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PubMed2005

Evaluation of bilaterally implanted adult subjects with the nucleus 24 cochlear implant system.

OBJECTIVE: To evaluate the speech perception benefits of bilateral implantation for subjects who already have one implant. STUDY DESIGN: Repeated measures. PATIENTS: Thirty adult cochlear implant users who received their second implant from 1 to 7 years with a mean of 3 years after their first device. Ages ranged from 29 to 82 years with a mean of 57 years. SETTING: Tertiary referral centers across the United Kingdom. MAIN OUTCOME MEASURES: Monosyllabic consonant-nucleus-consonant words and City University of New York sentences in quiet with coincident speech and noise and with the noise spatially separated from the speech by +/-90 degrees . RESULTS: At 9 months, results showed the second ear in noise was 13.9 +/- 5.9% worse than the first ear (p < 0.001); a significant binaural advantage of 12.6 +/- 5.4% (p < 0.001) over the first ear alone for speech and noise from the front; a 21 +/- 6% (p < 0.001) binaural advantage over the first ear alone when noise was ipsilateral to the first ear; no binaural advantage when noise was contralateral to the first ear. CONCLUSIONS: There is a significant bilateral advantage of adding a second ear for this group. We were unable to predict when the second ear would be the better performing ear, and by implanting both ears, we guarantee implanting the better ear. Sequential implantation with long delays between ears has resulted in poor second ear performance for some subjects and has limited the degree of bilateral benefit that can be obtained by these users. The dual microphone does not provide equivalent benefit to bilateral implants.

Adult↗
PubMed1980

Brain stem auditory evoked response development in the kitten.

The development of brain stem auditory evoked responses (BAERs), recorded from a surface electrode as short-latency, volume-conducted potentials, was studied in a series of kittens over a postnatal period ranging from birth to 60 days. Repeated, longitudinal observations on particular kittens were supplemented with observations on additional kittens during the first and second postnatal week to determine age of onset of the BAERs. The position of the animal and sound source within the recording chamber were held constant across recording sessions, as was click intensity except during recordings in which intensity effects were specifically studied. Click rates of 1, 10, 50 and 100/sec were routinely presented. Reference electrodes at the tongue, pinna and neck showed volume-conducted responses to the click stimuli and resulted in considerable distortion of the activity recorded by the vertex electrode; the forepaw, in contrast, showed no activity and a vertex-forepaw electrode configuration provided good resolution of the BAERs across development. A number of new observations were made. BAERs were first observed at 4 days of age, approximately the same age at which depth evoked potentials are first recorded in brain stem auditory nuclei. Initially the BAERs were diffuse, high threshold and fatigued rapidly, characteristics shared with depth evoked potentials in the early postnatal period. Over the first two weeks, the potentials showed marked decrease in threshold, increased resistance to fast click rates, and better definition of wave forms. All BAER components showed exponential decreases in latency. Because all of the brain stem evoked potentials could be recorded concurrently and longitudinally in the same subject a number of developmental comparisons were possible among the BAER components. Wave 1, related to the acoustic nerve in the adult cat, showed a developmental time course and adult latency similar to that reported for N1. Wave 2, related to the cochlear nucleus in the adult, showed a marked bimodality over the first month; wave 2a was a large amplitude clearly separated wave which gradually fused as an inconspicuous leading shoulder on wave 2b. Wave 2b developed with a time course and adult latency similar to that reported for the ventral cochlear nucleus. Wave 3, related to the region of the superior olivary complex in the adult, showed a clear but transient bimodality during the third week of development. Wave 5, related to the inferior colliculus in the adult, appeared later than waves 1-4 and showed a significantly slower rate of development than waves 1-4. These data indicate that differential developmental changes occur within the brain stem auditory pathway and that the BAERs provide a dynamic probe of concurrent maturational interactions.

Acoustic Stimulation↗
PubMed1992

Commissural connections mediate inhibition for the computation of interaural level difference in the barn owl.

In the barn owl (Tyto alba), the posterior nucleus of the ventral lateral lemniscus (VLVp) is the first site of binaural convergence in the pathway that processes interaural level difference (ILD), an important sound-localization cue. The neurons of VLVp are sensitive to ILD because of an excitatory input from the contralateral ear and an inhibitory input from the ipsilateral ear. A previously described projection from the contralateral cochlear nucleus, can account for the excitation. The present study addresses the source of the inhibitory input. We demonstrate with standard axonal transport methods that the left and right VLVps are interconnected via fibers of the commissure of Probst. We further show that the anesthetization of one VLVp renders ineffective the inhibition that is normally evoked by stimulation of the ipsilateral ear. Thus, one cochlear nucleus (driven by the ipsilateral ear) appears to provide inhibition to the ipsilateral VLVp by exciting commissurally-projecting inhibitory neurons in the contralateral VLVp.

Animals↗
PubMed2001

A longitudinal study of electrode impedance, the electrically evoked compound action potential, and behavioral measures in nucleus 24 cochlear implant users.

OBJECTIVE: The primary goal of this study was to examine changes that may occur in electrode impedance, electrically evoked compound action potential (EAP) threshold and slope of the EAP growth function, and behavioral measures of threshold T-level) and maximum comfort (C-level) over time in both adult and child cochlear implant users. Secondary goals were to determine whether changes in these measures are consistent between children and adults, and to determine whether behavioral measures (MAP T- and C-levels) and electrophysiologic measures (EAP thresholds) exhibit the same trends over time. DESIGN: Thirty-five children and 33 adults implanted with the Nucleus CI24M between November 1996 and August 1999 participated in this study. Subjects were included in this study if 1) they had used their implant for at least 1 yr after device connection, and 2) they had participated in the necessary data collection at a minimum number of the time intervals assessed in this study. EAP threshold, slope of the EAP growth function, and common ground electrode impedance measures were collected intraoperatively, at initial stimulation, and at several subsequent visits up to 2 yr post initial stimulation. MAP T- an d C-levels weremeasured at initial stimulation and at the same time intervals as described above. RESULTS: Changes in electrode impedance, EAP thresholds, and slope of the EAP growth function from measures made intraoperatively, at initial stimulation, and at 1 to 2 mo post initial stimulation were similar in both children and adults. Beyond the 1- to 2-mo visit, children exhibited significant increases in electrode impedance, EAP thresholds, slope, and MAP T-levels, whereas these samemeasures in adults remained relatively stable. EAP thresholds in children stabilized by the 3- to 8-mo visit, and electrode impedance stabilized by the 6- to 8-mo visit, while slope of the EAP growth function, MAP T-levels,and MAP C-levels werestable by 1 yr post initial stimulation. C-levels in adults increased up to 1 yr post initial stimulation; however, the amount of increase was much smaller than that seen in children. In both children and adults, longitudinal trends in EAP thresholds mirrored T-level more closely than C-level. CONCLUSIONS: The results of this study suggest that peripheral changes occur in many children that do not generally occur in adults within the first year of cochlear implant use. One implication of these results is that if EAP thresholds are to be used to assist in programming the speech processor for children, it is best to make those measures at the same time interval as device programming rather than using measures made intraoperatively or at the initial programming session to set MAP levels at later visits.

Acoustic Impedance Tests↗
PubMed1981

Glutamate and aspartate: alteration of thresholds and response patterns of auditory neurons.

Iontophoretic application of the excitant amino acids glutamate and aspartate onto neurons in the chinchilla cochlear nucleus results in a lowering of the threshold of response to auditory stimuli. Neurons that display 'on'-type phasic responses to toneburst stimuli may become tonic, sustained responders with iontophoretic application of glutamate or aspartate. The ability of either glutamate or aspartate to effect changes in thresholds and response patterns of cochlear nucleus neurons is further evidence that one of these amino acids may be the afferent transmitter of the auditory nerve. The effects seen with these excitant amino acids may also provide insight into the underlying synaptic events involved in the generation of a particular response pattern.

Animals↗
PubMed2002

Multichannel auditory brainstem implant: update on performance in 61 patients.

OBJECT: Neurofibromatosis Type 2 (NF2) has typically resulted in deafness after surgical removal of bilateral vestibular schwannomas (VSs). Cochlear implants are generally ineffective for this kind of deafness because of the loss of continuity in the auditory nerve after tumor removal. The first auditory brainstem implant (ABI) in such a patient was performed in 1979 at the House Ear Institute, and this individual continues to benefit from electrical stimulation of the cochlear nucleus complex. In 1992, an advanced multichannel ABI was developed and a series of patients with NF2 received this implant to study the safety and efficacy of the device. METHODS: At the time of first- or second-side VS removal, patients received an eight-electrode array applied to the surface of the cochlear nucleus within the confines of the lateral recess of the fourth ventricle. The device was activated approximately 6 weeks after implantation. and patients were tested every 3 months for the 1st year after the initial stimulation, and annually thereafter. The protocol included a comprehensive battery of psychophysical and speech perception tests. CONCLUSIONS: The multichannel ABI proved to be effective and safe in providing useful auditory sensations in most patients with NF2. The ABI improved patients' ability to communicate compared with the lipreading-only condition, it allowed the detection and recognition of many environmental sounds, and in some cases it provided significant ability to understand speech by using just the sound from the ABI (with no lipreading cues). Its performance in most patients has continued to improve for up to 8 years after implantation.

Adolescent↗
PubMed1991

A spectrotemporal analysis of DCN single unit responses to wideband noise in guinea pig.

Spectrotemporal receptive fields (STRFs) were estimated for chopper and pauser units recorded in guinea pig dorsal cochlear nucleus (DCN). Sixteen wideband, periodic noise stimuli, represented as time-frequency surfaces of energy density, were cross correlated in time with the unit's corresponding period histograms to determine if specific energy patterns tended to precede spike occurrence. The STRFs obtained were unique to the DCN, as compared to the ventral cochlear nucleus (VCN) [Clopton and Backoff, 1991, Hear. Res. 52, 329-344] in their degree of temporal and spectral complexity. Certain unit response types, classified from their peristimulus-time histograms (PSTHs) to tonebursts, were associated with distinctive patterns in the STRFs. All STRFs had at least one region of elevated energy density (peak region) closely preceding spike occurrence, which may reflect a short-pathway, primary excitatory input (or inputs) to the neuron. In addition, some units displayed low-energy regions (troughs) with greater temporal precedences on their STRFs, particularly when higher stimulus intensities were used. This analysis approach appears to have potential for investigating functional neural connectivity and predicting responses to novel complex stimuli, although specific implementations of the technique impose limitations on the interpretation of results.

Acoustic Stimulation↗
PubMed1996

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro.

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

Amino Acid Sequence↗
PubMed1996

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro.

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

Analysis of Variance↗
PubMed2004

GABA(B) receptor activation modulates GABA(A) receptor-mediated inhibition in chicken nucleus magnocellularis neurons.

Neurons of nucleus magnocellularis (NM), a division of avian cochlear nucleus that performs precise temporal encoding, receive glutamatergic excitatory input solely from the eighth nerve and GABAergic inhibitory input primarily from the ipsilateral superior olivary nucleus. GABA activates both ligand-gated Cl- channels [GABA(A) receptors (GABA(A)Rs)] and G protein-coupled receptors (GABA(B) receptors). The net effect of GABA(A)R-mediated input to NM is inhibitory, although depolarizing. Several studies have shown that this shunting, inhibitory GABAergic input can evoke action potentials in postsynaptic NM neurons, which could interfere with their temporal encoding. While this GABA-mediated firing is limited by a low-voltage-activated K+ conductance, we have found evidence for a second mechanism. We investigated modulation of GABA(A)R-mediated responses by GABA(B)Rs using whole cell recording techniques. Bath-applied baclofen, a GABA(B)R agonist, produced dose-dependent suppression of evoked inhibitory postsynaptic currents (eIPSCs). This suppression was blocked by CGP52432, a potent and selective GABA(B)R antagonist. Baclofen reduced the frequency but not the amplitude of miniature IPSCs (mIPSCs) and did not affect postsynaptic currents elicited by puff application of a specific GABA(A)R agonist muscimol, suggesting a presynaptic mechanism for the GABA(B)R-mediated modulation. Firing of NM neurons by synaptic stimulation of GABAergic inputs to NM was eliminated by baclofen. However, endogenous GABA(B)R activity in the presynaptic inhibitory terminals was not observed. We propose that presynaptic GABA(B)Rs function as autoreceptors, regulating synaptic strength of GABA(A)R-mediated inhibition, and prevent NM neurons from generating firing during activation of the inhibitory inputs.

Analysis of Variance↗
PubMed1988

Connections of the dorsal nucleus of the lateral lemniscus: an inhibitory parallel pathway in the ascending auditory system?

This study examines the dorsal nucleus of the lateral lemniscus (DNLL) and its afferent and efferent connections. In Nissl-stained material, DNLL has three parts: dorsal, ventral, and lateral. Although each part contains neurons with similar Nissl patterns, the subdivisions may be distinguished by the size, shape, and orientation of the cells. The lateral DNLL contains a mixture of DNLL neurons and cells from the sagulum. Afferent connections to DNLL were investigated with anterograde axonal transport techniques. Bilateral inputs to DNLL arise from the anteroventral cochlear nucleus and lateral superior olive, while unilateral inputs are provided by the ipsilateral medial superior olive and the contralateral DNLL. The inputs appear to have a tonotopic organization. Afferent fibers to DNLL form horizontal bands that are continuous both mediolaterally and rostrocaudally. All parts of DNLL do not share the same inputs, and a medial-to-lateral gradient in the labeling of some pathways is evident. To study the efferent connections of DNLL, both retrograde and anterograde axonal transport techniques were used. The DNLL projects to the inferior colliculus and the contralateral DNLL. The topography of these projections suggests that areas of similar tonotopic organization are connected. In the inferior colliculus, the projection is heaviest to the central nucleus and extends to the adjacent dorsal and caudal cortex, the rostral pole nucleus, and the ventrolateral nucleus. Axons from DNLL terminate along the fibrodendritic laminae of the central nucleus as bands that are prominent on the contralateral side, whereas those on the ipsilateral colliculus are more diffuse. The afferent and efferent connections of DNLL constitute a multisynaptic pathway, parallel to the other ascending pathways to the inferior colliculus. The other ascending pathways include the direct pathways from the cochlear nucleus to the inferior colliculus and the indirect pathways via the superior olivary complex. Ascending pathways are discussed as to their relationship to the subdivisions of the inferior colliculus, the laterality of their projections, and their banding patterns in the central nucleus. In contrast to the excitatory pathways to the inferior colliculus, the neurons in DNLL may use GABA as a neurotransmitter. Axons from the DNLL terminate in the inferior colliculus as bands that could have a unique inhibitory function. Thus, the multisynaptic, DNLL pathway may provide feed-forward inhibitory inputs to the inferior colliculus, bilaterally, and to the contralateral DNLL.

Animals↗
PubMed1994

Age-related changes in brainstem auditory neurotransmitters: measures of GABA and acetylcholine function.

This study was designed to determine if there are age-related alterations in the bio-synthetic enzyme glutamic acid decarboxylase (GAD), the degradative enzyme GABA-transaminase (GABA-T), and the uptake system for GABA in the central nucleus of the inferior colliculus (CIC), the cochlear nucleus (CN), and/or nuclei of the lateral lemniscus (NLL) of Fischer-344 rats. For purposes of comparison, the cholinergic neuronal system was studied in parallel in young adult (3-7 months), mature (15-17 months) and aged (24-26 months) rats. In young adults GAD activity was highest in the CIC (219 nmol/mg protein/h; N = 5), intermediate in NLL (82 nmol/mg protein/h), and lowest in CN (34 nmol/mg protein/h). Choline acetyltransferase (ChAT) activity was highest in NLL and CN, and approximately 35-40% lower in CIC. A more uniform pattern was observed with GABA-T activity. Reductions in GAD activity were seen in the CIC of mature (-31%) and aged (-30%) rats that were not graded with age when compared to young adult, P < 0.05 (N = 5). This effect was regionally selective, since the CN did not show any loss of GAD or ChAT activity. The neurotransmitter selectivity of this deficit in CIC is supported by the non-parallel changes in ChAT activity (-22%, aged vs. mature, P < 0.05) that occurred after the changes in GAD activity. In contrast to the loss of GABAergic biosynthetic capacity in aged CIC, high affinity uptake processes (Kd and Vmax) for 14C-GABA and 3H-D-aspartate were not significantly altered (P > 0.05). Similar to the CIC, the NLL showed remarkable age-related deficits, but these deficits were more substantial for the cholinergic system (ChAT activity: -56% aged vs. young adult, P < 0.05; GAD activity: -35% aged vs. mature). None of the areas examined showed a significant loss of GABA-T activity with aging. These data suggest: 1) Age-related loss of GABA-mediated inhibition in the CIC of Fischer-344 rats is not attributable to changes in uptake or degradation of GABA, but may be related loss of biosynthetic capacity (i.e. activity or quantity) of the GAD present; 2) processing centers of the central auditory pathway (i.e. CIC and NLL), but not necessarily primary (i.e. CN) integrative nuclei, demonstrate selective, age-related neurochemical deficits; and 3) age-related neurochemical changes in central auditory structures may contribute substantially to the abnormal perception of signals in noise and loss of speech discrimination observed in neural presbycusis.

4-Aminobutyrate Transaminase↗
PubMed1984

Excitability of auditory neurons in the dorsal and ventral cochlear nuclei of DBA/2 and C57BL/6 mice.

Extracellular response properties were studied in neurons of the dorsal and ventral divisions of the cochlear nucleus (DCN and VCN, respectively) of DBA/2 (DBA) and C57BL/6 (C57) mice. Mice of the former inbred strain show susceptibility to audiogenic seizures and have severe high frequency hearing loss when young; mice of the latter strain do not. Whereas afterdischarges had been readily observed in the inferior colliculus of DBA mice in a previous study, they were never observed in the cochlear nucleus. The incidence of nonmonotonic intensity functions, the slopes of intensity functions, and the incidence of inhibition in response areas indicated that inhibition was diminished in the DCN of DBA mice. However, in the VCN, these response properties did not differ between the two strains. There appeared to be an "amplification" of excitability (i.e., attenuation of inhibition) from VCN to DCN to inferior colliculus in DBA mice.

Acoustic Stimulation↗
PubMed1990

Central projections of cochlear nerve fibers in the alligator lizard.

The auditory (cochlear) ganglion cells of the alligator lizard (Gerrhonotus multicarinatus) give rise to two types of peripheral fibers: tectorial fibers, which contact hair cells covered by a tectorial membrane, and free-standing fibers, which contact hair cells without a tectorial membrane. To determine the central projections of these fibers, we applied intracellular and extracellular injections of horseradish peroxidase (HRP) to the peripheral component of the cochlear nerve. After histological processing with diaminobenzidine, individual cochlear nerve fibers could be traced through serial sections with the aid of a light microscope and drawing tube. The projection patterns formed two morphologically distinct groups. Neurons whose peripheral processes contacted tectorial hair cells in the cochlea projected to three divisions of the cochlear nucleus: nucleus magnocellularis lateralis (NML), nucleus magnocellularis medialis (NMM), and nucleus angularis lateralis (NAL). Neurons whose peripheral processes contacted free-standing hair cells projected primarily to the nucleus angularis medialis (NAM), although some also sent a single, thin branch to the NML; these neurons never projected to NAL or NMM. Morphometric comparisons of tectorial and free-standing fibers demonstrate that tectorial fibers have a larger axonal diameter, form a greater number of terminal swellings, and make proportionally more somatic contacts. By correlating the morphologically defined groups with previously reported physiologically defined groups, we conclude that different divisions of the cochlear nucleus are associated with separate frequency ranges and that stimuli in the different frequency ranges may be processed separately in the brain.

Acoustic Stimulation↗
PubMed2000

Cholinergic receptors: dual roles in transduction and plasticity.

The regional distributions and possible functions of nicotinic acetylcholine receptors (nAChRs) in the developing and adult auditory rat brain are reviewed. The predominant nAChR in the auditory brainstem is the alpha7 homomeric receptor. alpha7 mRNA and protein are expressed in selected regions of the cochlear nucleus (CN), inferior colliculus (IC), medial superior olive, lateral superior olive, ventral nucleus of the lateral lemniscus and superior paraolivary nucleus. Peak expression of mRNA and protein occurs by the second postnatal week in most auditory brainstem areas. In contrast, the alpha3 and beta4 nicotinic subunits are expressed in the embryo and early in postnatal development in the CN and IC, but not other brainstem nuclei. Of particular interest is the octopus cell region of the posteroventral cochlear nucleus (PVCN). alpha3 and beta4 are down-regulated in the octopus cell region about postnatal day 10, which is the age that alpha7 is at peak expression. NAChRs play important roles in transduction and in regulating intracellular calcium. The ability of the alpha7 receptor to synchronize synaptic activity and stabilize synapses makes it a prime candidate as a mechanism underlying homeostatic plasticity in the auditory system.

Animals↗
PubMed1994

Autoradiographic measurement of regional brainstem blood flow: occlusion of the anterior inferior cerebellar artery.

Autoradiography was used to measure regional brainstem blood flow in Wistar rats following permanent left anterior inferior cerebellar artery (AICA) occlusion. With the AICA occluded, blood flow to the left vestibular nucleus decreased 31% while flow to the left cochlear nucleus decreased 47% when compared to the right (unobstructed) side. In the rat, the median pontine branch of the basilar artery was found to provide the principal blood supply to the vestibular nucleus. Electrocochleography was also used to measure the action potential (AP), summating potential (SP) and cochlear microphonics (CM) during left AICA occlusion. The AP disappeared completely after at least 7 min, while the SP polarity changed from negative to positive. Findings also showed that CM2 did not disappear completely in pre-mortem animals.

Action Potentials↗
PubMedDate not supplied

Glycine immunoreactivity in the brainstem auditory and vestibular nuclei of the guinea pig.

An immunohistochemical study was performed on the brainstem of the guinea pig, using a specific antibody against glycine. Glycine-like immunoreactivity was observed in stellate and multipolar neurons in the cochlear nucleus, in the medial and lateral nuclei of the trapezoid body and in the ventromedial periolivary cell group. No immunoreactive neurons were found in the vestibular nuclei. Positive fibre tracts were observed mainly in dorsal acoustic stria and lateral lemniscus. The results are consistent with electrophysiological and anatomical data from the literature concerning the response pattern in the fusiform layer of the dorsal cochlear nucleus and the phenomenon of binaural inhibition in the superior olivary complex.

Animals↗
PubMed1999

Responses of neurons to click-pairs as simulated echoes: auditory nerve to auditory cortex.

When two identical sounds are presented from different locations with a short interval between them, the perception is of a single sound source at the location of the leading sound. This "precedence effect" is an important behavioral phenomenon whose neural basis is being increasingly studied. For this report, neural responses were recorded to paired clicks with varying interstimulus intervals, from several structures of the ascending auditory system in unanesthetized animals. The structures tested were the auditory nerve, anteroventral cochlear nucleus, superior olivary complex, inferior colliculus, and primary auditory cortex. The main finding is a progressive increase in the duration of the suppressive effect of the leading sound (the conditioner) on the response to the lagging sound (the probe). The first major increase occurred between the lower brainstem and inferior colliculus, and the second between the inferior colliculus and auditory cortex. In neurons from the auditory nerve, cochlear nucleus, and superior olivary complex, 50% recovery of the response to the probe occurred, on average, for conditioner and probe intervals of approximately 2 ms. In the inferior colliculus, 50% recovery occurred at an average separation of approximately 7 ms, and in the auditory cortex at approximately 20 ms. Despite these increases in average recovery times, some neurons in every structure showed large responses to the probe within the time window for precedence (approximately 1-4 ms for clicks). This indicates that during the period of the precedence effect, some information about echoes is retained. At the other extreme, for some cortical neurons the conditioner suppressed the probe response for intervals of up to 300 ms. This is in accord with behavioral results that show dominance of the leading sound for an extended period beyond that of the precedence effect. Other transformations as information ascended included an increased variety in the shapes of the recovery functions in structures subsequent to the nerve, and neurons "tuned" to particular conditioner-probe intervals in the auditory cortex. These latter are reminiscent of neurons tuned to echo delay in bats, and may contribute to the perception of the size of the acoustic space.

Acoustic Stimulation↗