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PubMed2001

Type III units in the gerbil dorsal cochlear nucleus may be spectral notch detectors.

Broadband sounds originating in the median plane are thought to be localized by neural processing of spectral notches introduced by the filtering action of the pinnae. Previous studies (Nelken, I., and E. D. Young. J. Neurophysiol. 71:2446-2462, 1994: Spirou, G. A., and E. D. Young. ibid. 66: 1750-1768. 1991) suggested that type IV units in decerebrate cat dorsal cochlear nucleus (DCN) are functional detectors of these spectral notches. Intracellular marking studies by Ding et al. (Ding, J., T. E. Benson, and H. F. Voigt. J. Neurophysiol. 82:3434-3457, 1999) have shown that type III units in gerbil arise from the DCN's principal output neurons, which are thought to have type IV unit properties in cat. A relative paucity of type IV units in the decerebrate gerbil (Davis. K. A., J. Ding, T. E. Benson, and H. F. Voigt. J. Neurophysiol. 75: 1411-1431, 1996) has motivated this study of spectral notch sensitivity in the gerbil DCN. Responses to notch noise stimuli were recorded from 15 gerbil type III units to investigate whether these units may function as spectral notch detectors. For narrow notch noise stimuli, all 15 units showed excitatory responses. For progressively wider notches, the discharge rate of 13/15 units became inhibited. As the maximum limits of notch width were approached, 11/15 units showed some degree of recovery from this inhibition. This response pattern in gerbil type III units possesses the salient features of notch noise responses in cat type IV units and implicates type III units in gerbilline spectral notch detection processes.

Acoustic Stimulation↗
PubMed2006

Evidence for a direct, short latency projection from the dorsal cochlear nucleus to the auditory thalamus in the guinea pig.

The auditory thalamus (medial geniculate body, MGB) receives its main ascending input from the inferior colliculus (IC), which was considered to be an obligatory relay for all auditory inputs to the MGB. However, recent anatomical evidence in the rat [ (Malmierca et al. 2002) J. Neurosci., 22, 10891-10897] has confirmed the presence of a direct pathway from the dorsal cochlear nucleus (DCN) to the medial MGB, bypassing the IC, as previously suggested in the chimpanzee [ (Strominger et al. 1977) J. Comp. Neurol., 172, 349-366]. We show that this direct pathway is also present in the guinea pig and apparently results in short latency responses in the thalamus. Injection of anterograde tracer into the DCN of five adult guinea pigs revealed terminal boutons and axonal swellings distributed throughout the medial MGB, but absent from all other MGB subdivisions. Electrophysiological recordings made from 39 adult guinea pigs (24 male & 15 female) showed neurons in the medial MGB responded with significantly shorter latencies to acoustic clicks (7.8 ms) than those from the ventral (11.0 ms), dorsal (14.4 ms), or shell (16.5 ms) MGB, consistent with the direct pathway from the DCN. The function of the direct pathway is not known but may be related to the fast responses and the role of the medial MGB in integrating combined somatosensory and auditory inputs. Short latency responses may be important in priming the auditory cortex to prepare it for rapid analysis and in recruiting the amygdala for rapid emotional responses such as fear.

Acoustic Stimulation↗
PubMed2006

Decorrelation sensitivity of auditory nerve and anteroventral cochlear nucleus fibers to broadband and narrowband noise.

Binaural neurons show remarkable sensitivity to temporal differences in the waveforms at the two ears. This ability obviously requires temporal coding of sound waveforms in the monaural afferents that converge on such binaural neurons. We introduce a new analysis to investigate how well responses of single monaural neurons support discrimination of decorrelations in waveforms. Spike trains from auditory nerve (AN) and anteroventral cochlear nucleus (AVCN) neurons of cats to many repetitions of a set of broadband and narrowband noise tokens were obtained. The normalized correlation between the noise tokens ranged from 0.99 to -1. A coincidence and signal detection analysis was used to perform a correlation discrimination task using the monaural spike trains. The correlation discrimination thresholds derived from AVCN neurons were lower than those derived from AN fibers and sometimes as low as human psychophysical just noticeable differences. Importantly, low detection thresholds required comparisons of spike trains at small internal delays. Bandwidth dependence of neural decorrelation thresholds agreed with psychophysical data when large internal delays contributed to the detection. We conclude that, in the context of correlation discrimination, coding by AVCN fibers is superior to that by AN fibers and that these discriminations require a distribution of internal or best delays in binaural processing that differs from the predictions from studies of discrimination in interaural time delays.

Acoustic Stimulation↗
PubMed1999

The representation of pure tones and noise in a model of cochlear nucleus neurons.

The limited dynamic range of the majority of auditory-nerve fibers represents a difficulty in accounting for normal hearing capabilities over the known psychoacoustic intensity range. The presence of noise is an additional complication because it will tend to saturate these fibers, thereby considerably reducing their dynamic range, i.e., the range of mean firing rates. In this study, simulations involving a model of auditory nerve and cochlear nucleus neurons were conducted using pure-tone stimuli in the presence of noise. The main focus is on the role of inhibition in regulating the activity of cells, improving their capability to represent signals in background noise. This concerns in particular those inhibitory neurons that receive input from a wide range of auditory-nerve fibers and respond with an onset chopper pattern. A detailed model of stellate cells is used. It allows several parameters such as the number, location, and strength of inputs to be manipulated. The fist part of this paper presents the model and its responses to pure-tone and noise stimuli presented separately. The model's capacity to generalize to tone/noise combinations is then tested. Responses to these stimuli are found to be qualitatively similar to neurophysiological findings. Model neurons exhibit appropriate shifts in their rate-level functions and their responses are inhibited or suppressed by tones outside their characteristic frequency. The model stellate cell is also found to display many of the temporal patterns reported in electrophysiological studies as a result of appropriate settings of certain parameters. Therefore, the model is sufficient to account for a larger number of findings and should serve as a basis for predicting responses to novel stimuli, or as a building block for modeling larger networks.

Audiometry, Pure-Tone↗
PubMed1992

A computer model of a cochlear-nucleus stellate cell: responses to amplitude-modulated and pure-tone stimuli.

A computer model of a ventral-cochlear-nucleus (VCN) stellate cell with chop-S type response properties is presented and evaluated. The model is based on a simplified model of spike generation preceded by a stage that simulates dendritic low-pass filtering. Input to the model is in the form of simulated auditory-nerve spikes produced by a model of the auditory periphery [Meddis and Hewitt, J. Acoust. Soc. Am. 89, 2866-2882 (1991)]. Outputs from the stellate-cell model are shown to qualitatively replicate a wide range of typical in vivo responses. These include: (a) realistic onset and steady-state rate-level functions, (b) "chopper"-type post-stimulus time histogram responses; (c) typical "chop-S"-type neuron responses characterized by a low coefficient of variance (CV less than 0.3) of interspike intervals as a function of time; (d) level-dependent amplitude-modulation transfer functions; (e) intrinsic oscillations in responses to pure-tone stimuli; (f) amplitude-modulation encoding over a wide dynamic range; and (g) frequency-limited phase locking to pure tones. It is shown that these responses can be explained primarily by the membrane properties of the cells. More specifically, how the model encodes signal amplitude modulation was studied and an explanation was suggested for the generation of the bandpass modulation transfer functions. Such functions are observed neurally in response to amplitude-modulated stimuli presented at moderate to high signal levels.

Animals↗
PubMed2003

The effects of early bilateral deafening on calretinin expression in the dorsal cochlear nucleus of aged CBA/CaJ mice.

The aim of this study was to test the hypothesis that calretinin (CR) levels in the aged mouse auditory brainstem depend upon hearing ability. Old animals with good hearing, and thus higher sound-evoked activity levels, were predicted to have higher levels of CR immunoreactivity than old animals with hearing loss. CR immunoreactivity was analyzed in the deep layer (layer III) of the dorsal cochlear nucleus (DCN) in CBA/CaJ mice that were bilaterally deafened at 3 months of age with kanamycin, and then aged until 24 months. This manipulation partially mimics the lack of sound-evoked auditory activity experienced by old C57BL/6J mice, who are deaf at 24 months of age (but show residual hearing at 15 months) and have lower levels of CR immunoreactivity than old CBA mice with normal hearing [Hear. Res. 158 (2001) 131]. Cell counts revealed that the density of CR+ cells in DCN layer III of the deafened CBA mice was statistically different from old intact CBA mice raised under identical conditions. Old deafened CBAs showed a decline of 47% in the mean density of CR+ cells compared to old hearing CBAs, thus supporting the hypothesis. Interestingly, while there tended to be fewer CR+ cells in the old deaf C57s as compared to young C57s and young and old CBAs with normal hearing, the difference was not statistically significant. It is possible that the residual hearing of C57 mice at 15 months may provide sufficient auditory input to maintain CR at levels higher than CBA mice that are deafened completely at 3 months of age, and are profoundly deaf for a much longer time (21 months).

Aging↗
PubMed1986

The distribution of spherical cells in the anteroventral cochlear nucleus of the guinea pig.

Several types of neuron are found in Nissl-stained sections of the anteroventral cochlear nucleus (AVCN). From these one group, the spherical cells (Osen, 1969), can be readily distinguished from the remaining small and multipolar forms. The rostral pole of the AVCN has previously been subdivided into the large and the small spherical cell areas (in several mammals). In the present study of the guinea pig AVCN, spatial distributions of cell density, size, and shape have been investigated. These have been used to test whether the subdivision made on the basis of morphological differences in the spherical cells is valid, or whether there is a gradual gradient in these features. This analysis has shown that although variations in cell size and shape are observed, the spherical cell area cannot be partitioned on these grounds. There is, however, a graded increase in spherical cell packing density towards the rostral pole of the AVCN, with proportionately fewer of the other cell types present.

Animals↗
PubMed1988

The spatial representation of frequency in the rat dorsal cochlear nucleus and inferior colliculus.

The spatial distribution of neural activity produced by tones was assessed in the rat dorsal cochlear nucleus (DCN) and inferior colliculus (IC), using the 2-deoxyglucose (2-DG) technique. Eight pure tones, spanning the range of reported single unit characteristic frequencies in the rat, were presented at 40 dB above behavioral threshold. The relationship between frequency of stimulation and location of neural activity within each nucleus was evaluated quantitatively. Based on the 2-DG uptake pattern across animals, a tonotopic axis in the transverse plane was defined for each nucleus. This axis transected the centers of regions of evoked 2-DG uptake for each frequency. There was an orderly relationship between stimulus frequency and the location of evoked neural activity along the axis. Each pure tone stimulus activated an approximately equal proportion of this axis, for all frequencies tested, in both the DCN and IC. This suggests the existence of equal 'spatial bandwidths, in rat central auditory structures, across its entire frequency range. Equal spatial bandwidths could facilitate signal analysis strategies which require interaction between neurons with closely-related CFs. In the horizontal plane, however, the proportion of stimulated tissue was not equal across frequency. High-frequency (greater than 8 kHz) tones produced increased neural activity along a much greater extent of the anterior-to-posterior axis of the IC than did low-frequency tones.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2002

Gene expression profiles of the rat cochlea, cochlear nucleus, and inferior colliculus.

High-throughput DNA microarray technology allows for the assessment of large numbers of genes and can reveal gene expression in a specific region, differential gene expression between regions, as well as changes in gene expression under changing experimental conditions or with a particular disease. The present study used a gene array to profile normal gene expression in the rat whole cochlea, two subregions of the cochlea (modiolar and sensorineural epithelium), and the cochlear nucleus and inferior colliculus of the auditory brainstem. The hippocampus was also assessed as a well-characterized reference tissue. Approximately 40% of the 588 genes on the array showed expression over background. When the criterion for a signal threshold was set conservatively at twice background, the number of genes above the signal threshold ranged from approximately 20% in the cochlea to 30% in the inferior colliculus. While much of the gene expression pattern was expected based on the literature, gene profiles also revealed expression of genes that had not been reported previously. Many genes were expressed in all regions while others were differentially expressed (defined as greater than a twofold difference in expression between regions). A greater number of differentially expressed genes were found when comparing peripheral (cochlear) and central nervous system regions than when comparing the central auditory regions and the hippocampus. Several families of insulin-like growth factor binding proteins, matrix metalloproteinases, and tissue inhibitor of metalloproteinases were among the genes expressed at much higher levels in the cochlea compared with the central nervous system regions.

Animals↗
PubMed1984

Baclofen reduces tone-evoked activity of cochlear nucleus neurons.

Recent evidence suggests that an excitant amino acid may be a neurotransmitter at acoustic nerve synapses in cochlear nucleus (CN). Release of excitant amino acids is reportedly reduced by baclofen, a lipophilic GABA-mimetic used to treat the spasticity of multiple sclerosis and spinal injury. Microiontophoresis of (-)baclofen suppressed spontaneous and tone-evoked activity in CN neurons. GABA inhibited the responses of most neurons responsive to (-)baclofen. However, iontophoresis of these two substances onto the same CN neuron resulted in dramatic differences in time course to maximum effect and to recovery. Onset and offset of (-)baclofen-induced firing reduction were gradual at all doses (currents), but even the highest doses rarely caused total suppression of firing. Inhibition of firing by GABA was abrupt, and total suppression was frequently observed over the range of doses used. GABA desensitization (fading) commonly occurred while the (-)baclofen response never faded. The same CN neurons were also suppressed by D-alpha-aminoadipate, which blocks certain excitatory amino acid receptors, while the GABA antagonist bicuculline had no effect on the (-)baclofen response. These findings support the hypothesis that an excitant amino acid may be a transmitter at acoustic nerve synapses in CN.

2-Aminoadipic Acid↗
PubMed1989

Response properties and tonotopical organization in the dorsal cochlear nucleus in rats.

Pure tone burst stimulation was used to study discharge properties and tonotopic organization in the dorsal cochlear nucleus (DCN) of rats anesthetized with alpha-chloralose urethane. An animal's head was fixed with a fine nail embedded in the skull in order to prevent the head from moving. Unit discharges of DCN neurons were recorded with micropipettes filled with fast green dye. Free field pure tone bursts were presented using a ribbon tweeter; it delivered stimuli at a maximum SPL of 96 dB. Electrode tracks were reconstructed through subsequent histological examinations. A total of 429 units were sampled in 51 animals. Of these, 230 units (54%) responded to tone bursts higher than 20 kHz with a mean latency of 6.3 ms. Tonotopical organization was clearly evident throughout the mediolateral axis of the DCN; high frequencies were represented medially and low frequencies laterally. Nearly constant frequencies were represented along the dorso-ventral axis and no systematic organization was observed along the rostrocaudal extension of the DCN. When tuning curves were measured in DCN neurons with excitatory and inhibitory responses, we found that inhibitory sidebands were usually located in a frequency range higher or lower than the excitatory response area and sometimes partially overlapped with it.

Acoustic Stimulation↗
PubMed1992

Amplitude-modulated tone encoding behavior of cochlear nucleus neurons: modeling study.

A recent study of amplitude-modulated (AM) tone encoding behavior of dorsal and posteroventral cochlear nucleus (DCN and PVCN) neurons by Kim et al. [Hear. Res. 45, 95-113, 1990] observed that certain neurons (e.g., pause/build type-III neurons and chop-S neurons) tended to exhibit band-pass modulation transfer functions (MTFs) and intrinsic oscillations (IOs) whereas certain other neurons (e.g., chop-T neurons) tended to exhibit low-pass MTFs and no IOs. The goal of the present study was to develop models of these response characteristics in an attempt to understand the underlying neuronal mechanisms. We hypothesized that chopper neurons corresponded to stellate cells and pause/build neurons corresponded to fusiform cells. We also hypothesized that, with right input combination, appropriate models of a single stellate and fusiform cell could account for band-pass and low-pass MTFs as well as the associated IOs. The neuron models developed by Arle and Kim [Biol. Cybern. 64, 273-283, 1991] for the stellate and fusiform cells were used in this study. The models are modified versions of MacGregor type neuron model incorporating cell-specific nonlinear voltage-dependent conductances. The AM tone excitation via the auditory nerve fibers was represented by a current at the soma of the neuron model, which consisted of dc, ac and a zero-mean Gaussian noise. The dc, ac and noise represent a high-frequency carrier beyond the neuron's phase-locking limit, an envelope, and randomness of the system, respectively. With systematic variation of dc, ac and noise amplitudes, we observed the following: the band-pass MTF behaviors of pause/build and chop-S neurons were reproduced by the fusiform cell model and the stellate cell model with a strong dc/noise ratio, respectively. The low-pass MTF behavior of a chop-T neuron was reproduced by the stellate cell model with a weak dc/noise ratio. It was observed that the stellate cell model was more susceptible to the noise, in the sense that an increase in noise tended to abolish the IO and change the MTF of the model from band-pass to low-pass more readily in the stellate cell model than in the fusiform cell model. Kim et al. (1990) observed a close correlation between the IO frequency and the best envelope frequency (BEF). In the models, a similar correlation was observed between the two measures for both the stellate and fusiform cell models.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed2001

Cellular and subcellular distribution of AMPA-type glutamate receptor subunits and metabotropic glutamate receptor 1alpha in the cochlear nucleus of the horseshoe bat (Rhinolophus rouxi).

Ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) selective glutamate receptors (GluRs) are the main mediators of fast excitatory neurotransmission and composed of a variable combination of four different subunits (GluR1-4). The metabotropic glutamate receptor 1alpha (mGluR1alpha) is involved in plastic synaptic events. Since horseshoe bats strongly depend on temporal cues for acoustic imaging by echolocation and exhibit prominent species specific specializations of the cochlear nucleus (CN), the subunit distribution of AMPA selective GluRs and the distribution of mGluR1alpha was studied at the light and electron microscopic level with preembedding immunocytochemistry. Immunoreactivity to GluR1 was low throughout the CN. All types of projection neurons of the ventral CN expressed distinct GluR2/3 and GluR4 immunoreactivity with GluR4-labeling especially prominent in multipolar and octopus cell-like neurons of the posteroventral CN. The AMPA and metabotropic receptor inventory of the laminated ventral subdivision of the dorsal CN (DCNv) agreed with that reported in other mammals, whereas the specialized dorsal non-laminated subdivision of DCN (DCNd) lacked the prominent labeling for GluR2/3 and mGluR1alpha that characterizes cartwheel cells of DCNv. Distinct GluR2/3 and GluR4 immunoreactivity combined with low expression of mGluR1alpha immunoreactivity was characteristic for fusiform cells of DCNv and DCNd. Tuberculoventral cells of both the deep DCNv and the DCNd exhibited light to moderate GluR2/3 and GluR4 immunoreactivity. The staining patterns in DCNd thus indicate a loss of cerebellar-like microcircuits and a conservation of frequency specific circuitry of the deep and fusiform cell layers of the mammalian DCN.

Animals↗
PubMed1997

Temporal encoding and transmitting of amplitude and frequency modulations in dorsal cochlear nucleus.

Amplitude- and frequency-modulated (AM and FM, respectively) tones have been considered as simplified models of natural sounds. The responses of auditory neurons can phase-lock to the modulation frequency (fm). The encoding and transmitting of such modulation phase-locking are interesting since there is no any fm physical peak in spectrum. In the present study, we approached these issues by recording the phase-locked responses of the dorsal cochlear nucleus (DCN) units in guinea pigs to different AM and FM tones. For AM noise tones without the spectral cues of fm, the unit's discharges still phase-locked to the envelope cycles, but it was generally weaker than to sinusoidal AM (SAM) tones. At 50% modulation depth (dm), the mean modulation gains of Pauser/ Buildup (P/B) units (n = 7) to AM noise tones was -0.61 dB whereas they had a 6.48 dB mean to SAM tones. Similar to the case of AM tones, phase-locking to sinusoidal FM (SFM) tones represented the time courses of frequency changes, and it could be separated and changeable corresponding to the frequency increasing and decreasing. There were differences between the phase-locking to SAM and SFM tones in an identical unit. Both ON and type I/III units tended to have stronger phase-locking to the SFM tones than to the SAM tones. The phase-locking to the possible demodulated fm components was further examined with different carrier frequencies (fc) and pure tones. The DCN units showed poor or no responses to modulation tones out of their response areas even in the low characteristic frequency (CF) units, but the low-CF units had clear phase-locking to pure tones at the similar fm ranges. The puretone phase-locking had a band-pass shape different from the low-pass shape of the auditory nerve fibers. These data suggest that the modulation phase-locking in the DCN units may be based on the temporal modulation cues and transmitted in the carrier place. The temporal integration of modulation information over the unit's response area as an across-frequency temporal processing model was discussed for modulation enhancement in the CN units.

Acoustic Stimulation↗
PubMed1987

Spectral and temporal response patterns of single units in the chinchilla dorsal cochlear nucleus.

Spectral and temporal response patterns to pure-tone stimuli were collected from single units in the dorsal cochlear nucleus of anesthetized chinchillas. The spectral response profiles were divisible into groups based on the balance of excitation and inhibition. Temporal responses were characterized in chloralose-anesthetized animals by collecting PST-histograms. There appeared to be no simple one-to-one relationship between a unit's spectral and its temporal response pattern. Excitatory spectral responses were generally sharply tuned areas resembling those of auditory nerve fibers. However, unlike the latter, the majority of these had chopper or pauser/buildup temporal responses. Inhibitory spectral responses were of two distinct types: one included lateral inhibitory areas flanking the tuned excitatory areas which occasionally invaded the latter creating a nonmonotonic excitatory response at the unit's characteristic frequency. The other included sharply tuned inhibitory areas. The characteristic frequencies of these units were found to be in close correspondence with those of sharply tuned excitatory units from the same penetration suggesting that these inhibitory units were tonotopically mapped in the same register as tuned excitatory units. The spectral response patterns were studied with three types of anesthesia: ketamine/xylazine, dial/urethane, and chloralose. In each of these groups the patterns were similar. However, the proportions of units showing inhibition was strongly dependent on the choice of anesthetic agent with chloralose yielding the highest proportions (59%) and ketamine/xylazine yielding the lowest (29%).

Action Potentials↗
PubMed1992

Serotonin modulates auditory information processing in the cochlear nucleus of the rat.

The effect of iontophoretic application of serotonin (5-HT) was studied in neurons of the cochlear nucleus in the rat. 5-HT inhibited the spontaneous activity in 71%, and the tone-evoked activity in 32% of the neurons. We also observed an excitatory effect, with a longer latency than that of the inhibition, in 40% of the neurons. In some neurons 5-HT had both inhibitory and excitatory effects. Neurons with different response types seem to have different sensitivities to 5-HT. As the effects of 5-HT were generally weaker than those of other putative neurotransmitters, it probably has only a small modulatory influence on auditory processing.

Acoustic Stimulation↗
PubMed1994

Stapedius reflex and EABR thresholds in experienced users of the Nucleus cochlear implant.

Evoked auditory brainstem responses (EABR) and stapedius reflex thresholds were established in 7 experienced users of the Nucleus cochlear implant. Even using biphasic 400 microseconds/phase clicks for the EABR, responses were observed in only 5 patients; no stapedius reflex (SR) was seen in 3 patients, 2 of whom had a history of middle-ear disorder. The EABR threshold varied widely between subjective threshold and uncomfortable loudness level (ULL) for the same stimulus. The average SR threshold was found somewhat more consistently at 66% of the dynamic range between threshold and ULL, but grossly overestimated the most comfortable level (MCL) in most cases. To obtain equal loudness at the same current level we suggest that broad clicks (300 microseconds/phase) be used for EABR measurements, thus compensating for the lower repetition rate of EABR stimulus compared with the device fitting stimulus.

Acoustic Stimulation↗
PubMedDate not supplied

Temporal relationship between the auditory brainstem response and focal responses of auditory nerve root and cochlear nucleus during development in the tammar wallaby (Macropus eugenii).

Thirty-two pouch-young tammar wallabies were used to discover the generators of the auditory brainstem response (ABR) during development by the use of simultaneous ABR and focal brainstem recordings. A click response from the auditory nerve root (ANR) in the wallaby was recorded from postnatal day (PND) 101, when no central auditory station was functional, and coincided with the ABR, a simple positive wave. The response of the cochlear nucleus (CN) was detected from PND 110, when the ABR had developed 1 positive and 1 negative peak. The dominant component of the focal ANR response, the N1 wave, coincided with the first half of the ABR P wave, and that of the focal CN response, the N1 wave, coincided with the later two thirds. In older animals, the ANR response coincided with the ABR's N1 wave, while the CN response coincided with the ABR's P2, N2 and P3 waves, with its contribution to the ABR P2 dominant. The protracted development of the marsupial auditory system which facilitated these correlations makes the tammar wallaby a particularly suitable model.

Animals↗