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PubMed1992

[The definition of "selectivity" in reduced speech discrimination as a sequela of peripheral perceptual damage].

The term "selectivity" means the ability to correctly select speech in the presence of other kinds of noise. In the literature the term "selectivity" is used very differently with regard to the different levels of the hearing system. In actual practice a separation of this term is important for possible audiometric topodiagnosis study. The problems of selectivity in hearing are discussed and different terms are proposed regarding the different planes of hearing system: 1. for the interaural-external ear transfer function, the terms "stereophonic outer ear effect" or "stereophonic outer ear efficiency" are used; 2. for the power of the inner ear, the term "ability of analysis"; 3. for the central auditory pathways, the term "chiasmatic selectivity" or "meshing selectivity"; 4. for the cortical centre, "active selectivity." The audiometric test batteries used clinically for topodiagnosis are discussed and two examples shown to illustrate the usefulness of the distorted Freiburg Word Test for detecting "analytical ability" in cases of inner ear damage.

Attention↗
PubMed1995

Distribution and origin of serotoninergic afferents to guinea pig cochlear nucleus.

The distribution of serotoninergic fibers in the guinea pig cochlear nucleus was studied with serotonin immunohistochemistry. In addition, the origin of the serotoninergic fibers was determined by combining the retrograde transport of wheat germ agglutinin-apohorseradish peroxidase (gold conjugated) with serotonin immunohistochemistry. Immunoreactivity was present in varicose and nonvaricose fibers that were unevenly distributed throughout the cochlear nucleus. The fibers were most prominent in the superficial layers of the dorsal cochlear nucleus and the anterior spherical cell area of the anteroventral cochlear nucleus. Although less prominent, serotonin-positive fibers were also present in the remaining part of the anteroventral cochlear nucleus and the posteroventral cochlear nucleus. A few positive fibers were present in the auditory nerve root and the dorsal and intermediate acoustic striae. Double-labeled cells were found throughout the rostral-caudal extent of the serotoninergic system from the caudal linear nucleus to the nucleus raphe pallidus. However, most were confined to the dorsal (52%) and median (18%) raphe nuclei. Some serotoninergic cell groups contained retrogradely labeled cells that were not serotonin immunoreactive, indicating nonauditory afferents to cochlear nucleus containing other neurotransmitter substances. Serotonin may tonically modulate auditory processing within the cochlear nucleus as well as influence certain ascending auditory pathways. Most of the serotonin in the cochlear nucleus comes from superior raphe nuclei that also project to basal ganglia motor systems and limbic structures. Therefore, the effect of serotonin on the cochlear nucleus may be related to level of arousal or behavioral state.

Animals↗
PubMedDate not supplied

Physiological differentiation within the auditory part of the thalamic reticular nucleus of the cat.

Spike trains of 153 single units were recorded in the caudoventral part of the thalamic reticular nucleus (RE) of 7 nitrous oxide anaesthetized cats. Functional properties defined by spontaneous activity pattern, studied by mean of auto renewal density histograms, were used to subdivide the units into 4 groups. Types I (18%), II (56%) and III (15%) were defined by an increasing bursting activity and Type IV (11%) by firing no bursts spontaneously. The responses to auditory stimuli confirmed that the caudoventral part of RE is tightly related to central auditory pathways. Responses to white noise bursts (200 ms duration) significantly let appear that Type I units responded in a high proportion (greater than 70%) until 80 ms after the stimulus onset, Type II units where mostly affected during the entire stimulus duration, and Type III units showed preferentially late responses. The units responsive to high frequencies (greater than 8 kHz) were mostly located in the dorsal and the units responsive to low frequencies (less than 2 kHz) in the anteroventral sector of auditory RE. However, only a loosely tonotopy is supported by this study. The neuronal circuitry within RE was shown to be stable when white noise bursts were delivered. Cross-correlograms indicated a large proportion of interconnected units (64%) and signs of mutual inhibition between neighboring RE units (11%). The hypothesis is discussed that the auditory RE exerts a fine control on the time-dependent analysis of the incoming auditory input to the cerebral cortex. The complex intranuclear connectivity suggests that the cell types correspond to distinct patterns of functional connections.

Acoustic Stimulation↗
PubMed1999

Central nervous system metabolic activity after cochlear implantation in the feline neonatal model.

OBJECTIVE: To determine the effects of deafening and cochlear implant stimulation on central nervous system (CNS) metabolic activity in the feline neonate model. BACKGROUND: Deafening of fetal animals has been shown to result in acute, profound depression of CNS glucose metabolism, in both auditory structures and the cerebral hemispheres. Preliminary studies have suggested that electrical stimulation of the auditory system may increase central nervous metabolic activity after deafening. The purpose of this study was to investigate this possibility. METHODS: This was a prospective, randomized, blinded, and controlled animal study of 13 random-source newborn kittens. It was set in an animal research facility for otologic disorders. OUTCOME: Deoxyglucose metabolism (assessed with autoradiograph densitometry) of brain cross-sections of normal, deafened, and deafened and cochlear-implanted animals after 6 weeks of auditory stimulation or deprivation. RESULTS: Chronic deafening did not result in a profound reduction in CNS metabolic activity. Cochlear implantation and electrical stimulation did not significantly raise the level of CNS metabolic activity within either auditory pathways or the cerebral hemispheres. CONCLUSIONS: Deafening is not associated with significant chronic reduction in CNS metabolic activity. Other parameters of CNS activity and maturation may be necessary to assess the effects of cochlear implantation and stimulation in animal models.

Animals↗
PubMed1984

The neuronal architecture of the inferior colliculus in the cat: defining the functional anatomy of the auditory midbrain.

This study defines anatomical subdivisions in Golgi-impregnated material from the inferior colliculus of the cat. The findings demonstrate that the inferior colliculus consists of a mosaic of morphologically distinct parts of neuropil. Each part is also characterized by a unique set of neuronal types. Each part of the inferior colliculus can be defined as tectal or tegmental on the basis of the fundamental pattern of dendritic branching. The main subdivisions of the auditory tectum are the central nucleus, the cortex, and the paracentral nuclei. The central nucleus is distinguished by its laminated neuropil composed of neurons with disc-shaped dendritic fields oriented in parallel arrays with the lemniscal axons. In contrast, the cortex is identified by its broad layers of loosely woven neuropil, which are orthogonal to those in the central nucleus and lack neurons with disc-shaped dendritic fields. The paracentral nuclei, so called because of their scattered arrangement around the central nucleus, are the commissural, dorsomedial, rostral pole, lateral, and ventrolateral nuclei. The main subdivisions of the auditory tegmentum are the pericollicular areas, the nucleus of the brachium of the inferior colliculus, and the sagulum. The pericollicular areas are intercollicular or subcollicular and separate the tectal division from the superior colliculus, central gray, and remaining portions of the tegmentum. The afferent projections to each tectal and tegmental subdivision, as observed in silver-degeneration experiments, distinguish the parcellations based on the Golgi findings. Subdivisions containing tectal cell types receive afferents predominantly from the auditory pathways, in contrast to subdivisions with tegmental cell types, which receive inputs from a wide variety of sources. This suggests a correlation between neuronal types and the nature of their inputs. This analysis of the subdivisions of the inferior colliculus differs from previous studies, especially those relying on Nissl stains. It is likely that subdivisions distinguished by the pattern of the neuropil differ functionally, since the structural components identified in the Golgi-impregnated material are essential parts of the synaptic organization of the auditory midbrain. Future physiological studies should benefit from approaches in which the cell types serve as the focus for the analysis.

Animals↗
PubMed2003

Heterogeneous presynaptic release probabilities: functional relevance for short-term plasticity.

We discuss a model of presynaptic vesicle dynamics, which allows for heterogeneity in release probability among vesicles. Specifically, we explore the possibility that synaptic activity is carried by two types of vesicles; first, a readily releasable pool and, second, a reluctantly releasable pool. The pools differ regarding their probability of release and time scales on which released vesicles are replaced by new ones. Vesicles of both pools increase their release probability during repetitive stimulation according to the buildup of Ca(2+) concentration in the terminal. These properties are modeled to fit data from the calyx of Held, a giant synapse in the auditory pathway. We demonstrate that this arrangement of two pools of releasable vesicles can account for a variety of experimentally observed patterns of synaptic depression and facilitation at this synapse. We conclude that synaptic transmission cannot be accurately described unless heterogeneity of synaptic release probability is taken into account.

Action Potentials↗
PubMed2003

Myelin-deficiency in the cochlear nerve of the 'bt' mutant hamster.

In a previous report, we showed abnormal auditory evoked potentials in the mutant hamster, 'black tremor (bt)', with significantly prolonged wave latencies of auditory brainstem responses and prolonged N1 latencies of compound action potentials, but normal cochlear microphonics. In this report, we present the results of morphological studies supporting the results of our electrophysiological studies of the auditory pathway in bt. Observation by transmission electron microscopy revealed an abnormal myelin sheath surrounding the spiral ganglion cells, and a thinner compact myelin sheath surrounding the axons in bt than in normal hamsters. The bt hamster has a myelin deficiency not only in the brainstem, but also in the cochlear nerve.

Animals↗
PubMed1985

The localization and origin of somatostatin-containing fibers in an auditory brainstem nucleus.

Immunoreactive perikarya and terminals have been visualized in the rat brainstem using antibodies raised against somatostatin 14 and somatostatin 28 (4-14) in a peroxidase antiperoxidase procedure. Labeling was observed in several areas of the brainstem, including the solitary nucleus, the dorsal lateral lemniscus, the ventral cochlear nucleus, several areas of the reticular formation, the nucleus of the spinal trigeminal tract, and the locus ceruleus. There was no obvious difference obtained with antisera against SS 14 and SS 28 (4-14). Several auditory areas were found to contain SS in discrete sub-groups of perikarya and terminals, suggesting the existence of one or more specific SS-containing auditory pathways. The origin of the SS-positive terminals in the medial nucleus of the trapezoid body was investigated using thermal and chemical lesions. It was concluded that these terminals originate from a specific SS-positive group of perikarya located either in or adjacent to the medial portion of the nucleus of the dorsal lateral lemniscus.

Animals↗
PubMed2002

The processing of temporal pitch and melody information in auditory cortex.

An fMRI experiment was performed to identify the main stages of melody processing in the auditory pathway. Spectrally matched sounds that produce no pitch, fixed pitch, or melody were all found to activate Heschl's gyrus (HG) and planum temporale (PT). Within this region, sounds with pitch produced more activation than those without pitch only in the lateral half of HG. When the pitch was varied to produce a melody, there was activation in regions beyond HG and PT, specifically in the superior temporal gyrus (STG) and planum polare (PP). The results support the view that there is hierarchy of pitch processing in which the center of activity moves anterolaterally away from primary auditory cortex as the processing of melodic sounds proceeds.

Acoustic Stimulation↗
PubMed1999

Expression of type 2 iodothyronine deiodinase in hypothyroid rat brain indicates an important role of thyroid hormone in the development of specific primary sensory systems.

Thyroid hormone is an important epigenetic factor in brain development, acting by modulating rates of gene expression. The active form of thyroid hormone, 3,5,3'-triiodothyronine (T3) is produced in part by the thyroid gland but also after 5'-deiodination of thyroxine (T4) in target tissues. In brain, approximately 80% of T3 is formed locally from T4 through the activity of the 5'-deiodinase type 2 (D2), an enzyme that is expressed mostly by glial cells, tanycytes in the third ventricle, and astrocytes throughout the brain. D2 activity is an important point of control of thyroid hormone action because it increases in situations of low T4, thus preserving brain T3 concentrations. In this work, we have studied the expression of D2 by quantitative in situ hybridization in hypothyroid animals during postnatal development. Our hypothesis was that those regions that are most dependent on thyroid hormone should present selective increases of D2 as a protection against hypothyroidism. D2 mRNA concentration was increased severalfold over normal levels in relay nuclei and cortical targets of the primary somatosensory and auditory pathways. The results suggest that these pathways are specifically protected against thyroid failure and that T3 has a role in the development of these structures. At the cellular level, expression was observed mainly in glial cells, although some interneurons of the cerebral cortex were also labeled. Therefore, the T3 target cells, mostly neurons, are dependent on local astrocytes for T3 supply.

Animals↗
PubMedDate not supplied

Auditory processing disorders and problems with hearing-aid fitting in old age.

The hearing handicap experienced by elderly subjects depends only partially on end-organ impairment. Not only the neural unit loss along the central auditory pathways contributes to decreased speech discrimination, but also learning processes are slowed down. Diotic listening in elderly people seems to fasten learning of discrimination in critical conditions, as in the case of sensitized speech. This fact, and the binaural gain through the binaural release from masking, stress the superiority, on theoretical grounds, of binaural over monaural hearing-aid fitting.

Acoustic Stimulation↗
PubMed2002

Context-dependent adaptive coding of interaural phase disparity in the auditory cortex of awake macaques.

In the ascending auditory pathway, the context in which a particular stimulus occurs can influence the character of the responses that encode it. Here we demonstrate that the cortical representation of a binaural cue to sound source location is profoundly context-dependent: spike rates elicited by a 0 degrees interaural phase disparity (IPD) were very different when preceded by 90 degrees versus -90 degrees IPD. The changes in firing rate associated with equivalent stimuli occurring in different contexts are comparable to changes in discharge rate that establish cortical tuning to the cue itself. Single-unit responses to trapezoidally modulated IPD stimuli were recorded in the auditory cortices of awake rhesus monkeys. Each trapezoidal stimulus consisted of linear modulations of IPD between two steady-state IPDs differing by 90 degrees. The stimulus set was constructed so that identical IPDs and sweeps through identical IPD ranges recurred as elements of disparate sequences. We routinely observed orderly context-induced shifts in IPD tuning. These shifts reflected an underlying enhancement of the contrast in the discharge rate representation of different IPDs. This process is subserved by sensitivity to stimulus events in the recent past, involving multiple adaptive mechanisms operating on timescales ranging from tens of milliseconds to seconds. These findings suggest that the cortical processing of dynamic acoustic signals is dominated by an adaptive coding strategy that prioritizes the representation of stimulus changes over actual stimulus values. We show how cortical selectivity for motion direction in real space could emerge as a consequence of this general coding principle.

Acoustic Stimulation↗
PubMed2003

Olivocochlear efferent suppression in classical musicians.

Suppression of transient-evoked otoacoustic emissions was recorded from 29 members of the Louisiana Philharmonic Orchestra and 28 non-musician control subjects matched for age and gender. Binaural broad band noise was used as the suppressor stimulus in a forward masking paradigm. Results showed musicians to have significantly more suppression than non-musicians for both the right and left ears. Two possible explanations for this functional difference between groups are that moderately loud music serves as a sound conditioning stimulus and that music can be a mechanism for strengthening central auditory pathways which may influence the olivocochlear reflex arc. Possible explanations for this are discussed and ear, gender, and age differences within each group are examined. Additionally, middle-ear muscle reflex thresholds were found to be higher in musicians than non-musicians at some frequencies in some conditions.

Acoustic Impedance Tests↗
PubMed2002

Protein kinase C regulates [3H]D-aspartate release in auditory brain stem nuclei.

We previously found that unilateral cochlear ablation altered transmitter release from glutamatergic synaptic endings in several brain stem auditory nuclei. To determine if this release activity could be regulated by protein kinase C (PKC), which has been associated with regulation of transmitter release, the electrically evoked release of [3H]d-aspartate ([3H]d-Asp) was quantified in vitro as an index of exocytosis from glutamatergic presynaptic endings in the major subdivisions of the cochlear nucleus (CN) and in the main nuclei of the superior olivary complex (SOC). Treating dissected tissues with a PKC activator, such as phorbol 12,13-diacetate (PDA) or phorbol 12,13-dibutyrate (PDBu) (3 microM), elevated the evoked release of [3H]d-Asp by 1.5- to 3.3-fold. The PKC inhibitor Ro31-8220 (50 nM) did not alter the evoked release but blocked the stimulatory effects of PDA and PDBu. These findings suggested that PKC could positively regulate transmitter release from glutamatergic presynaptic endings in brain stem auditory pathways. Seven days after unilateral cochlear ablation, when cochlear nerve endings had degenerated in the ipsilateral CN, PDBu elevated the evoked release bilaterally in each CN subdivision and SOC nucleus, implying that PKC could regulate glutamatergic release in the noncochlear pathways remaining in the ipsilateral CN and in the other pathways after unilateral hearing loss. After 145 postlesion days, Ro31-8220 blocked endogenous elevations in the evoked release in the ipsilateral SOC but did not alter the elevated or upregulated release in the other tissues. This suggested that the elevations of glutamatergic release activity in the ipsilateral SOC that appeared after unilateral cochlear ablation depended on endogenous activation of PKC.

Animals↗
PubMed1989

Dichotic listening in a child with a cerebral lesion: the "paradoxical" ipsilateral ear deficit.

Behavioral testing is reported for a child with a deep, left cerebral lesion that displaced the third ventricle across midline and compressed and elevated the lateral ventricle, presumably disrupting interhemispheric auditory pathways. The onset of the lesion was before 2 1/2 years of age. Cognitive screening tests indicated normal nonverbal (visual perceptual) and semantic (confrontation naming) abilities, but impaired syntactic skills. Audiological testing showed an isolated ipsilateral ear abnormality on dichotic testing only. All other audiological results, including monotic testing with the same speech materials, were normal. Results were comparable to findings in adults with similar lesions.

Auditory Pathways↗
PubMed1994

Auditory nerve terminals in the cochlear nucleus magnocellularis: differences between low and high frequencies.

Primary auditory nerve fibers were labelled in the barn owl by localized horseradish peroxidase (HRP) injections into the cochlear nucleus angularis. They were followed to their terminal sites in the hearing organ (basilar papilla), confirming that they were auditory, and to the cochlear nucleus magnocellularis. The terminal sites of low-frequency fibers within nucleus magnocellularis always included an area previously described as the lagenar part, i.e., an area receiving primary input which is probably only vestibular. Furthermore, a number of differences were recognized between these low-frequency (up to 0.64 kHz) and the high-frequency (1.8 kHz and above) auditory nerve projections to nucleus magnocellularis. Most importantly, the collaterals given off by low-frequency fibers into the nucleus typically showed multiple terminal branching, with both en passant and terminal bouton-like swellings. High-frequency fiber collaterals, in contrast, terminated unbranched in a single endbulb of Held. Nucleus magnocellularis is the first station in a brainstem auditory pathway processing stimulus timing information, coded through neuronal phase locking. The prominent difference in terminal shape found between its low- and high-frequency input fibers is interpreted as reflecting different requirements of the absolute temporal precision for significant phase locking. Terminals in the shape of endbulbs of Held are probably a specialization to improve the temporal precision of synaptic transmission, allowing phase locking to higher frequencies.

Acoustic Stimulation↗
PubMed2001

Modulation of a presynaptic hyperpolarization-activated cationic current (I(h)) at an excitatory synaptic terminal in the rat auditory brainstem.

1. A hyperpolarization-activated non-specific cation current, I(h), was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole-cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise I(h) in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of I(h) activation on synaptic transmission. 2. Presynaptic I(h) was activated by hyperpolarizing voltage-steps, with half-activation (V(1/2)) at -94 mV. Activation time constants were voltage dependent, showing an e-fold acceleration for hyperpolarizations of -32 mV (time constant of 78 ms at -130 mV). The reversal potential of I(h) was -29 mV. It was blocked by external perfusion of 1 mM CsCl but was unaffected by BaCl(2). 3. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V(1/2) of -74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half-activation was mimicked by external perfusion of a membrane-permeant analogue, 8-bromo-cAMP. 4. The bushy cell body I(h) showed similar properties to those of the synaptic terminal; V(1/2) was -94 mV and the reversal potential was -33 mV. Somatic I(h) was blocked by CsCl (1 mM) and was partially sensitive to BaCl(2). Somatic I(h) current density increased with postnatal age from 5 to 16 days old, suggesting that I(h) is functionally relevant during maturation of the auditory pathway. 5. The function of I(h) in regulating presynaptic excitability is subtle. I(h) had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic I(h) shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.

Animals↗