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PubMed1984

Frequency-following responses in the cat.

The frequency-following response was studied in five cats aged 1 week to 3 years. Animals studied had either monaural or binaural hearing. Responses were recorded differentially from vertex and circumaural electrodes to tonebursts at octave intervals from 0.5 to 4 kHz. Thresholds and dynamics of the responses with and without white-noise masking were investigated. Evidence was found to support the view that the cochlear microphonic makes a major contribution to the frequency-following response, while the double frequency-following response to the 0.5 kHz stimulus is neural.

Animals↗
PubMed1984

Analysis of the frequency following response in the cat.

The generators of the frequency following response (FFR) were characterized for three frequency ranges by studying changes in FFR response after lesioning the nuclei within the central brainstem auditory pathway. Responses to low frequency (200-500 Hz) stimulation demonstrated changes in the complexity of the FFR waveform in both time and frequency domains following lesions in the brainstem auditory pathway. The results indicate that the complexity of the low frequency FFR is due to activity from multiple sites within the auditory pathway. The intermediate frequency (700-1500 Hz) responses showed unpredictable amplitude changes following similar lesions and no conclusion could be drawn about the generators of the FFR in this frequency range. The responses to high frequency (3-8 Hz) stimulation showed no reduction in amplitude following serial lesioning. These results, combined with other experimental evidence presented, indicate that the high frequency FFR response originates from the cochlear microphonic. Different electrode configurations were used to evaluate the low frequency FFR. In contrast to multiple generator sources recorded with the standard vertex-mastoid electrode configuration, we were able to record a response contributed primarily by the inferior colliculi with a less peripherally sensitive electrode configuration (vertex-linked-pinnae) at low intensity stimulation. The fact that auditory brainstem nuclei contribute to the FFR in varying amounts depending on the electrode configuration may explain some of the conflicting characterizations of this response in the literature. Despite this difficulty, the FFR neural generators were identified and characterized in the low frequency range using our most sensitive electrode configuration (vertex-mastoid) and in the high frequency range where the single generator is the cochlear microphonic.

Animals↗
PubMed1984

Water permeability of the endolymph-perilymph barrier in the guinea pig cochlea.

The diffusional water permeability of the endolymph-perilymph barrier was determined in guinea pigs by perfusing the perilymphatic space with tritiated water and measuring the uptake of tritiated water into the endolymph. Assuming that the volume flow across the endolymph-perilymph barrier is negligible for short periods of perfusion, the water permeability of the barrier is approximately 130 times greater than its permeability to K+ in normal guinea pigs.

Animals↗
PubMed1984

The microstructure of quiet and masked thresholds.

Factors leading to the microstructure of the audiogram (a constant pattern of threshold maxima and minima as a function of frequency) are shown to influence masked thresholds, changing the shape of masking functions when both constant and variable tonal maskers are used. Simultaneous masking with broadband noise gradually reduces the difference between threshold maxima and minima until no further differences can be seen when masked thresholds are above 40-50 dB SPL. Nonsimultaneous masking with broadband noise reveals a changed microstructure when thresholds are elevated above 30-40 dB SPL. Thus in both simultaneous and nonsimultaneous masking the rate of threshold growth with increasing masker level is different for tones from threshold minima than for tones from threshold maxima. These psychophysical measures are related to measures of evoked cochlear emissions and the results are discussed in terms of the implications for understanding the cochlear mechanisms responsible for the microstructure and for the interpretation of psychophysical measures with low level stimuli.

Audiometry, Pure-Tone↗
PubMed1985

Diurnal cycle for spontaneous oto-acoustic emission frequency.

Spontaneous oto-acoustic emission frequency was measured in two subjects at different times on the same day. A statistically significant frequency decrease during the morning hours was observed for both subjects. Possible mechanisms (ear canal pressure, body temperature, androgen level) for this decrease are discussed.

Adult↗
PubMed1985

Tuning of the auditory brainstem OFF responses is complementary to tuning of the auditory brainstem ON response.

Continuous masking studies show a complementary pattern of effects on the auditory brainstem responses (ABRs) which are generated by the onset and by the offset of a midfrequency tone. The masking profiles of the two responses are almost opposite with a probe stimulus frequency of 32 kHz (16-32 kHz is the midfrequency region for the CBA/J mouse). The Offset and Onset ABR tuning curves (TCs) also reveal very different properties at the midfrequencies of 16, 20, 24 and 32 kHz. The Offset TC is exquisitely sensitive to masking by very low intensity stimuli at a narrow range of frequencies which are lower than the probe stimulus frequency. Continuous masking produces a well-tuned low frequency tip to the Offset TC. For Offset TCs generated in response to midfrequency tones, the Q+10 dB of this tip averages 8.3. Masking at this low frequency tip of the Offset TC has no observable effect on the Onset ABR. The Offset ABR is also sensitive to masking by a narrow range of frequencies which are higher than the probe stimulus frequency. This occurs at an intensity which also has no observable effect on the Onset ABR. The Q+10 dB of this high frequency tip averages 9.2. The average frequencies where these Offset TC tips occur fit the cubic difference formula (2f1-f2), which describes a distortion product of two-tone suppression. At low probe stimulus frequencies, there is only a high frequency Offset TC tip; at high stimulus frequencies, only a low frequency tip. The high frequency tip has a higher threshold and appears more susceptible to metabolic disturbance. The Offset ABR TC also has a peak which corresponds to the probe stimulus frequency. Continuous masking with the stimulus frequency produces nonmonotonic enhancement of the Offset ABR, while it simultaneously reduces the magnitude of the Onset ABR. The tuning of this Offset TC peak (measured as Q-10 dB) is almost always much sharper than the corresponding Onset TC tip in the same mouse. These values for midfrequency stimuli average 6.2 for the Onset, and 13.6 for the Offset TCs. This fine tuning of the Offset TC at the probe stimulus frequency occurs at SPLs from 50 to more than 90 dB.

Animals↗
PubMed1985

Role of suppressive interactions in the cochlear microphonic response to wide-band clicks.

Cochlear microphonic responses (CM) were recorded from the guinea pig cochlea with conventional differential electrodes implanted in the first and second turns. The CM frequency functions were determined by calculating the Fast Fourier Transform of responses to wide-band clicks. These frequency functions appeared to be dependent upon the spectrum of the click. This effect was interpreted as resulting from suppressive interactions between the various components of the click. It is suggested that such CM interactions are related to corresponding effects reported previously concerning cochlear nerve responses.

Acoustic Stimulation↗
PubMed1986

Direct measurement of longitudinal endolymph flow rate in the guinea pig cochlea.

The rate of longitudinal endolymph flow in the guinea pig cochlea has been measured with a novel tracer technique. The tracer we utilized was the tetramethylammonium (TMA) ion, the movement of which was monitored by ion-sensitive microelectrodes. Extremely small amounts of tracer were required as the electrodes could readily detect TMA concentrations in endolymph as low as 10 microM. TMA was introduced into scala media in the form of a small bolus, varying from 2-20 nl in volume. To examine whether longitudinal flow affects the dispersion of TMA in endolymph, we compared the characteristics of TMA spread to turn I following injection into turn II, with those of TMA spread to turn II following injection into turn I. The comparison of these data with an analytical model combining the processes of diffusion and volume flow demonstrates that the spread of tracer is dominated by passive diffusion processes with very little contribution from longitudinal endolymph flow. The rate of longitudinal endolymph flow between turn I and turn II was estimated to be less than 0.01 mm/min directed towards the basal turn. This value is considerably lower than recently published estimates using other techniques.

Cochlea↗
PubMed1986

The acoustic middle ear muscle reflex in albino rats.

The acoustic middle ear muscle reflex was studied in albino rats anesthetized with chloralose. The best frequency of the reflex and the threshold at this frequency were on average about 3 kHz and 57 dB SPL, respectively. The threshold increased as frequency increased above, and decreased below, the best frequency at a rate of about 20 dB/octave. Above about 12 kHz, the muscular response showed instability and habituation. Thresholds were similar between stapedius and tensor tympani reflexes and between ipsilateral and contralateral reflexes. The middle ear transmission loss due to the reflex was the greatest and nearly constant below about 1 kHz, where the loss was about 18 dB at the maximal stimulation. Above this frequency the loss decreased as frequency increased up to 20 kHz. Thus the reflex, unlike that in other animals, suppressed transmission over the whole range of reflex-eliciting frequencies. The transfer function of the reflex had a well damped low-pass characteristic with a cut-off frequency of about 20 Hz. From the above characteristics of the reflex, the role of the rat's tympanic muscles in improving ultrasonic hearing under ambient noises was suggested.

Animals↗
PubMed1986

Contrast enhancement in frequency spectra of cochlear microphonic responses to complex stimuli.

Cochlear microphonic responses were measured in pigeons and guinea pigs during stimulation with complex sounds. The acoustical stimuli had many component frequency spectra with a more or less undulating envelope. Enhancement of the peak-and-valley structure of the envelope occurred at high stimuli levels, especially if all frequency components in the stimulus had equal (cosine) phase. The observed effects could very well be modelled with a simple passive electronic network, as well as with an analytical expression that describes saturation of the cochlear microphonic at high stimulus levels.

Acoustic Stimulation↗
PubMed1986

Molecular mechanisms of drug-induced hearing loss.

Although the ototoxic actions of a variety of drugs have long been documented, the biochemical mechanisms underlying such toxicity largely remain to be established. For example, recent advances have provided us with information about the actions of salicylates (aspirin) and diuretics (furosemide) but we are not yet able to specify the mechanisms by which these drugs damage the cochlea. On the other hand, the considerable amount of biochemical and pharmacological data on the effects of aminoglycosides (streptomycin, neomycin, gentamicin and related compounds) has enabled us to formulate a rational hypothesis of their mechanism of action. We have previously presented evidence for an involvement of polyphosphoinositides in the ototoxic actions of aminoglycosides. Recent electrophysiological and pharmacokinetic studies have shown in addition that aminoglycosides occupy at least two distinct compartments in the course of their actions. Further studies of drug uptake in vitro and of drug toxicity in cochlear perfusions suggested the involvement of an active (energy-requiring) aminoglycoside transport system. These and other data are compatible with the following multi-step model of aminoglycoside toxicity: The initial step in the reaction sequence is an electrostatic interaction of aminoglycosides with the plasma membrane. The resulting displacement of calcium accounts for acute effects but the action is reversible and antagonized by divalent cations. An energy-dependent uptake process is required for the expression of toxicity. It can be prevented by select metabolic blockers. A crucial step in subsequent intracellular drug actions is the binding of aminoglycosides to phosphatidylinositol bisphosphate inhibiting its hydrolysis and preventing its physiological function.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminoglycosides↗
PubMed1986

Measurement of basilar membrane vibrations and evaluation of the cochlear condition.

The experimental procedure for measuring basilar membrane responses to acoustic signals is described. The surgical procedure developed for opening the cochlea with minimal trauma is presented. Each experiment included sound pressure level measurements to define the input signal, cochlear microphonic (CM) measurements to monitor the cochlear condition, interferometric measurements and histological evaluation of the cochleas. The characteristic frequency (CF) and the sensitivity at CF for the basilar membrane response is correlated with the change of CM response observed in six animals. It is demonstrated that both tuning characteristics are extremely sensitive to cochlear trauma as evidenced by changes of CM.

Animals↗
PubMed1986

Relationship between basilar membrane tuning and hair cell condition.

Basilar membrane tuning characteristics were measured in 15 cats using laser interferometry. The experimental procedures introduced varying degrees of cochlear trauma. Variability from animal to animal was also observed in the characteristic frequency (CF) of tuning, the sensitivity at CF and lower frequencies, and the sharpness of tuning. The changes in the tuning characteristics of the basilar membrane are correlated with the extent of outer hair cell (OHC) damage. These observations lead to the conclusion that the tuning properties in the CF region are predominantly determined by the mechanical properties of the OHC and not the basilar membrane.

Animals↗
PubMed1986

Differential susceptibility to noise-induced permanent threshold shift between albino and pigmented guinea pigs.

Evidence that reduced levels of cochlear melanin are associated with increased auditory sensitivity, increased levels of auditory fatigue and an increased susceptibility to noise-induced hearing loss led us to investigate the effects of noise exposure on the cochlear microphonic (CM) in albino and pigmented English shorthair guinea pigs. CMs were recorded from the round window prior to and at 90 min and 7 days after exposure to 45 min of 126 dB noise. Thresholds for the first detectable elicitation of the CM for four pure tones were determined and the output voltage of each cochlea was measured in 10 dB steps through intensity levels which produced a maximum voltage amplitude in the CM and voltage rollover. This analysis demonstrated that: albino guinea pigs displayed significantly lower auditory thresholds than did pigmented animals before exposure to noise; thresholds were elevated to comparable levels in both groups 90 min after noise exposure; pigmented guinea pigs showed a reliable recovery in CM thresholds 7 days after exposure to noise while thresholds in the albinos remained elevated to the same degree at both 90 min and 7 days after noise; 90 min after noise exposure, the maximum voltage output of albino cochleas was significantly less than that recorded from the cochleas of the pigmented guinea pigs. These results demonstrate that albino guinea pigs are more susceptible to the ototoxic effects of high intensity noise than pigmented guinea pigs. Converging evidence indicates that some aspects of cochlear function involve melanin pigment and that its absence may produce auditory abnormalities. Reduced melanin pigmentation may also contribute to such phenomena as noise-induced threshold shifts and individual differences in noise-induced hearing loss.

Albinism↗
PubMed1986

Effects of extracochlear direct current stimulation on the ensemble auditory nerve activity of cats.

The influence of direct current applied by round window stimulation on the whole nerve response of the auditory nerve of the cat has been studied. Effects on acoustically driven activity (CAP) and on the ensemble spontaneous activity of the nerve were observed. Stimulation with positive current suppressed driven and spontaneous activity. The strength and spread of suppressive effects was a function of the applied current level. After a period of positive electrical stimulation, driven and spontaneous activity rapidly returned to normal values. A rebound effect was sometimes observed, marked by a brief increase in spontaneous activity above the normal level. Negative current initially produced an increase in the amplitude of driven and spontaneous responses. Prolonged stimulation with negative current (greater than 30 s) resulted in a subsequent, graded reduction of neural activity, until a profound suppression of spontaneous and evoked neural activity was attained. The amplitude/latency relationship of CAPs was altered during passing of negative currents but not during passing of positive currents. Recovery from the suppression generated by negative currents was commonly prolonged for anything from a few seconds to many minutes; prolongation was dependent on stimulus amplitude, duration and duty cycles.

Acoustic Stimulation↗
PubMed1987

Far-field recordings of short latency auditory responses in the White Leghorn chick.

Far-field recordings of short latency auditory responses were characterized in the White Leghorn chick (Gallus domesticus, 2-3 weeks post hatch). Six to twelve positive peaks were normally present within the first 8 ms following the onset of a click stimulus (stimulus intensity = 99.0 +/- 6.3 dB pe SPL, 44.3 +/- 1.2 dB SL). Both cochlear microphonic (CM) and neural response components were distinguished based on functional criteria. Neural responses (P1a-P6b) were readily masked by white noise. CM responses were resistant to masking and inverted when click polarity was reversed. Systemic cooling produced shifts in neural response latencies such that later peaks (e.g. latencies greater than or equal to P3) were delayed to a greater extent than early peaks (P1a and P2a). The latencies of CM components were relatively unaffected by cooling. The mean latencies and amplitudes were described quantitatively for dominant positive peaks. The mean auditory response threshold for 18 animals was 54.5 +/- 7 dB pe SPL. Dominant later components (P3a and the P3b/P4a complex) were the first to disappear after lethal injection of anesthetic. These were followed immediately by earlier neural peaks. CM components were the last to disappear. A working hypothesis is advanced regarding the distinction between central (P3a, P3b and P4a) and peripheral (P1a, P2a) neural components.

Animals↗
PubMed1987

Rapidly fluctuating thresholds at the onset of experimentally-induced hydrops in the guinea pig.

Pigmented guinea pigs were chronically implanted bilaterally with a platinum electrode on each round window. After recovery the endolymphatic sac was destroyed and the duct blocked on one side only; the other side was employed as a control. The round window response thresholds on both sides were recorded several times per week over a three month period. There were three main results. A sensitivity loss of up to 20 dB was observed for frequencies between 250 Hz and 6.4 kHz within two weeks post-op. At the end of three months the threshold elevation for these frequencies was as much as 50 dB. On the other hand the thresholds for frequencies between 8 and 16 kHz remained within 10 dB of their pre-operative value for at least two months. The thresholds fluctuated with a shift of as much as 25 dB within 24 h. The threshold elevation was associated with a decrease in the latency, at threshold, of the round window AP response which at frequencies between 250 Hz and 6.4 kHz was as short as that for 8 kHz. This observation suggested that it was the base, only, of the cochlea which responded. The present study has indicated that experimentally induced endolymphatic hydrops in the guinea pig mimics well the progressive and fluctuating hearing loss characteristic of Ménière's disease.

Animals↗
PubMed1987

Effect of cisplatin on hair cell morphology and lateral wall Na,K-ATPase activity.

The dose-response ototoxic effects of cisplatin were studied in guinea pigs. Loss of Preyer reflex and suppression of the N1 amplitude occurred in cisplatin-treated animals and was described as dose-related. Drug-induced hair cell damage, as observed with scanning electron microscopy, occurred sporadically throughout the turns of the cochlea and the incidence increased with dose. Na,K-ATPase activity in the lateral wall tissues was not significantly different between treatment groups. The results reported here indicate that cisplatin ototoxicity was dose-dependent, but was not directly related to Na,K-ATPase activity in the lateral wall.

Animals↗