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Results for “Auditory Threshold”

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PubMed1989

Auditory neural activity evoked by pure-tone stimulation as a function of intensity.

The 2-deoxyglucose (2-DG) autoradiographic technique was employed to map activation of the central auditory pathway in the mongolian gerbil during stimulation with a 3.0 kHz tone at several intensities. In most auditory nuclei, the tone produced restricted areas in which 2-DG uptake was markedly higher than that of adjacent tissue, at locations consistent with the known tonotopic organization of the structure. The size of these regions changed relatively little with increasing stimulus intensity from 25 to 65 dB SPL (re 0.0002 dyne/cm2). At higher intensities, evoked uptake spreads into locations which represent frequencies above 3.0 kHz. In the inferior colliculus, relative 2-DG uptake decreased with increasing stimulus intensity in bands on either side of the 3.0 kHz region. These bands of reduced uptake became wider with increasing stimulus intensity from 25 to 85 dB SPL. The optical densities of auditory structures were normalized by the density of non-auditory white matter to derive optical density ratios. In the cochlear nuclei, optical density ratios in the 3.0 kHz region increased monotonically with increasing stimulus intensity, to a plateau at 45 dB SPL. In higher auditory nuclei, relative 2-DG uptake increased to a peak at 45 dB SPL and then declined at higher intensities.

Action Potentials↗

PubMed1988

Effects of maternal cocaine abuse on the neonatal auditory system.

Eighteen neonates born to cocaine-abusing mothers were tested for peripheral and brainstem auditory dysfunction using auditory brainstem responses (ABR). Their data were compared to ABRs from 18 normal neonates. The ABR data were analyzed to determine if ABR parameters were abnormal in neonates born to cocaine-abusing mothers in comparison to normal neonates. ABRs from neonates exposed to maternal cocaine abuse show prolonged interpeak latencies and prolonged absolute latencies. These abnormalities indicate neurologic impairment or dysfunction that warrants further audiologic and neurologic follow-up.

Audiometry, Evoked Response↗

PubMed1996

Evolution of transient evoked otoacoustic emissions in preterm newborns: a preliminary study.

Transient evoked otoacoustic emissions (TEOEs) were recorded in a cohort study of preterm neonates in order to study their basic properties as a function of gestational age. Their main properties were: (1) TEOEs spectrum did not vary with age; it was analogous to those of full term; (2) there was no statistically significant variation of the TEOEs amplitude with age. The maturation of outer hair cell properties appears to be complete at a gestational age of 29 weeks. Because a number of infants at risk for hearing loss are preterm babies, screening for TEOEs has to be performed in the neonatal care unit. To improve the accuracy and efficiency of the test, screening should take place as close to hospital discharge as possible, with the optimum time at 35 gestational weeks.

Analysis of Variance↗

PubMed1980

Ototoxicity of kanamycin in developing rats: relationship with the onset of the auditory function.

In order to test the relationship between the ototoxicity of kanamycin and the onset of the auditory function, two groups of developing rats were intoxicated with kanamycin before and after the period of onset of cochlear potentials (8th postnatal day). Kanamycin was shown to have a weak ototoxic effect before the 8th postnatal day and a strong ototoxic effect after this period. These results indicate a critical period of sensitivity to ototoxic antibiotics during auditory development.

Acoustic Stimulation↗

PubMed1982

Corresponding effects of acoustic fatigue on the cochlear microphonic and the compound action potential.

Moderate levels of acoustic fatigue, usually induced by a 100 dB SPL pure tone at a frequency appropriate to the location of intracochlear differential electrodes, have a surprising and paradoxical influence on the cochlear microphonic and the compound action potential of the auditory nerve. While the low-level microphonic becomes smaller, the low-level action potential becomes considerably larger and exhibits a shortened latency. The high-level microphonic and the high-level action potential are left virtually unchanged at these levels of fatigue. Nonetheless, if the duration or intensity of exposure is increased, both the high-level and low-level action potentials decrease as well. Assuming that the low-level microphonics are generated by outer hair cells, these data suggest that one relationship between outer hair cell function and neural function is inhibition at low intensities. Assuming that the high-level microphonic tends to be generated by inner hair cells, the decrease in the action potential at all intensities whenever the high-level microphonic potential is impaired suggests that the functional relationship between inner hair cells and auditory nerve function is excitatory.

Acoustic Stimulation↗

PubMed1985

Peripheral auditory adaptation and fatigue: a model oriented review.

A model is introduced for auditory adaptation based on the stochastic models that are widely documented for birth and death processes and used as a vehicle to review single unit and compound action potential studies as well as various models for adaptation and forward masking. It appears that such a model inherently incorporates a relation between the perstimulatory adaptation time constant, the poststimulatory recovery time constant and the ratio between adapted firing rate and onset firing rate. The knowledge of any two of these parameters allows the prediction of the third one. The model takes into account postsynaptic membrane properties. The model based on Markov assumptions is a linear one. Although onset firing rates depend on a nonlinear way upon stimulus level, the above mentioned time constants and the adapted rate to onset rate ratio are intensity independent. The idea is put forward and tested that a comparable situation exists for the depression effects produced by mild auditory fatigue. The apparent findings that time constants tend to be intensity dependent for auditory fatigue is explained on basis of an interaction effect of dependent exponential processes.

Action Potentials↗

PubMed1987

Development of brainstem auditory evoked responses in the hamster.

The hamster is relatively immature at birth compared with other mammalian species. Because of the amount of maturation that occurs postnatally, the development of the peripheral auditory system of the hamster has generated a great deal of interest. However, indices of the developing functional capabilities of the central auditory system of the hamster are lacking. The following study investigated the ontogeny of brainstem auditory evoked responses (BAERs) to clicks in young hamsters. The general maturational trends noted for other species were found for the hamster. The major peaks of the BAER decrease in latency at the same time relative to birth and at the same rate in the hamster as in the cat and rat. Some maturational events such as the onset of a recordable BAER, appearance of the late peaks and increase in amplitude of the late peaks occur later, relative to birth, in the hamster than in the other species. Development delays in the hamster make it an attractive model for studying postnatal development of the central auditory system.

Aging↗

PubMed1987

Qualitative and quantitative changes in the guinea pig organ of Corti after pure tone acoustic overstimulation.

A direct relationship between delivered sound energy and morphological overstimulation has been formulated in the so-called equal energy principle. The validity of this principle was systematically tested by studying the effects of acoustic overstimulation on the guinea pig cochlea by means of scanning electron microscopy (SEM) and light microscopy (LM) techniques. The animals in the present study were exposed to 3.85 kHz pure tones of various durations and intensities and allowed a recovery period of 4 weeks before the morphological changes in the cochlea were assessed. A 4-grade scale was devised in order to achieve a qualitative as well as a quantitative SEM evaluation of the cochlear damage. The results of the examinations by LM and SEM were compared and analysed statistically. No significant difference was observed between groups of animals exposed to a sound energy of 9.4 and 37.5 Pa2.h, irrespective of sound intensity (114 or 120 dB SPL). Groups of animals exposed to a higher energy level (150 Pa2.h) differed significantly from those exposed to higher and also lower sound energy levels. Groups exposed to different intensities (108, 114 and 120 dB SPL) did not differ significantly within this energy level.

Acoustic Stimulation↗

PubMed1989

Functional organization of the ventral division of the medial geniculate body of the cat: evidence for a rostro-caudal gradient of response properties and cortical projections.

The response properties to clicks, noise and tone bursts of 2152 single units located in the ventral division of the medial geniculate body were analysed as a function of their anatomical position. A particular spatial distribution of these properties was observed in the pars lateralis (LV) and ovoidea (OV). The distribution of different response characteristics changed along the rostro-caudal axis. Units located posteriorly were in majority either insensitive to simple acoustical stimuli or responded exclusively to pure tones, presenting generally a broad tuning and a loose tonotopic arrangement. Inhibitory response patterns were about as frequent as excitatory ones, response latencies were long on the average and widely distributed. Only a few units showed time-locking of their discharges in response to repetitive clicks. Most units had non-monotonic intensity functions. Going anteriorly, the distribution of response properties progressively changed: the number of units sensitive to various simple acoustical stimuli (pure tones and broad band stimuli together) increased, the tonotopic arrangement was more precise and more units were sharply tuned. Response patterns were in majority of the excitatory type, and latencies were shorter on the average and less dispersed. More units were precisely time-locked to repetitive clicks. The proportion of units with monotonic intensity functions increased. The origin of thalamo-cortical projections was studied with focal injections of wheat-germ agglutinin labeled with horseradish peroxidase in functionally defined loci of the various auditory cortical fields. An evolution of the density of labeled cells in LV and OV was observed along the same rostro-caudal axis for which a gradient of functional properties is described above. Thalamo-cortical projections to the primary auditory area and the anterior auditory field originated predominantly from the anterior half of LV, whereas the posterior auditory field received inputs from a wider rostro-caudal extend of LV including its posterior half.

Acoustic Stimulation↗

PubMed1992

Binaural responses in rat inferior colliculus following kainic acid lesions of the superior olive: interaural intensity difference functions.

The binaural responses of cells in the rat's inferior colliculus were determined following either unilateral or bilateral lesions of the superior olivary complex (SOC). The lesions were made by local injection of kainic acid through a glass micropipette lowered stereotaxically into the auditory brain stem. After a recovery period of at least two weeks, neural responses to pure tone pulses were recorded from the inferior colliculus with tungsten microelectrodes inserted into the central nucleus. Attention was focused on neurons that exhibited binaural suppression, ie., were excited by contralateral and inhibited by bilateral stimulation. Binaural suppression responses were still present following complete unilateral destruction of the superior olive either ipsilateral or contralateral to the recording site. Binaural responses were also present following bilateral lesions of the SOC. Furthermore, the shape and slope of interaural intensity difference functions were not statistically different for normal animals and animals with either unilateral or bilateral SOC lesions. No differences were found among lesion groups or between recording sites ipsilateral or contralateral to the lesions. These data suggest that the superior olive is not essential for binaural interaction and that supraolivary structures play a significant role in shaping binaural responses in the inferior colliculus.

Acoustic Stimulation↗

PubMed1993

Brainstem auditory evoked responses from birth to adulthood: development of wave amplitude.

Brainstem auditory evoked response (BAER) was recorded in children from birth to 6 years and adults to study the development of wave amplitude. The amplitudes of all BAER waves increased with age, the greatest changes occurring during early infancy. Adult values were reached at 6 months of age for wave I and 2 years for wave V. The two waves continued to increase above the adult values until the highest amplitude value was reached at 3 years for wave I and 5 years for wave V. Subsequently, the amplitudes decreased towards the values in adults. The V/I amplitude ratio, which was slightly than the adult value shortly after birth, decreased during the first year of life and reached the minimum value between 1 and 4 years. Thereafter, it increased towards the adult value. Throughout the maturational stages the ratio was smaller than in adults. The amplitude of wave V was relatively stable and its variation was much smaller than those of wave I and V/I amplitude ratio. Lower normative limits of these amplitude measures were given to provide a reference baseline for further study under the similar experimental conditions. We suggest that, in light of age-related differences of the V/I ratio, the normative criteria for the amplitude ratio should be established in relation to age. The amplitude of wave V is recommended to be useful in some clinical situations.

Adult↗

PubMed1993

Effects of multiple sclerosis brainstem lesions on sound lateralization and brainstem auditory evoked potentials.

Magnetic resonance (MR) imaging, brainstem auditory evoked potentials (BAEPs), and tests of interaural time and level discrimination were performed on sixteen subjects with multiple sclerosis (MS). Objective criteria were used to define MR lesions. Of the eleven subjects in whom no pontine lesions were detected and the one subject who had pontine lesions that did not encroach upon the auditory pathways, all had normal BAEPs and interaural level discrimination, although a few had abnormal interaural time discrimination. Of four subjects with lesions involving the pontine auditory pathway, all had both abnormal BAEPs and abnormal interaural time discrimination; one also had abnormal interaural level discrimination. Analysis of the data suggest the following: waves I and II are generated peripheral to the middle of the ventral acoustic stria (VAS); wave III is generated ipsilaterally in the region of the rostral VAS, caudal superior olivary complex (SOC) and trapezoid body (TB); and waves V and L are generated contralaterally, rostral to the SOC-TB. The region of the ipsilateral rostral SOC-TB is implicated as part of the pathway involved in the generation of waves V and L. Interaural time discrimination of both high and low frequency stimuli were affected by all brainstem lesions that encroached on auditory pathways. A unilateral lesion in the region of the LL affected interaural time discrimination for low-frequency stimuli less severely than bilateral lesions of the LL or a unilateral lesion of the VAS. The only interaural level discrimination abnormality occurred for a subject with a unilateral lesion involving the entire rostral VAS. It appears that detailed analysis of lesion locations coupled with electrophysiological and psychophysical data holds promise for testing hypotheses concerning the function of various human auditory brainstem structures.

Acoustic Stimulation↗

PubMed1993

Does type I afferent neuron dysfunction reveal itself through lack of efferent suppression?

We present here two patients and three control subjects to demonstrate the clinical utility of studying evoked otoacoustic emissions and their contralateral suppression, as an aid to the delineation of afferent neuron dysfunction and possible lack of efferent suppression. The key patients here who fail to show contralateral suppression of their very robust otoacoustic emissions, concomitantly show paradoxically absent auditory brainstem responses (ABRs) and absence of middle ear muscle reflexes despite normal audiograms in the 2 kHz region and normal tympanograms. One of these patients has nearly normal pure tone sensitivity up to 3 kHz. The other has normal sensitivity in the 2 kHz region, but poor sensitivity on either side of that frequency. In addition, the two patients of interest show absent masking level differences and inordinately poor speech discrimination. Three 'foils' are presented: one patient with poor hearing on either side of 2 kHz, one with Bell's Palsy, and the third with bilateral temporal lobe disease. These patients show respectively that (1) isolated normal hearing at 2 kHz, (2) absence of middle ear muscle reflexes and (3) conscious cortical awareness of sound do not contribute directly to this intriguing clinical state. We propose that these patients with absent ABRs suffer from an auditory nervous system dysfunction which disrupts access to the efferent system. This condition also disables whatever systems contribute to the neural synchrony inherent in recording compound far-field action potentials such as the ABR. There are a number of hypotheses to be considered here. One suggests that the key patients are deficient in synchronous activation of Type I afferent fibers to the degree that they cannot activate efferent feedback, or they may be able to use only so-called Type II afferent neurons to support their normal zones of pure tone sensitivity. A less likely consideration focuses on the notion that discharge of primary neurons might be in some way synchronized by an efferent system which in these patients is the primary source of deficit.

Acoustic Impedance Tests↗

PubMed1993

Psychophysical frequency modulation thresholds in a FM-bat, Tadarida brasiliensis.

Echolocating bats hunting flying insects discriminate complex temporal patterns of acoustic stimuli. For bats using frequency modulated sonar calls (FM bats), there are no behavioral data on the perception of sinusoidally frequency modulated (SFM) stimuli. Discrimination performance for SFM stimuli of varying modulation depth was measured in 4 Tadarida brasiliensis in a two-alternative, forced choice procedure. A center frequency of 40 kHz was modulated with rates between 10 and 2000 Hz. It was found that discrimination performance improved from a mean threshold modulation depth of 3.05 kHz at a modulation rate of 2000 Hz to 1.58 kHz at a modulation rate of 10 Hz. Psychoacoustical modulation depth thresholds of T. brasiliensis are thus distinctly larger than those observed in bat species emitting constant frequency (CF) components followed by an FM-sweep, in active echolocation experiments. The modulation thresholds of T. brasiliensis are discussed in connection with the ability of bats to discriminate insect wingbeats. A comparison between non-echolocating mammals and the FM bat T. brasiliensis shows that the ability to echolocate is not reflected in the modulation thresholds.

Acoustic Stimulation↗

PubMed1994

Responses of inferior colliculus neurons in C57BL/6J mice with and without sensorineural hearing loss: effects of changing the azimuthal location of an unmasked pure-tone stimulus.

Azimuth functions (discharge rates evoked by tone bursts as a function of stimulus azimuth) were obtained from neurons in the inferior colliculus (IC) of C57 mice aged 2, 7 and 12 months. Because of a gene that affects the cochlea, C57 mice exhibit high-frequency sensorineural hearing loss at 7 and 12 months. Azimuth functions were examined for differences that might be related to the decline in localization acuity that accompanies hearing loss in this strain. Irrespective of age group, nearly all neurons in the central area of the IC were sensitive to the azimuth of a best frequency (BF) stimulus, as revealed by azimuth functions in which firing rates varied by more than 50% from maximum to minimum at one or more intensities. The age groups were similar in many respects (e.g., there were no significant differences in the proportion of functions meeting the criterion for direction sensitivity, the proportion of neurons with direction sensitive functions over a range of intensities, azimuth function shapes, the locations or stability of 'borders' separating angles evoking high versus low discharge rates). However, in 7- and 12-month-olds: the proportion of IC neurons in which the strongest excitatory driving was evoked by ipsilateral stimulation was significantly larger; azimuth function borders were more likely to be 'reversed' (i.e., the high rates being evoked by the more ipsilateral angle); and a greater proportion of azimuth functions met the criterion for direction sensitivity only minimally. The findings suggest that binaural excitatory-inhibitory interactions are altered in IC neurons of hearing-impaired mice.

Acoustic Stimulation↗

PubMed1994

Chirp evoked otoacoustic emissions.

The principles of short frequency sweeps (chirps) and their application to evoke transiently evoked otoacoustic emissions (TEOAE) are developed in comparison to using standard click stimuli. In contrast to click stimuli, chirp signals have the advantage of stimulating a freely selectable frequency range. In addition, chirp signals contain more energy than a click stimulus with the same maximum amplitude. The effects of different stimuli on TEOAE were investigated in normal hearing and hearing-impaired subjects. Using wide-band chirp signals yields a better signal-to-noise ratio compared to click stimulation. In addition, the stimulation of selected regions of the basilar membrane with frequency-limited chirps evokes TEOAE with frequency components that lie within the stimulated frequency range. The characteristic fine structure of this spectrum was found to be independent of the stimulus applied. The utilization of chirp stimuli appears to be useful for evoking TEOAE in, e.g., clinical applications.

Acoustic Stimulation↗

PubMed1994

Responses of peripheral auditory neurons to two-tone stimuli during development: III. Rate facilitation.

Approximately 25% of peripheral auditory neurons having low characteristic frequencies (CFs) and broad tuning, and recorded from immature animals, responded to two-tone stimuli with increases in discharge rate greater than predicted by linear summation of the responses to probe tones (facilitation), in contrast to the two-tone suppression observed in adult animals and in more sharply tuned immature neurons. Facilitation was not seen after 81 gestational days and was not observed when test tones produced a substantial increase in rate when presented alone. The fact that some neural responses were facilitated under conditions of two-tone stimulation during the final stages of cochlear differentiation provides additional evidence that signal transduction is fundamentally different in neonatal kittens than in adults. A linear model of basilar membrane mechanics coupled to nonlinear neural processes is proposed which can account for the production of facilitation.

Acoustic Stimulation↗

PubMed1995

Decreased inhibition to lateral superior olive neurons in young and aged Sprague-Dawley rats.

Lateral superior olive (LSO) neurons in young and aged Sprague-Dawley rats have functional properties consistent with a limited contralateral inhibition, which is markedly different from other animals. An unusually low proportion of LSO cells (36/113) exhibited contralateral inhibition (and ipsilateral excitation, IE), while over 25% of LSO units exhibited excitatory responses to contralateral stimuli. Inhibition of most IE LSO neurons was evident only when the contralateral intensity was greater than the ipsilateral intensity, resulting in a marked shift in sensitivity to interaural intensity differences (IID). The firing rate of IE neurons was also affected more by a change in intensity of ipsilateral compared to contralateral stimuli. The shift in the IID sensitivity and the relative decrease in effectiveness of contralaterally driven inhibition in Sprague-Dawley rat LSO neurons could be due to decreased inhibitory inputs from the MNTB principal cells, increased contralateral excitatory effects and/or increased ipsilateral excitatory effects. Age-related decreases in the numbers of MNTB neurons observed anatomically is not reflected in a change in LSO function. The Sprague-Dawley rat may be a useful model for the effect of reduced inhibition in the superior olivary complex on auditory behavior.

Acoustic Stimulation↗