Marginal shell of the anteroventral cochlear nucleus: single-unit response properties in the unanesthetized decerebrate cat

The marginal shell of the anteroventral cochlear nucleus (AVCN) is anatomically different from its central core. We investigated 38 single units in the shells of 10 cats and contrasted them with 62 single units in the cores of 15 cats. The sites of all shell units were localized with the use of reconstructed electrode tracks. The shell units were divided into acoustically well-driven (68%) and weakly/not-driven (32%) subgroups. The shell units mostly exhibited low spontaneous rates (SRs). Among the well-driven shell units, a large majority (68%) exhibited wide dynamic ranges (> or = 50 dB) to tones, noise, or both, with some ranges as wide as 89 dB. In contrast, a large majority (80%) of the core units exhibited narrow dynamic ranges (< 50 dB) to tones and noise. The poststimulus time histograms (PSTHs) of the well-driven shell units included pause-build (29%), onset (24%), and unusual (33%) types, whereas those of the core units included mainly primary-like (47%) and chopper (29%) types. The excitatory-inhibitory areas (EIAs) of the well-driven shell units included types I/III (47%), III (22%), IV (13%), and II (9%), whereas those of the core units included mainly types III (52%) and I/III (32%). On the basis of Fisher's exact tests, we conclude that the shell and core neural groups of the AVCN are significantly different regarding all of the following physiological characteristics: SR, maximum driven rate, threshold and dynamic range to tones and noise, frequency response area, PSTH type, latency, and EIA type. Wide dynamic ranges of the well-driven shell units suggest that they may play a role in encoding absolute intensity of acoustic stimulus.     

Spatiotemporal tuning of low-frequency cells in the anteroventral cochlear nucleus

Low-frequency cells in the anteroventral cochlear nucleus (AVCN) can be sensitive to changes in the spatiotemporal pattern of discharges across their auditory nerve (AN) inputs (). This sensitivity suggests that these cells may be tuned to particular spatiotemporal patterns, or features, in the discharge patterns of populations of AN fibers. To evaluate and characterize this sensitivity, we developed a technique whereby the physiological responses of AVCN cells to wide-band noise were analyzed using the simulated response of a population of AN fibers to the same noise stimulus. By averaging the simulated two-dimensional spatiotemporal pattern of AN activity that preceded each AVCN discharge, it was possible to derive a two-dimensional reverse-correlation function that characterized the spatiotemporal tuning of each AVCN cell. The derived spatiotemporal tuning pattern represented a feature in the AN population response that was most likely to precede discharges of the AVCN cell. To test the spatiotemporal tuning characterizations, we used these patterns to predict the responses of cells to noise stimuli statistically independent from the stimuli used to characterize the cells. This technique provides a general tool for the study of any neural system that involves the analysis of spatiotemporal input patterns.     

Temporal aspects of the effects of cooling on responses of single auditory nerve fibers

During an investigation of the effects of cochlear cooling on frequency tuning and input/output relations of single auditory nerve fibers in gerbil (Ohlemiller and Siegel (1994) Hear. Res. 80, 174-190), cooling-related changes in post-stimulus time histogram (PSTH) shape and phase-locking to tonebursts were characterized in a small sample of neurons. Local cochlear cooling by 5-10 degrees C below normal core temperature did not alter overall PSTH shape, although some evidence was found for a reduction in the time constants of rapid and short term rate adaptation. The relative contributions of rapid and short term response components appeared unaltered. Effects of cooling on phase-locking were assessed by calculating the synchronization index for responses to intense ( > 70 dB SPL) tonebursts at 0.5, 1.0, and 2.0 kHz. Synchronization filter functions exhibited modest reductions in both magnitude and the upper frequency limit of phase-locking. The effects of cooling on the temporal character of responses appear distinct from those of a simple reduction in stimulus intensity. Results are interpreted in terms of cooling-related changes in responses of cochlear hair cells and afferent neurons, and suggest that temperature artifacts are unlikely to underlie reported species differences in PSTH shape and phase-locking.     

Influence of centrifugal pathways on forward masking of ventral cochlear nucleus neurons

When responses to one part of a sequence of auditory signals reduce the responses to a subsequent portion of the signal, "forward masking" results. Although forward masking occurs in the auditory nerve, that observed in the ventral cochlear nucleus (VCN) more closely resembles psychophysical forward masking. In contrast to the auditory nerve in which the amount of forward masking is proportional to the amount of excitation produced by the masker, most VCN neurons show a poor correlation between forward masking and excitation produced by the masker, indicating a more complex interaction between responses to adjacent signals. This study tested the hypothesis that one component of forward masking is produced by inputs from centrifugal neural connections to the VCN. The centrifugal pathways were interrupted with knife-cut lesions medial to the CN. Responses of single units obtained 60 minutes after the lesions were compared to those obtained before the lesions. In primarylike, sustained chopper and on units the lesions resulted in a reduction in forward masking and enhanced recovery. In contrast, lesions resulted in increased masking in primarylike-notch and low-intensity chopper units. The relationship between masker-elicited excitation and forward masking became more monotonic for transient choppers and on units, approaching that observed for auditory nerve fibers. These effects are probably the result of removal of both inhibitory and excitatory inputs, ultimately reflecting a balance of excitation and inhibition to each neural population in the VCN.     

Neural encoding of single-formant stimuli in the ventral cochlear nucleus of the chinchilla

Responses of the principal unit types in the ventral cochlear nucleus of the chinchilla were studied with a single-formant stimulus set that covered fundamental frequency (f0) from 100 Hz to 200 Hz and formant center frequency (F1) from 256 to 782 Hz. Temporal coding for f0 and F1 was explored for 95 stimulus combinations of f0 (n = 5) and F1 (n = 19) in primarylike, onset and chopper unit categories. Several analyses that explored temporal coding were employed including: autocorrelation, interspike interval analysis, and synchronization to each harmonic of f0. In general, the representation of f0 is better in onset and chopper units than in primarylike units. Nearly all units in the cochlear nucleus showed a gain in phase locking to the envelope (f0) of the single-formant stimulus relative to the auditory nerve. The fundamental is represented directly in neural discharges of units in the cochlear nucleus with an interval code (also Cariani and Delgutte, 1996; Rhode, 1995). The formant is represented in the temporal domain in primarylike units, though some chopper and onset units also possess the ability to code F1 through discharge synchrony. Onset-I units, which are associated with the octopus cells, exhibited the strongest phase locking to f0 of any unit types studied. The representation of f0 and F1 in the temporal domain is weak or absent in some units. All-order-interspike interval distributions computed for populations of units show preservation of temporal coding for both f0 and F1. Results are in agreement with earlier amplitude modulation studies that showed nearly all cochlear nucleus unit types phase lock to the signal envelope better than auditory nerve fibers over a considerable range of signal amplitudes.     

Modeling inhibition of type II units in the dorsal cochlear nucleus

Type II units in the dorsal cochlear nucleus (DCN) are characterized by vigorous but nonmonotonic responses to best frequency tones as a function of sound pressure level, and relatively weak responses to noise. A model of DCN neural circuitry was used to explore two hypothetical mechanisms by which neurons may be endowed with type II unit response properties. Both mechanisms assume that type II units receive excitatory input from auditory nerve (AN) fibers and inhibitory input from an unspecified class of cochlear nucleus interneurons that also receive excitatory AN input. The first mechanism, a lateral inhibition (LI) model, supposes that type II units receive inhibitory input from a number of narrowly tuned interneurons whose best frequencies (BFs) flank the BF of the type II unit. Tonal stimuli near BF result in only weak inhibitory input, but broadband stimuli recruit enough lateral inhibitors to greatly weaken the type II unit response. The second mechanism, a wideband inhibition (WBI) model, supposes that type II units receive inhibitory input from interneurons that are broadly tuned so that they respond more vigorously to broadband stimuli than to tones. Physiological and anatomical evidence points to the possible existence of such a class of neurons in the cochlear nucleus. The model extends an earlier computer model of an iso-frequency DCN patch to multiple frequency slices and adds a population of interneurons to provide the inhibition to model type II units (called 12-cells). The results show that both mechanisms accurately simulate responses of type II units to tones and noise. An experimental paradigm for distinguishing the two mechanisms is proposed.     

Effects of stimulus duration on responses of neurons in the chinchilla inferior colliculus

The effects of the stimulus duration (10 to 300 ms) on the responses of chinchilla inferior colliculus neurons to pure tones were studied in 41 units. The responses of the majority of the neurons (90%) were classified as sustained, onset, pause with onset peak and pause without onset peak response patterns. Three neurons were found to have response to the stimulus offset (offset response pattern). One neuron responded to the sound with the decrease of the spontaneous discharge rate (inhibitory response pattern). The responses restricted within the stimulus duration could be simply predicted from the peristimulus time histogram (PSTH) to the longer duration. The leading part of the PSTH to the longer stimulus duration resembled that to the shorter stimulus duration. The function of the spike number versus duration was correlated with the PSTH patterns. The response following the stimulus offset (including inhibitory response) could vary with the stimulus duration nonmonotonically and show a band-pass or band-reject property. Overall, four (about 10%) of the neurons could be regarded as duration-tuned units. The duration selectivity could be understood by the interaction between the ongoing and the offset process of the neurons.     

Response map properties of units in the dorsal cochlear nucleus of barbiturate-anesthetized gerbil (Meriones unguiculatus)

The response map scheme introduced by Evans and Nelson (1973) and modified by others, including Davis et al. (1996) for use with gerbils, has been used primarily for classifying units recorded in the cochlear nucleus of unanesthetized decerebrate preparations. Units lacking spontaneous activity (SpAc) have been classified as either type I/III or type II units based on the relative strength of their responses to broad-band noise compared to their responses to best-frequency (BF) tones. The relative noise index (rho), a ratio of these responses after SpAc is subtracted out, provides a convenient measure of this relative strength. In this paper, responses of 320 units recorded in the dorsal cochlear nucleus (DCN) of barbiturate-anesthetized gerbils to short-duration BF tones and broad-band noise were recorded. Since 87.5% of these units lacked SpAc, their response maps resembled those of type II and type I/III units. Units were characterized by rho and the normalized slope (m) of a best line fit to the BF rate versus level plot starting from the sound level corresponding to the first inflection point of the rate curve (typically its maximum value or the start of its sloping saturation). The distributions of rho and m values do not form distinct clusters as they do for units in the decerebrate preparation. Thus, the criteria developed for classifying DCN units in the decerebrate preparation do not appear appropriate for units in the barbiturate-anesthetized preparation. Deposits of horseradish peroxidase were used to locate 52 units. Most of the low SpAc units, 56% with poor noise responses (5/9) and nearly 70% with strong noise responses (25/36), and nearly all of the high SpAc units (6/7), were located either within or below the fusiform cell layer.     

Ultrastructural differences among afferent synapses on cochlear hair cells: correlations with spontaneous discharge rate

The major class of cochlear afferent fibers, the type-I or radial-fiber (RF) population, has been subdivided into three functional groups according to spontaneous discharge rate (SR): those with low SR have the highest acoustic thresholds, high SR fibers have the lowest thresholds and medium SR fibers are of intermediate sensitivity (Liberman [1978] J. Acoust. Soc. Amer. 63:442-455). Existing evidence from intracellular labeling studies at the light microscopic level (Liberman [1982a] Science 216:1239-1241) suggests that a single cochlear inner hair cell makes synaptic contact with representatives of all three functional groups; however, low and medium SR fibers are spatially segregated from high SR fibers around the hair cell circumference, and low and medium SR fibers are smaller in caliber than those with high SR. The present study extends to the ultrastructural level the structure-function correlations available via intracellular labeling. Analysis is based on serial section reconstruction of the synaptic contacts between 11 radial fibers of known SR and their target hair cells. Results suggest systematic differences in synaptic ultrastructure among fibers of the three SR groups: with decreasing SR, the size and complexity of the synaptic body (a presynaptic specialization characteristic of the peripheral afferent synapses in all hair cell systems and some other peripheral receptors) tend to increase, as does the associated number of synaptic vesicles. The possible functional significance of these trends is discussed in the context of other known structural and functional differences among the three SR groups.     

Effects of wide band inhibitors in the dorsal cochlear nucleus. II. Model calculations of the responses to complex sounds

In a previous modeling study of signal processing in the dorsal cochlear nucleus [Reed and Blum, J. Acoust. Soc. Am 96, 1442-1453 (1997)] it was shown that inclusion of a wideband inhibitor (WBI) greatly improved the fit between model response maps and the experimental response maps of type IV units to pure tones. In this study we examine the effect of the WBI on the responses to complex sound stimuli such as broadband noise (BBN), notch noise, noise bands, and band/notch combinations. A new and more realistic model for auditory nerve (AN) response in the presence of different levels of noise is used. It is shown that one can explain and understand the qualitative features of virtually all the published data on type II and type IV unit responses to BBN, notch noise and noise bands. The monotone decreasing response of the maximum firing rate of type II units to noise bands of increasing width that is observed experimentally occurs in the model due to the increasing inhibition of type II cells by the WBI. Similarly, the various nonmonotone patterns of maximum firing rate of type IV units to noise bands of increasing width is shown to arise from the complex and highly nonlinear effects of inhibition from the type II to type IV and the WBI to type IV cells and the nonlinear direct excitation from the AN to the type IV cells. A number of experiments using double notches, double noise bands, or notch-noise band pairs are suggested which, by comparison with model results, would allow one to infer probable connectional patterns between type II and type IV units and between the WBI units and the type IV units.     

Postnatal morphologic changes and glial fibrillary acidic protein immunoreactivity in the anteroventral cochlear nucleus of the acoustically-deprived gerbil

This study investigated the morphological changes and glial fibrillary acidic protein immunoreactivity (GFAP-IR) in the anteroventral cochlear nucleus (AVCN) of acoustically-deprived gerbils during postnatal development. The mongolian gerbil, Meriones unguiculatus, had been acoustically deprived on the right side or left side by a surgical ligation of the external auditory canal at postnatal day 12-14. No discernible microcysts were located in the ipsilateral AVCN at one, three, six and nine months after monaural ligation. Also, no discernible microcysts were located in the contralateral AVCN at one and three months after monaural ligation. Numerous microcysts were located in the contralateral AVCN at six months after monaural ligation and were slightly reduced in number at nine months after monaural ligation. Some of the microcysts closely apposed to and connected with the blood vessels through a leakage route or channel. A foamy region was found in the superficial granule cell cap of the AVCN. The foamy region became evident in the ipsilateral AVCN at three months after monaural ligation. However, the foamy region became evident in the contralateral AVCN at three and nine months after monaural ligation. Vacuoles were mainly found in the neuronal cells at the junction of the superficial and deep layers in the AVCN. These vacuoles were found in the contralateral AVCN at one, three, six, and nine months after monaural ligation. However, vacuoles were found in the ipsilateral AVCN only at three months after monaural ligation. Morphological changes of the myelin sheath were found to be more severe in the contralateral AVCN than in the ipsilateral. GFAP-IR was located in the superficial layer of the contralateral AVCN at three and nine months after monaural ligation. However, GFAP-IR was found in the superficial and deep layers of the ipsilateral AVCN at three and nine months after monaural ligation. GFAP-IR was also found in the superficial layers of the ipsilateral AVCN at six months after monoaural ligation. Microcysts are presumably derived from the detachment of the myelin sheath from the retracted axons, protrusion of the myelin sheath, and disruption of the myelin sheath. The major conclusions were that (1) microcysts were greatly reduced following acoustical ligation during postnatal development, and (2) blood vessels and GFAP-immunoreactive astrocytes may be involved in the depletion of microcysts for maintaining the homeostasis of the microenvironment in the cochlear nuclei.     

A model for the responses of low-frequency auditory-nerve fibers in cat

A computational model was developed for the responses of low-frequency auditory-nerve (AN) fibers in cat. The goal was to produce realistic temporal response properties and average discharge rates in response to simple and complex stimuli. Temporal and average-rate properties of AN responses change as a function of sound-pressure level due to nonlinearities in the auditory periphery. The input stage of the AN model is a narrow-band filter that simulates the mechanical tuning of the basilar membrane. The parameters of this filter vary continuously as a function of stimulus level via a feedback mechanism, simulating the compressive nonlinearity associated with the mechanics of the basilar membrane. A memoryless, saturating nonlinearity and two low-pass filters simulate transduction and membrane properties of the inner hair cell (IHC). A diffusion model for the IHC-AN synapse introduces adaptation. Finally, a nonhomogeneous Poisson process, modified by absolute and relative refractoriness, provides the output discharge times. Responses to several different stimuli are presented. These responses illustrate nonlinear temporal response properties that cannot be achieved with linear models for AN fibers.     

Muscimol suppression of the dorsal cochlear nucleus impairs frequency discrimination in rats

The cochlear nucleus is composed of three sub-nuclei: the dorsal (DCN), anteroventral (AVCN) and posteroventral cochlear nucleus (PVCN). In addition to connections between these sub-nuclei, each nucleus receives frequency specific tonotopically organised input from the cochlea. Evidence suggests that connections from the DCN to the AVCN are inhibitory and organised tonotopically but the functional significance of this pathway has yet to be elucidated. The possible role of this pathway in frequency discrimination using a T-maze behavioural paradigm and DCN suppression was examined. Five rats were trained on a two choice frequency discrimination task. Once frequency difference limens for 10-30% performance above chance were determined, rats had cannulae implanted bilaterally over the DCN. After recovery rats were tested on the behavioural task with nothing, saline and the GABA agonist muscimol injected into the DCN via the cannulae. Muscimol alone significantly reduced the rats ability to perform the task. This performance decrease was attributed to an inability to discriminate high frequency and not low frequency tones suggesting that place and not temporal coding of sound was compromised by DCN suppression. These results are consistent with the hypothesis that inhibitory drive from the DCN to AVCN may be crucial for the fine tuning of frequency information.     

Ingestive responses to homeostatic challenges in rats with ablations of the anterolateral neocortex.

Because rats with either anterolateral neocortical (AN) or lateral hypothalamic (LH) damage initially display similar feeding and drinking deficits and recovery patterns, the present study examined the possibility that anterolateral neocortical ablations would also produce similar chronic ingestive impairments to glucoprivic and hydrational challenges. 73 male Sprague-Dawley rats received AN or dorsoposterior neocortical lesions or served as unoperated controls. Ss with AN ablations exhibited normal feeding responses to food deprivation and glucoprivation induced by insulin (4–26 U/kg, sc) or 2-deoxydextroglucose (2-DG [125 or 250 mg/kg, ip]), but their response to 500 mg/kg or 2-DG was impaired. These Ss also drank normally in response to hypertonic saline injections and following water deprivation but only if food was available during the test session. Results indicate that, although the anterolateral neocortex and LH are anatomically related, these brain regions appear to be functionally dissimilar in terms of the regulation of ingestion. (39 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Computer simulation of shared input among projection neurons in the dorsal cochlear nucleus

Computer simulations of a network model of an isofrequency patch of the dorsal cochlear nucleus (DCN) were run to explore possible mechanisms for the level-dependent features observed in the cross-correlograms of pairs of type IV units in the cat and nominal type IV units in the gerbil DCN. The computer model is based on the conceptual model (of a cat) that suggests two sources of shared input to DCN's projection neurons (type IV units): excitatory input for auditory nerves and inhibitory input from interneurons (type II units). Use of tonal stimuli is thought to cause competition between these sources resulting in the decorrelation of type IV unit activities at low levels. In the model, P-cells (projection neurons), representing type IV units, receive inhibitory input from I-cells (interneurons), representing type II units. Both sets of model neurons receive a simulated excitatory auditory nerve (AN) input from same-CF AN fibers, where the AN input is modeled as a dead-time modified Poisson process whose intensity is given by a computationally tractable discharge rate versus sound pressure level function. Subthreshold behavior of each model neuron is governed by a set of normalized state equations. The computer mode has previously been shown to reproduce the major response properties of both type IV and type II units (e.g., rate-level curves and peri-stimulus time histograms) and the level-dependence of the functional type II-type IV inhibitory interaction. This model is adapted for the gerbil by simulating a reduced population of I-cells. Simulations were carried out for several auditory nerve input levels, and cross-correlograms were computed from the activities of pairs of P-cells for a complete (cat model) and reduced (gerbil model) population of I-cells. The resultant correlograms show central mounds (CMs), indicative of either shared excitatory or inhibitory input, for both spontaneous and tone-evoked driven activities. Similar to experimental results, CM amplitudes are a non-monotonic function of level and CM widths decrease as a function of level. These results are consistent with the hypothesis that shared excitatory input correlates the spontaneous activities of type IV units adn shared inhibitory input correlates their driven activities. The results also suggest that the decorrelation of the activities of type IV units can result from a reduced effectiveness of the AN input as a function of increasing level. Thus, competition between the excitatory and inhibitory inputs is not required.     

Remodeling of neuronal membranes as an early response to deafferentation. A freeze-fracture study

The early effects of deafferentation on the postsynaptic membrane beneath the end bulb of Held in the anteroventral cochlear nucleus (AVCN) were studied with the freeze-fracture technique. Three distinct responses were seen on the external membrane leaflet after cochlear ablation. Within 12 h the number of nonaggregate particles increased 147% by the addition of new particles to the membrane. The increase in number of nonaggregate particles continued until 4 days after cochlear ablation. The other responses occurred later, after degenerative changes were present in the end bulb. Between 1 and 2 days after cochlear ablation, the number of perisynaptic aggregates surrounding the postsynaptic active zone decreased to 10% of normal numbers. By 4 days, all perisynaptic aggregates had disappeared from the membrane. Coated vesicles may be involved in removing these aggregates. Between 1 and 3 days, the number of junctional aggregates decreased, but the size of the aggregates increased, apparently as a result of coalescence of nearby junctional aggregates. The total number of particles in junctional aggregates in the membrane was not altered during the first 6 days after cochlear ablation. The three separate responses suggest the existence of at least three different types of intramembranous particles on the external leaflet of the principal cell membrane, with each type dependent upon different cues for its maintenance in the membrane.     

Antidromic responses of single units from the spiral ganglion

1. Antidromic responses of single units in the guinea pig spiral ganglion were recorded in response to shocks to the auditory nerve root. The orthodromic responses of these units were also recorded in response to sound. The aim of this study was 1) to classify units according to their response patterns to shocks and to sound and 2) to propose anatomic types that might correlate with these responses. The four classes of units were as follows: type I, olivocochlear (OC), long-latency: locked, and long-latency: jittering. 2. Type I units responded antidromically to shocks with little jitter and short latency. Their responses to sound were also of short latency and had irregular interspike intervals. Some of these units had complex spike waveforms. These units likely correspond to type I primary afferent neurons, the majority population of spiral ganglion cells. 3. One-third of the OC units responded to shocks, with little jitter and intermediate latency (2 ms). OC unit responses to sound were of long latency and had regular interspike intervals. These units likely correspond to efferent neurons that originate in the superior olivary complex of the brain and end on outer hair cells in the cochlea. 4. Long-latency: locked units responded to shocks with little jitter and long latency (4-11 ms). Many of these units had complex spike waveforms and most did not respond to high-level noise bursts. Long-latency: locked units may correspond to type II spiral ganglion neurons. 5. Long-latency: jittering units responded to shocks with a jitter of several milliseconds and long latency. Some of these units responded to sound in a pattern reminiscent of OC units. These units may constitute a subgroup of OC units that respond to shocks via activation of the reflex pathway from the cochlea to the superior olive and back out to the cochlea. 6. Further data were collected on the type I response to shocks. Antidromic spikes lacked the inflections seen on the waveforms that are typically seen on orthodromic spikes. Type I shock responses depended on shock level and duration and were reduced when a click preceded the shock by approximately 2 ms. Several type I characteristics depended on the rate of spontaneous discharge: for units of low and medium spontaneous rates (when compared with units of high rates), the shock thresholds were lower, shock latencies were longer, and the probability of firing repetitive spikes to a single shock was higher.(ABSTRACT TRUNCATED AT 400 WORDS)     

Single olivocochlear neurons in the guinea pig. I. Binaural facilitation of responses to high-level noise

Single medial olivocochlear (MOC) neurons were recorded from the cochlea of the anesthetized guinea pig. We used tones and noise presented monaurally and binaurally and measured responses for sounds up to 105 dB sound pressure level (SPL). For monaural sound, MOC neuron firing rates were usually higher for noise bursts than tone bursts, a situation not observed for afferent fibers of the auditory nerve that were sampled in the same preparations. MOC neurons also differed from afferent fibers in having less saturation of response. Some MOC neurons had responses that continued to increase even at high sound levels. Differences between MOC and afferent responses suggest that there is convergence in the pathway to olivocochlear neurons, possibly a combination of inputs that are at the characteristic frequency (CF) with others that are off the CF. Opposite-ear noise almost always facilitated the responses of MOC neurons to sounds in the main ear, the ear that best drives the unit. This binaural facilitation depends on several characteristics that pertain to the main ear: it is higher in neurons having a contralateral main ear (contra units), it is higher at main-ear sound levels that are moderate (approximately 65 dB SPL), and it is higher in neurons with low discharge rates to main-ear stimuli. Facilitation also depends on parameters of the opposite-ear sound: facilitation increases with noise level in the opposite ear until saturating, is greater for continuous noise than noise bursts, and is usually greater for noise than for tones. Using optimal opposite-ear facilitators and high-level stimuli, the firing rates of olivocochlear neurons range up to 140 spikes/s, whereas for moderate-level monaural stimuli the rates are <80 spikes/s. At high sound levels, firing rates of olivocochlear neurons increase with CF, an increase that may compensate for the known lower effectiveness of olivocochlear synapses on outer hair cells responding to high frequencies. Overall, our results demonstrate a high MOC response for binaural noise and suggest a prominent role for the MOC system in environments containing binaural noise of high level.