Intensity coding in the auditory periphery of the cat: Responses of cochlear nerve and cochlear nucleus neurons to signals in the presence of bandstop masking noise

The dynamic range over which fine intensity discrimination is possible has been reported to be largely unaffected by limitation of the spread of neuronal activity to neighbouring frequency regions by bandstop noise masking.We have therefore examined the responses of cochlear nerve and nucleus neurons to tone and noise signals in the presence of a bandstop masking noise designed to be comparable to that employed in the psychophysical experiments. Under these conditions, the vast majority of cochlear nerve fibres were saturated by sound levels at which some 50% of our sample of cochlear nucleus neurons still responded to signal level differences. The extended dynamic ranges of these cochlear nucleus neurons was shown to be a result of activation, by the masking noise, of the lateral inhibitory side-bands ‘biassing’ the neuron's discharge. A small proportion of cochlear fibres, having low spontaneous discharge rates and showing strong two-tone suppression effects, demonstrated analogous but not so pronounced effects. It is unclear in what form information on the level of stimuli under these conditions is transmitted by the majority of apparently saturated cochlear nerve fibres, but several possible mechanisms are discussed.     

Tuning characteristics of cochlear nucleus units in response to electrical stimulation of the cochlea

The responses of single units in the ventral cochlear nucleus of acute anesthetized guinea pigs were studied with continuous sinusoidal electrical stimuli presented through a multi-electrode implant in the scala tympani. Implants had two or four electrodes along the axis of the scala with 1 mm separations. Best frequencies were consistently in the 100 Hz range (50-250 Hz) with thresholds of about 0.063 mA peak-to-peak. Tuning curves were usually symmetrical with slopes of 3-4 dB/octave, both below and above the best frequency. The relative sharpness of the tuning curves, as measured by Q10dB, averaged 0.2. Dynamic ranges as determined by the intensity-rate functions for the various frequencies were 2-15 dB. No significant difference was found between tuning characteristics of units in response to stimulation via the apical or basal pair of implant electrodes. The findings suggest some limitations on the applicability of independent stimulating channels in multi-electrode implants.     

Response properties of type II and type III units in dorsal cochlear nucleus

Type II and type III units in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats are those with excitatory responses to best frequency (BF) tones at all levels above threshold. They are distinguished from type IV units which give predominantly inhibitory responses to tones. Type II and type III units are distinguished from one another by their rates of spontaneous activity. Type II units have low rates of spontaneous activity (below 2.5 spikes/s), whereas type HI units are spontaneously active at rates up to 95 spikes/s. In this paper we show that segregation of units according to spontaneous activity produces a segregation of several other properties as well. A typical type II unit responds vigorously to BF tones (median maximum rate of 242 spikes/s), has a slightly nonmonotonic rate versus level function, and responds weakly or not at all to broadband noise. These units tend to be located in the deep layer of the DCN. Type III units tend to respond to BF tones at moderate rates (median maximum driven rate of 124 spikes/s), have monotonie or nonmonotonic rate versus level functions, and respond to noise at rates comparable to their tone rates. They are distributed somewhat more uniformly in all DCN layers, but a majority were found in the fusiform cell and molecular layers. Most evidence indicates that type II units are recorded from inhibitory interneurons in the DCN.     

Threshold sensitivity of the auditory pathway: Effects of worsening signal-to-noise ratio in the auditory nerve

Perfusions of scala tympani with high potassium solutions increase spontaneous activity of auditory nerve fibres without affecting their threshold sensitivity. In these circumstances, however, the signal-to-noise ratio of the auditory nerve response to a given stimulus is worsened. Recordings from cochlear nucleus neurones during such perfusions indicate that this worsening is critical for threshold sensitivity at higher levels in the auditory pathway.     

Spatial organization of the reciprocal connections between the cat dorsal and anteroventral cochlear nuclei

We are studying the interconnections between the anteroventral cochlear nucleus (AVCN) and the dorsal cochlear nucleus (DCN). Biotinylated dextran was injected into the DCN, where the best frequency of responses was also recorded. Ventrotubercular neurons in AVCN were labeled, along with cochlear nerve fibers and the axons of cells in DCN. In AVCN, a central band of labeled cochlear nerve axons and large endbulbs was labeled. Bordering this band was a ‘fringe' of smaller tuberculoventral axonal endings forming pericellular nests. Most AVCN neurons projecting to DCN were stellate, elongate, or giant cells, located in the posterior division of AVCN, regardless of the DCN injection site. About 75% of the labeled AVCN cells lay within the bands of labeled cochlear nerve fibers. Another 15% were in the outer fringes on either side of these bands, while 10% were outside the bands and the fringes. These findings suggest that most AVCN neurons projecting to the DCN conform to the tonotopic map. A significant portion of the ventrotubercular neurons occupy side-bands in AVCN. Reciprocally, the tuberculoventral tract forms a robust fringe of axonal endings flanking the central bands. The neuronal and axonal bands and side-bands may underlie excitatory and inhibitory signal transformations.     

Frequency threshold curves and simultaneous masking functions in single fibres of the guinea pig auditory nerve

Tuning curves for simultaneous masking were measured electrophysiologically, in single fibres of the guinea pig auditory nerve. The masking paradigm used was an analogy of that used in the determination of psychophysical tuning curves in man. The resulting tuning curves ran nearly parallel to the corresponding neural frequency threshold curve, over all except the high-frequency portion of the tuning curve. There, the masking functions had a shallower slope than the excitatory frequency threshold curve. The frequency at which the slope became shallower had a close and consistent dependence on fibre characteristic frequency, reaching a value of 1.2 times fibre characteristic frequency in high-frequency fibres. Analysis of firing rates during the threshold determination gave information about the mechanism of the masking, and the results were supported by theoretical analysis. The results give information useful for the interpretation of psychophysical tuning curves, determined by simultaneous masking, in man.     

Responses of chopper units in the ventral cochlear nucleus of the anaesthetised guinea pig to clicks-in-noise and click trains

Auditory brainstem responses (ABRs) have been measured with clicks, clicks masked by noise, click trains and pseudorandom maximum length sequences (MLS) of clicks. To investigate the neuronal populations contributing to the ABR under these stimulation conditions, we measured the extracellular responses of ventral cochlear nucleus (VCN) units in the urethane-anaesthetised guinea pig. We studied 23 chopper, 7 primary-like and 7 onset units. This report focuses on the responses from chopper units. The probability of discharge for chopper units increased with increasing click level reaching nearly 100% in many units, over a range of about 20–30 dB. Following each response to a click there was a 5–10 ms suppression of the spontaneous or noise evoked activity. As the level of the noise was increased over a range of 20–30 dB, the response to the clicks gradually decreased leading to a complete abolition of the click response at high noise levels. In a few units, low level noise produced a facilitation of the response to single clicks. In response to constant level equally spaced click trains, discharge probability increased with increasing minimum pulse interval (MPI), approaching 100% for MPIs of 4–8 ms in some units. The recovery afforded by the gaps in the MLS train often resulted in higher discharge probability for MLS than click trains with the same MPI, while response probabilities for MLS and click trains were similar when compared at equivalent average click rates. At short MPIs (0.5 and 1.0 ms), peri stimulus time histograms in response to click trains resembled those to best frequency (BF) tones and noisebursts, with chopping peaks unrelated to unit BF. VCN units show highly synchronised and reliable responses to click trains, MLS trains and clicks masked by noise. The decrease in discharge rate and increase in latency of chopper units with decreasing click level, increasing click rate and increasing masker level parallel the peak amplitude and latency changes observed in the auditory brainstem response.     

Responses of cochlear nucleus units to electrical stimulation through a cochlear prosthesis: channel interaction

The responses of 39 single units in the ventral cochlear nucleus of acute anesthetized guinea-pigs were studied with continuous electrical stimuli presented through a dual-channel implant in the scala tympani. Implants had four electrodes placed along the axis of the cochlea with 1 mm separations. With a specific pair (either apical or basal) of stimulating electrodes, about half of the units responded when current was flowing apically, while the rest responded to current in the opposite direction. No obvious relation existed between the effective polarity of the basal and apical pairs of electrodes. Two response types were observed while stimulating through both pairs simultaneously. Seventy-nine percent of the units responded to the sum of the current waveforms presented through the two pairs. Twenty-one percent responded to the difference between the waveforms. Both types of responses were observed for suprathreshold as well as for some intensities of stimulation that alone were subthreshold. The type of response was not dependent on the absolute threshold or threshold difference between the two pairs. For equal peak-intensities of stimuli, two-channel stimulation evoked larger responses than single-channel stimulation, provided the two channels were in their effective polarities. Responses to dual-channel stimulation were consistently larger than the summed responses to the two individual single-channel stimuli. The observed responses to the dual-channel stimulation indicate that the adequate stimulus was determined by the linear combination of fields produced by the individual channels in the vicinity of the stimulating electrodes before the auditory nerve is stimulated, and that excitation takes place in a spatially restricted area of the auditory nerve.     

Influence of temperature on tuning of primary-like units in the guinea pig cochlear nucleus

The effect of temperature changes on the response area of primary-like single units recorded extracellularly in the anteroventral cochlear nucleus of the guinea pig was studied for cochlear temperatures in the range 27.4–40.3°C, while maintaining a normal rectal temperature of 38.5°C.A new approach to the cochlear nucleus was developed which involved opening of the temporal bone overlying the flocculus cavity and aspirating a small portion of the paraflocculus.Irrespective of characteristic frequency CF (0.8–16 kHz), hypothermia caused a reversible elevation of CF thresholds (1.6 dB/°C), a small loss of the sharpness of threshold tuning and reductions of the saturation and mean spontaneous rates. The CFs were unaffected, in contrast to single units in birds and cold-blooded animals.From a comparison of the effects of temperature on different species it is proposed that the processes that define CF and sharpness of tuning are different within a cochlea, and that these processes can be differentially influenced.     

Usefulness of radiological findings for predicting cochlear implantation outcomes in children with cochlear nerve deficiency: a pilot study

Conclusion: Children with CND received limited benefits from CIs and their results varied. The size of the vestibulocochlear nerve relative to the facial nerve could potentially be used as a predicator for CI outcomes in children with CND.

Objective: This study aimed to (1) retrospectively review the outcomes of cochlear implants (CIs) in children with cochlear nerve deficiency (CND) and (2) evaluate the clinical usefulness of radiological findings as predictors for post-implantation outcomes.

Methods: Study participants included 10 children with bilateral CND and profound sensorineural hearing loss. The preoperative magnetic resonance imaging and temporal bone computed tomography scans were evaluated. Auditory processing capability and speech perception performance were measured with Categories of Auditory Performance (CAP) and Speech Intelligibility Rating (SIR) scales. Aided hearing thresholds with CI were measured. The relationships between CI outcomes and the sizes of vestibulocochlear nerve and cochlear nerve canal (CNC) were analysed.

Results: Although post-operative CAP scores and hearing thresholds significantly improved in children with CND, their results were worse than those measured in implanted children with normal cochlear nerve. No significant correlation was found between the CI outcomes and the vestibulocochlear nerve diameters or the CNC diameters in children with CND. However, children with larger vestibulocochlear-nerve-to-facial-nerve-ratios got better results.     

光学成像甘氨酸对蜗核和前庭核神经元兴奋的抑制作用

目的 从神经细胞群的水平研究甘氨酸是否对蜗核(cochlear nucleus,CN)与前庭核(vestibular nucleus,VN)神经元核团的兴奋起抑制作用以及其作用方式。方法 从新生小鼠(1-3天)制备活体(in vitro)脑干组织切片,并用电压敏感染料(voltage-sensitive dye)进行染色,采用多位点光学记录系统(optical imaging)观察电刺激位听神经(第8颅神经,nⅧth)后传入兴奋的传导。结果 光学成像显示电刺激nⅧth后兴奋传导至脑干的CN核团和VN核团,并且光学方法可同步记录256个记录单元(elements)中神经兴奋传导的二维动态过程(n=35)。用甘氨酸受体桔抗剂-马钱子碱(50μM)浸泡组织切片后,在CN和VN中的光学信号与脑片在标准人工脑脊液中相比较有明显的增强(n=10)。马钱子碱对峰样快反应信号与慢反应信号的增强效应不同,在CN和VN的不同核团之间也有差异。在CN核团,快反应信号与慢反应信号增强幅度分别为154±34％(n=23,elements)和271±91％(n=23,elements),马钱子碱的增强效应与标准ACSF相比有显著性统计学差异(P<0.05)。在VN核团,慢反应信号增强幅度为149±56％(n=17,elements),也有显著性统计学差异(P<0.05)。但是,在VN核团,快反应信号增强幅度仅为102±14％(n=17,elements),与对照组相比较无显著性统计学差异(P>0.05)。另外,     

Processing of noise by single units of the inferior colliculus of the bat Rhinolophus ferrumequinum

For inferior colliculus units the response patterns and the thresholds for pure tones and noise of variable bandwidth were determined. In a threshold—bandwidth plot the noise thresholds usually fell along two regression lines whose point of intersection established the size of the neuronal critical bandwidth (nCB). The relevance of the small nCBs (0.2–0.4 kHz) obtained for the frequency range of the constant frequency part of the orientation call is discussed. No fixed relation was found either between the nCBs and the neuronal critical ratios or between the size of nCB and the width of the tuning curve 3 dB above threshold of the best frequency.     

HRCT指导下人工耳蜗植入术面神经垂直段定位及保护

人工耳蜗植入手术最常用的是经乳突-面隐窝进路术式[1]。其关键步骤是打开位于面神经垂直段外侧的面隐窝,以充分暴露后鼓室,找到蜗窗。传统的术中面神经垂直段定位可根据砧骨所指示的方向至二腹肌嵴确定大致走形,但文献报道及临床手术中发现面神经变异情况并非罕见,常见的解剖变异有骨管裂缺、分支变异、行程异常等[2]。     

Forward masking of auditory nerve and brainstem responses using high-pass noise maskers

The effects of varying the low-frequency cutoff of a high-pass noise used as a forward masker for a broadband click (40 dB HL) are compared for wave V of the auditory brainstem response and N₁ from the ear canal. Derived responses were also obtained using a waveform subtraction technique. N, amplitude and wave V latency were most sensitive to the changes in the masker low-frequency cutoff. Results suggest that N₁ is biased toward the high frequencies, most likely because of its requirements for high neural synchrony, and is not necessarily related to the maximum area of excitation. Wave V, on the other hand, seems to be less dependent upon highly synchronous events and is more closely related to activity in the mid-to-lower frequencies. Complex behavior of wave V was observed as a function of ΔT which suggests that temporal as well as spatial factors can interact in their contributions to wave V.     

Isolation and chemical analysis of neuron soma samples from the cat cochlear nucleus

Pieces of neuron somata were dissected from the cochlear nuclei of two cats and analyzed for activity of malic dehydrogenase, an important enzyme of oxidative metabolism. Based on location, size and shape the somata were identified as those of spherical bushy cells, octopus cells, and fusiform cells. Somata of cerebellar Purkinje cells were analyzed for comparison. The results suggest that the large cochlear nucleus neurons have malic dehydrogenase activities higher than those of most other neurons so far studied. Also, the octopus and fusiform somata had higher malic dehydrogenase activities than those of the spherical bushy cells.     

Lateral inhibition in the anteroventral cochlear nucleus of the cat: a microiontophoretic study

The spontaneous firing rates of non-prepotential (NPP) units of the anteroventral cochlear nucleus are quite low so it has not been possible to determine whether side band tones are inhibitory when presented alone. Microiontophoretically-applied excitatory amino acids can be used to excite non-spontaneous cells directly. Using this technique it can be shown that side band tone bursts 1/2 to 3/4 octave above the characteristic frequency (CF) of a NPP unit inhibit the amino acid-induced firing. Side band tones which inhibited the amino acid-induced firing were beyond the tuning curve. Side band tones within the tuning curve produced excitation. Both, however, usually reduced the activity evoked by a CF tone burst (i.e., two-tone interaction). The data suggests that lateral inhibition and two-tone interactions are separate phenomena in the auditory system and that lateral inhibition may play a critical role in determining the shape of the tuning curve of NPP units.     

Development of hyperactivity after hearing loss in a computational model of the dorsal cochlear nucleus depends on neuron response type

Cochlear damage can change the spontaneous firing rates of neurons in the dorsal cochlear nucleus (DCN). Increased spontaneous firing rates (hyperactivity) after acoustic trauma have been observed in the DCN of rodents such as hamsters, chinchillas and rats. This hyperactivity has been interpreted as a neural correlate of tinnitus. In cats, however, the spontaneous firing rates of DCN neurons were not significantly elevated after acoustic trauma. Species-specific spontaneous firing rates after cochlear damage might be attributable to differences in the response types of DCN neurons: In gerbils, type III response characteristics are predominant, whereas in cats type IV responses are more frequent. To address the question of how the development of hyperactivity after cochlear damage depends on the response type of DCN neurons, we use a computational model of the basic circuit of the DCN. By changing the strength of two types of inhibition, we can reproduce salient features of the responses of DCN neurons. Simulated cochlear damage, which decreases the activity of auditory nerve fibers, is assumed to activate homeostatic plasticity in projection neurons (PNs) of the DCN. We find that the resulting spontaneous firing rates depend on the response type of DCN PNs: PNs with type III and type IV-T response characteristics may become hyperactive, whereas type IV PNs do not develop increased spontaneous firing rates after acoustic trauma. This theoretical framework for the mechanisms and circumstances of the development of hyperactivity in central auditory neurons might also provide new insights into the development of tinnitus.