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PubMed1985

Auditory structure and function in the bird middle ear: an evaluation by SEM and capacitive probe.

The anatomic features of the middle ear in five avian species were identified in the scanning electron microscope. Various aspects of the conductive apparatus were quantitatively measured in a number of specimens from each species. These included the tympanic membrane, columella footplate and oval window area; the length of the columella, extra-stapedius and Platner's ligament: and the angular relations between the columella and the tympanic membrane, extra-stapedius and footplate. The velocity vs frequency response curve, measured from the tip of the concave tympanic membrane, and corrected to a constant stimulus level of 100 dB SPL, was obtained for the neonatal chick and parakeet for frequencies between 0.2 and 10.0 kHz with a capacitive probe. In both species this curve resembled a bandpass filter whose best frequency was in the range of 1.5-3.0 kHz. The low and high frequency roll-off was 6 and 16 dB per octave, respectively. Displacement of the TM in the chick was measured at several frequencies between 70 and 120 dB SPL and was found to be linear. The results of displacement measured from the tip of the tympanum and from a location 1.0 mm more central on the drum membrane revealed a large difference in displacement over all frequencies. A comparison between the shape of the audibility curve and the tympanic membrane velocity function in the parakeet revealed that both curves were nearly the same for the mid-range frequency region. These findings add to our understanding of middle-ear function in the avian ear.

Age Factors↗
PubMed1985

Growth of suppression in the cochlear potentials.

Measurement of two-tone effects in the cochlear microphonic and summating potential indicates that the growth of suppression is different for these two cochlear potentials. Whereas the CM response to the fundamental is reduced 10 dB for each 10 dB increase in suppressor level, the SP decreases at a faster rate; approximately 20 dB per 10 dB increase. Slopes of functions for the CM response to the second harmonic are similar to those for the dc component. Since these results are consistent with the notion that suppression operates by attenuating the input to the CM generator, they are consonant with a mechanical origin of suppression.

Animals↗
PubMed1985

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.

Animals↗
PubMed1985

Evoked acoustic emissions and cochlear microphonics in the mustache bat, Pteronotus parnellii.

In the echolocating bat, Pteronotus parnellii, otoacoustic responses at a frequency of 62 kHz are measurable in the external ear canal during continuous and after transient acoustic stimulation. These responses are interpreted to represent emissions from the cochlea. They can reach an amplitude as large as 70 dB SPL and occur in the frequency range most important for echolocation, namely on the average about 700 Hz above the constant frequency component of the orientation calls. A sharp maximum of the amplitude of cochlear microphonic potentials at about 62 kHz could be correlated with the emission frequency. In one bat an evoked otoacoustic response changed to a spontaneous otoacoustic emission. The frequency and amplitude of the evoked otoacoustic responses reversibly decreased after exposure for 1 min to continuous sounds of more than 85 dB SPL with frequencies of about 2.5-7.5 kHz above the emission frequency. Similar effects occurred during anaesthesia or cooling. A possible relation between the existence of otoacoustic emissions and morphological specializations of the cochlea is discussed.

Acoustic Impedance Tests↗
PubMed1986

Steady-state evoked responses to sinusoidally amplitude-modulated sounds recorded in man.

Steady-state potentials evoked in response to binaural, sinusoidally amplitude-modulated (AM) pure tones and broadband noise signals were recorded differentially from position F4 and the ipsilateral mastoid on the human scalp. The responses elicited by the AM stimuli were approximately periodic waveforms whose energy was predominantly at the modulation frequency of the stimulus. The magnitude of responses was between 0.1 and 4 microV for modulation frequencies between 2 and 400 Hz imposed on a 1-kHz carrier signal. The magnitude of the responses increased linearly with log modulation depth for low (4 Hz) and high (80 Hz) modulation rates. The response magnitude also increased linearly with the mean intensity of the sound for intensities up to 60 dB above the subject's pure tone threshold; at higher levels the response saturated. The relationship between response magnitude and modulation frequency (the modulation transfer function) was a lowpass function for both pure tone and broadband noise carrier signals. The modulation transfer functions were similar to those obtained from human psychophysical measurements where spectral cues are either unavailable or not used by the subject. The responses also contained a significant component at the second harmonic of the modulation frequency. The magnitude of this component was greatest at modulation rates between 5 and 20 Hz. The responses elicited by ipsilateral and contralateral monaural stimulation were approximately equal in magnitude, and binaural stimulation produced a potential 30% greater than the individual monaural responses. It is suggested that the evoked response represents the entrained neural activity to temporal amplitude fluctuations, and reflects the psychophysically measured performance of the auditory system for the detection and analysis of amplitude modulation.

Adult↗
PubMed1986

Searching for neural correlates of the hearing sensation fluctuation strength in the auditory cortex of squirrel monkeys.

Sounds with slow (less than 20 Hz) fluctuations may elicit the hearing sensation fluctuation strength. For AM tones, neural correlates of fluctuation strength were searched in the auditory cortex of unanesthetized squirrel monkeys. To enable a comparison of psychophysical and physiological data, the 'modulation' of the peristimulus time histogram was fitted by a sinusoidal function. The dependence of the amplitude of this function on modulation frequency, modulation depth and sound pressure level was often comparable to the dependence of fluctuation strength on the same stimulus parameters. In particular, as a function of modulation frequency, the neural data also show a bandpass characteristic at low modulation frequencies as was found for the hearing sensation fluctuation strength.

Animals↗
PubMed1986

Detection of pure-tone amplitude modulation as a function of sensation level from 8 to 14 kHz.

Amplitude modulation detection thresholds were obtained for pure-tone stimuli of 8, 10, 12 and 14 kHz at 5 dB intensity increments from 10 to 65 dB sensation level. Performance at 8 and 10 kHz was a non-monotonic function of sensation level for all four subjects with the largest difference limen measured near 30 dB sensation level and optimal performance at the highest sensation level (60 dB). Weber fractions at 12 and 14 kHz appear dependent on each subject's high frequency hearing profile; i.e., the difference limens remain high and either increase or remain essentially constant at high sensation levels only when the frequencies tested are near a particular subject's upper limit of hearing.

Auditory Threshold↗
PubMed1986

Are nonlinearities observed in firing rates of auditory-nerve afferents reflections of a nonlinear coupling between the tectorial membrane and the organ of Corti?

Shear motion between the reticular lamina and the tectorial membrane at the location of the inner hair cells, studied on a network model of the cochlea with a nonlinear coupling between the two structures, exhibits the same type of nonlinearities as seen in the firing rates of auditory-nerve afferents. They include wave form distortions, phase changes and nonmonotonic dependence of the output amplitude on the cochlear input amplitude.

Animals↗
PubMed1987

Development of the middle latency response in an animal model and its relation to the human response.

Although the clinical use of the middle latency response (MLR) in adults is fairly straightforward, its use is complicated by maturational changes that continue throughout the first decade of life. In order to telescope the time period of this long developmental course, we have approached the study of MLR maturation using the gerbil as an animal model. The course of MLR obtained over the temporal lobe development was characterized in the Mongolian gerbil ranging in age from 10 days to 3 months of life. The adult gerbil MLR consists of two positive peaks (A and C) at 11 and 25 ms, respectively, and a negative component (B) at 16 ms. These components emerge in a systematic fashion as a function of age. The present work supports a strong age effect of increased MLR detectability in the gerbil, similar to findings reported for humans. Wave A was infrequently detected in young animals, but when present, it occurred at adult latencies. The latency of waves B and C decreased systematically with age. The amplitude of all components increased with age, similar to findings in humans. The fact that adult-like thresholds were obtained shortly after birth indicates that when present, MLRs may be a good index of hearing threshold. Effects of stimulating across a wide range of intensities were described. The gerbil model appears appropriate for the study of development of the central auditory system function.

Age Factors↗
PubMed1987

A comparison of two methods for the measurement of neural sensitivity to interaural intensity differences.

Two alternative methods for the measurement of neural sensitivity to interaural intensity differences (IIDs) were used to obtain IID-sensitivity functions for samples of excitatory-inhibitory (EI) neurons from the central nucleus of the inferior colliculus and the primary auditory cortex of the cat. In one, the EMI-constant method, intensity was held constant in the ear providing excitatory input and varied above and below that level in the other ear. In the alternative (ABI-constant) method, intensity at the two ears was varied symmetrically about a constant base intensity, in a manner roughly approximating the pattern of changes that occur when a free-field stimulus is moved in azimuth from the median sagittal plane. For neurons with monotonic or near-monotonic rate-intensity functions for the excitatory ear, the two methods generated IID-sensitivity functions that were identical or near-identical by a number of quantitative criteria. For neurons with non-monotonic rate-intensity functions, however, the IID functions generated by the two methods were very different: those produced by the EMI-constant method were monotonic, whereas those generated by the ABI-constant method were non-monotonic and sharply peaked. The advantages and disadvantages of the two methods, and the implications of the results for the neural encoding of IIDs and for the azimuthal sensitivity of EI neurons with non-monotonic rate-intensity functions, are discussed.

Animals↗
PubMed1987

Variations of cochlear microphonic potential after sectioning efferent fibers to the cochlea.

Cochlear microphonic (CM) potentials were recorded, in guinea pig, with differential electrodes before and after sectioning the medial efferent innervation at the level of the brainstem. Sectioning the crossed part of the medial efferent innervation did not change the CM whatever the frequency or level of stimulation used. Sectioning the medial--crossed and uncrossed--efferent fibers diminished CM amplitude at frequencies above 2 kHz. Thus, the ipsilateral medial efferent tract seems to be involved, through a tonic action, in controlling outer hair cell micromechanics.

Acoustic Stimulation↗
PubMed1988

T complex hemispheric asymmetries: effects of stimulus intensity.

The T complex component of the human auditory evoked potential (AEP) is thought to be produced in auditory cortex, on the posterior lateral surface of the temporal lobe. Recorded over temporal scalp, it consists of an 80-90 ms positive peak, Ta, and a 120-140 negative peak, Tb. As part of an effort to develop the clinical usefulness of the T complex in assessing auditory cortical function, we studied the effects of change in monaural stimulus intensity (20-80 dB SL) on T complex latency, amplitude, and hemispheric differences in normal adults. Ta and Tb peak latencies decreased as stimulus intensity increased. These latency changes were not dependent on ear or hemisphere. Right hemisphere Ta latency was shorter with contralateral than with ipsilateral stimulation; while left hemisphere Ta latency was not dependent on the ear stimulated. Tb latency was shorter over the left hemisphere, and over the contralateral hemisphere. Ta-b amplitude increased as stimulus intensity increased. This amplitude change was not dependent on ear or hemisphere. Ta-b amplitudes were larger over the right hemisphere and over the contralateral hemisphere. Hemispheric asymmetries were not significantly affected by stimulus intensity.

Adolescent↗
PubMed1988

Intensity coding and the dynamic range problem.

The psychophysical data on intensity discrimination indicate that certain schemes are unlikely as general intensity codes at the level of the auditory nerve and indirectly suggest that the most likely code is one based upon the firing rates of frequency-localized groups of fibers. A detection-theory analysis of a rate-based intensity code indicates that information from very few fibers can, if the information is appropriately combined, account for psychophysical discrimination even at high intensities. This suggests that fibers with similar CFs can code intensity over a wide range and that complex spectra can be represented at the level of the auditory nerve by a rate-CF code over the dynamic range of hearing. The analysis also indicates, however, a substantial discrepancy between the psychophysical data on the dependence of discrimination thresholds on level and the predicted discrimination behavior of a representative population of auditory nerve fibers. Thus, if intensity coding is based on localized firing rate, this fundamental psychophysical behavior does not result solely from peripheral processes.

Differential Threshold↗
PubMed1988

Neuronal discharge rate is unsuitable for encoding sound intensity at the inferior-colliculus level.

Rate-intensity functions from single neurons in the central nucleus of the inferior colliculus (ICC) of the cat in response to tone bursts and continuous noise were recorded. Only 6% of 64 quantitatively studied neurons had monotonic functions in response to tone bursts, 12.5% in response to continuous noise. The other neurons had functions with a single peak which could occur at any super-threshold level tested (3-80 dB), or with multiple peaks. In 78% of the neurons the rate-intensity functions in response to tones and noise were of different shape. We interpret this stimulus dependency of the shape of rate-intensity functions of most neurons as evidence against sound-intensity coding only on the basis of the shape of rate-intensity functions at the ICC level. Rate-intensity functions averaged from neurons with similar characteristic frequencies or from all neurons of our sample indicate a constant average discharge over at least 60 dB sound intensity. This excludes sound-intensity coding by means of an average neuronal discharge rate in the ICC.

Air Pressure↗
PubMed1988

Time is traded for intensity in the bat's auditory system.

Disparities in time and intensity are the two chief cues animals use for localizing a sound source in space. Echolocating bats belonging to the family Molossidae emit brief, ultrasonic signals for orientation that sweep downward about an octave over the duration of the pulse. Due to acoustic shadowing and the directional properties of the ears, pronounced interaural intensity disparities are created that vary as a function of azimuth. However, due to the small headwidth of these animals, azimuthal changes create small interaural time disparities that are at most 30 microseconds. The experiments in this report are concerned with the binaural processing of time and intensity disparities using brief FM signals that simulate the animal's natural echolocation calls. Binaural neurons receiving excitation from one ear and inhibition from the other (E-I neurons) were recorded from the inferior colliculus of Mexican free-tailed bats. The majority of units sampled were highly sensitive for temporal disparities of 100-300 microseconds, and a few had significant changes in discharge probability when interaural time was changed by 10-20 microseconds. However, all E-I neurons were also sensitive to intensity disparities. With only one exception, all E-I neurons traded time for intensity. On the average, each decibel difference in intensity could be compensated for by advancing or delaying the inhibitory sound by 47 microseconds. The main conclusion is that the auditory system processes interaural disparities by transforming level differences at the two ears into latency differences. Thus the discharge probability of each binaural neuron is determined largely by the arrival times of the discharges from the excitatory and inhibitory ears. In view of the substantial time-intensity trading ratios, the small interaural time disparities produced by azimuthal locations off the midline play no role in shaping the response properties of these neurons. Specific examples of how time-intensity trades can translate into a high spatial selectivity are presented.

Animals↗
PubMed1988

Rhythmic discharge properties of caudal cochlear nucleus neurons during postnatal development in cats.

Action potentials recorded extracellularly from neurons within the caudal cochlear nuclei of developing cats exhibited distinctive temporal characteristics (i.e., rhythmic responses) in response to long-duration acoustic stimuli including both tone and noise bursts. Unlike the homogeneous response characteristics of auditory nerve fibers, cochlear nucleus neurons exhibited many variations in rhythmic discharge patterns. The majority of neurons within the caudal CN of kittens younger than 10 days of age responded rhythmically to long-duration acoustic stimuli, however, the percentage of neurons responding rhythmically steadily decreased thereafter, and by the end of the second postnatal week most tonically-responding neurons maintained sustained steady-state discharge rates throughout stimulation. Discharges of neurons recorded during the transitional ages (around 13 days) were rhythmic at low sensation levels and exhibited adultlike sustained patterns at higher levels. Using constant sensation level stimuli (re individual neuron thresholds), burst frequencies remained essentially constant during the period of development in which rhythmic responses were observed. Intervals separating discharge bursts decreased as stimulus intensities increased for all neurons studied during the relevant period, but were not related in an orderly way to stimulus frequency. The effects of intensity on response periodicity were not mimicked by altering the amount of neurotransmitter present at the postsynaptic cell through microiontophoresis of excitatory amino acids and their antagonists onto the surface of neurons within the caudal CN. In addition, some immature neurons which responded phasically to acoustic stimuli responded rhythmically during the simultaneous presentation of acoustic stimuli and neuroexcitatory agents (i.e., glutamate). These results suggest that the source of the rhythmicity is not intrinsic to neurons in the caudal CN. Based on these and other observations we conclude that the most probable source of response periodicity observed early in development is the domination of inner hair cell output by efferent projections of the olivocochlear bundle, the temporal discharge patterns of which are also periodic.

Age Factors↗
PubMed1988

Single-tone intensity discrimination based on auditory-nerve rate responses in backgrounds of quiet, noise, and with stimulation of the crossed olivocochlear bundle.

We use simple statistical models of the firing patterns of high, medium, and low spontaneous rate auditory-nerve fibers to study mechanisms which determine the overall dynamic range of the auditory periphery. The models relate experimentally measured rate response properties of fibers with best frequency (BF) near 8.0 kHz to their ability to encode changes in BF tone level by changes in discharge rate in backgrounds of quiet and noise, with and without electrical stimulation of the crossed olivocochlear bundle (COCB). Application of the models to the BF tone rate responses of auditory-nerve fibers in backgrounds of quiet shows that optimum processing of the rate responses of fibers with BF near 8.0 kHz yields performance in the intensity discrimination task meeting or exceeding that of human subjects over an 80 dB range of levels. By defining a statistical measure of dynamic range, we confirm the results of Costalupes et al. (1984) demonstrating that masking noise shifts the dynamic range of auditory-nerve fibers to higher stimulus levels, thus preventing rate saturation. However, model analysis shows that masking noise also produces large reductions of dynamic range as well as large increases in the minimum intensity difference that can be encoded by the rate responses of single and ensembles of fibers. Electrical stimulation of the COCB can restore auditory-nerve fiber dynamic range and sensitivity to changes in BF tone level in noise backgrounds, in some cases to roughly that observed in backgrounds of quiet.

Animals↗
PubMed1990

The effect of a low-frequency masker on loudness.

Intense low-frequency tones produce masking-period patterns in which the masking of a higher-frequency tone burst varies by up to 25 dB over the period of the masker. In this experiment, observers were asked to match the loudness of partially masked test-tone bursts in one ear by adjusting the level of unmasked bursts presented to the other ear. It was found that the variation in masked threshold over the period of the masker also affects loudness matches. This effect on loudness, although it decreases in size with increasing level above masked threshold, persists even 25 dB above masked threshold.

Acoustic Stimulation↗