Biomedical subjects

D W Schwarz

Publications and source records attributed to D W Schwarz.

At least 19 recordsLinked to original sources

PubMed1995

Melody recognition and musical interval perception by deaf subjects stimulated with electrical pulse trains through single cochlear implant electrodes.

The perception of musical pitch was investigated in postlinguistically deaf subjects with cochlear implants. Stimuli consisted of sequences of biphasic electrical pulse trains at rates which represented the tones of the equal-tempered musical scale, delivered at equalized comfortable loudness levels to selected single bipolar electrodes along the array of the Nucleus cochlear implant. Seventeen subjects correctly identified a mean of 44% of rhythmically intact familiar tunes, presented in an open-set paradigm. Three subjects were tested with a closed set of melodies without rhythmic cues. The results showed relatively higher recognition scores at lower pulse rates, although melody recognition remained possible up to rates of approximately 600-800 pulses per second. Stimulation of apical electrodes yielded higher recognition scores than of basal electrodes. The perception of musical intervals, defined as frequency ratios between two trains of stimulus pulse rates, was investigated in an interval intonation labeling experiment, for intervals ranging from a minor 3rd to a major 6th. Within a range of low pulse rates, subjects defined the intervals mediated by electrical pulse rate by the same ratios which govern musical intervals of tonal frequencies in normal-hearing listeners. It may be concluded that temporal cues are sufficient for the mediation of musical pitch, at least for the lower half of the range of fundamental frequencies commonly used in music.

Adult↗

PubMed1995

Electrical resonances in central auditory neurons.

In the auditory pathway, signal processing depends on the filter functions of neurons. We used frequency analysis to investigate the contributions of intrinsic membrane properties to the input-output relationships in neurons. The whole-cell tight-seal recording technique in brain slices of chicks was used to study neurons at four levels of the auditory pathway. Neurons displayed resonant peaks in their voltage responses to injected sinusoidal currents that swept through a specified frequency range. Higher resonant frequencies tended to predominate at relatively lower stations in the auditory pathway (approximately 100 Hz in the nucleus magnocellularis, 24 Hz in the nucleus laminaris, 6 Hz in the nucleus ovoidalis). Field L neurons (cortex homologue) displayed low pass filter characteristics without resonance. We propose that the subthreshold membrane resonances amplify synaptic inputs at specific frequencies and contribute to the chick's ability to decode temporal sound parameters.

Animals↗

PubMed1995

Plasticity in human directional hearing.

Interaural time difference (ITD), the main cue for localization of low-frequency sound in azimuth, is widely thought to be evaluated according to Jeffress' model. This theory proposes that each of an array of neurons detects coinciding input from both ears, conducted along axonal delay lines, with the azimuth angle corresponding to the activation of selected neurons. Thus, sound source localization is assumed to depend on axon conduction velocities, a relatively fixed parameter. Clinical experience suggests that directional hearing is adaptable. We investigated if sound localization in azimuth could adapt plastically to altered ITDs. We equipped binaural insert hearing aids with adjustable electronic delay lines. Subjects with normal hearing were required to wear these devices during all waking hours for several days. Localization of an invisible sound source was measured in an anechoic room before and at various intervals after introduction of a constant delay in one ear between 171 and 684 mus. Test sounds were high-pass, low-pass and broad-band noises. Introduction of a delay in one ear lead to an immediate displacement of the perceived sound location towards the opposite side. Within hours of exposure, the displacement was reduced, and further normalization of the perceived localization occurred over several days. After removal of the delays sound localization normalized rapidly. We conclude that ITD alterations can lead to plastic adaptation of directional hearing, which cannot rely exclusively on fixed axon conduction velocities. Our results suggest additional mechanisms for directional hearing on the basis ITD.

Acoustic Stimulation↗

PubMed1995

Connections of the superior olive in the chicken.

The avian superior olive (OS) is known to be a station in the auditory pathway, although its anatomic connections remain uncertain. The afferent and efferent connections of OS neurons in the chicken were identified with wheat germ agglutinin conjugated to horse radish peroxidase (WGA-HRP) injected into the OS nucleus. Projections to the OS originate bilaterally in the cochlear nuclei (nucleus angularis) and the nucleus laminaris. Anterogradely labelled axon terminals were found in the ipsilateral nucleus magnocellularis, the contralateral intermediate nucleus of the lateral lemniscus, and the shell portion of the central nucleus of the inferior colliculus. Retrograde transport of [3H]-glycine from the OS was also charted. Glycine-transporting cells were found ipsilaterally in the nucleus angularis and the nucleus laminaris. Neuronal soma in a newly identified nucleus of the trapezoid body (NTB) were found to actively concentrate glycine, although the neurons probably do not synapse within the OS. Anatomically, the avian OS would appear to be part of the interaural intensity difference pathway; however, our data and published information are insufficient to establish a homology to the human lateral superior olive.

Animals↗

PubMed1995

Retrograde labelling of auditory brainstem neurons following tritiated glycine injection into the inferior colliculus of the chicken, Gallus domesticus.

We investigated retrograde labelling with tritiated glycine injected into the inferior colliculus of the chicken. Tracer deposits were placed in the central nucleus of the inferior colliculus at positions yielding unit activity with best frequencies between 0.4 and 4 kHz. Most neuron systems known to project to the inferior colliculus in birds were unlabelled, whereas strongly labelled cells were found in three nuclei, only on the ipsilateral side. The nucleus lemnisci lateralis pars ventralis contained numerous small glycine-transporting cells. The superior olivary nucleus contained few such cells of similar size in its peripheral region. The nucleus of the trapezoid body contained a group of larger labelled neurons. The observed specificity suggests that we labelled glycinergic neurons projecting to the inferior colliculus.

Animals↗

PubMed1994

Mode of firing and rectifying properties of nucleus ovoidalis neurons in the avian auditory thalamus.

1. We studied neurons of the nucleus ovoidalis, the principal auditory thalamic relay nucleus of the chicken, with tight-seal whole-cell recording techniques in in vitro slice preparations. Nucleus ovoidalis, marked by anterograde labeling of afferents from the inferior colliculus, consists of a clearly delineated group of densely packed, multipolar cells of approximately uniform diameter. We measured a wide range of non covarying resting potentials (-60 +/- 9 mV, mean +/- SD) and input resistances (277 +/- 168 M omega). All neurons discharged overshooting fast spikes. The observed electrophysiological properties may have a decisive role in the transfer of sensory signals. 2. We grouped neurons on the basis of their firing patterns, in response to intracellular injections of depolarizing current pulses from various membrane potentials. The majority of neurons (86%) displayed weakly adapting, tonic firing. A smaller group of neurons (14%) exhibited qualitative changes in firing modes. They fired repetitively when the stimulus pulse was superimposed on relatively depolarized levels, usually including the resting potential. DC-hyperpolarization led to burst responses consisting of fast action potentials on top of slow potentials. 3. In all neurons, application of 300 nM tetrodotoxin blocked the action potentials and reduced a depolarization-activated inward rectification, observed during 1-s current pulses in a range of membrane potentials depolarized from rest. This rectification is interpreted as a partial result of a persistent Na+ current. 4. During the applications of tetrodotoxin in neurons with burst firing capability, two other slow potentials were visible in isolation. Depolarizing current pulses evoked slow, transient depolarizations at the onset whereas rebound slow potentials occurred on termination of hyperpolarizing current pulses. The slow potentials were blocked by application of 0.5 mM Ni2+ and are likely a result of a low threshold Ca2+ current, such as a T-current. 5. A distinctive property of all ovoidalis neurons was a hyperpolarization-activated inward rectification. Application of Cs+ (3 mM) but not Ba2+ (3 mM), tetraethylammonium (10 mM), or 4-aminopyridine (4 mM) reversibly blocked the current that produced this rectification. The activation time constants for the current varied between approximately 50 and 400 ms and were voltage dependent in some neurons. Thus the hyperpolarization-activated current (IH), responsible for thalamic sleep mechanisms in mammals, also is present in a submammalian thalamus. 6. We suggest that the voltage and time dependencies of the persistent Na+ current and IH participate in generation of the sub- and suprathreshold temporal activity patterns in the neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Subthreshold frequency selectivity in avian auditory thalamus.

1. We studied the frequency responses of neurons in the nucleus ovoidalis (OV), the principal thalamic auditory relay nucleus of the chicken, in the subthreshold range of membrane potentials. The frequency response is the impedance amplitude profile evident in the voltage response to a broadband stimulus. The stimulus was a deterministic periodic current input of small amplitude, sweeping through a specified frequency range. We used whole-cell, tight-seal recording techniques in slices to study the voltage responses and membrane properties in current and voltage clamp. 2. Generally, low-frequency resonant humps with peak impedances of approximately 6 Hz characterized the frequency responses of OV neurons. This resonance was the principal determinant for frequency selectivity in the majority of OV neurons expressing only a tonic mode of firing. 3. The 6-Hz resonance was voltage dependent and most distinct where the activation ranges of a hyperpolarization activated inward current (IH) and a persistent Na+ current tend to overlap. The potential range for optimal resonance often included the resting potential. 4. Application of the Na+ current antagonist, tetrodotoxin, blocked the persistent Na+ current and most of the resonant hump at depolarized levels but did not affect the resonant peak along the frequency axis. Thus the persistent Na+ current may serve to amplify the resonance. 5. Extracellular application of Cs+, but not Ba2+, blocked a voltage sag during pulsed hyperpolarization as well as the IH current. Application of Cs+ also eliminated the 6-Hz resonance. An IH seems, therefore, instrumental for the resonance. 6. A minority of neurons that expressed low-threshold Ca2+ spikes and burst firing at hyperpolarized states displayed voltage oscillations at 2-4 Hz, spontaneously or in response to pulsatile stimuli. Application of Ni2+ blocked the oscillations and the low-threshold spikes, presumably produced by a T-type Ca2+ current. The resonance at 6 Hz, however, was only slightly affected by Ni2+. A T-type current, therefore, is critical for the 2- to 4-Hz oscillations. 7. Membrane resonance may dominate the power spectrum of subthreshold potential fluctuations. The resonance demonstrated in vitro may be stabilized by experimental procedures; its frequency may be different and more variable in vivo. Resonances in thalamic neurons may play a role in auditory signal processing in birds.

Animals↗

PubMed1993

Phrenic nerve reinnervation of the cat's larynx: a new technique with proven success.

Reinnervation of the posterior cricoarytenoid muscle (PCA) should provide vocal cord abduction on inspiration, and passive adduction to enable phonation. Previous investigators have shown that reinnervation is possible, but results have not been clinically encouraging. When reinnervation was successful, the question remained whether it was provided by the transplanted nerve or by the ingrowth of adjacent nerves. In this study the phrenic nerve was transplanted directly into the PCA in a series of 12 cats. Fibrin glue was used to overcome nerve trauma and to prevent retraction of the nerve from the PCA. Laryngoscopy, electromyography, and retrograde labeling of the phrenic motoneurons provided evidence of functional reinnervation in 9 cats. Partial or complete failure in the remaining 3 was due to retraction of the nerve from the muscle. These results appear to justify trials of the procedure in humans.

Animals↗

PubMed1993

A survey of auditory brainstem nuclei in the chicken (Gallus domesticus) with cytochrome oxidase histochemistry.

The chicken's auditory brainstem nuclei from the cochlear nuclei to the nuclei of the lateral leminiscus were studied with cytochrome oxidase histochemistry. A strong reactivity in the cochlear and laminar nuclei was confirmed. Additional structures displaying high activity levels include the superior olive and both partitions of the nucleus intermedius lemnisci lateralis. Unilateral cochlea removal led to a strong reduction of activity in the cochlear nuclei and the nucleus laminaris, whereas there was no remarkable effect in higher brainstem centers. After bilateral cochlea extirpation auditory structures still displayed higher enzyme levels than most other nuclei. These observations point to the extraordinary metabolic activity in the ascending auditory pathway which is largely independent of sensory input from the auditory nerve.

Animals↗

PubMed1993

A survey of the auditory midbrain, thalamus and forebrain in the chicken (Gallus domesticus) with cytochrome oxidase histochemistry.

The chicken's central auditory nuclei from the inferior colliculus to field L in the forebrain were studied with cytochrome oxidase histochemistry. All stations of the ascending pathway displayed high activity levels, including the inferior colliculus, the nucleus ovoidalis of the thalamus, and field L1 to L3 and the hyperstriatum ventrale caudale which correspond to primary and secondary auditory cortex. In the inferior colliculus a moderately active external nucleus could be distinguished from a more intensely stained central and superficial nucleus. In the central nucleus there was a lateral shell displaying stronger neuropil reactivity than a central core. Unilateral cochlea removal caused no remarkable effect in tectum and thalamus. The auditory forebrain contralateral to the lesion displayed reduced CO reactivity compared with the ipsilateral side. After bilateral cochlea extirpation auditory structures still displayed higher enzyme levels than most other nuclei.

Animals↗

PubMed1992

Location of motoneurons innervating the middle ear muscle of the chicken, (Gallus domesticus).

The motoneuron pool for the musculus columellae, the avian equivalent to the m. stapedius, was identified by retrograde labeling with WGA-HRP. It consists of a discrete group of approximately 65 neurons located along the dorsolateral border in the ventral subnucleus of the facial nuclear complex. Other facial motoneurons were only labeled when diffusion of the tracer into neighbor structures was not excluded. The dorsal subnucleus of the facial nerve innervates the m. depressor mandibulae.

Animals↗

PubMed1992

Cochlear efferent neurons projecting to both ears in the chicken, Gallus domesticus.

Different retrograde neuroanatomical tracers were injected into each cochlea of adult chicken. The number of cells labeled in the cochlear efferent cell group found bilaterally within the caudal pontine reticular formation depended upon the tracer, with True Blue and Fluoro Gold yielding maximal average counts of 332 efferent neurons per injection. Double labeling of less than 1% of these cells was possible with the combination of True Blue and Diamidino Yellow. Thus the contribution of efferent neurons with axon collaterals projecting to both ears is not fundamentally different in birds and other vertebrates.

Animals↗

PubMed1992

Sound delay lines in the nucleus laminaris of the chicken.

Delays of neurophonic potentials (NP) induced by monaural sound stimuli were measured across the three dimensions in the nucleus laminaris (NL) of the anesthetized chicken. Peak latencies and delays in cross-correlograms changed with recording distance. An orderly delay line was observed across the NL thickness, that is, along dendritic trees of individual fusiform cells (FC), where phase lags increased dorso-ventrally during ipsi- and in the opposite direction during contralateral stimuli. Delays along isofrequency FC arrays were variable, with delay ranges being smaller for ipsilateral than for contralateral sound stimuli. Net delays for contralateral sounds were directed medio-laterally and differences between ipsi- and contralateral delays covered, roughly, intercochlear time differences (ITD). The observed delays are thought to contribute to sound localization and frequency analysis.

Animals↗

PubMed1992

The distribution of neurons labelled retrogradely with [3H]-D-aspartate injected into the colliculus inferior of the cat.

It is important to know if the transmission of sound signals through the inferior colliculus is mediated by the transmitters glutamate or aspartate because of pharmacological consequences for auditory perception. In order to identify candidate's neurons, the retrograde transport for [3H]-D-aspartate, injected into the left inferior colliculus, was studied in cats. Labelled cells were found in the dorsal and intermediate lateral lemniscal nuclei, mainly on the contralateral side. The cochlear nuclei, superior olivary nuclei and the auditory cortex were not labelled in brains containing other labelled neurons at greater distances from the injection site. Labelled cells were found in the reticular formation and adjacent nucleus coeruleus, the parabrachial nuclei, raphe nuclei (magnus, dorsalis and centralis superior), nucleus prepositus hypoglossi, lateral hypothalamus and hippocampal CA1.

Animals↗

PubMed1992

Can central neurons reproduce sound waveforms? An analysis of the neurophonic potential in the laminar nucleus of the chicken.

Extracellular field potentials in response to pure tones, clicks and noise were recorded with microelectrodes from the laminar nucleus (NL) of the anesthetized chicken. Slow "on" and "off" potentials reversed polarity with recording depth, indicating that synaptically activated dendrites of fusiform cells were dorsal for ipsilateral and ventral for contralateral stimuli. Oscillations at sound frequency were found to be maintained for the duration of the stimulus (neurophonic potential, or NP). In contrast to slow "on" and "off" potentials, NPs were gradually shifted in phase as the electrodes penetrated the NL from dorsal to ventral. Neurophonic oscillation frequencies obtained with clicks and noise were equal to best pure tone frequencies yielding maximal NP amplitudes. Autocorrelation functions calculated from steady state NPs in response to pure tone stimuli indicated a presence of sine waves in noise, and power spectra typically consisted of single frequency components. NPs, slow "on" and slow "off" potentials were sharply tuned over similar frequency ranges and tuning tended to be sharper for higher frequencies. The NP represents an electrical replica of the sound waveform which may be present across the fusiform cell membrane.

Acoustic Stimulation↗

PubMed1990

Spectral response patterns of auditory cortex neurons to harmonic complex tones in alert monkey (Macaca mulatta).

1. The auditory cortex in the superior temporal region of the alert rhesus monkey was explored for neuronal responses to pure and harmonic complex tones and noise. The monkeys had been previously trained to recognize the similarity between harmonic complex tones with and without fundamentals. Because this suggested that they could preceive the pitch of the lacking fundamental similarly to humans, we searched for neuronal responses relevant to this perception. 2. Combination-sensitive neurons that might explain pitch perception were not found in the surveyed cortical regions. Such neurons would exhibit similar responses to stimuli with similar periodicities but differing spectral compositions. The fact that no neuron with responses to a fundamental frequency responded also to a corresponding harmonic complex missing the fundamental indicates that cochlear distortion products at the fundamental may not have been responsible for missing fundamental-pitch perception in these monkeys. 3. Neuronal responses can be expressed as relatively simple filter functions. Neurons with excitatory response areas (tuning curves) displayed various inhibitory sidebands at lower and/or higher frequencies. Thus responses varied along a continuum of combined excitatory and inhibitory filter functions. 4. Five elementary response classes along this continuum are presented to illustrate the range of response patterns. 5. "Filter (F) neurons" had little or no inhibitory sidebands and responded well when any component of a complex tone entered its pure-tone receptive field. Bandwidths increased with intensity. Filter functions of these neurons were thus similar to cochlear nerve-fiber tuning curves. 6. "High-resolution filter (HRF) neurons" displayed narrow tuning curves with narrowband widths that displayed little growth with intensity. Such cells were able to resolve up to the lowest seven components of harmonic complex tones as distinct responses. They also responded well to wideband stimuli. 7. "Fundamental (F0) neurons" displayed similar tuning bandwidths for pure tones and corresponding fundamentals of harmonic complexes. This response pattern was due to lower harmonic complexes. This response pattern was due to lower inhibitory sidebands. Thus these cells cannot respond to missing fundamentals of harmonic complexes. Only physically present components in the pure-tone receptive field would excite such neurons. 8. Cells with no or very weak responses to pure tones or other narrowband stimuli responded well to harmonic complexes or wideband noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1989

Transmitter neurochemistry of the efferent neuron system innervating the labyrinth.

It is likely that several mechanisms contribute to the efferent control of cochlear and vestibular function. Different effects are probably mediated by different neuronal transmitters. In spite of a number of transmitter candidates, it is still widely assumed that the entire efferent system can be globally characterized as cholinergic. We attempted to label retrogradely identified efferent neurons in the brainstem with a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine (ACh) synthesizing enzyme. Only a portion of the vestibular efferents could thus be shown to be cholinergic in the rat. Medial cochlear efferents, terminating under outer hair cells, may also be cholinergic since they stain intensely for acetylcholine esterase (AChE) after pre-treatment with the AChE inhibitor diisopropylfluorophosphate (DFP). The lateral cochlear efferents terminating under inner hair cells, as well as more than half of the vestibular efferent neuron population, reacted negatively with either method designed to identify cholinergic neurons. Half of the lateral olivo-cochlear neuron population filled retrogradely with tritiated gamma-amino butyric acid [( 3H]-GABA). These cells were similar in size and distribution to neurons staining for the GABA synthesizing enzyme glutamic acid decarboxylase (GAD). Retrograde transport of [3H]-aspartate from the inner ear to the brainstem was seen in half of the lateral olivocochlear population, as well as in part of the efferent vestibular population in group E and in the caudal pontine reticular nucleus (CPR). Since various peptides have also been located in efferent neurons, this system is chemically diversified. Several distinct mechanisms of efferent control with presumably differing functions must, therefore, exist.

Acetylcholinesterase↗

PubMed1989

Electrophysiology of the electrically and mechanically damaged cochlea.

Electrical and mechanical stimuli were used in an attempt to cause cochlear deafness in a preparation with a rich supply of afferent cochlear neurons. Hearing sensitivity was assessed by electrocochleography and neuron survival was estimated by evaluating electrically induced auditory brainstem responses (EABR). Charge balanced sinusoidal alternating currents between 1 and 30 kHz for up to 15 hours produced a limited high frequency hearing loss when applied through the intact round window. A similar permanent threshold shift (PTS) could be induced by mechanical irritation with a scala tympani electrode through a round window fenestration. There is a summation of electrical and mechanical damage; however, complete deafness never occurred and the EABR provided no evidence for a major retrocochlear damage. These results suggest that deafness associated with perilymph leakage or induced during certain types of ear surgery should not be accepted as inevitable.

Animals↗