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The medial nucleus of the trapezoid body (MNTB) is an important source of inhibition during the computation of sound location. It transmits fast and precisely timed action potentials at high frequencies; this requires an efficient calcium clearance mechanism, in which plasma membrane calcium ATPase 2 (PMCA2) is a key component. Deafwaddler ( dfw 2J ) mutant mice have a null mutation in PMCA2 causing deafness in homozygotes ( dfw 2J / dfw 2J ) and high-frequency hearing loss in heterozygotes (+/ dfw 2J ). Despite the deafness phenotype, no significant differences in MNTB volume or cell number were observed in dfw 2J homozygous mutants, suggesting that PMCA2 is not required for MNTB neuron survival. The MNTB tonotopic axis encodes high to low sound frequencies across the medial to lateral dimension. We discovered a cell size gradient along this axis: lateral neuronal somata are significantly larger than medially located somata. This size gradient is decreased in +/ dfw 2J and absent in dfw 2J / dfw 2J The lack of acoustically driven input suggests that sound-evoked activity is required for maintenance of the cell size gradient. This hypothesis was corroborated by selective elimination of auditory hair cell activity with either hair cell elimination in Pou4f3 DTR mice or inner ear tetrodotoxin (TTX) treatment. The change in soma size was reversible and recovered within 7 days of TTX treatment, suggesting that regulation of the gradient is dependent on synaptic activity and that these changes are plastic rather than permanent. NEW & NOTEWORTHY Neurons of the medial nucleus of the trapezoid body (MNTB) act as fast-spiking inhibitory interneurons within the auditory brain stem. The MNTB is topographically organized, with low sound frequencies encoded laterally and high frequencies medially. We discovered a cell size gradient along this axis: lateral neurons are larger than medial neurons. The absence of this gradient in deaf mice lacking plasma membrane calcium ATPase 2 suggests an activity-dependent, calcium-mediated mechanism that controls neuronal soma size., (Copyright © 2017 the American Physiological Society.)

Interaural time differences (ITDs) are the primary cue animals, including humans, use to localize low-frequency sounds. In vertebrate auditory systems, dedicated ITD processing neural circuitry performs an exacting task, the discrimination of microsecond differences in stimulus arrival time at the two ears by coincidence-detecting neurons. These neurons modulate responses over their entire dynamic range to sounds differing in ITD by mere hundreds of microseconds. The well-understood function of this circuitry in birds has provided a fruitful system to investigate how inhibition contributes to neural computation at the synaptic, cellular, and systems level. Our recent studies in the chicken have made significant progress in bringing together many of these findings to provide a cohesive picture of inhibitory function.

Glutamatergic synaptic transmission is essential for binaural auditory processing in birds and mammals. Using whole cell voltage clamp recordings, we characterized the development of synaptic ionotropic glutamate receptor (iGluR) function from auditory neurons in the chick nucleus laminaris (NL), the first nucleus responsible for binaural processing. We show that synaptic transmission is mediated by AMPA- and N-methyl-d-aspartate (NMDA)-type glutamate receptors (AMPA-R and NMDA-R, respectively) when hearing is first emerging and dendritic morphology is being established across different sound frequency regions. Puff application of glutamate agonists at embryonic day 9 (E9) revealed that both iGluRs are functionally present prior to synapse formation (E10). Between E11 and E19, the amplitude of isolated AMPA-R currents from high-frequency (HF) neurons increased 14-fold. A significant increase in the frequency of spontaneous events is also observed. Additionally, AMPA-R currents become faster and more rectifying, suggesting developmental changes in subunit composition. These developmental changes were similar in all tonotopic regions examined. However, mid- and low-frequency neurons exhibit fewer spontaneous events and evoked AMPA-R currents are smaller, slower, and less rectifying than currents from age-matched HF neurons. The amplitude of isolated NMDA-R currents from HF neurons also increased, reaching a peak at E17 and declining sharply by E19, a trend consistent across tonotopic regions. With age, NMDA-R kinetics become significantly faster, indicating a developmental switch in receptor subunit composition. Dramatic increases in the amplitude and speed of glutamatergic synaptic transmission occurs in NL during embryonic development. These changes are first seen in HF neurons suggesting regulation by peripheral inputs and may be necessary to enhance coincidence detection of binaural auditory information.

During a critical period prior to hearing onset, cochlea ablation leads to massive neuronal death in the mouse anteroventral cochlear nucleus (AVCN), where cell survival is believed to depend on glutamatergic input. We investigated the development of spontaneous miniature excitatory postsynaptic currents (mEPSCs) in AVCN neurons using whole cell patch-clamp techniques during [postnatal day 7 (P7)] and after (P14, P21) this critical period. We also examined the effects of unilateral cochlea ablation on mEPSC development. The two main AVCN neuron types, bushy and stellate cells, were distinguished electrophysiologically. Bushy cell mEPSCs became more frequent and faster between P7 and P14/P21 but with little change in amplitude. Dendritic filtering of mEPSCs was not detected as indicated by the lack of correlation between 10 and 90% rise times and decay time constants. Seven days after cochlea ablation at P7 or P14, mEPSCs in surviving bushy cells were similar to controls, except that rise and decay times were positively correlated (R = 0.31 and 0.14 for surgery at P7 and P14, respectively). Consistent with this evidence for a shift of synaptic activity from the somata to the dendrites, SV2 staining (a synaptic vesicle marker) forms a ring around somata of control but not experimental bushy cells. In contrast, mEPSCs of stellate cells showed few significant changes over these ages with or without cochlea ablation. Taken together, mEPSCs in mouse AVCN bushy cells show dramatic developmental changes across this critical period, and cochlea ablation may lead to the emergence of excitatory synaptic inputs impinging on bushy cell dendrites.

Neurons of nucleus magnocellularis (NM), a division of avian cochlear nucleus that performs precise temporal encoding, receive glutamatergic excitatory input solely from the eighth nerve and GABAergic inhibitory input primarily from the ipsilateral superior olivary nucleus. GABA activates both ligand-gated Cl- channels [GABA(A) receptors (GABA(A)Rs)] and G protein-coupled receptors (GABA(B) receptors). The net effect of GABA(A)R-mediated input to NM is inhibitory, although depolarizing. Several studies have shown that this shunting, inhibitory GABAergic input can evoke action potentials in postsynaptic NM neurons, which could interfere with their temporal encoding. While this GABA-mediated firing is limited by a low-voltage-activated K+ conductance, we have found evidence for a second mechanism. We investigated modulation of GABA(A)R-mediated responses by GABA(B)Rs using whole cell recording techniques. Bath-applied baclofen, a GABA(B)R agonist, produced dose-dependent suppression of evoked inhibitory postsynaptic currents (eIPSCs). This suppression was blocked by CGP52432, a potent and selective GABA(B)R antagonist. Baclofen reduced the frequency but not the amplitude of miniature IPSCs (mIPSCs) and did not affect postsynaptic currents elicited by puff application of a specific GABA(A)R agonist muscimol, suggesting a presynaptic mechanism for the GABA(B)R-mediated modulation. Firing of NM neurons by synaptic stimulation of GABAergic inputs to NM was eliminated by baclofen. However, endogenous GABA(B)R activity in the presynaptic inhibitory terminals was not observed. We propose that presynaptic GABA(B)Rs function as autoreceptors, regulating synaptic strength of GABA(A)R-mediated inhibition, and prevent NM neurons from generating firing during activation of the inhibitory inputs.

Using whole cell patch-clamp recordings, we pharmacologically characterized the voltage-gated Ca2+ channel (VGCC) currents of chicken nucleus magnocellularis (NM) neurons using barium as the charge carrier. NM neurons possessed both low- and high-voltage-activated Ca2+ channel currents (HVA I(Ba2+)). The N-type channel blocker (omega-conotoxin-GVIA) inhibited more than half of the total HVA I(Ba2+), whereas blockers of L- and P/Q-type channels each inhibited a small fraction of the current. Metabotropic glutamate receptor (mGluR)-mediated modulation of the HVA I(Ba2+) was examined by bath application of glutamate (100 microM), which inhibited the HVA I(Ba2+) by an average of 16%. The inhibitory effect was dose dependent and was partially blocked by omega-conotoxin-GVIA, indicating that mGluRs modulate N and other type HVA I(Ba2+). The nonspecific mGluR agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarbosylic acid (1S,3R-ACPD), mimicked the inhibitory effect of glutamate on HVA I(Ba2+). Group I-III mGluR agonists showed inhibition of the HVA current with the most potent being the group III agonist L(+)-2-amino-4-phosphonobutyric acid. 1S,3R-ACPD (200 microM) had no effect on K+ or Na+ currents. The firing properties of NM neurons were also not altered by 1S,3R-ACPD. We propose that the inhibition of VGCC currents by mGluRs limits depolarization-induced Ca2+ entry into these highly active NM neurons and regulates their Ca2+ homeostasis.

Whenever the head turns, the vestibuloocular reflex (VOR) produces compensatory eye movements to help stabilize the image of the visual world on the retina. Uncompensated slip of the visual world across the retina results in a gradual change in VOR gain to minimize the image motion. VOR gain changes naturally during normal development and during recovery from neuronal damage. We ask here whether visual slip is necessary for the development of the chicken VOR (as in other species) and whether it is required for the recovery of the VOR after hair cell loss and regeneration. In the first experiment, chickens were reared under stroboscopic illumination, which eliminated visual slip. The horizontal and vertical VORs (h- and vVORs) were measured at different ages and compared with those of chickens reared in normal light. Strobe-rearing prevented the normal development of both h- and vVORs. After 8 wk of strobe-rearing, 3 days of exposure to normal light caused the VORs to recover partially but not to normal values. In the second experiment, 1-wk-old chicks were treated with streptomycin, which destroys most vestibular hair cells and reduces hVOR gain to zero. In birds, vestibular hair cells regenerate so that after 8 wk in normal illumination they appear normal and hVOR gain returns to values that are normal for birds of that age. The treated birds in this study recovered in either normal or stroboscopic illumination. Their hVOR and vVOR and vestibulocollic reflexes (VCR) were measured and compared with those of untreated, age-matched controls at 8 wk posthatch, when hair cell regeneration is known to be complete. As in previous studies, the gain of the VOR decreased immediately to zero after streptomycin treatment. After 8 wk of recovery under normal light, the hVOR was normal, but vVOR gain was less than normal. After 8 wk of recovery under stroboscopic illumination, hVOR gain was less than normal at all frequencies. VCR recovery was not affected by the strobe environment. When streptomycin-treated, strobe-recovered birds were then placed in normal light for 2 days, hVOR gain returned to normal. Taken together, the results of these experiments suggest that continuous visual feedback can adjust VOR gain. In the absence of appropriate visual stimuli, however, there is a default VOR gain and phase to which birds recover or revert, regardless of age. Thus an 8-wk-old chicken raised in a strobe environment from hatch would have the same gain as a streptomycin-treated chicken that recovers in a strobe environment.

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.

Avian auditory and vestibular hair cells regenerate after damage by ototoxic drugs, but until recently there was little evidence that regenerated vestibular hair cells function normally. In an earlier study we showed that the vestibuloocular reflex (VOR) is eliminated with aminoglycoside antibiotic treatment and recovers as hair cells regenerate. The VOR, which stabilizes the eye in the head, is an open-loop system that is thought to depend largely on regularly firing afferents. Recovery of the VOR is highly correlated with the regeneration of type I hair cells. In contrast, the vestibulocolic reflex (VCR), which stabilizes the head in space, is a closed-loop, negative-feedback system that seems to depend more on irregularly firing afferent input and is thought to be subserved by different circuitry than the VOR. We examined whether this different reflex also of vestibular origin would show similar recovery after hair cell regeneration. Lesions of the vestibular hair cells of 10-day-old chicks were created by a 5-day course of streptomycin sulfate. One day after completion of streptomycin treatment there was no measurable VCR gain, and total hair cell density was approximately 35% of that in untreated, age-matched controls. At 2 wk postlesion there was significant recovery of the VCR; at this time two subjects showed VCR gains within the range of control chicks. At 3 wk postlesion all subjects showed VCR gains and phase shifts within the normal range. These data show that the VCR recovers before the VOR. Unlike VOR gain, recovering VCR gain correlates equally well with the density of regenerating type I and type II vestibular hair cells, except at high frequencies. Several factors other than hair cell regeneration, such as length of stereocilia, reafferentation of hair cells, and compensation involving central neural pathways, may be involved in behavioral recovery. Our data suggest that one or more of these factors differentially affect the recovery of these two vestibular reflexes.

Neurons of the cochlear nucleus, nucleus magnocellularis (NM), of young chicks require excitatory afferent input from the eighth nerve for maintenance and survival. One of the earliest changes seen in NM neurons after deafferentation is an increase in intracellular calcium concentration ([Ca2+]i). This increase in [Ca2+]i is due to loss of activation of metabotropic glutamate receptors (mGluR) that activate second-messenger cascades involved in [Ca2+]i regulation. Because mGluRs are known to act via the phospholipase C and adenylate cyclase signal transduction pathways, the goal of this study was to determine the roles of protein kinases A (PKA) and C (PKC) activities in the regulation of NM neuron [Ca2+]i by eighth nerve stimulation. Additionally, we sought to determine the relationship between increased [Ca2+]i and cell death as measured by propidium iodide incorporation. [Ca2+]i of individual NM neurons in brain stem slices was monitored using fura-2 ratiometric fluorescence imaging. NM field potentials were monitored in experiments in which the eighth nerve was stimulated. Five hertz orthodromic stimulation maintained NM neuron [Ca2+]i at approximately 110 nM for 180 min. In the absence of stimulation, NM neuron [Ca2+]i increased steadily to a mean of 265 nM by 120 min. This increase was attenuated by superfusion of PKC activators phorbol-12,13-myristate acetate (100 nM) or dioctanoylglycerol (50 microM) and by activators of PKA: 1 mM 8-bromoadenosine-3',5'-cyclophosphate sodium (8-Br-cAMP), 50 microM forskolin or 100 microM Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine. Inhibition of PKA (100 microM Rp-cAMPS) or PKC (50 nM bisindolymaleimide or 10 microM U73122) during continuous orthodromic stimulation resulted in an increase in NM neuron [Ca2+]i that exceeded 170 and 180 nM, respectively, by 120 min. Nonspecific kinase inhibition with 1 microM staurosporine during stimulation resulted in an [Ca2+]i increase that was greater in magnitude than that seen with either PKA or PKC inhibition alone, equal to that seen in the absence of stimulation, but much smaller than that seen with inhibition of mGluRs. In addition, manipulations that resulted in a [Ca2+]i increase >/=250 nM resulted in an increase in number and percentage of propidium iodide-labeled NM neurons. These results suggest that eighth nerve activity maintains [Ca2+]i of NM neurons at physiological levels in part via mGluR-mediated activation of PKA and PKC and that increases in [Ca2+]i due to activity deprivation or interruption of the PKA and PKC [Ca2+]i regulatory mechanisms are predictive of subsequent cell death.

The role of mitochondria and the endoplasmic reticulum in buffering [Ca2+]i in response to imposed calcium loads in neurons of the chick cochlear nucleus, nucleus magnocellularis (NM), was examined. Intracellular calcium concentrations were measured using fluorometric videomicroscopy. After depolarization with 125 mM KCl, NM neurons demonstrate an increase in [Ca2+]i that returns to near-basal levels within 6 min. Addition of the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP) dissipated the mitochondrial membrane potential, as evidenced by increased fluorescence when cells were loaded with rhodamine-123. Two micromolar CCCP had minimal effect on baseline [Ca2+]i. However, 2 or 10 microM CCCP interfered with the ability of NM cells to buffer [Ca2+]i in response to KCl depolarization without significantly affecting peak [Ca2+]i. Oligomycin also interfered with postdepolarization regulation of [Ca2+]i, but blocked late (7-8 min postdepolarization) increases in [Ca2+]i caused by CCCP. Thapsigargin had no effect on baseline, peak, or postdepolarization [Ca2+]i in NM cells. These results suggest that normal mitochondrial membrane potential and ATP synthesis play an important role in buffering [Ca2+]i in response to imposed calcium loads in NM neurons. Furthermore, the endoplasmic reticulum does not appear to play a significant role in either of these processes. Thus increases in mitochondrial number and function noted in NM cells after deafferentation may represent an adaptive response to an increased cytosolic calcium load.

1. Neurons in the cochlear nucleus, nucleus magnocellularis (NM), of embryonic and neonatal chicks are dependent on eighth nerve activity for their maintenance and survival. Removing this input results in the death of 20-40% of the NM neurons and profound changes in the morphology and metabolism of surviving neurons. 2. One of the first changes in NM neurons after an in vivo cochlea removal is an increase in intracellular calcium concentration ([Ca2+]i). Increased [Ca2+]i has been implicated in a number of neuropathologic conditions. 3. In this study, we orthodromically and antidromically stimulated NM neurons in an in vitro brain stem slice preparation and monitored NM field potentials while simultaneously assessing the [Ca2+]i of NM neurons using fura-2. 4. During continuous orthodromic stimulation, [Ca2+]i of NM neurons remained constant at 80 nM. In the absence of stimulation, NM neuron [Ca2+]i increased steadily to 230 nM by 90 min. Antidromic and contralateral stimulation produced a [Ca2+]i increase in NM neurons that was similar in magnitude but slightly more rapid than that observed in the absence of stimulation. 5. Addition of the metabotropic glutamate receptor (mGluR) antagonists (R,S)-alpha-methyl-4-carboxyphenylglycine or 2-amino-3-phosphonopropionic acid to the superfusate during continued orthodromic stimulation resulted in a dose-dependent, rapid, and dramatic increase in NM neuron [Ca2+]i without affecting the postsynaptic field potentials recorded from NM. 6. The ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovalerate eliminated NM field potentials during continued orthodromic stimulation but did not result in an increase in [Ca2+]i. 7. Continuous superfusion of trans-(+/-)-aminocyclopentane dicarboxylate, but not glutamate, prevented the increase in [Ca2+]i in the absence of stimulation. 8. These results suggest that NM neurons rely on eighth nerve activity-dependent activation of a mGluR to maintain physiological [Ca2+]i. Removal of this mGluR activation results in an increase in [Ca2+]i that may contribute to the early stages of degeneration and eventual death of these neurons.

1. Although auditory and vestibular hair cells are known to regenerate after aminoglycoside intoxication in birds, there is only sparse evidence that the regenerated hair cells are functional. To address this issue, we examined the relation of hair cell regeneration to recovery of the vestibuloocular reflex (VOR), whose afferent signal originates at hair cells in the vestibular epithelium. Hair cell damage was produced by treating white Leghorn chicks (Gallus domesticus, 4-8 days posthatch) with streptomycin sulfate in normal saline (1,200 mg.kg-1.day-1 im) for 5 days. 2. In the 1st wk after treatment, the VOR gain was essentially 0, and hair cell density as assessed by light microscopy was approximately 40% of normal. Between the 1st and 3rd wk after treatment, the VOR was present. Although VOR gain varied considerably from one chick to another, it increased, on average, between the 1st and 3rd wk, as did the average hair cell density. At the end of 8-9 wk, the gain and phase of the VOR had returned to normal values, as had the average density of hair cells. 3. Therefore, despite the catastrophic initial effect of hair cell loss on the VOR, recovered hair cells appeared to restore the VOR completely. Average hair cell density increased with average VOR gain. VOR gain correlated better with recovery of type 1 hair cells than with recovery of type II hair cells. 4. In contrast to hair cell density, the appearance of the vestibular epithelia as assessed by hair cell stereocilia in scanning electron micrographs was a poor indicator of VOR gain. In both treated and control birds, epithelia with the same appearance could have quite different VOR gains, suggesting a variation in the functional viability of the hair cells. 5. This observation suggests that several factors, such as the repair of stereocilia, the efficacy of hair cell synapses on afferent fibers, and the extent of compensation by central vestibular pathways, may affect the recovery of VOR gain. However, our data suggest that hair cell regeneration plays an important role in this recovery.

1. Fura-2 imaging was used to measure the effects of glutamate on caffeine-sensitive Ca2+ stores in neurons of the avian cochlear nucleus, n. magnocellularis (NM). 2. On average, 100-mM caffeine stimulated a 250-nM increase in intracellular calcium ion concentration {[Ca2+]i} in Ca(2+)-free media; 1-mM glutamate significantly attenuated caffeine-stimulated Ca2+ responses. 3. The metabotropic glutamate receptor agonist, ACPD, also inhibited the caffeine-stimulated rise in [Ca2+]i. 4. Glutamate has an important role in regulating Ca2+ stores in NM neurons. Glutamate-deprivation (viz. cochlear removal) results in a rise in [Ca2+]i that may, in part, be the result of release from Ca2+ stores. We hypothesize that Ca(2+)-induced Ca2+ release stores (CICRs) may be involved in deprivation-induced cell death.

1. The objective of this study was to precisely evaluate the relationship between the threshold of neurons in the anteroventral cochlear nucleus (AVCN) and the properties of distortion product otoacoustic emissions (DPOAEs). Response areas of multiunit clusters in the AVCN and DPOAEs in the ear canal were measured alternately in the adult gerbil during furosemide-induced changes of the endocochlear potential. Stimulus frequencies of the probe tones for DPOAE measurement were in the range of f1 = 1.7-7.6 kHz and f2 = 2.0-9.0 kHz; the ratio f2:f1 was always 1.18. Stimulus amplitudes were varied in 5-dB steps from 30 to 80 dB SPL, with either equal amplitudes (L1 = L2) or unequal, with L1 set 10 dB above L2. Multiunit response areas were determined from cluster responses to a series of 100-ms tone bursts presented with a pseudo-random sequence in frequency and intensity. 2. Changes in the multiunit discharge properties after 50-75 mg/kg furosemide injection were as follows: the best frequency (BF) threshold increased from initial values in the range of 20-30 dB SPL to 50-80 dB SPL at 10-20 min postinjection and then recovered fully by 60-90 min. The spontaneous discharge activity decreased to zero before any changes in the frequency threshold curve were observed and did not return to initial values for several hours. Likewise, total discharge rates of stimulus elicited responses were reduced and tended to stay reduced even after BF threshold had fully recovered. 3. From the DPOAE measurements, the changes observed in the cubic distortion tone (CDT, 2f1-f2) emission after furosemide injection were as follows: at high levels of the probe tones, changes in the emission intensities generally stayed within a 10-dB range. The CDT amplitudes for low stimulus levels, however, were typically reduced by up to 40 dB, but recovered (depending on the furosemide dosage) by approximately 60-90 min. 4. At low to moderate stimulus levels of 40-60 dB SPL, there was a near perfect, minute-by-minute covariation of the ear canal CDT amplitude and the BF threshold measured in the AVCN. A 10-dB increase in threshold was associated with a 5- to 7-dB decrease in the CDT emission. 5. The optimum stimulus parameter set for the noninvasive estimation of cochlear performance from the CDT response was for stimulus amplitudes L1 = 50, L2 = 40 dB SPL. 6. This experiment demonstrates that CDT emissions at low stimulus levels are very good predictors of the thresholds of cochlear afferents, but this validity is lost for BF thresholds greater than approximately 60-70 dB SPL. 7. The ear canal CDT amplitude is better correlated with the BF threshold sensitivity of neuronal response areas in the AVCN than with the spontaneous discharge rate or absolute above-threshold discharge rates.