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PubMed2001

Functional magnetic resonance imaging of human pontine auditory pathway.

The purpose of this study is to visualize brainstem auditory pathways by functional magnetic resonance imaging (fMRI). Eighteen healthy volunteers (age 28 to 42 years) with normal hearing function underwent fMRI examination on a 1.5 Tesla imaging system (Philips, Best, The Netherlands) with periodic click stimulation. Blood oxygen level dependent images were obtained using a three-dimensional EPI sequence with shifted echo technique (principles of echo shifting with a train of observations). Control scans without click stimulation were obtained in the identical setting. Cross correlation activation maps were calculated using a postprocessing tool (Philips). They were matched with anatomic slices of identical orientation and thickness. Five of 18 subjects were excluded because of motion artifacts. In 4/13 significant activation was observed at the root entry zone of the ipsilateral acoustic nerve corresponding to the cochlear nuclei. In 11/13 subjects, significant activation was found in the same slice contralaterally close to the floor of the 4th ventricle, corresponding to the expected region of the superior olivary nucleus. Activation of the rostral parts of the auditory pathway (inferior colliculus, medial geniculate body) was not found. In the absence of the stimulus no activation occurred in these structures. It was concluded that activation of the brainstem auditory pathways by click stimuli can be visualized by fMRI.

Acoustic Stimulation↗
PubMed2005

Avian superior olivary nucleus provides divergent inhibitory input to parallel auditory pathways.

The avian auditory brainstem displays parallel processing, a fundamental feature of vertebrate sensory systems. Nuclei specialized for temporal processing are largely separate from those processing other aspects of sound. One possible exception to this parallel organization is the inhibitory input provided by the superior olivary nucleus (SON) to nucleus angularis (NA), nucleus magnocellularis (NM), and nucleus laminaris (NL) and contralateral SON (SONc). We sought to determine whether single SON neurons project to multiple targets or separate neuronal populations project independently to individual target nuclei. We introduced two different fluorescent tracer molecules into pairs of target nuclei and quantified the extent to which retrogradely labeled SON neurons were double labeled. A large proportion of double-labeled SON somata were observed in all cases in which injections were made into any pair of ipsilateral targets (NA and NM, NA and NL, or NM and NL), suggesting that many individual SON neurons project to multiple targets. In contrast, when injections involved the SONc and any or all of the ipsilateral targets, double labeling was rare, suggesting that contralateral and ipsilateral targets are innervated by distinct populations of SON neurons arising largely from regionally segregated areas of SON. Therefore, at the earliest stages of auditory processing, there is interaction between pathways specialized to process temporal cues and those that process other acoustic features. We present a conceptual model that incorporates these results and suggest that SON circuitry, in part, functions to offset interaural intensity differences in interaural time difference processing.

Animals↗
PubMed1994

Primary auditory pathway and reticular activating system dysfunction in Alzheimer's disease.

Patients with Alzheimer's disease (AD) have pathologic involvement of several important components of the primary auditory pathway, including the inferior colliculus, medial geniculate body, primary auditory cortex, and secondary auditory cortex. The main components of the brainstem auditory evoked response (BAER) and middle latency response (MLR) reflect the function of portions of the primary auditory pathway, including those affected pathologically in AD. The amplitude of the P1 component of the MLR reflects the degree of neuronal activity of midbrain portions of the ascending reticular activating system (ARAS) with cortical cholinergic projections. To determine whether there is dysfunction of the primary auditory pathway and ARAS in AD, we compared simultaneous BAER and MLR component latency and amplitude measurements in patients with mild-moderate AD (n = 35) and age-matched healthy elderly controls (n = 34). There were significant latency delays in brainstem transmission time (BAER I-V interpeak latency; p < 0.05) and in primary auditory cortex evoked potential generation (MLR Pa latency; p < 0.05) in the AD group compared with controls. In addition, there was a significant reduction in the P1 component amplitude of the MLR in the AD group (p < 0.01). These results indicate dysfunction of the primary auditory pathway and ARAS in patients with mild-moderate AD and support the hypothesis that impairment of auditory function and of arousal are intrinsic features of AD.

Aged↗
PubMed1981

The anatomy and pathology of the central auditory pathways.

The anatomy of the central auditory pathways from the cochlear nucleus to the auditory cortex is reviewed. The common pathologies affecting these pathways are described and the neurological, ophthalmological and vestibular symptoms and signs, which are valuable in diagnosing and siting the level of these lesions are discussed.

Auditory Cortex↗
PubMed1981

Patterns of habituation to electrical stimulation of reticular and auditory pathways.

Patterns of habituation of the orienting response to electrical stimulation of midbrain, thalamus, and cortical components of the primary auditory pathway and the ascending reticular system were studied in awake, freely mobile cats. Individual orienting responses to high-frequency stimulation of structures in the auditory pathway could not be distinguished from orienting responses to stimulation of reticular structures. However, with repeated presentations of the stimulus, the orienting response to auditory pathway stimulation habituated significantly faster than did the orienting response to stimulation of structures in the ascending reticular system. Within the auditory pathway, orienting to stimulation of the inferior colliculus, the medial geniculate body, or primary auditory cortex did not result in distinguishable patterns of habituation. Stimulation of parietal cortex resulted in habituation that occurred significantly slower than habituation to auditory cortex stimulation, and significantly faster than habituation to stimulation of the midbrain reticular formation or the centre median nucleus of the thalamus. These results are discussed with regard to a hypothetical arousal system that includes the brain stem reticular formation, the intralaminar nuclei of the thalamus, and parts of the parietal cortex.

Animals↗
PubMed1992

Visual projections routed to the auditory pathway in ferrets: receptive fields of visual neurons in primary auditory cortex.

How does cortex that normally processes inputs from one sensory modality respond when provided with input from a different modality? We have addressed such a question with an experimental preparation in which retinal input is routed to the auditory pathway in ferrets. Following neonatal surgical manipulations, a specific population of retinal ganglion cells is induced to innervate the auditory thalamus and provides visual input to cells in auditory cortex (Sur et al., 1988). We have now examined in detail the visual response properties of single cells in primary auditory cortex (A1) of these rewired animals and compared the responses to those in primary visual cortex (V1) of normal animals. Cells in A1 of rewired animals differed from cells in normal V1: they exhibited larger receptive field sizes and poorer visual responsivity, and responded with longer latencies to electrical stimulation of their inputs. However, striking similarities were also found. Like cells in normal V1, A1 cells in rewired animals exhibited orientation and direction selectivity and had simple and complex receptive field organizations. Furthermore, the degree of orientation and directional selectivity as well as the proportions of simple, complex, and nonoriented cells found in A1 and V1 were very similar. These results have significant implications for possible commonalities in intracortical processing circuits between sensory cortices, and for the role of inputs in specifying intracortical circuitry.

Animals↗
PubMed1981

Ipsilateral auditory pathway suppression in dichotic CV speech perception: evidence from error analysis.

The exact nature of ipsilateral auditory pathway suppression under dichotic stimulation remains controversial. To help resolve this controversy, the distinctive feature confusions (blend vs nonblend errors) made on a dichotic listening CV test by two hemispherectomees were analyzed. Results support ipsilateral auditory pathway suppression under dichotic CV stimulation. Ipsilateral auditory pathway suppression is discussed in terms of stimulus specificity, subject, and task difference.

Adult↗
PubMed1979

Somatostatin along the auditory pathway.

Several structures associated with the auditory pathway have been examined by radioimmunoassay for their content of somatostatin. Of these, the medial geniculate body had the highest content, followed by the cochlear nucleus, inferior colliculus, auditory cortex and cochlea. Cochlear perilymph had no detectable somatostatin-like immunoreactivity.

Animals↗
PubMed1987

Functional and morphometrical maturation of the brainstem auditory pathway.

The correlation between the functional and morphological maturation of the auditory pathway was studied in preterm and term infants, children and adults. As to the auditory brainstem response (ABR), peak latencies and I-V interpeak latencies (central transmission) gradually decreased during the third trimester and the first 2 years postnatally. The calculated pontine auditory conduction velocity (PACV) showed dramatic development, which may indicate a more precise auditory function. The PACV value at the age of 2-4 years was almost the same as that of adults. In a histomorphometrical study, the density of nerve cells in the cochlear nucleus and the inferior colliculus was found to decrease with age, that in the inferior colliculus decreasing more slowly. Myelination in the lateral lemniscus proceeded from the late fetal to the infantile period, and the myelin sheaths of large diameter increased mainly in the infantile period. Thus, on studying ABR in combination with the quantitative histomorphometrical investigation, the development of PACV was found to be related to the maturation of nerve cells in the upper nuclei corresponding to each latency as well as myelination of small and large fibers in the auditory pathway. PACV, which can be calculated by studying ABR and magnetic resonance imaging, may be used to assess more accurately the brainstem function in individual patients.

Adolescent↗
PubMed1995

Differential effect of adaptation on the ipsi- and contralateral auditory brainstem pathways.

Auditory brainstem potential components II and V are delayed in the contralateral recording in comparison with that ipsilateral to the stimulus. Wave III is recorded earlier contralaterally. The effect of increasing stimulus repetition rate on the ipsilateral/contralateral latency differences in these components was examined. There is a progressive increase in latency from Wave I to Wave V ipsilaterally with an increase in stimulus rate; however, contralaterally there is no further increase in latency after Wave III. At 40 Hz stimulus rate, therefore, the ipsi/contra difference in the latency of Wave V disappears, suggesting that there is a differential effect of peripheral and central adaptation on the ipsilateral and contralateral auditory pathways.

Adult↗
PubMed1998

Spontaneous regeneration and recovery of hearing function of the central auditory pathway in young rats.

Spontaneous regeneration of the auditory pathway after transection of the ventral cochlear tract of the brain stem to pons was examined in young rats by the anterograde tracing method with wheat germ agglutinin-conjugated horseradish peroxidase. Care was taken to cut the tract as sharply as possible to minimize traumatic injuries. About 59% of the young rats examined showed successful regeneration. Functional recovery of hearing by the regenerated fibers was confirmed by the auditory brain stem response. Thus, the present findings contradict the widely held view that the mammalian central auditory system cannot be restored following damage.

Animals↗
PubMedDate not supplied

Middle latency auditory evoked potentials improve the detection of abnormalities along auditory pathways in multiple sclerosis patients.

Brain-stem and middle latency auditory evoked potentials (BAEPs and MLAEPs) have been studied in 34 multiple sclerosis (MS) patients. We were able to detect a central nervous system auditory pathway involvement in 17 (50%) of the patients: 38% by BAEPs alone (I-V inter-peak latency) and 47% by MLAEPs alone (Na and Pa peak latency). Five patients had abnormal MLAEPs with normal BAEPs whereas the opposite was detectable in only 1 patient. In addition, most MLAEP parameters in the MS group statistically differed from those obtained in the control group. Therefore, our results demonstrated that the auditory pathway impairment could frequently be located at a rostral level along the auditory radiation. In conclusion, even if only Na and Pa components were considered, MLAEPs succeeded in improving the sensitivity of the auditory evoked potential examination without increasing the false positive rate.

Acoustic Stimulation↗
PubMed1990

Gentamicin induced dose related alterations in lipid levels and lipid peroxidation in central auditory pathway of the rat.

A study was carried out on the effect of gentamicin on the central auditory pathway in rats. Rats were administered graded doses of gentamicin (60 mg, 100 mg and 160 mg/kg b.wt., i.m.) daily for 21 days. Levels of total lipids, phospholipids and cholesterol showed dose related increment in the central auditory pathway. On the other hand, the rate of lipid peroxidation was inhibited in all regions of the central auditory pathway of the rat.

Animals↗
PubMed1999

Activating separate ascending auditory pathways produces different human thalamic/cortical responses.

When auditory nerve function is lost due to surgical removal of bilateral acoustic tumors in cases of neurofibromatosis type 2, a sense of hearing may be restored by means of an auditory brainstem implant (ABI), which electrically stimulates the cochlear nucleus. Electrically evoked auditory brainstem responses recorded from ABI subjects exhibit a variety of waveforms due to the presence or absence of different components. Evidently, ABI stimulation activates different ascending auditory pathways in different individuals. This study examined whether such differences at the brainstem level are associated with corresponding differences at higher levels. Multichannel recordings of electrically evoked middle-latency and late auditory responses were obtained from two ABI subjects whose very different electrically evoked auditory brainstem responses represent distinct categories of waveform morphology. The waveforms of both types of response were qualitatively similar in that for each condition tested there were corresponding main peaks and troughs. Quantitatively, however, there were differences in the scalp distributions and magnitudes of all components present. One subject had distributions suggesting bilateral activation and an N1-P2 complex of large amplitude, whereas the other subject had distributions suggesting unilateral activation contralateral to the side of stimulation and an N1-P2 complex of small amplitude. The differences suggest that activation of different ascending pathways in the auditory system results in different spatial and temporal patterns of neural activity in the thalamic and/or cortical auditory areas.

Adult↗
PubMed1999

Activity influences on neuronal connectivity within the auditory pathway.

HYPOTHESIS: Sensorineural hearing loss may be associated with altered patterns of neuronal connections within the central auditory pathway. STUDY DESIGN: The cat auditory system was used to address the relative importance of impulse traffic within the auditory nerve in maintaining central nervous system connections. METHODS: Acute, unilateral deafness was induced by tetrodotoxin intoxication of cochlear hair cells. Analysis focused on the structural patterns of contact between auditory nerve endings called end bulbs of Held and their target neurons. Specifically, end bulb morphology and target cell size within the cochlear nucleus were examined. Highly specialized synaptic contacts at this junction provide a powerful site for study. RESULTS: The principal finding was that abolished activity in the auditory nerve caused nerve endings to assume a different shape, typified by more branching and smaller terminal swellings. The new shape is one typically associated with only a subpopulation of fibers in normal-hearing cats--those exhibiting a high-threshold, low-spontaneous activity profile. This result implies that abolished activity alters patterns of nerve fiber contact with second-order neurons. Tetrodotoxin produced differential effects on subpopulations of target neurons within the brainstem and is interpretable on the basis of "weighing" synaptic inputs. Second-order neurons that receive large axosomatic inputs from their parent fiber were significantly smaller than neurons that receive small, axodendritic terminals. Thus, attenuated auditory activity may produce differential effects across the auditory pathway, thereby disrupting the normal balance of inputs into synaptic stations. CONCLUSIONS: Impulse traffic is a critical factor in the interaction between the ear and central auditory stations and appears necessary for the maintenance of key synapses. As hearing disorders with impaired comprehension may be modeled by studies of auditory deafferentation, these observations extend the possibility that changes in central neuronal connections underlie reduced capabilities for processing restored auditory input.

Animals↗
PubMed1992

Mapping of c-fos expression elicited by pure tones stimulation in the auditory pathways of the rat, with emphasis on the cochlear nucleus.

C-fos expression was mapped in the auditory pathways of rats, stimulated acoustically with pure tones. In the cochlear nucleus, two clusters of c-fos-like immunoreactive neurons, located respectively in the caudal part of the dorsal cochlear nucleus and in the granular cell region, did not show clear systematic shift in their position as a function of the tones frequency. On the other hand, more rostrally in the dorsal cochlear nucleus, a cluster of c-fos-like positive neurons moved progressively from dorsal to ventral for decreasing tones frequency. In the posteroventral cochlear nucleus, another cluster of c-fos-like positive neurons was observed, whose position also varied with tones frequency. Surprisingly, no or very rare c-fos-like immunoreactive neurons were present in the anteroventral cochlear nucleus and in the superior olivary complex. In the inferior colliculus, however, c-fos-like immunoreactive neurons formed clear isofrequency contours, shifting from dorsolateral to ventromedial for increasing tones frequency. In the medial geniculate body c-fos-like immunostaining was restricted to the medial and dorsal divisions while the ventral division was free of labeling. The cause of this differential labeling along the auditory pathways is at present unknown but may eventually provide clues as to physiological differences in parallel auditory pathways.

Acoustic Stimulation↗
PubMed1998

Descending auditory pathways in the adult male zebra finch (Taeniopygia guttata).

Here, we examine the connectivity of two previously identified telencephalic stations of the auditory system of adult zebra finches, the neostriatal "shelf" that underlies the high vocal center (HVC) and the archistriatal "cup" adjacent to the robust nucleus of the archistriatum (RA). We used different kinds of neuroanatomical tracers to visualize the projections from the shelf to the HVC. In addition, we show that the shelf projects to the cup and that the cup projects to thalamic, midbrain, and pontine nuclei of the ascending auditory pathway. Our observations extend to songbirds anatomical features that are found in the auditory pathways of a nonoscine bird, the pigeon (Wild et al. [1993] J. Comp. Neurol. 337:32-62), and we suggest that the descending auditory projections found in mammals may also be a general property of the avian brain. Finally, we show that the oscine song control system is closely apposed to auditory pathways at many levels. Our observations may help in understanding the evolution and organization of networks for vocal communication and vocal learning in songbirds.

Animals↗
PubMed2004

Role of auditory stimulation in maturation of the auditory pathway.

OBJECTIVE: To compare the maturation of the auditory pathway, as shown by electrical brainstem auditory potentials (EABRs), in ears with and without prior auditory stimulation. MATERIAL AND METHODS: Electrophysiological data were collected prospectively from ears which had received cochlear implants. Implant-evoked (Imp)EABRs were recorded. Thirty children, implanted after January 2000, were selected according to a strict inclusion/exclusion protocol. All the children had received a 22-channel Nucleus cochlear implant (CI24 series). Intraoperatively, ImpEABRs were recorded using the Medelec Synergy Evoked Response system in conjunction with Nucleus Neural Response Telemetry software. The ImpEABR latencies of waves eII, eIII and eV and the morphology of wave eV were assessed. RESULTS: ImpEABRs alter during the first 12 months of life. The latency becomes shorter during this period and the morphology of wave eV alters from a broad shape to a more distinct waveform. This appears to occur independently, even in the absence of auditory stimulation. CONCLUSION: The development of electrical brainstem auditory potentials is not dependent on auditory stimulation.

Acoustic Stimulation↗