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PubMed2002

Effect of efferent-induced changes on acoustical reflex.

The efferent auditory pathway is known to alter the cochlear electrical properties, at low stimulus levels. This study aims to identify the effect of activation of the efferent auditory pathway through contralateral wide- and narrow-band noise at high sound levels. Acoustical reflex threshold and acoustical reflex amplitude (at ART + 10 dB) were obtained at three frequencies (500 Hz, 1 kHz, 2 kHz) in 60 normally hearing subjects, in the presence and absence of contralateral wide-and narrow-band noise (centred around reflex activating stimuli) at 30 dB SL. Results showed a consistent reduction in amplitude, and an increase in threshold, for 1- and 2-kHz tones in the presence of contralateral wideband but not narrow-band noise. Results are discussed with reference to the electrical properties of the cochlea, and the response properties of the efferent auditory pathway and cochlear nucleus.

Adolescent↗
PubMed2000

Traces of learning in the auditory localization pathway.

One of the fascinating properties of the central nervous system is its ability to learn: the ability to alter its functional properties adaptively as a consequence of the interactions of an animal with the environment. The auditory localization pathway provides an opportunity to observe such adaptive changes and to study the cellular mechanisms that underlie them. The midbrain localization pathway creates a multimodal map of space that represents the nervous system's associations of auditory cues with locations in visual space. Various manipulations of auditory or visual experience, especially during early life, that change the relationship between auditory cues and locations in space lead to adaptive changes in auditory localization behavior and to corresponding changes in the functional and anatomical properties of this pathway. Traces of this early learning persist into adulthood, enabling adults to reacquire patterns of connectivity that were learned initially during the juvenile period.

Animals↗
PubMed1994

Long latency evoked potentials in a case of corpus callosum agenesia.

Following monoaural stimulation, long latency auditory evoked potentials (LLAEPs) recorded from contralateral temporal areas have a shorter latency and larger amplitude than those recorded from the ipsilateral temporal areas. This observation agrees with the operational model drawn up in 1967 by Kimura, which assumes that only anatomically prevailing crossed auditory pathways are active during dichotic hearing, while direct pathways are inhibited. The inputs may then be conveyed to the contralateral cortex, from where they finally reach the ipsilateral temporal areas by means of interhemispheric commissures. It is this mechanism which may underline the right ear advantage for verbal stimuli and the left ear advantage for melodies observed when administering dichotic listening tasks. With the aim of verifying this hypothesis, we recorded temporal LLAEPs in a 21 year-old woman suffering from complex partial seizures, whose CT scan and MRI showed corpus callosum agenesia. Our data support the hypothesis that ipsilateral pathways are greatly inhibited by the contralateral pathways, and therefore auditory stimuli can be supposed to reach the contralateral auditory cortex from where they are transferred through the corpus callosum to the ipsilateral auditory cortex.

Acoustic Stimulation↗
PubMed2002

Neurophysiology of cochlear implant users II: comparison among speech perception, dynamic range, and physiological measures.

OBJECTIVE: The overall objective of this study was to relate electrically evoked potentials recorded from different levels of the auditory pathway with behavioral measures obtained from adult cochlear implant subjects. The hypothesis was that adult recipients of cochlear implants who have open-set speech perception and those recipients with no open-set speech perception would differ in their neurophysiologic responses recorded at one or more levels of the auditory pathway. DESIGN: The subjects were 11 adults implanted with the Clarion cochlear implant. The electrical auditory brainstem response (EABR, Wave V), electrical auditory middle latency response (EAMLR, Na-Pa complex), and the electrical late auditory response (ELAR, N1-P2 complex), were recorded from three intra-cochlear electrodes. The stimuli used to record the evoked potentials varied in rate and amplitude. Behavioral measures (between threshold and upper limit of comfortable loudness) were used to define the subject's dynamic range at the different stimulus rates. Word and sentence recognition tests evaluated subjects' speech perception in quiet and noise. Evoked potential and behavioral measures were examined for statistical significance using analysis of variance for repeated measures and correlational analyses. RESULTS: Subjects without open-set speech recognition demonstrated 1) poorly formed or absent evoked potential responses, 2) reduced behavioral dynamic ranges, 3) lack of change in the size of the dynamic range with a change in stimulus rate, and 4) longer periods of auditory deprivation. The variables that differentiated the best performers included 1) presence of responses at all three levels of the auditory pathway, with large normalized amplitudes for the EAMLR, 2) lower evoked potential thresholds for the Na-Pa complex, 3) relatively large dynamic ranges, and 4) changes in the size of the dynamic range with changes in stimulus rate. CONCLUSIONS: In this study, the inability to follow changes in the temporal characteristics of the stimulus was associated with poor speech perception performance. Results also illustrate that variability in speech perception scores of cochlear implant recipients relates to neurophysiologic responses at higher cortical levels of the auditory pathway. Presumably, limited neural synchrony for elicitation of electrophysiologic responses underlies limited speech perception. Results confirm that neural encoding with electrical stimulation must provide sufficient physiologic responses of the central nervous system to perceive speech through a cochlear implant.

Adult↗
PubMed2006

No hearing loss associated with the use of artemether-lumefantrine to treat experimental human malaria.

Artemisinin derivatives are becoming the first-line treatment for uncomplicated malaria in areas with widespread resistance to chloroquine. Although generally safe and well tolerated, it has been suggested from animal experiments, and more recently from one human study with artemether-lumefantrine, that these compounds are potentially neurotoxic, affecting particularly the brainstem auditory pathways. We report here the auditory analyses of 15 volunteers who underwent an experimental human malaria infection and were treated with artemether-lumefantrine. The subjects underwent audiological examination before the start of the study, during infection, and after treatment. Examination included standard tone audiometry, high frequency tone audiometry and auditory brainstem response (ABR). No effects on hearing loss that were deemed to be caused by drug treatment were found using tone audiometry. ABR analysis similarly failed to demonstrate any auditory pathway damage in the volunteers after treatment. We have thus not found any clear evidence of a detrimental effect on the auditory system by artemether-lumefantrine treatment in uncomplicated malaria. Our results support the continued implementation of artemisinin derivatives in the fight against drug-resistant malaria.

Adolescent↗
PubMed1991

Neonatal conductive hearing loss does not compromise brainstem auditory function and structure in rhesus monkeys.

The effect of conductive hearing loss on the maturation of the auditory pathway was evaluated using the auditory brainstem response (ABR) in rhesus monkeys. Ten newborn rhesus monkeys were assigned to control (N = 4), unilateral hearing loss (N = 3), or bilateral hearing loss (N = 3) groups. Hearing loss was created by surgically excising a 3 mm section of the external auditory canal and suturing the canal. Auditory brainstem responses to click stimuli were recorded prior to and after the surgical procedure and bi-monthly or monthly for a 14 month follow-up period. Results showed that after surgery all ears developed an estimated 30-50 dB conductive hearing loss which was retained throughout the follow-up period. Contrary to expectations, the latencies of the ABR component waves decreased with age in all ears. When adjusted for hearing level, there were no differences between ears in maturation of the component waves of the ABR. These data suggest that, in primates, a conductive hearing loss does not affect the maturation of those aspects of the auditory pathway reflected in the ABR. Furthermore, the conductive losses were not accompanied by any discernible change in the neuronal sizes of brainstem auditory neurons or the volume of the cochlear nuclei.

Animals↗
PubMed1999

Mechanisms of experience-dependent plasticity in the auditory localization pathway of the barn owl.

Sound localization is a computational process that requires the central nervous system to measure various auditory cues and then associate particular cue values with appropriate locations in space. Behavioral experiments show that barn owls learn to associate values of cues with locations in space based on experience. The capacity for experience-driven changes in sound localization behavior is particularly great during a sensitive period that lasts until the approach of adulthood. Neurophysiological techniques have been used to determine underlying sites of plasticity in the auditory space-processing pathway. The external nucleus of the inferior colliculus (ICX), where a map of auditory space is synthesized, is a major site of plasticity. Experience during the sensitive period can cause large-scale, adaptive changes in the tuning of ICX neurons for sound localization cues. Large-scale physiological changes are accompanied by anatomical remodeling of afferent axons to the ICX. Changes in the tuning of ICX neurons for cue values involve two stages: (1) the instructed acquisition of neuronal responses to novel cue values and (2) the elimination of responses to inappropriate cue values. Newly acquired neuronal responses depend differentially on NMDA receptor currents for their expression. A model is presented that can account for this adaptive plasticity in terms of plausible cellular mechanisms.

Animals↗
PubMed2001

Morphology and physiology of neurons in the ventral nucleus of the lateral lemniscus in rat brain slices.

The ventral nucleus of the lateral lemniscus (VNLL) is a prominent neuronal group that lies within the auditory pathway connecting the auditory lower brainstem and midbrain. Previous physiologic studies showed that VNLL neurons respond mainly to contralaterally presented sounds and display various firing patterns. To understand better the role that VNLL neurons play in transmitting and processing of auditory information, we examined the morphology of VNLL neurons and their cellular physiology in young rat brain slices. We made whole-cell patch-clamp recordings and labeled cells intracellularly with neurobiotin to investigate the relation between morphologic neuronal types, intrinsic membrane properties, and postsynaptic responses. VNLL neurons fell into two distinct morphologic groups, i.e., bushy cells and stellate cells, based on their dendritic patterns. Stellate cells were grouped further into stellate I, II, and elongate cells according to soma shape, dendritic branches, and orientation. Bushy cells showed an onset firing pattern and a nonlinear current-voltage relationship. All three subtypes of stellate cells had a linear current-voltage relationship, but exhibited different firing patterns. Stellate I cells showed regular and onset-pause firing patterns, whereas stellate II cells showed adapting and elongate cells showed burst firing patterns. Bushy cells and stellate cells responded to stimulation of the lateral lemniscus with excitatory and/or inhibitory synaptic potentials. These results suggest that the VNLL is a heterogeneous neuronal group and that it contains many channels for processing different kinds of auditory information. Neuronal morphology and intrinsic membrane properties contribute to the behavior of individual neurons.

Animals↗
PubMed2003

Acute audiogenic stress-induced activation of CRH neurons in the hypothalamic paraventricular nucleus and catecholaminergic neurons in the medulla oblongata.

Strong c-fos expression was induced in neuronal cells of several brain nuclei and the auditory cortex by a short duration auditory stimulation (white noise) in rats. By double immunostaining, Fos-immunoreactive cell nuclei appeared in corticotropin-releasing hormone (CRH)-containing neurons in the hypothalamic paraventricular nucleus, but not in CRH neurons elsewhere in the brain including the central nucleus of the amygdala. Among brain catecholaminergic neurons, only cells in the medulla oblongata (in the A1/C1and A2/C2 cell groups) established double immunostaining for Fos and tyrosine hydroxylase. Sound stimulus in rats with unilateral tympanotomy and plugging the airways resulted in side differences of Fos immunoreactivity in neurons of the auditory pathways and the auditory cortex, but the effect was bilateral in hypothalamic and amygdaloid nuclei. The present data provide evidence for the participation of CRH-synthesizing neurons in hypothalamus and medullary catecholaminergic neurons in the central organization of responses to audiogenic stress stimuli.

Acoustic Stimulation↗
PubMed2007

The pattern of auditory brainstem response wave V maturation in cochlear-implanted children.

OBJECTIVE: Maturation of acoustically evoked brainstem responses (ABR) in hearing children is not complete at birth but rather continues over the first two years of life. In particular, it has been established that the decrease in ABR wave V latency can be modeled as the sum of two decaying exponential functions with respective time-constants of 4 and 50 weeks [Eggermont, J.J., Salamy, A., 1988a. Maturational time-course for the ABR in preterm and full term infants. Hear Res 33, 35-47; Eggermont, J.J., Salamy, A., 1988b. Development of ABR parameters in a preterm and a term born population. Ear Hear 9, 283-9]. Here, we investigated the maturation of electrically evoked auditory brainstem responses (EABR) in 55 deaf children who recovered hearing after cochlear implantation, and proposed a predictive model of EABR maturation depending on the onset of deafness. The pattern of EABR maturation over the first 2 years of cochlear implant use was compared with the normal pattern of ABR maturation in hearing children. METHODS: Changes in EABR wave V latency over the 2 years following cochlear implant connection were analyzed in two groups of children. The first group (n=41) consisted of children with early-onset of deafness (mostly congenital), and the second (n=14) of children who had become profoundly deaf after 1 year of age. The modeling of changes in EABR wave V latency with time was based on the mean values from each of the two groups, allowing comparison of the rates of EABR maturation between groups. Differences between EABRs elicited at the basal and apical ends of the implant electrode array were also tested. RESULTS: There was no influence of age at implantation on the rate of wave V latency change. The main factor for EABR changes was the time in sound. Indeed, significant maturation was observed over the first 2 years of implant use only in the group with early-onset deafness. In this group maturation of wave V progressed as in the ABR model of [Eggermont, J.J., Salamy, A., 1988a. Maturational time-course for the ABR in preterm and full term infants. Hear Res 33, 35-47; Eggermont, J.J., Salamy, A., 1988b. Development of ABR parameters in a preterm and a term born population. Ear Hear 9, 283-9] of normal hearing children: a sum of two decaying exponential functions, one showing an early rapid decrease in latency and the other a slower decrease. Remarkably, the time-constants fell well within the ranges described by Eggermont and Salamy (i.e., 3.9 and 68 weeks), consistent with the time-course of the neurophysiological mechanisms presumably involved in auditory pathway maturation during the first 2 years of life: i.e., myelination and increased synaptic efficacy. In contrast, relatively little change in wave V was evident in children with late-onset deafness. In agreement with the notion that EABR maturation follows an apex-to-base gradient as described for ABR, we observed that wave V latencies were longer for the basal than the apical end of the implant electrode array and remained so throughout the study period, whatever the time of onset of deafness. CONCLUSIONS: The findings in the early-onset of deafness group support the theory that auditory pathways remain "frozen" during the period of sensory deprivation until cochlear implant rehabilitation restores the normal chronology of maturational processes. In children with late-onset deafness, however, some maturational processes may occur before the onset of deafness, and thus less additional maturation is required during the first two years of implant use resulting in no significant EABR latency changes being observed in this period. The results suggest that the rehabilitation-induced plasticity of the auditory pathways is, in case of late auditory deprivation, unlikely to result in neurophysiological outcomes similar to those observed in children with early auditory deprivation. SIGNIFICANCE: Changes in EABR wave V latency over the first 2 years of cochlear implant use were found to be well fitted by the sum of two decaying exponential functions in children with early-onset deafness. This is in line with the maturation of ABR wave V latency in normal-hearing children over the first two years of life. Further studies are needed to assess whether the differences observed in terms of auditory pathways maturation are associated with consistent differences in terms of language development.

Adolescent↗
PubMed2004

Immunocytochemical distribution of Met-enkephalin-Arg6-Gly7-Leu8 (Met-8) in the auditory system of the rat.

Methionine-enkephalin-Arg(6)-Gly(7)-Leu(8) (Met(8)) is known to act as a neurotransmitter or neuromodulator and it has been implicated in pain, cardiovascular and motor mechanisms, but its role in audition is currently unknown. In the present study we have applied an immunocytochemical technique and describe the distribution of cell bodies and fibers containing Met(8) in the auditory pathway of the rat. The main finding is that we found either Met(8)-immunoreactive fibers or cell bodies or both in virtually all nuclei of the rat auditory system except for the medial superior olive and the ventral division of the medial geniculate body in which we did not find any immunoreactivity for Met(8). This suggests that the neuropeptide Met(8) is widely distributed throughout the auditory system of the rat. Our results suggest that Met(8) could play at least two roles in hearing. It seems to be involved in the processing of the descending auditory pathway, and it may be implicated in the multisensory integration of auditory information that takes place in the non-lemniscal auditory pathway.

Animals↗
PubMed1975

[Functional organization of pathways transmitting auditory signals in the somatosensory zone of the cat cerebral cortex].

The functional properties of the auditory projections to the somatosensory zones S2 and S were studied by recording evoked potentials in anesthetized and vigil unrestrained cats. The thresholds of evoked potentials recorded from SI and SII were higher by 15--35 db than those from AI. No tonotopical localization was found in SI and SII. Signals about pure tones of different frequencies were conducted to SI and SII via area AI. The signals about clicks were ascending to SI and SII not only through this pathway, but also through other ones. It suggested from the time constants analysis of the first positive wave of the evoked potentials that the interneuronal organization of the cortical auditory projections to AI is not so comples as that to SI and SII. The differences in amplitudes of evoked potential recorded in SI indicate that the head projection areas receive higher portion of the auditory projections. This is confirmed by the morphological evidence.

Animals↗
PubMed1995

Electrical resonances in central auditory neurons.

In the auditory pathway, signal processing depends on the filter functions of neurons. We used frequency analysis to investigate the contributions of intrinsic membrane properties to the input-output relationships in neurons. The whole-cell tight-seal recording technique in brain slices of chicks was used to study neurons at four levels of the auditory pathway. Neurons displayed resonant peaks in their voltage responses to injected sinusoidal currents that swept through a specified frequency range. Higher resonant frequencies tended to predominate at relatively lower stations in the auditory pathway (approximately 100 Hz in the nucleus magnocellularis, 24 Hz in the nucleus laminaris, 6 Hz in the nucleus ovoidalis). Field L neurons (cortex homologue) displayed low pass filter characteristics without resonance. We propose that the subthreshold membrane resonances amplify synaptic inputs at specific frequencies and contribute to the chick's ability to decode temporal sound parameters.

Animals↗
PubMed1997

A monosynaptic GABAergic input from the inferior colliculus to the medial geniculate body in rat.

The goal was to investigate possible monosynaptic GABAergic projections from the inferior colliculus (IC) to thalamocortical neurons of the medial geniculate body (MGB) in the rat. Although there is little evidence for such a projection in other sensory thalamic nuclei, a GABAergic, ascending auditory projection was reported recently in the cat. In the present study, immunohistochemical and tract-tracing methods were used to identify neurons in the IC that contain GABA and project to the MGB. GABA-positive projection neurons were most numerous in the central nucleus and less so in the dorsal and lateral cortex. They were rare in the lateral tegmental system and brachium of the IC. The dorsal nucleus of the lateral lemniscus also contained GABA-positive projection neurons. In brain slices, stimulation of the brachium produced monosynaptic inhibitory postsynaptic potentials in morphologically identified thalamocortical relay neurons. The inhibitory potentials cannot originate locally, because they persisted when ionotropic glutamatergic transmission was blocked. Typically, brachium stimulation elicited a GABAA-mediated inhibitory potential followed by an excitatory potential and a longer latency GABAB-mediated inhibitory potential. We conclude that the GABA-containing neurons of the IC make short-latency, monosynaptic inputs to the thalamocortical projection neurons in the MGB. Such inputs may distinguish the main auditory pathway from indirect or tegmental auditory pathways as well as from other sensory systems. Monosynaptic inhibitory inputs to the medial geniculate may be important for the regulation of firing patterns in thalamocortical neurons.

Animals↗
PubMed1985

The auditory pathology of brain death as revealed by auditory evoked potentials.

A case of brain death is reported in which the auditory brainstem response, middle latency component, and slow vertex response were recorded before and after the cessation of cortical activity, and in which histological examination of the temporal bone and central auditory pathways was performed post mortem. Brain death of at least 48 hours' duration was demonstrated by neurological examination, flat electroencephalographic recording, and persistent absence of auditory evoked responses. The postmortem examination was performed 3 hours after death. The pathological studies of the whole length of the auditory pathway revealed total autolysis of the organ of Corti, marked cell loss in the dorsal cochlear nucleus, and moderate cell loss in other nuclei of the central auditory pathway. It should be considered that total autolysis of the organ of Corti and severe cell loss of the cochlear nucleus may occur if the auditory brainstem response becomes absent in comatose patients.

Adult↗
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↗
PubMed2001

Evidence of peripheral auditory activity modulation by the auditory cortex in humans.

At the auditory periphery, the medial olivocochlear system is assumed to be involved in complex sound processing and may be influenced by feedback from higher auditory nuclei. Indeed, the descending auditory pathway includes fibers coming from the auditory cortex that are anatomically well positioned to influence the superior olivary complex, and thus the medial efferent system. The aim of the present study was to verify the hypothesis of an implied influence of the auditory cortex on the peripheral auditory system. In three rare cases of patients presenting with intractable temporal lobe epilepsy, Heschl's gyrus (i.e. the temporal superior gyrus) was surgically removed in the right hemisphere in two patients and in the left hemisphere in a third patient, in order to minimize epilepsy attacks, as preoperative stereoencephalography had shown the epileptic focus or tumor to be situated in those locations. In all three cases, several weeks after the operation the medial olivocochlear system was clearly less functional on both sides, but especially on the side contralateral to the resection. In healthy controls, no such pattern was obtained. In four other epileptic patients, who were operated unilaterally at the anterior temporal pole, amygdala and hippocampus with the temporal gyrus partially spared, efferent suppression grew stronger in the ear ipsilateral to surgery. These results revealed that, in humans, the primary and secondary auditory cortex play a role in modulating auditory periphery activity through direct or indirect efferent fibers. In accordance with previous findings, this descending influence may improve the auditory afferent message by adapting the hearing function according to cortical analysis of the ascending input.

Adult↗