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PubMed1982

Noise impairment in the guinea pig. I. Changes in electrical evoked activity along the auditory pathway.

Changes in the cochlear microphonics (CM), auditory nerve action potential (AP), and evoked responses from the inferior colliculus (IC-ER) and auditory cortex (AC-ER) of the guinea pig were assessed after exposure to white noise of 115 dB for 30 min. Both continuous and intermittent (200 ms noise and 200 ms pause) exposures were used. In comparison with the pre-exposure level, CM isopotential curves were shifted by 1.1 +/- 0.5 dB (means +/- S.E.) on the average in the range of 0.5-8 kHz (recorded at the round window). The amplitude-intensity function of the click-evoked auditory nerve action potential decreased by 8.4 +/- 1.2 dB, that of the inferior colliculus evoked response by 20.9 +/- 3.7 dB, and the amplitude-intensity function of the auditory cortex evoked potential decreased by 6.2 +/- 4.7 dB. A similar reduction in the amplitude was found after both continuous and intermittent noise exposure. In contrast to the decrease in amplitudes of evoked potentials, the latency-intensity functions of the individual waves of potentials evoked along the auditory pathway did not change when compared at the same click intensity before and after the exposure. The results suggest that individual auditory nuclei are impaired by the noise to different extents and that the impairment does not increase linearly up to the auditory cortex.

Action Potentials↗

PubMed1976

Electrophysiologic assessment of the lower portion of the auditory pathway in the human subject. A preliminary report on auditory evoked eighth nerve and brain stem potentials including their relation to the electrocochleogram.

The electrophysiologic approaches to the assessment of electrical activity of the various portions of the human auditory pathway are reviewed. A method of detection of click evoked eighth nerve and brain stem responses with the use of surface electrodes is described. Preliminary observations in the normal subject with different stimulus parameters and in patients with unilateral deafness are presented. Simultaneous recordings of the eighth nerve and brain stem responses with the cortical evoked response and with the extratympanic electrocochleogram are described. The technique of click evoked eighth nerve and brain stem potentials offers us an additional method of objective assessment of auditory function in a human subject.

Audiometry↗

PubMed1978

The organization of central auditory pathways in a reptile, Iguana iguana.

The present experiments were designed to trace the central auditory pathways in an extant reptile, the New Worlkd lizard--Iguana iguana, utilizing anterograde axonal degeneration stained by the Fink-Heimer ('67) method and the retrograde axonal transport of horseradish peroxidase (LaVail and LaVail, '74). Beginning with the projections of the auditory portion of the VIIIth nerve, the ascending pathways were traced through successive relay nuclei to the telencephalon. The auditory portion of the VIIIth nerve projects to two nuclei in the dorsomedial medulla-nucleus angularis and nucleus magnocellularis medialis. These two nuclei together with a third cll group, nucleus magnocellularis lateralis (intercalated between nucleus angularis and nucleus magnocellularis medialis), have been referred to as the auditory tubercle in previous studies (cf. Miller, '75). The axonal degeneration following large lesions of the auditory tubercle and small lesions of nucleus angularis demonstrated the second order auditory pathways. Fibers leave nucleus angularis ventrally and travel to the ventral surface of the medulla where they cross the midline and ascend to the midbrain in pathways resembling the trapezoid body and the lateral lemniscus of mammals. Along these pathways, terminal arborizations of some fibers were seen in three lower brainstem nuclei while other fibers ascent to the midbrain and terminate in the central nucleus of the torus semicircularis. Experiments in which horseradish peroxidase injections were made in the torus semicircularis demonstrated that nucleus angularis is a primary source of second order auditory fibers to the midbrain and, in addition, that two of the lower brainstem targets of the auditory tubercle project to the torus semicircularis. These lower brainstem pathways were shown to be associated with the auditory system by electrophysiologically recording sound-evoked responses from clusters of cells in the torus semicircularis. Ascending fibers arising from the central nucleus of the torus semicircularis were followed rostrally where they entered the dorsal thalamus and terminated throughout nucleus medialis. Finally, a thalamotelencephalic auditory pathway was traced from nucleus medialis into the lateral forebrain bundle. Terminations of this pathway from nucleus medialis were seen in the medial dorsal ventricular ridge and in the striatum. It was concluded that the ascending auditory pathways of the iguana bear a remarkable resemblance to both the mammalian and avian auditory pathways from the level of the first order neurons in the VIIIth nerve to the level of the telencephalon. At the same time, there are important specializations of the auditory system in birds and mammals such as the development of particular lower brainstem nuclei. Nevertheless, a basic plan for the organization of the auditory system in terrestrial vertebrates can be recognized which invites comparisons with the vertebrate classes that remained in aquatic habitats...

Animals↗

PubMed1994

Parvalbumin, calbindin D-28k, and calretinin immunoreactivity in the ascending auditory pathway of horseshoe bats.

In the subcortical auditory system of Rhinolophus rouxi, antibodies directed against the calcium-binding proteins parvalbumin, calbindin D-28k, and calretinin yield partly overlapping and partly complementary labeling patterns which are described in detail for each nucleus. The most general features of the labeling patterns are that: 1) Parvalbumin is a potent marker for large and heterogeneous populations of cells and puncta (presumed axon terminals) throughout the auditory pathway. 2) Immunostaining with the monoclonal calbindin-antiserum was typically absent or sparse in most auditory brainstem centers, but prominent in auditory nerve fibers and in cells of the medial geniculate body (MGB). 3) Calretinin label is abundant but more restricted to subsets of auditory nuclei or subpopulations of cells than parvalbumin. 4) Calcium-binding proteins are useful markers to define particular subregions or cell types in auditory nuclei: for example, i) different labeling patterns are obtained within the nuclei of the lateral lemniscus and adjacent tegmental zones; ii) in the inferior colliculus both calbindin- and calretinin-antisera yield similar regional specific staining patterns, but label different cell types; iii) subregions of the medial geniculate body have characteristic profiles of calcium-binding proteins; and iv) analyses of different nuclei showed that there is no simple common denominator for cells characterized by the expression of particular calcium-binding proteins, nor does labeling correspond in a straightforward way with specific functional systems. 5) there are profound differences between the calbindin labeling patterns seen in Rhinolophus and those in other mammals.

Animals↗

PubMed1983

Measurement of frequency selectivity of single neurons in the central auditory pathway.

A large sample of threshold tuning curves of single units from various levels of the auditory pathway was used in order to examine the relationship between best frequency and a measure of sharpness of tuning. This measure is simply the tuning curve bandwidth at 20 dB above threshold on a square-root scale of frequency. The transformation was found to be largely independent of unit best frequency, suggesting that it is suitable for comparing sharpness of tuning between samples. No difference in sharpness of tuning was found between samples of units from nuclei in the lemniscal auditory pathway, although units from the anterior auditory field showed broader tuning than those in the lemniscal pathway.

Animals↗

PubMed1990

Functional changes in the brainstem auditory pathways following open-heart surgery.

Measurements of the auditory brainstem responses (ABR) were used to assess the functional integrity of the brainstem auditory pathways following open-heart surgery. Recording of ABR was performed in 31 patients before and after open-heart surgery. The values of ABR measurements were compared to those of a control group of normal-hearing subjects. Postoperative aberrations in ABR were observed in 12 patients. Brainstem transmission time measured as wave I to wave V interpeak latency (I-V IPL) was prolonged in 6 patients and shortened in 8 patients. Concomitant impairment in hearing function was not present, and therefore, these changes most probably reflected a functional alteration in the neurological status of the brainstem. Possible explanations of the postoperative changes in I-V IPL are discussed, however, further studies are needed to clarify the pathogenesis of these changes. Measurement of ABR may prove to be an important adjunct in the overall assessment of the neurological status following open-heart surgery.

Adult↗

PubMed1995

The auditory pathway in cat corpus callosum.

The cortical auditory fields of the two hemispheres are interconnected via the corpus callosum. We have investigated the topographical arrangement of auditory callosal axons in the cat. Following circumscribed biocytin injections in the primary (AI), secondary (AII), anterior (AAF) and posterior (PAF) auditory fields, labelled axons have been found in the posterior two-thirds of the corpus callosum. Callosal axons labelled by small individual cortical injections did not form a tight bundle at the callosal midsagittal plane but spread over as much as one-third of the corpus callosum. Axons originating from different auditory fields were roughly topographically ordered, reflecting to some extent the rostro-caudal position of the field of origin. Axons from AAF crossed on average more rostrally than axons from AI; the latter crossed more rostrally than axons from PAF and AII. Callosal axons originating in a discrete part of the cortex travelled first in a relatively tight bundle to the telo-diencephalic junction and then dispersed progressively. In conclusion, the cat corpus callosum does not contain a sector reserved for auditory axons, nor a strictly topographically ordered auditory pathway. This observation is of relevance to neuropsychological and neuropathological observations in man.

Animals↗

PubMed1984

Effects of reaction time performance on single-unit activity in the central auditory pathway of the rhesus macaque.

The activity of single units at various locations in the central auditory pathway of rhesus macaques was recorded during the monkeys' performance and nonperformance in an auditory reaction time task. Evoked unit responses during performance were compared with those observed during passive delivery of identical stimuli. Single units were recorded from the cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate nucleus, and auditory cortex. Significant effects of task performance on unit discharge patterns were observed at all levels of the central auditory pathway: Spontaneous discharge rates in the more peripheral auditory nuclei tended to be higher during performance. Evoked discharge that occurred relatively late during a stimulus presentation (greater than 75 msec after stimulus onset) was increased during performance, compared with the nonperformance condition, in nuclei above the cochlear nucleus. The initial latency of evoked discharge was increased during performance for subcortical nuclei but was decreased for units in auditory cortex. These results suggest that the effects of performance may be mediated by a tonic increase in the excitability of auditory units which operates primarily at peripheral auditory stations, and a descending, stimulus-evoked increase in excitability which primarily influences the cells of higher auditory nuclei. At the cortical level, these changes lead to increased signal-to-noise ratio of the evoked response during performance in the auditory task.

Acoustic Stimulation↗

PubMed2005

Fibroblast growth factor-2 immunoreactivity is present in the central and peripheral auditory pathways of adult rats.

Recent findings have pointed out the role of neurotrophic factors in the survival and maintenance of neurons of the auditory system. Basic fibroblast growth factor (bFGF, FGF-2) is a potent neurotrophic molecule whose actions can be seen in the central and peripheral nervous systems. In the present study, FGF-2 immunoreactivity was analyzed in the auditory pathways of the adult rat, employing a well-characterized polyclonal antibody against FGF-2. In the cochlea, FGF-2 immunoreactivity was observed in the inner and outer hair cells of the organ of Corti, spiral ganglion neurons, spiral limbus, and stria vascularis. Stereological methods employing optical fractionator revealed the presence of 84.5, 15, and 0.5% of spiral ganglion neurons possessing FGF-2 immunoreactivity of strong, moderate, and weak intensity, respectively. In the central auditory pathways, FGF-2 immunoreactivity was found in the cytoplasm of the neurons of the cochlear nuclei, trapezoid body nuclei, medial geniculate nucleus, and inferior colliculus. The two-color immunoperoxidase method showed FGF-2 immunoreactivity in the nuclei of astrocytes throughout the central auditory pathway. Computer-assisted microdensitometric image analysis revealed higher levels of specific mean gray values of FGF-2 immunoreactivity in the trapezoid body and ventral cochlear nucleus and also in the spiral ganglion and inner hair cells. Sections incubated with FGF-2 antibody preabsorbed with human recombinant FGF-2 showed no immunoreaction in the majority of the studied regions, exhibiting only a slight immunoreactive product in the hair cells of the organ of Corti. Furthermore, no changes in immunoreactivity were observed in sections incubated with FGF-2 antiserum preincubated with human recombinant acidic FGF (FGF-1). The findings suggest that FGF-2 may exert paracrine and autocrine actions on neurons of the central and peripheral auditory systems and may be of importance in the mechanism of hearing diseases.

Animals↗

PubMed2004

Functional magnetic resonance imaging of activation in subcortical auditory pathway.

OBJECTIVES: Functional magnetic resonance imaging (fMRI) has been used to investigate activation of the auditory cortex; however, assessment of activation in the subcortical auditory pathway has been challenging. The aim of this study was to examine neural correlates of cortical and subcortical auditory activation evoked by pure-tone stimulus using silent fMRI. STUDY DESIGN: Prospective analysis. METHODS: Seventeen normal-hearing volunteers (7 male, 10 female; age range, 14-37 yrs) underwent silent fMRI. An audiometer was used to deliver pure tones of 1000 Hz to the left ear. Pure tones were presented at hearing thresholds determined in the scanner. Brain regions showing increased activation during pure-tone stimulus presentation were mapped and auditory activations exceeding P <.001 were included in the analysis. RESULTS: Pure-tone stimuli evoked bilateral activation in cortical regions of the transverse and superior temporal gyri and the planum temporale. Activation in subcortical structures included the medial geniculate body, inferior colliculus, lateral lemniscus, superior olivary complex, and cochlear nucleus. CONCLUSIONS: Silent functional magnetic resonance imaging findings documented the feasibility of detecting activation elicited by pure tone along the cortical and subcortical auditory pathway. The use of this technique in the assessment of disorders with auditory dysfunction merits further investigation.

Acoustic Stimulation↗

PubMed1981

Sprouting in the avian brainstem auditory pathway: dependence on dendritic integrity.

The brainstem auditory pathway of the chicken were used to examine the relationship between the maintenance of dendrites following denervation and the successful reinnervation (sprouting) by surviving afferents. In the system the third-order cells in n. laminaris receive spatially segregated binaural innervation from n. magnocellularis. Afferents from the ipsilateral n. magnocellularis innervate the dendrites on the dorsal aspect of n. laminaris cells, while afferents from contralateral magnocellular neurons innervate ventral dendrites via the crossed dorsal cochlear tract. Denervation of the ventral dendrites of n. laminaris cells by transection at the midline results in rapid and severe atrophy of the denervated dendrite. Unilateral cochlea removal induces transneuronal degeneration of 30-45% of the ipsilateral magnocellular cells, thereby partially denervating one dendrite of the n. laminaris cells on each side of the brain. In animals with long-standing transections of the crossed dorsal cochlear tract there is no evidence of sprouting the fibers from the ipsilateral n. magnocellularis when the projections of the surviving magnocellular neurons are traced with degeneration methods after a secondary cochlea removal. However, when dendrites of n. laminaris are partially denervated dendrites do not disappear. Furthermore, secondary lesions of the crossed dorsal cochlear tract or secondary cochlea removal reveal that these denervated dendrites are reinnervated by the afferents from the opposite n. magnocellularis which are normally restricted to the opposite dendrite of the n. laminaris cells.

Animals↗

PubMed1993

[Effect of auditory deprivation on maturation of auditory pathways in the rat].

To examine the influence of an auditory deprivation on the maturation of the central auditory pathways we reared newborn rats until the age of 21 days in a sound-proof chamber and additionally sutured their pinnae. At the age of 21 days we reopened the outer ear channels and placed the young rats into a normal environment. Every three days we recorded brain stem auditory evoked potentials with clicks, 1-, 8- and 16 kHz tone pulses until the age of 45 days, at the age of 60 days for the last time. Additionally all animals underwent middle latency response audiometry. Only directly after reopening the ear channels the thresholds for 8 and 16 kHz were worse in the deprived animals than in the normal ones. For clicks and 1 kHz and all testings on the other days the mean threshold response of the deprived rats did not differ from that of the normal animals. Testing with clicks, the interpeak latencies (I-IV), that is the brain stem transmission time, were prolonged significantly between the 24th and the 36th day of life. Measuring with 1 kHz tone pulses we found a difference from the 30th to the 36th day. Testing with 8 and 16 kHz tone pulses the brainstem transmission time was significantly prolonged between the 21st and 33rd day of life. The middle latency responses showed large variances, there was no significant difference. After the 36th day of life all latencies had normalised.

Aging↗

PubMed1995

Functional magnetic resonance imaging of the central auditory pathway following speech and pure-tone stimuli.

Functional magnetic resonance imaging (FMRI) is a new noninvasive technique for imaging cerebral function. Studies of the human central auditory pathway examined responses in eight normal hearing volunteers following auditory stimuli, including narrative speech and pure-tone audiometry. The activation demonstrated by FMRI is modeled on an increase in regional blood flow with increased neuronal activity. The FMRI signals represent deoxyhemoglobin concentration changes in capillaries within the region of the brain that is activated. Brain activation was imaged in the superior temporal gyrus during text reading and pure tones. Activation in both text and pure-tone presentation did not vary with the intensity of the auditory stimulus and elicited a dominant response in the left temporal lobe. These observations demonstrate the capability of FMRI to correlate anatomic and functional relationships in the human central auditory pathway.

Acoustic Stimulation↗

PubMed1991

Neurogenesis in the brain auditory pathway of a marsupial, the northern native cat (Dasyurus hallucatus).

Neurogenesis in the auditory pathway of the marsupial Dasyurus hallucatus was studied. Intraperitoneal injections of tritiated thymidine (20-40 microCi) were made into pouch-young varying from 1 to 56 days pouch-life. Animals were killed as adults and brain sections were prepared for autoradiography and counterstained with a Nissl stain. Neurons in the ventral cochlear nucleus were generated prior to 3 days pouch-life, in the superior olive at 5-7 days, and in the dorsal cochlear nucleus over a prolonged period. Inferior collicular neurogenesis lagged behind that in the medial geniculate, the latter taking place between days 3 and 9 and the former between days 7 and 22. Neurogenesis began in the auditory cortex on day 9 and was completed by about day 42. Thus neurogenesis was complete in the medullary auditory nuclei before that in the midbrain commenced, and in the medial geniculate before that in the auditory cortex commenced. The time course of neurogenesis in the auditory pathway of the native cat was very similar to that in another marsupial, the brushtail possum. For both, neurogenesis occurred earlier than in eutherian mammals of a similar size but was more protracted.

Aging↗

PubMed2004

Pathophysiological changes of the central auditory pathway after blunt trauma of the head.

It is the aim of the present paper to correlate clinical symptoms of auditory dysfunction (tinnitus, hyperacusis, hearing loss) one year on average after a blunt trauma of the head with objective audiological test results (otoacoustic emission and auditory brainstem response testing, impedance audiometry) and to compare these findings to controls without history of head trauma. Thirty-one patients (24-56 years) were included. They were largely female (n = 26). The clinical and otolaryngological examination (including otoscopy) of all patients revealed no pathological abnormalities. Pure-tone audiograms were normal with one exception (pre-existing noise-induced hearing loss) as well as tympanograms. The main auditory symptoms were tinnitus (n = 9), hyperacusis (n = 2) and a reported transient hearing loss immediately after the trauma (n = 16) (which had improved at the time of examination). The results of testing the central auditory pathway showed that the transiently evoked otoacoustic emissions (otoemissions) revealed statistically significant differences between amplitude differences of all patients as well as patients with tinnitus and controls in the linear, but not in the non-linear stimulation mode. A complete loss of stapedial reflex responses was found in 12 of the patients and a partial (irregular) loss (in at least more than two frequencies) in four additional patients. Auditory brainstem responses (ABR) were normal in all patients, but 76% had lowered loudness discomfort levels (LDL). Blunt trauma of the head can lead to auditory dyfunction, probably as a result of diffuse axonal injury of the central auditory pathway. An initial sensorineural hearing loss after the trauma (as a result of the inner ear fluid concussion) was transiently reported only. Auditory symptoms play a minor role in the so-called "postconcussive syndrome," but should be considered and evaluated fully.

Adult↗

PubMed1999

Integrity of the auditory pathway in young children with congenital and postmeningitic deafness.

This study compares the functional integrity of the auditory pathways of congenitally deaf and postmeningitically deaf children. We used the electrical auditory brain stem response evoked by promontory stimulation to assess 49 profoundly deaf children before cochlear implantation. The age at implantation ranged from 21 months to 15 years (mean 4.5 years). The onset of deafness was either congenital or up to the age of 2 years (mean 5 months). The cause of deafness was meningitis in 19 children (39%) and congenital in 30 (61%). The number of children with identifiable waveform components (eV, eIII, and eII) was significantly greater in the congenitally deaf group. We also analyzed the amplitudes, the latencies, and 4 parameters of the amplitude input-output functions. All the statistically significant differences were in favor of better responses in the congenitally deaf children. These results suggest that the functional status of the peripheral neurons of the auditory pathways may be more intact in congenitally deaf children than in postmeningitic children.

Adolescent↗

PubMed2005

Effect of radiotherapy on retro-cochlear auditory pathways.

OBJECTIVES: The cochlea may be damaged by modern conventional radiotherapy (RT) for head and neck cancers when the ear is included in the radiation field. It is unclear however, if the retro-cochlear auditory pathways are affected as well, which has clinical significance in cochlear implantation. This study aims to investigate the integrity of the retro-cochlear auditory pathways in patients who had received RT for nasopharyngeal carcinoma. STUDY DESIGN: Prospective study. METHODS: Patients who were newly diagnosed with nasopharyngeal carcinoma and treated by RT alone were studied. Evoked response audiometry and PTAs were carried out prior to and after RT (at 3, 18, and 48 months postRT). In addition, evoked response audiometry was also performed during the 3rd, 5th, and 7th week of RT. Waves 1 to 5, 1 to 3, and 3 to 5 latencies were measured. The values recorded during and postRT were compared with those recorded before RT. In addition, a subset of ears that demonstrated postRT sensorineural hearing loss were identified so that their respective wave 1 to 5 interwave latencies could be similarly compared. Wilcoxon signed ranks test was used in the statistical analysis. To confirm that the cochlea and internal auditory meatus receive significant doses of radiation, the RT treatment plans of nine other nasopharyngeal carcinoma patients treated by the same RT technique were analyzed to derive dose-volume histograms of these structures. RESULTS: Twenty-seven patients (20 males and 7 females) with a mean age of 51.2 (range 36-75) years participated in the study. There was no statistically significant difference in waves 1 to 5, 1 to 3, and 3 to 5 interwave latencies recorded during RT and postRT as compared with those recorded before RT (P > .05). Pre- and postRT wave 1 to 5 latencies of the 16 ears that had postRT hearing deterioration were also not statistically significant (P = .366). The mean radiation doses delivered to the cochlea and internal auditory meatus ranged from 24.1 to 62.2 Gy and 14.4 to 43.4 Gy, respectively. CONCLUSION: This study suggests in patients who have had RT for nasopharyngeal carcinoma, the retro-cochlear auditory pathways are functionally intact even in the longer term.

Adult↗

PubMed1988

BAEPs in hereditary motor-sensory neuropathies. Involvement of central brain-stem parts of the auditory pathway.

Brain-stem auditory evoked potentials (BAEPs) were tested in 34 patients with hereditary motor and sensory neuropathy (HMSN), most of which with HMSN type I with the autosomal dominant type of inheritance. In only a few cases a recessive or sporadic form of the disease was diagnosed, and one patient was rated as HMSN type III. In 44% complicated forms with signs of CNS involvement (cerebellar manifestation, nystagmus or cranial nerve lesions) were ascertained. BAEPs tests revealed slightly, albeit non-significantly prolonged wave III latency while wave V latency was significantly prolonged and so was the interpeak interval of waves III-V. One third of the records showed a prominent decrease in wave V amplitude.

Adolescent↗