minor contributions to the AP are better represented in TD tuning curves.

Animals↗

PubMed1992

Spatial processing within the mustache bat echolocation system: possible mechanisms for optimization.

1. The directionality of an echolocation system is determined by the acoustic properties of both the emitter and receiver, i.e., by the radiation pattern of the emitted pulse and the directionally of the external ears. We measured the directionality of the echolocation system of the greater mustache bat (Pteronotus parnellii) at the 30 kHz, 60 kHz and 90 kHz harmonics of its echolocation pulse by summing, at points throughout the frontal sound field, the echo attenuation due to the spread of pulse energy and the attenuation due to the spread of pulse energy and the attenuation due to the directionality of its external ears. The pulse radiation pattern at the 3 harmonics was measured by comparing the output of a microphone moved throughout the frontal sound field against a second reference microphone at the center of the field. External ear directionality at the 3. harmonics was measured by presenting free-field sounds throughout the frontal sound field, and recording the intensity thresholds of cochlear microphonic potentials, and the intensity thresholds of monaural neurons in the inferior colliculus tuned to one of the 3 harmonics. 2. When compared with ear directionality, the echolocation system was found to be more directional for the center of the sound field in several respects. At all harmonics, attenuation of sounds originating in the peripheral part of the field was increased by 10 to 13 dB. Areas of maximum sound intensity contracted toward the center of the field. Also, the isointensity contours of the echolocation system were more radially symmetrical about the center of the field. 3. At 60 kHz, sound intensity along the azimuth within the echolocation system was nearly constant 26 degrees to either side of the center of the field. This suggests that the radiation pattern of the echolocation pulse and the directionality of the external ears complement one another to produce an acoustic environment at the center of the sound field in which stimulus intensity is stabilized to allow more effective analysis of various aspects of the echolocation target. In particular, we suggest that this intensity stabilization may allow the bat to more effectively resolve the interaural intensity differences it uses to localize prey. 4. Predictions of the azimuthal spatial tuning of binaurally sensitive neurons in the inferior colliculus within the echolocation system were compared with their spatial tuning when only ear directionality is considered.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1979

Influence of thyroid state and improved hypoxia tolerance on noise-induced cochlea damage.

Guinea pigs were exposed to pure tone noise (2.7 kHz, 130 dB, 1 h) and cochlear microphonic potentials were measured 24 h after exposure. There is the possibility to modify the resulting noise-induced cochlea damage by regulating the function of the thyroid gland to alter the rate of metabolism. A hypofunction of the thyroid gland during sound exposure lessens, an over-function aggravates the damage. After gradual adaptation of the animals to a simulated 10,000 m altitude, the electrophysiologically demonstratable noise-induced damage was reduced. This might be explained by the greater hypoxia tolerance and perhaps additional better oxygen supply to the receptor cells.

Altitude↗

PubMed1979

Effects of modern loop diuretics on the inner ear a quantitative evaluation using computer technics.

The effects of some modern high ceiling loop diuretics on the guinea-pig's inner ear are tested. Short-time experiments are run registrating the cochlear microphonic potentials (CMP) before and after i.v. bolus injections of a diuretic. A 100-point matrix program is performed by automatic sampling of the CMPs after stimulating by five frequencies at 20 sound pressure levels each. Data can be plotted in different scales within 10 min after the end of a whole-day experiment. The data are on-line processed by a IBM 1800 computer.

Animals↗

PubMed1978

[Influence of thyroid state on noise-induced cochlea damage (author's transl)].

Guinea pigs were exposed to pure tone noise (2.7 kHz, 130 dB, 1 h) and cochlear microphonic potentials were measured after 5 days. It is possible, to modify the resulting damage by experimentally altering the rate of metabolism by regulating the function of the thyroid gland. (table: see text) A hypofunction of the thyroid gland during sound exposure lessens, an over-function aggravates the damage. This effect possibly results from the influence on the metabolism exerted by the adenylic system. This conclusion leads to new viewpoints concerning prophylaxis, therapy and metaphylaxis in cases of noise deafness.

Adenylyl Cyclases↗

PubMedDate not supplied

Frequency selectivity in the auditory periphery: similarities between damaged and developing ears.

Single fiber tuning curves (stimulus frequency versus neural threshold curves) were obtained from 198 auditory nerve fibers in 24 kittens between birth and the 16th postnatal day and from 74 auditory nerve fibers in adult cats. Three developmental stages during which adult-like frequency-resolving capacity was acquired were identified. During the early stage of postnatal development, all auditory nerve fibers were essentially untuned and responded to a narrow range of low to middle frequency tone bursts presented at intensities exceeding 110 dB sound pressure level (SPL) re 20 muPa. In the intermediate stage, which occurred during the second postnatal week, auditory nerve fibers tuned to low- and mid-range frequencies acquired adult-like frequency-resolving capacity. Fibers tuned to high frequencies, which were recorded later in development than those tuned to lower frequencies, were as sharply tuned as their adult counterparts, but exhibited a low contrast between thresholds at characteristic frequency (tip) and lower (tail) frequencies (ie, low tip-to-tail ratios). Adult-like tuning curves were observed during the third stage, primarily as a consequence of the acquisition of adult-like tip-to-tail ratios. Our understanding of the cochlear mechanism(s) by which frequency selectivity is produced in adult animals has recently been enhanced by a combined anatomy and physiology investigation conducted by Liberman and Dodds, in which clear anatomic foci of cochlear damage were identified in cats with functionally characterized hearing loss. Similarly, descriptions of anatomic differentiation in the feline auditory end-organ correlate with functional measures of peripheral auditory system development. In this report, anatomic and physiologic similarities between developing and damaged ears are considered in an attempt to better characterized the process whereby normal frequency selectivities and thresholds are developed. Our findings support the notion that anatomic changes in the cochlea during development, primarily the development of adult-like anatomic relations between the tectorial membrane and sensory cells, underlie the acquisition of adult-like auditory nerve fiber tuning.

Age Factors↗

PubMed1986

Spontaneous, click-, and toneburst-evoked otoacoustic emissions from normal ears.

Evoked and spontaneous otoacoustic emissions were recorded bilaterally in a group of normal subjects (n = 14) using clicks and tonebursts at four frequencies (0.5, 1, 1.5, and 3 kHz). All ears (n = 28) demonstrated evoked emissions, but not to every stimulus type. The 0.5-kHz toneburst evoked emissions in only 10 (36%) ears, the 1.5-kHz toneburst in all ears, and the remaining stimuli in at least 80% of ears. Two distinct patterns of evoked emissions were identified. Five (18%) ears showed short, broadband click-evoked emissions lasting less than 20 ms after stimulus onset. In these ears, toneburst-evoked emissions were often more prominent than click-evoked emissions and no spontaneous emissions were detected. Twenty-three (82%) ears showed click-evoked emissions lasting longer than 20 ms poststimulus onset. Spectral analysis of these emissions demonstrated several (2-10) narrow frequency peaks. Highly similar peaks were present in the spectra of toneburst-evoked emissions within the range of toneburst spectra. Spontaneous emissions were recorded in 12 of the 23 ears. In these ears, at the frequencies of spontaneous emissions, prominent peaks in both click- and toneburst-evoked emission spectra were always present. Otoacoustic emission characteristics correlated significantly between the ears of individual subjects inferring that a symmetrical cochlear mechanism generates otoacoustic emissions.

Acoustic Stimulation↗