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PubMed1997

Speech recognition with the MPEAK and SPEAK speech-coding strategies of the Nucleus Cochlear Implant.

The Spectra 22 Speech Processor and spectral peak (SPEAK) speech-coding strategy provide Nucleus 22 Channel Cochlear Implant (Cochlear, Corp., Englewood, Colo.) users with a better understanding of speech than the previous Mini Speech Processor and Multipeak (MPEAK) speech-coding strategy. On the NU-6 Monosyllabic Word Test, subjects scored 13% higher for words and 14% higher for phonemes with the SPEAK strategy than with the MPEAK strategy. On the Connected Speech Test and the CID Everyday Sentence Test, subjects scored 32% and 37% higher, respectively, with SPEAK than with MPEAK.

Adult↗
PubMed2002

Elevated fusiform cell activity in the dorsal cochlear nucleus of chinchillas with psychophysical evidence of tinnitus.

Chinchillas with psychophysical evidence of chronic tinnitus were shown to have significantly elevated spontaneous activity and stimulus-evoked responses in putative fusiform cells of the dorsal cochlear nuclei (DCN). Chinchillas were psychophysically trained and tested before and after exposure to a traumatic unilateral 80 dB (sound pressure level) 4 kHz tone. Before exposure, two groups were matched in terms of auditory discrimination performance (noise, and 1, 4, 6, and 10 kHz tones). After exposure, a single psychophysical difference emerged between groups. The exposed group displayed enhanced discrimination of 1 kHz tones (p = 0.00027). Postexposure discrimination of other stimuli was unaffected. It was hypothesized that exposed animals experienced a chronic subjective tone (i.e., tinnitus), resulting from their trauma, and that features of this subjective tone were similar enough to 1 kHz to affect discrimination of 1 kHz objective signals. After psychophysical testing, single-unit recordings were obtained from each animal's DCN fusiform cell layer. Putative fusiform cells of exposed animals showed significantly (p = 0.0136) elevated spontaneous activity, compared with cells of unexposed animals. Putative fusiform cells of exposed animals showed a greater stimulus-evoked response to tones at 1 kHz (p = 0.0000006) and at characteristic-frequency (p = 0.0000009). This increased activity was more pronounced on the exposed side. No increase in stimulus-evoked responses was observed to other frequencies or noise. These parallel psychophysical and electrophysiological results are consistent with the hypothesis that chronic tonal tinnitus is associated with, and may result from, trauma-induced elevation of activity of DCN fusiform cells.

Acoustic Stimulation↗
PubMed2006

Basis of electrical stimulation of the cochlea and the cochlear nucleus.

Sensorineural hearing loss is the most common form of deafness in humans. In patients with a severe-profound sensorineural hearing loss therapeutic intervention can only be achieved by direct electrical stimulation of the auditory nerve via a cochlear implant, or - in cases where a cochlear implant is not a surgical option - neurons within the central auditory pathway via an auditory brainstem implant. This paper reviews the basis of electrical stimulation of these structures with an emphasis on pathophysiology and safety.

Animals↗
PubMed1990

Speech perception with the nucleus cochlear implant in children trained with the auditory/verbal approach.

Five children between the ages of 6 to 10 years using the Nucleus 22-electrode cochlear implant were tested on a battery of speech-perception tests. None of the five children used any sign language before implantation, and all received extensive training in auditory/verbal rehabilitation postimplantation. Performance ranged from 81 to 97 percent correct on the Matrix Test, from 24 to 80 percent correct on the WIPI test, from 33 to 73 percent correct on a 10-choice vowel recognition test, from 19 to 50 percent correct on a 13-choice consonant recognition test, from 2 to 46 percent words correct on a PB-K word list, from 4 to 71 percent words correct on a recorded BKB sentence test, and from 8 to 71 percent words correct on a live-voice BKB sentence test. In four of the five children, significant lipreading enhancement was observed on consonant recognition and two prelinguistically deafened children demonstrated some open-set word recognition.

Child↗
PubMed2003

Primary afferent and cochlear nucleus contributions to extracellular potentials during tone-bursts.

Gross electrical responses to tone bursts were measured in the guinea pig with electrodes located in scala tympani (ST) and scala vestibuli (SV) of the cochlea, on the central portion of the VIIIth nerve fibres in the internal auditory meatus, and on the surface of the cochlear nuclear complex (CN). Intracochlear perfusion of pharmacological blockers of neural and postsynaptic activity as well as aspiration of parts or all of the CN were used to dissect the origin of the many components of the gross responses. It was shown that single-ended recordings from either ST or SV or those derived from the sum of the ST and SV responses not only contain mixed responses from the auditory nerve fibres and cochlear hair cells, but are contaminated or modified by neural activity central to the internal auditory meatus, probably in various parts of the CN. Differential recordings between ST and SV were relatively uncontaminated by such activity. Recordings from central locations were largely uncontaminated by potentials from cochlear hair cells. These results suggest that a revised and extended system of nomenclature for the different components of the gross cochlear potentials is necessary, and interpretation of such potentials needs to take into account multiple central as well as peripheral generators.

Acoustic Stimulation↗
PubMed2006

Specific plasticity responses to unilaterally decreased or increased hearing intensity in the adult cochlear nucleus and beyond.

Variations of sensory activation in strength and pattern are known to affect structure and function of the mammalian brain. Whereas such malleability is readily granted to forebrain structures at early developmental stages, acceptance of experience-dependent structural plasticity has been slow for the adult brainstem. Over the past years we have identified consequences of cochlear ablation, noise trauma, or electrical intracochlear stimulation on neurons and circuitry of the auditory brainstem of the adult rat. We found that loss of sensory activation as well as a substitution for it entail specific molecular, ultrastructural, and morphological changes to central auditory neurons. Here, we make a first attempt to compare these different patterns of central remodeling. We tentatively suggest that after hearing loss or intracochlear stimulation responses of the central neural network in the adult brainstem suit the concept of functional adaptation.

Animals↗
PubMed1997

Frequency tuning and spontaneous activity in the auditory nerve and cochlear nucleus magnocellularis of the barn owl Tyto alba.

Single-unit recordings were obtained from the brain stem of the barn owl at the level of entrance of the auditory nerve. Auditory nerve and nucleus magnocellularis units were distinguished by physiological criteria, with the use of the response latency to clicks, the spontaneous discharge rate, and the pattern of characteristic frequencies encountered along an electrode track. The response latency to click stimulation decreased in a logarithmic fashion with increasing characteristic frequency for both auditory nerve and nucleus magnocellularis units. The average difference between these populations was 0.4-0.55 ms. The average most sensitive thresholds were approximately 0 dB SPL and varied little between 0.5 and 9 kHz. Frequency-threshold curves showed the simple V shape that is typical for birds, with no indication of a low-frequency tail. Frequency selectivity increased in a gradual, power-law fashion with increasing characteristic frequency. There was no reflection of the unusual and greatly expanded mapping of higher frequencies on the basilar papilla of the owl. This observation is contrary to the equal-distance hypothesis that relates frequency selectivity to the spatial representation in the cochlea. On the basis of spontaneous rates and/or sensitivity there was no evidence for distinct subpopulations of auditory nerve fibers, such as the well-known type I afferent response classes in mammals. On the whole, barn owl auditory nerve physiology conformed entirely to the typical patterns seen in other bird species. The only exception was a remarkably small spread of thresholds at any one frequency, this being only 10-15 dB in individual owls. Average spontaneous rate was 72.2 spikes/s in the auditory nerve and 219.4 spikes/s for nucleus magnocellularis. This large difference, together with the known properties of endbulb-of-Held synapses, suggests a convergence of approximately 2-4 auditory nerve fibers onto one nucleus magnocellularis neuron. Some auditory nerve fibers as well as nucleus magnocellularis units showed a quasiperiodic spontaneous discharge with preferred intervals in the time-interval histogram. This phenomenon was observed at frequencies as high as 4.7 kHz.

Acoustic Stimulation↗
PubMed2002

Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats.

Studies of visual system development have suggested that competition driven by activity is essential for refinement of initial topographically diffuse neuronal projections into their precise adult patterns. This has led to the assertion that this process may shape development of topographic connections throughout the nervous system. Because the cat auditory system is very immature at birth, with auditory nerve neurons initially exhibiting very low or no spontaneous activity, we hypothesized that the auditory nerve fibers might initially form topographically broad projections within the cochlear nuclei (CN), which later would become topographically precise at the time when adult-like frequency selectivity develops. In this study, we made restricted injections of Neurobiotin, which labeled small sectors (300-500 microm) of the cochlear spiral ganglion, to study the projections of auditory nerve fibers representing a narrow band of frequencies. Results showed that projections from the basal cochlea to the CN are tonotopically organized in neonates, many days before the onset of functional hearing and even prior to the development of spontaneous activity in the auditory nerve. However, results also demonstrated that significant refinement of the topographic specificity of the primary afferent axons of the auditory nerve occurs in late gestation or early postnatal development. Projections to all three subdivisions of the CN exhibit clear tonotopic organization at or before birth, but the topographic restriction of fibers into frequency band laminae is significantly less precise in perinatal kittens than in adult cats. Two injections spaced > or = 2 mm apart in the cochlea resulted in labeled bands of projecting axons in the anteroventral CN that were 53% broader than would be expected if they were proportional to those in adults, and the two projections were incompletely segregated in the youngest animals studied. Posteroventral CN (PVCN) projections (normalized for CN size) were 36% broader in neonates than in adults, and projections from double injections in the youngest subjects were nearly fused in the PVCN. Projections to the dorsal division of the CN were 32% broader in neonates than in adults when normalized, but the dorsal CN projections were always discrete, even at the earliest ages studied.

Aging↗
PubMed1985

Non-N-methyl-D-aspartate receptors mediating synaptic transmission in the avian cochlear nucleus: effects of kynurenic acid, dipicolinic acid and streptomycin.

We have examined the effects of a number of excitatory amino acid antagonists on transmission at the cochlear nerve-nucleus magnocellularis synapse in the chicken. Using an in vitro preparation and bath application of drugs, we studied the effects of kynurenic acid and several related substances, streptomycin and a selective N-methyl-D-aspartate receptor antagonist, DL-alpha-aminosuberate. The last compound had no effect on evoked transmission. Of the various kynurenic acid-related compounds tested, only kynurenic and dipicolinic acid selectively altered responses in nucleus magnocellularis. Quinolinic acid, a kynurenic acid analogue that is structurally akin to dipicolinic acid but which acts selectively at N-methyl-D-aspartate receptors, was without effect. The effect of kynurenic acid was solely inhibitory, completely blocking postsynaptic responses with a potency dependent on the frequency of nerve stimulation. No such frequency dependence was seen with dipicolinic acid although this compound also completely suppressed evoked responses. In addition dipicolinic acid potentiated postsynaptic responses at concentrations only slightly lower than those causing inhibition. Streptomycin inhibited responses in nucleus magnocellularis but this effect seems to result partially from the ability of the drug to inhibit presynaptic calcium influx. Our finding that selective antagonists of N-methyl-D-aspartate receptors were ineffective while antagonists of both receptor types, such as kynurenic and dipicolinic acids, inhibited evoked responses reinforces the conclusion that postsynaptic receptors mediating transmission at this synapse are of the non-N-methyl-D-aspartate type [Nemeth et al. (1983) Neurosci. Lett. 40, 39-44].(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2000

Neuronal death, not axonal degeneration, results in significant gliosis within the cochlear nucleus of adult chickens.

Injury to the central nervous system initiates a series of events that leads to neuronal cell death and glial activation. Astrocytes respond to damage and disease by becoming hyperplastic and hypertrophied. This 'reactive gliosis' is also accompanied by the upregulation of the intermediate filament protein glial fibrillary acidic protein, the release of growth factors and the formation of the glial scar. However, the signaling cascades which regulate these events, and the molecular mechanisms that give rise to this diverse response, have not been fully elucidated. For example, the role played by degenerating neurons vs. degenerating axons in the activation of astrocytes remains to be determined. To investigate the influence of neuronal cell death vs. axonal degeneration on gliosis, the current study examines the astrocyte response to cochlea removal in two different breeds of adult chickens, one of which exhibits neuronal cell death within the brainstem nucleus magnocellularis (NM) following the lesion and one which does not. Our results indicate that degeneration of NM neurons leads to large increases in both glial proliferation and hypertrophy, while eighth nerve degeneration without NM cell death results in very small increases in glial proliferation.

Animals↗
PubMed2003

Primary innervation of the avian and mammalian cochlear nucleus.

The auditory nerve of birds and mammals exhibits differences and similarities, but given the millions of years since the two classes diverged from a common ancestor, the similarities are much more impressive than the differences. The avian nerve is simpler than that of mammals, but share many fundamental features including principles of development, structure, and physiological properties. Moreover, the available evidence shows that the human auditory nerve follows this same general organizational plan. Equally impressive are reports that homologous genes in worms, flies, and mice exert the same heredity influences in man. The clear implication is that animal studies will produce knowledge that has a direct bearing on the human condition.

Animals↗
PubMed1997

Phase locking to high frequencies in the auditory nerve and cochlear nucleus magnocellularis of the barn owl, Tyto alba.

The auditory system of the barn owl is an important model for temporal processing on a very fast time scale and for the neural mechanisms and circuitry underlying sound localization. Phase locking has been shown to be the behaviorally relevant temporal code. This study examined the quality and intensity dependence of phase locking in single auditory nerve fibers of the barn owl to define the input to the known brainstem circuit for temporal processing. For direct comparison in the same individuals, recordings were also obtained from the relevant next higher center, the nucleus magnocellularis (NM). Phase locking was regularly seen at sound pressure levels (SPL) below those eliciting an increase in spike rate, thus providing an additional cue for signal detection. The quality of phase locking, expressed as vector strength, decreased with increasing frequency. Auditory nerve fibers showed an unusual step-like decline with a prominent plateau in the mid-frequency range (1.5-3 kHz), indicating that some specialization enables the owl to halt the deterioration and extend phase locking to frequencies up to 10 kHz, above the range commonly observed in other species. Phase locking in the NM was consistently inferior to that of auditory-nerve fibers at frequencies above 1 kHz, suggesting that the synapse plays a limiting role in temporal precision. The response delays, or group delays, derived from the phase-versus-frequency functions of auditory nerve fibers were not consistent with the unusual spatial frequency representation in the owl cochlea. This questions the common assumption that group delays reflect cochlear wave travel times.

Acoustic Stimulation↗
PubMed1990

A computer interface for psychophysical and speech research with the Nucleus cochlear implant.

A computer interface has been designed and implemented that allows presentation of biphasic pulse stimuli to patients with the Nucleus Ltd./Cochlear Corporation cochlear implant. The one version of the interface connects to a standard parallel output port of a PC or AT compatible computer, and another version plugs directly into a standard PC/XT bus slot. The host computer sends a stream of bytes to the parallel port that specifies the configuration of the desired output pulses. Upon receipt of the data, the interface generates the appropriate burst sequence that is delivered to the patient's external transmitter coil. The coded information is interpreted by the internal receiver that delivers the pulse to the specified electrodes at the specified amplitude and pulse width. This interface makes it possible to interleave pulses on two or more electrode pairs, to modulate the amplitude or timing of a pulse sequence, or to sweep a stimulus across the electrode array. Investigators can achieve stimulus control with this interface that allows them to conduct psychophysical, electrophysiological, and speech experiments not possible through the patient's speech processor or with available clinical interfaces.

Cochlear Implants↗