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.

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.

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.

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.

Frequency selectivity of central auditory neurons without inner ear

A group of neurons in the inferior colliculus (IC) of the chicken displayed frequency selectivity after surgical removal of both cochleae. Characteristic neuronal frequencies were evident in the following three measures. i) The cells fired spontaneously with discrete preferred interspike intervals; ii) Impulse responses of these neurons to electrical stimuli of cochlear nerves displayed oscillations at the preferred frequencies; iii) When the cochlear nerves were stimulated with a random pulse sequence, a reverse correlation analysis showed that the cells preferred the same frequencies in the stimulus input. Preferred frequencies observed thus far covered over 4 octaves of the auditory range. These "oscillating cells" were found only in a small rostromedial area in the IC. Neuronal frequency selectivity may serve a temporal analysis of sound and underlie sound identification with certain cochlear implants. It may also support interaural crosscorrelation necessary for directional hearing.

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.

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.

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.

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.

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.

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)

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.

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.

Secondary vestibular and neck position signals in the vestibular nuclei of alert rhesus monkeys performing active head movements

Rhesus monkeys were trained to track a visual target with head and eye movements in order to study central vestibular neurons under natural conditions. Single unit recordings of cells in the vestibular nuclei were obtained during active head rotations in the horizontal plane, and also during passive copies of these self-induced movements. Most cells exhibited secondary responses immediately following the primary vestibular responses to active or passive rapid head movements. They were of opposite polarity to the primary responses, and generally rate enhancements of secondary responses were of greater amplitude than rate suppressions. In addition, vestibular nuclei cells also encoded tonic neck position. The corresponding signal consisted of a variation in the basal discharge rate as a function of neck, and thus head, position. These observations prove for the first time that dynamic and static response characteristics recorded earlier from lesioned brains and under anaesthesia are, at least qualitatively, representative for the normally behaving vestibulomotor system in primates.

Perception of the missing fundamental in nonhuman primates

In preparation for neurophysiological experiments aimed at mechanisms of pitch perception, four rhesus monkeys were trained to press a button when the fundamental frequencies (missing or present) of two complex tones in a tone pair matched. Both tones were based on a five-component harmonic series. Zero to three of the lowest components could be missing in the first tone, while the second (comparison) tone contained all five harmonics. The range of fundamentals tested varied from 200 to 600 Hz. Three monkeys learned to match tones missing their fundamentals to comparison harmonic complexes with the same pitch, whereas the fourth monkey required the physical presence of the fundamental. Consideration of several cues available to the monkeys suggests that the animals could perceive the missing fundamental.

Histopathology of chloroform-induced inner ear damage

Inner ear function loss was caused in guinea pigs and rats by injecting chloroform into the middle ear. After symptoms for cochlear and vestibular deficit had been registered, the animals were permitted to survive for one day to five months. Ear histopathology was then studied in celloidin sections. In both species, hair cells and afferent nerve fibers were intact at all survival times. The acute stage of functional loss in guinea pigs was associated with inner ears of normal histological appearance. Within days after chloroform injection a severe otitis media developed which led to fibrous occlusion of the round window and eventually to new bone growth in the middle ear space around the otic capsule. A secondary labyrinthitis was also observed, resulting in endolymphatic hydrops at longer survival times. Different histopathological changes were seen in rats. The tectorial membrane appeared swollen in all cases, the swelling being more severe in more apical turns at longer survival times. It is concluded that only secondary sequela of the initial functional insult can be detected by standard light microscopic histopathology. Chloroform does not cause a chemical labyrinthectomy as previously assumed, although it is severely ototoxic.

Retrograde transport of [3H]-D-aspartate label by cochlear and vestibular efferent neurons

[3H]-D-aspartic acid was injected into the inner ear of rats. After a six hour survival time, labeled cells were found at all locations known to contain efferent cochlear or vestibular neurons. Most labeled neurons were found in the ipsilateral lateral superior olivary nucleus (LSO), although both ventral nuclei of the trapezoid body (VTB), group E, and the caudal pontine reticular nucleus (CPR) just adjacent to the ascending limb of the facial nerve also contained labeled cells. Because not all efferent neurons in the rat could be previously shown to be cholinergic, aspartate and glutamate are efferent transmitter candidates.

Retrograde transport of [3H]-GABA by lateral olivocochlear neurons in the rat

Injection of [3H]-gamma aminobutyric acid (GABA) into the perilymphatic space of the rat's inner ear resulted in retrograde labeling of a portion of the small efferent olivocochlear neurons within the lateral superior olivary nucleus (LSO). These cells were of similar size as LSO neurons stained immunohistochemically with antibodies to the GABA synthesizing enzyme glutamic acid decarboxylase (GAD). They were of fusiform shape, but smaller than principal LSO cells, which did not stain for GAD and did not accumulate [3H]-GABA. Other efferent cochlear and vestibular neurons were not labeled.

Sound code for cochlear implants

This is a review of how various commercially available cochlear prostheses encode speech and complex tones. Arguments in favor of temporal coding through relatively few channels are presented. Advantages of analogue over pulsatile stimuli are explained. Parameters responsible for perception of vowels, consonants, musical pitch and loudness are didactically analyzed. Active rehabilitation is considered an integral part of the coding effort.

A complex tone code in the auditory cortex

In the search for an objective measure of a biologically meaningful encoding scheme for cochlear implants, we recorded responses to tones from the alert auditory cortex. Neurons in the primary auditory field (A1) are typically characterized by filter characteristics resembling those of eighth nerve afferent fibers (filter neurons). In contrast, a different class of neurons has been found thus far only outside of the cochleotopic A1 array. These cells are sharply tuned to pure tones, and corresponding fundamental frequencies of harmonic complexes (F0 neurons). Thus these cells are specific for various tones of the same pitch. Sharpened pure- and complex-tone tuning can be accounted for by lateral inhibition. Time patterns for tonal and noise responses can vary dramatically.

Electrophysiological evaluation of chloroform-induced inner ear damage

Local placement of chloroform in either the external or the middle ear has been previously reported to induce a chemical labyrinthectomy. In order to examine the value of this effect as a research tool, we injected chloroform into the middle ears of guinea pigs and rats. Cochlear damage was assessed by electrocochleography (ECochG) and auditory brainstem response (ABR) audiometry. Both species developed complete deafness within a few hours after instillation of the chloroform. The deafness was permanent in the guinea pigs, whereas there was a partial recovery of auditory function in the rats. The survival rate of the auditory nerve fibers was estimated by measuring the ABR evoked by electrical stimulation via the scala tympani (EABR). A normal EABR recruitment pattern suggested that the main chloroform effect was located peripheral to the afferent axons. In conclusion, chloroform must be considered a severely ototoxic agent when applied locally.

Vestibular nuclear neuron activity during active and passive head movement in the alert rhesus monkey

Responses of single neurons were recorded in the medial and descending vestibular nuclei (MVN and DVN) and in the deep cerebellar nuclei of three juvenile rhesus monkeys (Macaca mulatta). Neuronal activity was measured during both passive sinusoidal and nonsinusoidal whole body rotation (peak velocities were under 90 degrees/s) and during active head movements. Although the active head movements occasionally exceeded 300 degrees/s, most exhibited peak velocities of less than 200 degrees/s. A total of 133 units sensitive to horizontal head rotation were recorded, and of these, 38 were held for sufficient time to obtain both passive and active head movement data. Comparison of the neuronal firing patterns obtained during active and passive head movements revealed no apparent differences. Thus neurons that were observed to burst or pause during saccades with the head fixed continued to do so when the head was free. Both the sensitivity to head velocity and the "inferred" spontaneous firing rate were compared during active and passive head movements by plotting rate-velocity curves for both conditions. When the data points were fitted with linear regression lines, no statistically significant differences in either sensitivity or spontaneous rate were found. The present study provides no evidence that efferent vestibular activity alters the properties of afferent vestibular neurons during active head movements, as has previously been suggested (21). Furthermore, neurons in the rostral portions of the vestibular nuclei in primates encode head velocity based entirely on labyrinthine information. Neither neck proprioceptors nor an efference copy of the head movement motor program seem to contribute significantly to the firing patterns observed.

Projection of afferents from individual vestibular sense organs to the vestibular nuclei in the pigeon

The projection of individual labyrinthine sensory organs to the brain stem was studied by autoradiography, employing discrete [3H]leucine injections into the sensory epithelia. Within the vestibular nuclei, separate partly overlapping termination areas for each end organ were found in the superior and descending vestibular nuclei, whereas projection territories in the medial, ventrolateral and tangential nuclei overlapped extensively. A few lagenar fibres terminated in the external cuneate nucleus. Semicircular canals and utricular macula also project to the lateral cerebellar nucleus and the reticular formation. For each semicircular canal a projection system could be traced to distinct subgroups of the extraocular motoneuron pools.

Cholinergic innervation of the rat's labyrinth

Efferent vestibular and cochlear neurons were identified in the rat's brain stem by retrograde labelling with True Blue (TB) or wheat germ agglutinin - horseradish peroxidase (WGA-HRP) injected into the utricle. Such cells were found at the same locations described in 1983 by White and Warr (ipsilateral superior olivary nucleus (LSO), bilateral latero-ventral nucleus of the trapezoid body (LTz) bilateral group E medial and lateral to the genu facialis) and, in addition, bilaterally in the caudal pontine reticular nucleus (CPR) at the level of the descending facial nerve. Cholinergic neurons were identified by counterstaining sections containing TB filled perikarya for acetylcholinesterase (AChE) following pretreatment with diisopropylfluorophosphate (DFP) or choline acetyltransferase (ChAT), by immunohistochemistry with highly specific monoclonal antibodies. Many, but not all, vestibular efferent cell bodies located in group E were shown to be cholinergic. These and other recently published data suggest that the efferent octavus system may consist of a number of chemically distinct cell groups.

Bechterew decompensation

A right labyrinthectomy was performed in rats 5 months after a left labyrinthectomy. Spontaneous compensation of balance functions after both operations was assessed by observing nystagmus, rolling about the longitudinal axis, circular walking and head tilt. Decompensation, induced by brief halothane-NO anaesthesia, released predominantly symptoms characteristic for the period after the first labyrinthectomy. Bechterew symptoms could, however, also be decompensated. It is concluded that Bechterew compensation does not re-establish balance in the central vestibular system. Dysbalance and vertigo of vestibular origin is thus conceivable in patients after all peripheral vestibular function has been lost.