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PubMed1994

Mesencephalic projections of the cochlear nucleus in the frog, Rana esculenta.

The lectin Phaseolus vulgaris leucoagglutinin was injected into chochlear nucleus in order to study the trajectory and projections of the auditory pathway in the frog. A strong contralateral and a weak ipsilateral fiber bundle representing the lemniscus lateralis could be followed to the torus semicircularis. The following areas of termination could be discerned in the mesencephalon: (1) Each of the five subnuclei of the torus semicircularis received fibers from the cochlear nucleus, the largest number of fibers terminated in the principal nucleus. (2) Nuclei of the mesencephalic tegmentum were supplied by fibers from the cochlear nucleus on both sides. (3) The nucleus isthmi was innervated by a few small caliber fibers, and (4) the nucleus visceralis secundarius was richly supplied by large caliber fibers. Two ill-defined neuron groups in the mesencephalic tegmentum, the nucleus reticularis isthmi (5) and (6) the nucleus profundus mesencaphali were conspicuously outlined by the terminals of auditory fibers. Their positions suggested homology to the mammalian nucleus of the lemniscus lateralis. (7) A few fibers and boutons were labelled in the fifth and sixth layers of the optic tectum. Retrogradely labelled cells were found in the nucleus laminaris of the torus semicircularis and in the nuclei of mesencephalic tegmentum. Our results indicate that the frog auditory pathway is more complex at the level of the secondary fiber projections than has been previously recognized. The retrogradely labelled neurons suggest a descending control of the cochlear nucleus from higher structures of the auditory pathway.

Affinity Labels↗
PubMedDate not supplied

Pathways from auditory cortex to the cochlear nucleus in guinea pigs.

The inferior colliculus (IC) and superior olivary complex (SOC) are important sources of descending pathways to the cochlear nucleus. The IC and SOC are also targets of direct projections from the auditory cortex but it is not known if cortical axons contact the cells that project to the cochlear nucleus. Multi-labeling techniques were used to address this question in guinea pigs. A fluorescent anterograde tracer was injected into temporal cortex to label corticofugal axons. Different fluorescent tracers were injected into one or both cochlear nuclei to label olivary and collicular cells. The brain was subsequently processed for fluorescence microscopy and the IC and SOC were examined for apparent contacts between cortical axons and retrogradely labeled cells. The results suggest that cortical axons contact cochlear nucleus-projecting cells in both IC and SOC. In both regions, contacts were more numerous on the side ipsilateral to the injected cortex. In the IC, the contacted cells projected ipsilaterally or contralaterally to the CN. In the SOC, the contacted cells projected ipsilaterally, contralaterally or bilaterally to the CN. We conclude that auditory cortex is in a position to modulate descending pathways from both the IC and SOC to the cochlear nucleus.

Animals↗
PubMed2000

A site of auditory experience-dependent plasticity in the neural representation of auditory space in the barn owl's inferior colliculus.

The barn owl's optic tectum contains a map of auditory space that is based, in part, on a map of interaural time difference (ITD). Previous studies have shown that this ITD map is shaped by auditory experience. In this study, we investigated whether the plasticity responsible for experience-induced changes in ITD tuning in the tectum occurs within the tectum itself or at an earlier stage in the auditory pathway. We altered auditory experience in young owls by implanting an acoustic filtering device in one ear that caused frequency-dependent changes in sound timing and level. We analyzed the representation of ITD in normal and device-reared owls in two nuclei in the ascending pathway: the external nucleus of the inferior colliculus (ICX), the primary source of ascending auditory input to the tectum, and the lateral shell of the central nucleus of the inferior colliculus (ICCls), the primary source of input to the ICX. In the ICX, device rearing caused adaptive, frequency-dependent changes in ITD tuning, as well as changes in frequency tuning. These changes in tuning were similar to changes that occurred in the optic tectum in the same owls. In contrast, in the ICCls, tuning for ITD and frequency was unaffected by device rearing. The data indicate that plasticity at the level of the ICX is largely responsible for the adaptive adjustments in ITD tuning and frequency tuning that are observed in the optic tecta of owls raised with abnormal auditory experience.

Acoustic Stimulation↗
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↗
PubMed2000

Intrauterine growth retardation and brainstem auditory-evoked response in preterm infants.

Intrauterine growth retardation is frequently associated with intrauterine undernutrition, and can deleteriously affect brain function. Twenty-eight premature small for gestational age infants were compared with 28 premature appropriate for gestational age infants to determine whether intrauterine growth retardation was associated with abnormalities in the auditory pathway in the early neonatal period. The auditory pathway was studied between 4-18 wk of life by analysis of brainstem auditory-evoked potentials elicited by a 10/s 75 decibel above normal adult hearing level (dB nHL) click stimulus presented at the infants' ears. Peak latencies of components I, III and V, and interpeak latencies I-III, III-V and I-V, yielded no statistically significant differences between groups. The present study indicates that intrauterine growth-retarded premature infants may not have abnormalities of brainstem auditory-evoked response in the early neonatal period.

Age Factors↗
PubMed1996

Possible application of functional imaging of the human auditory system in the study of acclimatization and late onset deprivation.

After some period of experience with a single hearing aid, speech recognition performance may increase for material presented to the aided ear. Conversely, performance may decline for material presented to the unaided ear. Improved performance for the normally aided ear beyond that observed at the initial fitting of the hearing aid has been described as the acclimatization effect. The decline in speech recognition for material presented to the unaided ear has been described as the late onset auditory deprivation effect. For both the acclimatization and deprivation effects, the observed changes in performance are not considered to be a consequence of a change in the functional status of the cochlea. Rather, the benefits and decrements in speech recognition performance presumably reflect functional changes or reorganization in the central auditory pathway. In nonhuman species, changes in central auditory function can be examined by physiological recordings directly from various structures along the auditory pathway. However, these techniques are invasive and inappropriate for studying possible changes in central function for the human auditory system. The purpose of this review is to describe noninvasive "imaging" techniques appropriate for use with human subjects and the ways they could be applied to objectively identify physiological changes that might be associated with either acclimatization or late onset deprivation effects. Currently, few of these techniques have been applied to the study of acclimatization and late onset auditory deprivation. Possible application of these techniques to assess the differential performance changes for material presented to the normally aided and normally unaided ear will be discussed.

Acoustic Stimulation↗
PubMed2000

Sound lateralization and interaural discrimination. Effects of brainstem infarcts and multiple sclerosis lesions.

Subjects with brainstem lesions due to either an infarct or multiple sclerosis (MS) underwent two types of binaural testing (lateralization testing and interaural discrimination) for three types of sounds (clicks and high and low frequency narrow-band noise) with two kinds of interaural differences (level and time). Two major types of abnormalities were revealed in the lateralization performances: perception of all stimuli, regardless of interaural differences (time and/or level) in the center of the head (center-oriented), or lateralization of all stimuli to one side or the other of the head (side-oriented). Similar patterns of abnormal lateralization (center-oriented and side-oriented) occurred for MS and stroke patients. A subject's pattern of abnormal lateralization testing was the same regardless of the type of stimulus or type of interaural disparity. Lateralization testing was a more sensitive test than interaural discrimination testing for both types of subjects. Magnetic resonance image (MRI) scanning in three orthogonal planes of the brainstem was used to detect lesions. A semi-automated algorithm superimposed the auditory pathway onto each MRI section. Whenever a lesion overlapped the auditory pathway, some binaural performance was abnormal and vice versa. Given a lateralization test abnormality, whether the pattern was center-oriented or side-oriented was mainly determined by lesion site. Center-oriented performance was principally associated with caudal pontine lesions and side-oriented performance with lesions rostral to the superior olivary complex. For lesions restricted to the lateral lemniscus and/or inferior colliculus, whether unilateral or bilateral, just noticeable differences (JNDs) were nearly always abnormal, but for caudal pontine lesions JNDs could be normal or abnormal. MS subjects were more sensitive to interaural time delays than interaural level differences particularly for caudal pontine lesions, while stroke patients showed no differential sensitivity to the two kinds of interaural differences. These results suggest that neural processing of binaural stimuli is multilevel and begins with independent interaural time and level analyzers in the caudal pons.

Adult↗
PubMed2004

Cochlear implantation in adults with prelingual deafness. Part II. Underlying constraints that affect audiological outcomes.

OBJECTIVES/HYPOTHESIS: To discuss the underlying physiological and anatomical constraints on audiological performance of late-implanted prelingually deafened adult cochlear implant patients. STUDY DESIGN: Retrospective review. METHODS: Published literature on the topic of auditory pathway responses to prolonged congenital deafness was reviewed. In particular, the authors sought to identify the anatomical and physiological changes that take place in both the peripheral and central auditory pathways in response to prolonged deafness, as well as how they are altered by chronic electrical stimulation. RESULTS: The currently available evidence suggests that the colonization of the auditory cortex by other sensory modalities is the main limiting factor in postimplantation performance, not the pathological degenerative changes of the auditory nerve, cochlear nucleus, or auditory midbrain. CONCLUSION: The reviewed evidence, although circumstantial, suggests that emphasizing aurally based educational programs before (with hearing aids) and after cochlear implantation could reduce the cortical colonization phenomenon and potentially improve postimplantation audiological performance of patients with long-term prelingual deafness.

Adult↗
PubMed1982

Auditory brainstem evoked responses in autistic children.

Auditory brainstem evoked responses (ABRs) were studied in 16 autistic children. Three children had severe delays in wave I latency, indicating defective functioning of the peripheral auditory pathway. The remaining subjects also had delayed wave I latency but only for right ear stimulation at the lowest stimulus intensity. Eight autistic children (and no control subjects) had ABR transmission time values 3 SDs beyond the normal mean, suggesting auditory processing defects peripheral to or within the brainstem auditory pathway. These findings (1) may have no causal relationship to the child's autistic handicaps, (2) may represent distortions in auditory input that impair the learning of language, and (3) may reflect an earlier state in which abnormal input directly caused maldevelopment of forebrain systems necessary for language and cognitive function.

Adolescent↗
PubMed1995

Brainstem lesions and click lateralization in patients with multiple sclerosis.

The ability to lateralize dichotic clicks with either interaural time delays (ITD) or interaural level differences (ILD) was tested in seven multiple sclerosis (MS) subjects who had normal audiograms. Along with the psychoacoustical tests, magnetic resonance images (MRI) of the subjects' brainstem were obtained. After matching each MRI section with the corresponding section of a computerized atlas of the brainstem, the parts of the auditory pathway affected by each MS lesion were determined. Of the seven subjects two performed normally with both types of interaural asymmetry and had no brainstem lesions involving the auditory pathway. Two subjects performed normally only with level differences, but perceived all the dichotic clicks with different ITDs in the center of the head; both had lesions involving the trapezoid body. Three subjects could not perform normally with either task, perceiving the clicks to the sides and never in the center for both ITDs and ILDs; all three had unilateral lesions of the lateral lemniscus. A multi-level decision making model is proposed to account for these results.

Audiometry↗
PubMed1991

[Changes in the early auditory evoked potentials during premature infancy, infancy and early childhood].

We report on the derivation of the early acoustic evoked potentials (EAEP) in 498 healthy normotrophic infants between the 32nd post-conceptional week and the end of the 3rd year of life. The changes of the wave latencies and interpeak intervals in different age periods until they reached the constant values found in adults are followed by 1528 potential derivations. The varying rate of maturation of the auditory pathway in the brain stem can be followed up well by the potential patterns. The time around the 34th post-conceptional week is characterized by a maturation episode of central and peripheral auditory pathway portions. The extrauterine matured normotrophic preterm infant shows statistically significantly longer latencies of waves III and V and a longer interpeak interval I-V in comparison with normotrophic term neonates. Gestational age, birth weight and Apgar score of the preterm infant had no influence on the behaviour of the EAEP. After the 38th post-conceptional week practically only the maturation of the central auditory pathway portion takes place: the main portion of the latency shortenings of III and V and IPI I-V is applicable to the 1st year of life. The maturation is an individually characterized progressive process of varying speed with no fixed age for its completion. Thus the adult wave V latency was reached in 8% of the 4- to 6-month-old children, and in 57% by the end of the 1st year of life. The adult IPI I-V values were reached in 42% of the 10- to 12-month-old children.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1976

Intracellular study of synaptic events related to phase-locking responses of cat cochlear nucleus cells to low frequency tones.

In this study intracellular recording techniques were used to study the synaptic events related to phase-locking of cochlear nucleus cells to low frequency stimuli. A variable degree of phase-locking was noted even with units of the same low characteristic frequency. With low frequency phase-locking units an excitatory postsynaptic potential (EPSP) occurred in response to each period of the frequency stimulus, but the probability of an action potential occurring decreased as the frequency of the stimulus was raised. Complex units were described which phase-locked at lower frequencies of stimulation but did not at higher frequencies where the temporal pattern of firing to tone burst stimulation changed as well. Results are discussed as they relate to the frequency following response recorded with gross electrodes in the lower auditory pathway and the relationship to frequency coding in the auditory pathway.

Animals↗
PubMed1998

Immaturity of electrophysiological response of the neonatal auditory brainstem to high repetition rates of click stimulation.

Changes in brainstem auditory evoked response (BAER) with varying stimulus repetition primarily represent neural processes concerning the efficacy of synaptic transmission in the brainstem auditory pathway. In this study the BAER to different rates of clicks was recorded from 16 term neonates. The results were compared with those from 16 adults to examine whether the degree of maturation of synaptic transmission of the neonatal brainstem auditory pathway parallels that of general function of the pathway. All BAER wave latencies and interpeak intervals increased linearly and wave amplitudes reduced with increasing click rate. The absolute rate-dependent changes in BAER measures were much greater in the neonates than in the adults, reflecting a significant immaturity in the efficacy of synaptic transmission in the neonatal auditory brainstem and in the ability of the neonatal brainstem to process rapid acoustic stimulation. When the data obtained at higher click rates at various age groups were analyzed as percentages, using the BAER measurements at conventionally used slow rate (21/s) of clicks as the denominators, the changing rates (%), or relative changes, of most BAER measures at higher rates in the neonates were still greater than those in the adults. Therefore, the rate-dependent BAER changes in the neonates are relatively less mature than general aspects of the BAER, reflected by the BAER elicited with conventionally used slow rates of clicks. These findings suggest that synaptic efficacy in the neonatal brainstem auditory pathway is relatively less mature than general function of the pathway and thus may be more susceptible to unfavourable perinatal conditions.

Acoustic Stimulation↗
PubMed1993

Experience-dependent plasticity in the inferior colliculus: a site for visual calibration of the neural representation of auditory space in the barn owl.

The optic tectum (homolog of the superior colliculus) contains mutually aligned neural maps of auditory and visual space. During development, the organization of the auditory map is guided by spatial information provided by vision: barn owls raised wearing prismatic spectacles, which optically shift the visual field and the visual map in the optic tectum, develop an auditory map that is shifted by an approximately equivalent amount, such that alignment between the two maps is preserved (Knudsen and Brainard, 1991). In this study we investigated whether this shift in the auditory map is intrinsic to the optic tectum or whether it reflects plasticity at an earlier stage in the auditory pathway. Owls were raised wearing prismatic spectacles that displaced the visual field by 23 degrees to the left or right. This manipulation alters the normal correspondence between locations in the visual field and interaural time difference (ITD), the primary cue for the azimuth of a sound source. In normal owls and in owls with at least 150 d of prism experience, extracellular unit recordings were used to assess the representations of ITD at anatomically and physiologically defined sites in the optic tectum and in the two prior stages of the auditory pathway, the external and central nuclei of the inferior colliculus (ICx and ICc). In the optic tectum of normal owls, the values of ITD to which units responded most strongly (best ITDs) varied systematically with the azimuths of unit visual receptive fields (VRFs). In the prism-reared owls, best ITDs were shifted from normal toward the values of ITD produced by sounds at the locations of the units' optically displaced VRFs. In the ICx of prism-reared owls, the representation of ITD also was shifted from normal, by an amount and in a direction that could completely account for the shift in ITD measured in the optic tectum. At some sites in the ICx, the shift in ITD tuning was apparent within the first 7-8 msec of the response; shifted tuning at such short latencies argues that the altered representation of ITD in the ICx reflects plasticity in the ascending auditory pathway, and is not the result of descending activity from higher auditory centers. In the ICc, which immediately precedes the ICx in the ascending pathway, the representation of ITD was normal. The results indicate that the visual instruction of auditory spatial tuning of neurons in the optic tectum reflects plasticity at the level of the ICx, the site where the auditory map of space is first synthesized.

Acoustic Stimulation↗
PubMed1987

Different analysis of frequency and amplitude modulations of a continuous tone in the human auditory cortex: a neuromagnetic study.

We have measured auditory evoked magnetic fields to intermittent frequency and amplitude modulations (FMs and AMs) of a continuous tone in 6 healthy humans. The stimuli were presented in pairs separated by 500 ms in four different combinations (FM-AM, FM-FM, AM-FM and AM-AM). Both modulations elicited neuromagnetic responses of similar waveforms: the largest deflection, N100m (magnetic counterpart of the electric N100), was preceded by a low amplitude P60m and followed by P200m. For stimuli of different types, the decrease of N100m from the first to the second response was less than expected from the recovery cycle of the responses, estimated from the pairs of similar stimuli. We interpret these results as evidence for different processing of amplitude and frequency modulations in the auditory pathways up to the level of supratemporal auditory cortex.

Auditory Cortex↗
PubMed1996

Auditory evoked potentials in multiple sclerosis: correlation with magnetic resonance imaging.

The present study addresses issues regarding the location of neural sources (i.e. generators) of human auditory evoked potentials (AEPs), and the pattern of neural conduction in the auditory pathway. AEPs were recorded from fifteen patients with multiple sclerosis (MS) and compared to normals. The recordings included auditory brainstem responses (ABRs), mid-latency responses (MLRs), and long-latency responses (LLRs). AEP latency abnormalities were related to the locus of demyelinating lesions, as determined by magnetic resonance imaging (MRI) scans. The data demonstrated several anatomical patterns relating abnormal ABR wave intervals and abnormal MRI signals. From these patterns specific loci for ABR neural sources in the brainstem might be postulated. In addition, the earlier the ABR waves, the more unilateral the abnormalities appeared, suggesting bilateral sources for later waves. The MLRs were highly correlated with ABR wave V and were associated with greater abnormality in MRI signals in midbrain and forebrain regions. In general, patients with abnormal LLRs also had widespread AEP and MRI abnormalities, supporting a multiple source approach for the N1 wave of the LLRs. The observation that LLRs were only abnormal in the presence of bilateral ABR abnormalities suggests a cross wiring which would serve as a compensatory mechanism for unilateral disturbances. The AEP data showed dissociation between early and late wave abnormalities, thus supporting parallel channels for neural conduction in the central auditory system. Such a model calls for some degree of independence of AEP generators along the auditory pathway.

Adult↗
PubMedDate not supplied

Auditory processing in patients with temporal lobe epilepsy.

UNLABELLED: Temporal epilepsy, one of the most common presentation of this pathology, causes excessive electrical discharges in the area where we have the final station of the auditory pathway. Both the anatomical and functional integrity of the auditory pathway structures are essential for the correct processing of auditory stimuli. AIM: to check the Auditory Processing in patients with temporal lobe epilepsy regarding the auditory mechanisms of discrimination from sequential sounds and tone patterns, discrimination of the sound source direction and selective attention to verbal and nonverbal sounds. METHOD: eight individuals with temporal lobe epilepsy were assessed, after excluding those with non-confirmed diagnosis or with the focus of discharges not limited to this lobe. The evaluation was carried out through special auditory tests: Sound Localization Test, Duration Pattern Test, Digits Dichotic Test and Non-Verbal Dichotic Test. Their performances were compared to the performances of individuals without neurological diseases (case-control study). RESULTS: similar performances were observed between patients with temporal lobe epilepsy and the control group regarding the auditory mechanism of sound source direction discrimination. Comparing the other auditory mechanisms assessed, the patients with temporal lobe epilepsy presented worse results. CONCLUSION: individuals with temporal lobe epilepsy had more deficits in auditory processing than those without cortical damage.

Acoustic Stimulation↗
PubMed1975

Central auditory deficits and temporal-lobe lesions.

The capacity of auditory pathways from the weak ear (ear with the suppressed score in a dichotic listening test) to transmit speech information to the cortical processing areas was examined on 10 patients with unilateral temporal-lobe lesions. Three lines of evidence were developed to suggest that auditory pathways from the weak ear and speech information presented to the weak ear are not completely suppressed during dichotic stimulation. Speech in the weak ear frequently interacts with competing speech information in the strong ear causing contrasting articulatory features from the two ears to be blended into a single response. Two models of unilateral central auditory deficits are discussed: ipsilateral-suppression and transmission-line.

Auditory Pathways↗