Powerful, onset inhibition in the ventral nucleus of the lateral lemniscus

A computational model of auditory chirp-velocity sensitivity and amplitude-modulation tuning in inferior colliculus neurons

We demonstrate a model of chirp-velocity sensitivity in the inferior colliculus (IC) that retains the tuning to amplitude modulation (AM) that was established in earlier models. The mechanism of velocity sensitivity is sequence detection by octopus cells of the posteroventral cochlear nucleus, which have been proposed in physiological studies to respond preferentially to the order of arrival of cross-frequency inputs of different amplitudes. Model architecture is based on coincidence detection of a combination of excitatory and inhibitory inputs. Chirp-sensitivity of the IC output is largely controlled by the strength and timing of the chirp-sensitive octopus-cell inhibitory input. AM tuning is controlled by inhibition and excitation that are tuned to the same frequency. We present several example neurons that demonstrate the feasibility of the model in simulating realistic chirp-sensitivity and AM tuning for a wide range of characteristic frequencies. Additionally, we explore the systematic impact of varying parameters on model responses. The proposed model can be used to assess the contribution of IC chirp-velocity sensitivity to responses to complex sounds, such as speech.

An Anatomical and Physiological Basis for Flexible Coincidence Detection in the Auditory System

Animals navigate the auditory world by recognizing complex sounds, from the rustle of a predator to the call of a potential mate. This ability depends in part on the octopus cells of the auditory brainstem, which respond to multiple frequencies that change over time, as occurs in natural stimuli. Unlike the average neuron, which integrates inputs over time on the order of tens of milliseconds, octopus cells must detect momentary coincidence of excitatory inputs from the cochlea during an ongoing sound on both the millisecond and submillisecond time scale. Here, we show that octopus cells receive inhibitory inputs on their dendrites that enhance opportunities for coincidence detection in the cell body, thereby allowing for responses both to rapid onsets at the beginning of a sound and to frequency modulations during the sound. This mechanism is crucial for the fundamental process of integrating the synchronized frequencies of natural auditory signals over time.

A Computational Model of Auditory Chirp-Velocity Sensitivity and Amplitude-Modulation Tuning in Inferior Colliculus Neurons

We demonstrate a model of chirp-velocity sensitivity in the inferior colliculus (IC) that retains the tuning to amplitude modulation (AM) that was established in earlier models. The mechanism of velocity sensitivity is sequence detection by octopus cells of the posteroventral cochlear nucleus, which have been proposed in physiological studies to respond preferentially to the order of arrival of cross-frequency inputs of different amplitudes. Model architecture is based on coincidence detection of a combination of excitatory and inhibitory inputs. Chirp-sensitivity of the IC output is largely controlled by the strength and timing of the chirp-sensitive octopus-cell inhibitory input. AM tuning is controlled by inhibition and excitation that are tuned to the same frequency. We present several example neurons that demonstrate the feasibility of the model in simulating realistic chirp-sensitivity and AM tuning for a wide range of characteristic frequencies. Additionally, we explore the systematic impact of varying parameters on model responses. The proposed model can be used to assess the contribution of IC chirp-velocity sensitivity to responses to complex sounds, such as speech.

Why is the perceptual octave stretched? An account based on mismatched time constants within the auditory brainstem

This paper suggests an explanation for listeners' greater tolerance to positive than negative mistuning of the higher tone within an octave pair. It hypothesizes a neural circuit tuned to cancel the lower tone that also cancels the higher tone if that tone is in tune. Imperfect cancellation is the cue to mistuning of the octave. The circuit involves two neural pathways, one delayed with respect to the other, that feed a coincidence-sensitive neuron via excitatory and inhibitory synapses. A mismatch between the time constants of these two synapses results in an asymmetry in sensitivity to mismatch. Specifically, if the time constant of the delayed pathway is greater than that of the direct pathway, there is a greater tolerance to positive mistuning than to negative mistuning. The model is directly applicable to the harmonic octave (concurrent tones) but extending it to the melodic octave (successive tones) requires additional assumptions that are discussed. The paper reviews evidence from auditory psychophysics and physiology in favor-or against-this explanation.

The ion channels and synapses responsible for the physiological diversity of mammalian lower brainstem auditory neurons

The auditory part of the brainstem is composed of several nuclei specialized in the computation of the different spectral and temporal features of the sound before it reaches the higher auditory regions. There are a high diversity of neuronal types in these nuclei, many with remarkable electrophysiological and synaptic properties unique to these structures. This diversity reflects specializations necessary to process the different auditory signals in order to extract precisely the acoustic information necessary for the auditory perception by the animal. Low threshold Kv1 channels and HCN channels are expressed in neurons that use timing clues for auditory processing, like bushy and octopus cells, in order to restrict action potential firing and reduce input resistance and membrane time constant. Kv3 channels allow principal neurons of the MNTB and pyramidal DCN neurons to fire fast trains of action potentials. Calcium channels on cartwheel DCN neurons produce complex spikes characteristic of these neurons. Calyceal synapses compensate the low input resistance of bushy and principal neurons of the MNTB by releasing hundreds of glutamate vesicles resulting in large EPSCs acting in fast ionotropic glutamate receptors, in order to reduce temporal summation of synaptic potentials, allowing more precise correspondence of pre- and post-synaptic potentials, and phase-locking. Pre-synaptic calyceal sodium channels have fast recovery from inactivation allowing extremely fast trains of action potential firing, and persistent sodium channels produce spontaneous activity of fusiform neurons at rest, which expands the dynamic range of these neurons. The unique combinations of different ion channels, ionotropic receptors and synaptic structures create a unique functional diversity of neurons extremely adapted to their complex functions in the auditory processing.

The Calyx of Held: A Hypothesis on the Need for Reliable Timing in an Intensity-Difference Encoder

The calyx of Held is the preeminent model for the study of synaptic function in the mammalian CNS. Despite much work on the synapse and associated circuit, its role in hearing remains enigmatic. We propose that the calyx is one of the key adaptations that enables an animal to lateralize transient sounds. The calyx is part of a binaural circuit that is biased toward high sound frequencies and is sensitive to intensity differences between the ears. This circuit also shows marked sensitivity to interaural time differences, but only for brief sound transients ("clicks"). In a natural environment, such transients are rare except as adventitious sounds generated by other animals moving at close range. We argue that the calyx, and associated temporal specializations, evolved to enable spatial localization of sound transients, through a neural code congruent with the circuit's sensitivity to interaural intensity differences, thereby conferring a key benefit to survival.

Supra-Threshold Hearing and Fluctuation Profiles: Implications for Sensorineural and Hidden Hearing Loss

An important topic in contemporary auditory science is supra-threshold hearing. Difficulty hearing at conversational speech levels in background noise has long been recognized as a problem of sensorineural hearing loss, including that associated with aging (presbyacusis). Such difficulty in listeners with normal thresholds has received more attention recently, especially associated with descriptions of synaptopathy, the loss of auditory nerve (AN) fibers as a result of noise exposure or aging. Synaptopathy has been reported to cause a disproportionate loss of low- and medium-spontaneous rate (L/MSR) AN fibers. Several studies of synaptopathy have assumed that the wide dynamic ranges of L/MSR AN fiber rates are critical for coding supra-threshold sounds. First, this review will present data from the literature that argues against a direct role for average discharge rates of L/MSR AN fibers in coding sounds at moderate to high sound levels. Second, the encoding of sounds at supra-threshold levels is examined. A key assumption in many studies is that saturation of AN fiber discharge rates limits neural encoding, even though the majority of AN fibers, high-spontaneous rate (HSR) fibers, have saturated average rates at conversational sound levels. It is argued here that the cross-frequency profile of low-frequency neural fluctuation amplitudes, not average rates, encodes complex sounds. As described below, this fluctuation-profile coding mechanism benefits from both saturation of inner hair cell (IHC) transduction and average rate saturation associated with the IHC-AN synapse. Third, the role of the auditory efferent system, which receives inputs from L/MSR fibers, is revisited in the context of fluctuation-profile coding. The auditory efferent system is hypothesized to maintain and enhance neural fluctuation profiles. Lastly, central mechanisms sensitive to neural fluctuations are reviewed. Low-frequency fluctuations in AN responses are accentuated by cochlear nucleus neurons which, either directly or via other brainstem nuclei, relay fluctuation profiles to the inferior colliculus (IC). IC neurons are sensitive to the frequency and amplitude of low-frequency fluctuations and convert fluctuation profiles from the periphery into a phase-locked rate profile that is robust across a wide range of sound levels and in background noise. The descending projection from the midbrain (IC) to the efferent system completes a functional loop that, combined with inputs from the L/MSR pathway, is hypothesized to maintain "sharp" supra-threshold hearing, reminiscent of visual mechanisms that regulate optical accommodation. Examples from speech coding and detection in noise are reviewed. Implications for the effects of synaptopathy on control mechanisms hypothesized to influence supra-threshold hearing are discussed. This framework for understanding neural coding and control mechanisms for supra-threshold hearing suggests strategies for the design of novel hearing aid signal-processing and electrical stimulation patterns for cochlear implants.

Corelease of Inhibitory Neurotransmitters in the Mouse Auditory Midbrain

The central nucleus of the inferior colliculus (ICC) of the auditory midbrain, which integrates most ascending auditory information from lower brainstem regions, receives prominent long-range inhibitory input from the ventral nucleus of the lateral lemniscus (VNLL), a region thought to be important for temporal pattern discrimination. Histological evidence suggests that neurons in the VNLL release both glycine and GABA in the ICC, but functional evidence for their corelease is lacking. We took advantage of the GlyT2-Cre mouse line (both male and female) to target expression of ChR2 to glycinergic afferents in the ICC and made whole-cell recordings in vitro while exciting glycinergic fibers with light. Using this approach, it was clear that a significant fraction of glycinergic boutons corelease GABA in the ICC. Viral injections were used to target ChR2 expression specifically to glycinergic fibers ascending from the VNLL, allowing for activation of fibers from a single source of ascending input in a way that has not been previously possible in the ICC. We then investigated aspects of the glycinergic versus GABAergic current components to probe functional consequences of corelease. Surprisingly, the time course and short-term plasticity of synaptic signaling were nearly identical for the two transmitters. We therefore conclude that the two neurotransmitters may be functionally interchangeable and that multiple receptor subtypes subserving inhibition may offer diverse mechanisms for maintaining inhibitory homeostasis.SIGNIFICANCE STATEMENT Corelease of neurotransmitters is a common feature of the brain. GABA and glycine corelease is particularly common in the spinal cord and brainstem, but its presence in the midbrain is unknown. We show corelease of GABA and glycine for the first time in the central nucleus of the inferior colliculus of the auditory midbrain. Glycine and GABA are both inhibitory neurotransmitters involved in fast synaptic transmission, so we explored differences between the currents to establish a physiological foundation for functional differences in vivo In contrast to the auditory brainstem, coreleased GABAergic and glycinergic currents in the midbrain are strikingly similar. This apparent redundancy may ensure homeostasis if one neurotransmitter system is compromised.

Octopus Cells in the Posteroventral Cochlear Nucleus Provide the Main Excitatory Input to the Superior Paraolivary Nucleus

Auditory streaming enables perception and interpretation of complex acoustic environments that contain competing sound sources. At early stages of central processing, sounds are segregated into separate streams representing attributes that later merge into acoustic objects. Streaming of temporal cues is critical for perceiving vocal communication, such as human speech, but our understanding of circuits that underlie this process is lacking, particularly at subcortical levels. The superior paraolivary nucleus (SPON), a prominent group of inhibitory neurons in the mammalian brainstem, has been implicated in processing temporal information needed for the segmentation of ongoing complex sounds into discrete events. The SPON requires temporally precise and robust excitatory input(s) to convey information about the steep rise in sound amplitude that marks the onset of voiced sound elements. Unfortunately, the sources of excitation to the SPON and the impact of these inputs on the behavior of SPON neurons have yet to be resolved. Using anatomical tract tracing and immunohistochemistry, we identified octopus cells in the contralateral cochlear nucleus (CN) as the primary source of excitatory input to the SPON. Cluster analysis of miniature excitatory events also indicated that the majority of SPON neurons receive one type of excitatory input. Precise octopus cell-driven onset spiking coupled with transient offset spiking make SPON responses well-suited to signal transitions in sound energy contained in vocalizations. Targets of octopus cell projections, including the SPON, are strongly implicated in the processing of temporal sound features, which suggests a common pathway that conveys information critical for perception of complex natural sounds.

Perinatal nicotine exposure impairs the maturation of glutamatergic inputs in the auditory brainstem

KEY POINTS

Chronic perinatal nicotine exposure causes abnormal auditory brainstem responses and auditory processing deficits in children and animal models. The effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem was investigated in granule cells in the ventral nucleus of the lateral lemniscus, which receive a single calyx-like input from the cochlear nucleus. Perinatal nicotine exposure caused a massive reduction in the amplitude of the excitatory input current. This caused a profound decrease in the number and temporal precision of spikes in these neurons. Perinatal nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons.

ABSTRACT

Maternal smoking causes chronic nicotine exposure during early development and results in auditory processing deficits including delayed speech development and learning difficulties. Using a mouse model of chronic, perinatal nicotine exposure we explored to what extent synaptic inputs to granule cells in the ventral nucleus of the lateral lemniscus are affected by developmental nicotine treatment. These neurons receive one large calyx-like input from octopus cells in the cochlear nucleus and play a role in sound pattern analysis, including speech sounds. In addition, they exhibit high levels of α7 nicotinic acetylcholine receptors, especially during early development. Our whole-cell patch-clamp experiments show that perinatal nicotine exposure causes a profound reduction in synaptic input amplitude. In contrast, the number of inputs innervating each neuron and synaptic release properties of this calyx-like synapse remained unaltered. Spike number and spiking precision in response to synaptic stimulation were greatly diminished, especially for later stimuli during a stimulus train. Moreover, chronic nicotine exposure delayed the developmental downregulation of functional nicotinic acetylcholine receptors on these neurons, indicating a direct action of nicotine in this brain area. This presumably direct effect of perinatal nicotine exposure on synaptic maturation in the auditory brainstem might be one of the underlying causes for auditory processing difficulties in children of heavy smoking mothers.

Inhibition in the auditory brainstem enhances signal representation and regulates gain in complex acoustic environments

Inhibition plays a crucial role in neural signal processing, shaping and limiting responses. In the auditory system, inhibition already modulates second order neurons in the cochlear nucleus, e.g. spherical bushy cells (SBCs). While the physiological basis of inhibition and excitation is well described, their functional interaction in signal processing remains elusive. Using a combination of in vivo loose-patch recordings, iontophoretic drug application, and detailed signal analysis in the Mongolian Gerbil, we demonstrate that inhibition is widely co-tuned with excitation, and leads only to minor sharpening of the spectral response properties. Combinations of complex stimuli and neuronal input-output analysis based on spectrotemporal receptive fields revealed inhibition to render the neuronal output temporally sparser and more reproducible than the input. Overall, inhibition plays a central role in improving the temporal response fidelity of SBCs across a wide range of input intensities and thereby provides the basis for high-fidelity signal processing.

En1 is necessary for survival of neurons in the ventral nuclei of the lateral lemniscus

The ventral nuclei of the lateral lemniscus (VNLL) are part of the central auditory system thought to participate in temporal sound processing. While the timing and location of VNLL neurogenesis have been determined, the genetic factors that regulate VNLL neuron development are unknown. Here, we use genetic fate-mapping techniques to demonstrate that all glycinergic and glycinergic/GABAergic VNLL neurons derive from a cellular lineage that expresses the homeobox transcription factor Engrailed 1 (En1). We also show that En1 deletion does not affect migration or adoption of a neuronal cell fate but does lead to VNLL neuron death during development. Furthermore, En1 deletion blocks expression of the transcription factor FoxP1 in a subset of VNLL neurons. Together, these data identify En1 as a gene important for VNLL neuron development and survival. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1266-1274, 2016.

Heterogeneity of Intrinsic and Synaptic Properties of Neurons in the Ventral and Dorsal Parts of the Ventral Nucleus of the Lateral Lemniscus

The ventral nucleus of the lateral lemniscus (VNLL) provides a major inhibitory projection to the inferior colliculus (IC). Neurons in the VNLL respond with various firing patterns and different temporal precision to acoustic stimulation. The present study investigates the underlying intrinsic and synaptic properties of various cell types in different regions of the VNLL, using in vitro electrophysiological recordings from acute brain slices of mice and immunohistochemistry. We show that the biophysical membrane properties and excitatory input characteristics differed between dorsal and ventral VNLL neurons. Neurons in the ventral VNLL displayed an onset-type firing pattern and little hyperpolarization-activated current (I_h). Stimulation of lemniscal inputs evoked a large all-or-none excitatory response similar to Calyx of Held synapses in neurons in the lateral part of the ventral VNLL. Neurons that were located within the fiber tract of the lateral lemniscus, received several and weak excitatory input fibers. In the dorsal VNLL onset-type and sustained firing neurons were intermingled. These neurons showed large I_h and were strongly immunopositive for the hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1) subunit. Both neuron types received several excitatory inputs that were weaker and slower compared to ventrolateral VNLL neurons. Using a mouse model that expresses channelrhodopsin under the promotor of the vesicular GABA transporter (VGAT) suggests that dorsal and ventral neurons were inhibitory since they were all depolarized by light stimulation. The diverse membrane and input properties in dorsal and ventral VNLL neurons suggest differential roles of these neurons for sound processing.

Broadband onset inhibition can suppress spectral splatter in the auditory brainstem

In vivo intracellular responses to auditory stimuli revealed that, in a particular population of cells of the ventral nucleus of the lateral lemniscus (VNLL) of rats, fast inhibition occurred before the first action potential. These experimental data were used to constrain a leaky integrate-and-fire (LIF) model of the neurons in this circuit. The post-synaptic potentials of the VNLL cell population were characterized using a method of triggered averaging. Analysis suggested that these inhibited VNLL cells produce action potentials in response to a particular magnitude of the rate of change of their membrane potential. The LIF model was modified to incorporate the VNLL cells’ distinctive action potential production mechanism. The model was used to explore the response of the population of VNLL cells to simple speech-like sounds. These sounds consisted of a simple tone modulated by a saw tooth with exponential decays, similar to glottal pulses that are the repeated impulses seen in vocalizations. It was found that the harmonic component of the sound was enhanced in the VNLL cell population when compared to a population of auditory nerve fibers. This was because the broadband onset noise, also termed spectral splatter, was suppressed by the fast onset inhibition. This mechanism has the potential to greatly improve the clarity of the representation of the harmonic content of certain kinds of natural sounds.

Acoustic signal characteristic detection by neurons in ventral nucleus of the lateral lemniscus in mice

Under free field conditions, we used single unit extracellular recording to study the detection of acoustic signals by neurons in the ventral nucleus of the lateral lemniscus (VNLL) in Kunming mouse (Mus musculus). The results indicate two types of firing patterns in VNLL neurons: onset and sustained. The first spike latency (FSL) of onset neurons was shorter than that of sustained neurons. With increasing sound intensity, the FSL of onset neurons remained stable and that of sustained neurons was shortened, indicating that onset neurons are characterized by precise timing. By comparing the values of Q10 and Q30 of the frequency tuning curve, no differences between onset and sustained neurons were found, suggesting that firing pattern and frequency tuning are not correlated. Among the three types of rate-intensity function (RIF) found in VNLL neurons, the proportion of monotonic RIF is the largest, followed by saturated RIF, and non-monotonic RIF. The dynamic range (DR) in onset neurons was shorter than in sustained neurons, indicating different capabilities in intensity tuning of different firing patterns and that these differences are correlated with the type of RIF. Our results also show that the best frequency of VNLL neurons was negatively correlated with depth, supporting the view point that the VNLL has frequency topologic organization.

Development and modulation of intrinsic membrane properties control the temporal precision of auditory brain stem neurons

Passive and active membrane properties determine the voltage responses of neurons. Within the auditory brain stem, refinements in these intrinsic properties during late postnatal development usually generate short integration times and precise action-potential generation. This developmentally acquired temporal precision is crucial for auditory signal processing. How the interactions of these intrinsic properties develop in concert to enable auditory neurons to transfer information with high temporal precision has not yet been elucidated in detail. Here, we show how the developmental interaction of intrinsic membrane parameters generates high firing precision. We performed in vitro recordings from neurons of postnatal days 9–28 in the ventral nucleus of the lateral lemniscus of Mongolian gerbils, an auditory brain stem structure that converts excitatory to inhibitory information with high temporal precision. During this developmental period, the input resistance and capacitance decrease, and action potentials acquire faster kinetics and enhanced precision. Depending on the stimulation time course, the input resistance and capacitance contribute differentially to action-potential thresholds. The decrease in input resistance, however, is sufficient to explain the enhanced action-potential precision. Alterations in passive membrane properties also interact with a developmental change in potassium currents to generate the emergence of the mature firing pattern, characteristic of coincidence-detector neurons. Cholinergic receptor-mediated depolarizations further modulate this intrinsic excitability profile by eliciting changes in the threshold and firing pattern, irrespective of the developmental stage. Thus our findings reveal how intrinsic membrane properties interact developmentally to promote temporally precise information processing.

Large somatic synapses on neurons in the ventral lateral lemniscus work in pairs

In the auditory system, large somatic synapses convey strong excitation that supports temporally precise information transfer. The information transfer of such synapses has predominantly been investigated in the endbulbs of Held in the anterior ventral cochlear nucleus and the calyx of Held in the medial nucleus of the trapezoid body. These large synapses either work as relays or integrate over a small number of inputs to excite the postsynaptic neuron beyond action potential (AP) threshold. In the monaural system, another large somatic synapse targets neurons in the ventral nucleus of the lateral lemniscus (VNLL). Here, we comparatively analyze the mechanisms of synaptic information transfer in endbulbs in the VNLL and the calyx of Held in juvenile Mongolian gerbils. We find that endbulbs in the VNLL are functionally surface-scaled versions of the calyx of Held with respect to vesicle availability, release efficacy, and synaptic peak currents. This functional scaling is achieved by different calcium current kinetics that compensate for the smaller AP in VNLL endbulbs. However, the average postsynaptic current in the VNLL fails to elicit APs in its target neurons, even though equal current suffices to generate APs in neurons postsynaptic to the calyx of Held. In the VNLL, a postsynaptic A-type outward current reduces excitability and prevents AP generation upon a single presynaptic input. Instead, coincidence detection of inputs from two converging endbulbs is ideal to reliably trigger APs. Thus, even large endbulbs do not guarantee one-to-one AP transfer. Instead, information flow appears regulated by circuit requirements.

Temporal properties of responses to sound in the ventral nucleus of the lateral lemniscus

Besides the rapid fluctuations in pressure that constitute the "fine structure" of a sound stimulus, slower fluctuations in the sound's envelope represent an important temporal feature. At various stages in the auditory system, neurons exhibit tuning to envelope frequency and have been described as modulation filters. We examine such tuning in the ventral nucleus of the lateral lemniscus (VNLL) of the pentobarbital-anesthetized cat. The VNLL is a large but poorly accessible auditory structure that provides a massive inhibitory input to the inferior colliculus. We test whether envelope filtering effectively applies to the envelope spectrum when multiple envelope components are simultaneously present. We find two broad classes of response with often complementary properties. The firing rate of onset neurons is tuned to a band of modulation frequencies, over which they also synchronize strongly to the envelope waveform. Although most sustained neurons show little firing rate dependence on modulation frequency, some of them are weakly tuned. The latter neurons are usually band-pass or low-pass tuned in synchronization, and a reverse-correlation approach demonstrates that their modulation tuning is preserved to nonperiodic, noisy envelope modulations of a tonal carrier. Modulation tuning to this type of stimulus is weaker for onset neurons. In response to broadband noise, sustained and onset neurons tend to filter out envelope components over a frequency range consistent with their modulation tuning to periodically modulated tones. The results support a role for VNLL in providing temporal reference signals to the auditory midbrain.

Development of on-off spiking in superior paraolivary nucleus neurons of the mouse

The superior paraolivary nucleus (SPON) is a prominent cell group in the auditory brain stem that has been increasingly implicated in representing temporal sound structure. Although SPON neurons selectively respond to acoustic signals important for sound periodicity, the underlying physiological specializations enabling these responses are poorly understood. We used in vitro and in vivo recordings to investigate how SPON neurons develop intrinsic cellular properties that make them well suited for encoding temporal sound features. In addition to their hallmark rebound spiking at the stimulus offset, SPON neurons were characterized by spiking patterns termed onset, adapting, and burst in response to depolarizing stimuli in vitro. Cells with burst spiking had some morphological differences compared with other SPON neurons and were localized to the dorsolateral region of the nucleus. Both membrane and spiking properties underwent strong developmental regulation, becoming more temporally precise with age for both onset and offset spiking. Single-unit recordings obtained in young mice demonstrated that SPON neurons respond with temporally precise onset spiking upon tone stimulation in vivo, in addition to the typical offset spiking. Taken together, the results of the present study demonstrate that SPON neurons develop sharp on-off spiking, which may confer sensitivity to sound amplitude modulations or abrupt sound transients. These findings are consistent with the proposed involvement of the SPON in the processing of temporal sound structure, relevant for encoding communication cues.

An in vivo investigation of inferior colliculus single neuron responses to cochlear nucleus pulse train stimulation

The auditory brain stem implant (ABI) is being used clinically to restore hearing to patients unable to benefit from a cochlear implant (CI). Speech perception outcomes for ABI users are typically poor compared with most CI users. The ABI is implanted either on the surface of or penetrating through the cochlear nucleus in the auditory brain stem and uses stimulation strategies developed for auditory nerve stimulation with a CI. Although the stimulus rate may affect speech perception outcomes with current stimulation strategies, no studies have systematically investigated the effect of stimulus rate electrophysiologically or clinically. We therefore investigated rate response properties and temporal response properties of single inferior colliculus (IC) neurons from penetrating ABI stimulation using stimulus rates ranging from 100 to 1,600 pulses/s in the rat. We found that the stimulus rate affected the proportion of response types, thresholds, and dynamic ranges of IC activation. The stimulus rate was also found to affect the temporal properties of IC responses, with higher rates providing more temporally similar responses to acoustic stimulation. Suppression of neural firing and inhibition in IC neurons was also found, with response properties varying with the stimulus rate. This study demonstrated that changes in the ABI stimulus rate results in significant differences in IC neuron response properties. Due to electrophysiological differences, the stimulus rate may also change perceptual properties. We suggest that clinical evaluation of the ABI stimulus rate should be performed.

Duration tuning across vertebrates

Signal duration is important for identifying sound sources and determining signal meaning. Duration-tuned neurons (DTNs) respond preferentially to a range of stimulus durations and maximally to a best duration (BD). Duration-tuned neurons are found in the auditory midbrain of many vertebrates, although studied most extensively in bats. Studies of DTNs across vertebrates have identified cells with BDs and temporal response bandwidths that mirror the range of species-specific vocalizations. Neural tuning to stimulus duration appears to be universal among hearing vertebrates. Herein, we test the hypothesis that neural mechanisms underlying duration selectivity may be similar across vertebrates. We instantiated theoretical mechanisms of duration tuning in computational models to systematically explore the roles of excitatory and inhibitory receptor strengths, input latencies, and membrane time constant on duration tuning response profiles. We demonstrate that models of duration tuning with similar neural circuitry can be tuned with species-specific parameters to reproduce the responses of in vivo DTNs from the auditory midbrain. To relate and validate model output to in vivo responses, we collected electrophysiological data from the inferior colliculus of the awake big brown bat, Eptesicus fuscus, and present similar in vivo data from the published literature on DTNs in rats, mice, and frogs. Our results support the hypothesis that neural mechanisms of duration tuning may be shared across vertebrates despite species-specific differences in duration selectivity. Finally, we discuss how the underlying mechanisms of duration selectivity relate to other auditory feature detectors arising from the interaction of neural excitation and inhibition.

Noise-induced hyperactivity in the inferior colliculus: its relationship with hyperactivity in the dorsal cochlear nucleus

Intense noise exposure causes hyperactivity to develop in the mammalian dorsal cochlear nucleus (DCN) and inferior colliculus (IC). It has not yet been established whether the IC hyperactivity is driven by hyperactivity from extrinsic sources that include the DCN or instead is maintained independently of this input. We have investigated the extent to which IC hyperactivity is dependent on input from the contralateral DCN by comparing recordings of spontaneous activity in the IC of noise-exposed and control hamsters before and after ablation of the contralateral DCN. One group of animals was binaurally exposed to intense sound (10 kHz, 115 dB SPL, 4 h), whereas the control group was not. Both groups were studied electrophysiologically 2-3 wk later by first mapping spontaneous activity along the tonotopic axis of the IC to confirm induction of hyperactivity. Spontaneous activity was then recorded at a hyperactive IC locus over two 30-min periods, one with DCNs intact and the other after ablation of the contralateral DCN. In a subset of animals, activity was again mapped along the tonotopic axis after the time course of the activity was recorded before and after DCN ablation. Following recordings, the brains were fixed, and histological evaluations were performed to assess the extent of DCN ablation. Ablation of the DCN resulted in major reductions of IC hyperactivity. Levels of postablation activity in exposed animals were similar to the levels of activity in the IC of control animals, indicating an almost complete loss of hyperactivity in exposed animals. The results suggest that hyperactivity in the IC is dependent on support from extrinsic sources that include and may even begin with the DCN. This finding does not rule out longer term compensatory or homeostatic adjustments that might restore hyperactivity in the IC over time.

Inferior colliculus responses to dual-site intralamina stimulation in the ventral cochlear nucleus

A major limitation of the present auditory brainstem implant (ABI) is its inability to access the tonotopic organization of the ventral cochlear nucleus (VCN). A previous study by our group indicated that stimulation of single sites within a given VCN frequency region did not always elicit frequency-specific responses within the central nucleus of the inferior colliculus (CIC) and in some cases did not elicit a response at all. For this study, we hypothesized that sequential stimulation (with a short interpulse delay of 320 μsec) of two VCN sites in similar frequency regions would enhance responsiveness in CIC neurons. Multiunit neural recordings in response to pure tones were obtained at 58 VCN and 164 CIC sites in anesthetized rats. Among the 58 VCN sites, 39 pairs of sites with similar characteristic frequencies were chosen for electrical stimulation. Each member of a VCN pair was electrically stimulated individually, followed by sequential stimulation of the pair, while recording CIC responses. On average, CIC sites were found to respond to dual-site VCN stimulation with significantly lower thresholds, wider dynamic ranges, a greater extent of activation with increasing current levels, and a higher degree of frequency specificity compared with single-site stimulation. Although these effects were positive for the most part, in some cases dual-site stimulation resulted in increased CIC thresholds and decreased dynamic ranges, extent of activation, and frequency specificity. The results suggest that multisite stimulation within VCN isofrequency laminae using penetrating electrodes could significantly improve ABI stimulation strategies and implant performance.

Going native: voltage-gated potassium channels controlling neuronal excitability

In this review we take a physiological perspective on the role of voltage-gated potassium channels in an identified neuron in the auditory brainstem. The large number of KCN genes for potassium channel subunits and the heterogeneity of the subunit combination into K(+) channels make identification of native conductances especially difficult. We provide a general pharmacological and biophysical profile to help identify the common voltage-gated K(+) channel families in a neuron. Then we consider the physiological role of each of these conductances from the perspective of the principal neuron in the medial nucleus of the trapezoid body (MNTB). The MNTB is an inverting relay, converting excitation generated by sound from one cochlea into inhibition of brainstem nuclei on the opposite side of the brain; this information is crucial for binaural comparisons and sound localization. The important features of MNTB action potential (AP) firing are inferred from its inhibitory projections to four key target nuclei involved in sound localization (which is the foundation of auditory scene analysis in higher brain centres). These are: the medial superior olive (MSO), the lateral superior olive (LSO), the superior paraolivary nucleus (SPN) and the nuclei of the lateral lemniscus (NLL). The Kv families represented in the MNTB each have a distinct role: Kv1 raises AP firing threshold; Kv2 influences AP repolarization and hyperpolarizes the inter-AP membrane potential during high frequency firing; and Kv3 accelerates AP repolarization. These actions are considered in terms of fidelity of transmission, AP duration, firing rates and temporal jitter. An emerging theme is activity-dependent phosphorylation of Kv channel activity and suggests that intracellular signalling has a dynamic role in refining neuronal excitability and homeostasis.

Inhibitory projections from the ventral nucleus of the lateral lemniscus and superior paraolivary nucleus create directional selectivity of frequency modulations in the inferior colliculus: a comparison of bats with other mammals

This review considers four auditory brainstem nuclear groups and shows how studies of both bats and other mammals have provided insights into their response properties and the impact of their convergence in the inferior colliculus (IC). The four groups are octopus cells in the cochlear nucleus, their connections with the ventral nucleus of the lateral lemniscus (VNLL) and the superior paraolivary nucleus (SPON), and the connections of the VNLL and SPON with the IC. The theme is that the response properties of neurons in the SPON and VNLL map closely onto the synaptic response features of a unique subpopulation of cells in the IC of bats whose inputs are dominated by inhibition. We propose that the convergence of VNLL and SPON inputs generates the tuning of these IC cells, their unique temporal responses to tones, and their directional selectivities for frequency modulated (FM) sweeps. Other IC neurons form directional properties in other ways, showing that selective response properties are formed in multiple ways. In the final section we discuss why multiple formations of common response properties could amplify differences in population activity patterns evoked by signals that have similar spectrotemporal features.

An in vivo investigation of first spike latencies in the inferior colliculus in response to multichannel penetrating auditory brainstem implant stimulation

The cochlear nucleus (CN) is the first auditory processing site within the brain and the target location of the auditory brainstem implant (ABI), which provides speech perception to patients who cannot benefit from a cochlear implant (CI). Although there is variance between ABI recipient speech performance outcomes, performance is typically low compared to CI recipients. Temporal aspects of neural firing such as first spike latency (FSL) are thought to code for many speech features; however, no studies have investigated FSL from CN stimulation. Consequently, ABIs currently do not incorporate CN-specific temporal information. We therefore systematically investigated inferior colliculus (IC) neuron's FSL response to frequency-specific electrical stimulation of the CN in rats. The range of FSLs from electrical stimulation of many neurons indicates that both monosynaptic and polysynaptic pathways were activated, suggesting initial activation of multiple CN neuron types. Electrical FSLs for a single neuron did not change irrespective of the CN frequency region stimulated, indicating highly segregated projections from the CN to the IC. These results present the first evidence of temporal responses to frequency-specific CN electrical stimulation. Understanding the auditory system's temporal response to electrical stimulation will help in future ABI designs and stimulation strategies.

Glycinergic inhibition creates a form of auditory spectral integration in nuclei of the lateral lemniscus

For analyses of complex sounds, many neurons integrate information across different spectral elements via suppressive effects that are distant from the neurons' excitatory tuning. In the mustached bat, suppression evoked by sounds within the first sonar harmonic (23-30 kHz) or in the subsonar band (<23 kHz) alters responsiveness to the higher best frequencies of many neurons. This study examined features and mechanisms associated with low-frequency (LF) suppression among neurons of the lateral lemniscal nuclei (NLL). We obtained extracellular recordings from neurons in the intermediate and ventral nuclei of the lateral lemniscus, observing different forms of LF suppression related to the two above-cited frequency bands. To understand the mechanisms underlying this suppression in NLL neurons, we examined the roles of glycinergic and GABAergic input through local microiontophoretic application of strychnine, an antagonist to glycine receptors (GlyRs), or bicuculline, an antagonist to gamma-aminobutyric acid type A receptors (GABA(A)Rs). With blockade of GABA(A)Rs, neurons showed an increase in firing rate to best frequency (BF) and/or LF tones but retained LF suppression of BF sounds. For neurons that displayed LF suppression tuned to 23-30 kHz, the suppression was eliminated or nearly eliminated by GlyR blockade. In contrast, GABA(A)R blockade did not eliminate nor had any consistent effect on suppression tuned to these frequencies. We conclude that LF suppression tuned in the 23- to 30-kHz range results from neuronal inhibition within the NLL via glycinergic inputs. For neurons displaying suppression tuned <23 kHz, neither GlyR nor GABAR blockade altered LF suppression. We conclude that such suppression originates at a lower auditory level, perhaps a result of cochlear mechanisms. These findings demonstrate that neuronal interactions within NLL create a particular form of LF suppression that contributes to the analysis of complex acoustic signals.

The ventral nucleus of the lateral lemniscus of the gerbil (Meriones unguiculatus): organization of connections with the cochlear nucleus and the inferior colliculus

The spatial organization of projections from the ventral cochlear nucleus (VCN) to the ventral nucleus of the lateral lemniscus (VNLL) and from the VNLL to the central nucleus of the inferior colliculus (CNIC) was investigated by using neuroanatomical tracing methods in the gerbil. In order to label cells in the VNLL that project to the CNIC, focal injections of biotinylated dextran amine (BDA) were made into different CNIC regions. Retrogradely labeled cells were distributed throughout the dorsal-to-ventral axis of the VNLL in all cases. In contrast, the distribution of labeled cells across the lateral-to-medial dimension of the VNLL was related to the location of the injection site along the dorsolateral to ventromedial (frequency) axis of the CNIC. Cells projecting to dorsolateral (low-frequency) regions of the CNIC were located peripherally in the VNLL, mainly laterally and caudally, whereas those projecting to ventromedial (high-frequency) regions of the CNIC tended to be clustered centrally. Projections to the VNLL were labeled anterogradely following injections of BDA in the VCN. The distribution of terminal fields in the VNLL closely paralleled the topographic arrangement of cells projecting to the CNIC; projections from ventrolateral (low-frequency) areas of the VCN terminated mainly along the lateral and caudal borders of the VNLL, whereas projections from dorsomedial (high-frequency) areas terminated in more central regions. The results demonstrate a topographic organization of the major afferent and efferent connections of the gerbil VNLL.

Timing of sound-evoked potentials and spike responses in the inferior colliculus of awake bats

Neurons in the inferior colliculus (IC), one of the major integrative centers of the auditory system, process acoustic information converging from almost all nuclei of the auditory brain stem. During this integration, excitatory and inhibitory inputs arrive to auditory neurons at different time delays. Result of this integration determines timing of IC neuron firing. In the mammalian IC, the range of the first spike latencies is very large (5-50 ms). At present, a contribution of excitatory and inhibitory inputs in controlling neurons' firing in the IC is still under debate. In the present study we assess the role of excitation and inhibition in determining first spike response latency in the IC. Postsynaptic responses were recorded to pure tones presented at neuron's characteristic frequency or to downward frequency modulated sweeps in awake bats. There are three main results emerging from the present study: (1) the most common response pattern in the IC is hyperpolarization followed by depolarization followed by hyperpolarization, (2) latencies of depolarizing or hyperpolarizing responses to tonal stimuli are short (3-7 ms) whereas the first spike latencies may vary to a great extent (4-26 ms) from one neuron to another, and (3) high threshold hyperpolarization preceded long latency spikes in IC neurons exhibiting paradoxical latency shift. Our data also show that the onset hyperpolarizing potentials in the IC have very small jitter (<100 micros) across repeated stimulus presentations. The results of this study suggest that inhibition, arriving earlier than excitation, may play a role as a mechanism for delaying the first spike latency in IC neurons.

Modification of membrane excitability of neurons in the rat's dorsal cortex of the inferior colliculus by preceding hyperpolarization

The inferior colliculus (IC) is among the largest nuclei in the central auditory system and is considered to be a major integration center in the auditory pathway. To understand how IC contributes to auditory processing, we investigated the effects of preceding hyperpolarization on membrane excitability and firing behavior of neurons located in the dorsal cortex of the inferior colliculus (ICD). We made whole-cell patch clamp recordings from ICD neurons (n=96) in rat brain slices. We classified ICD neurons into three types, i.e. sustained-regular, sustained-adapting and buildup, according to their responses to depolarizing current injection. Nearly 91% of the neurons had sustained firing throughout the period of current injection, showing either regular or adapting pattern. About 9% of the neurons exhibited a buildup pattern, in which sustained firing started after a long delay. Rebound depolarization and spikes after hyperpolarization were seen in 51.7% of the sustained neurons, but were not seen in buildup neurons. When depolarizing current was preceded by a hyperpolarizing current, various forms of the modification on membrane excitability were observed. For non-rebound neurons, the membrane excitability was either suppressed or unchanged after pre-hyperpolarization. The first spike latency lengthened in neurons whose firing changed to a buildup pattern, shortened in those whose firing changed to a pauser pattern, and remained unchanged in those whose discharge pattern remained sustained. For rebound neurons, the firing rate decreased in neurons whose firing pattern was changed to onset or pauser, increased in neurons whose firing was changed to adapting, or remained unchanged in neurons whose firing became irregular. The first spike latency was shortened in all the rebound cells. The results suggest that intrinsic membrane properties can play an important role in integration of excitatory and inhibitory inputs and thereby in determination of the output of ICD neurons.

Inferior colliculus responses to multichannel microstimulation of the ventral cochlear nucleus: implications for auditory brain stem implants

Multichannel techniques were used to assess the frequency specificity of activation in the central nucleus of the inferior colliculus (CIC) produced by electrical stimulation of localized regions within the ventral cochlear nucleus (VCN). Data were recorded in response to pure tones from 141 and 193 multiunit clusters in the rat VCN and the CIC, respectively. Of 141 VCN sites, 126 were individually stimulated while recording responses in the CIC. A variety of CIC response types were seen with an increase in both electrical and acoustic stimulation levels. The majority of sites exhibited monotonic rate-level types acoustically, whereas spike rate saturation was achieved predominantly with electrical stimulation. In 20.6% of the 364 characteristic frequency aligned VCN-CIC pairs, the CIC sites did not respond to stimulation. In 26% of the 193 CIC sites, a high correlation was observed between acoustic tuning and electrical tuning obtained through VCN stimulation. A high degree of frequency specificity was found in 58% of the 118 lowest threshold VCN-CIC pairs. This was dependent on electrode placement within the VCN because a higher degree of frequency specificity was achieved with stimulation of medial, central, and posterolateral VCN regions than more anterolateral regions. Broadness of acoustic tuning in the CIC played a role in frequency-specific activation. Narrowly tuned CIC sites showed the lowest degree of frequency specificity on stimulation of the anterolateral VCN regions. These data provide significant implications for auditory brain stem implant electrode placement, current localization, power requirements, and facilitation of information transfer to higher brain centers.

Rethinking tuning: in vivo whole-cell recordings of the inferior colliculus in awake bats

Tuning curves were recorded with patch electrodes from the inferior colliculus (IC) of awake bats to evaluate the tuning of the inputs to IC neurons, reflected in their synaptic tuning, compared with the tuning of their outputs, expressed in their discharge tuning. A number of unexpected features were revealed with whole-cell recordings. Among these was that most neurons responded to tones with inhibition and/or subthreshold excitation over a surprisingly broad frequency range. The synaptic tuning in many cells was at least 1.5-2.0 octaves wide and, on average, was more than twice as wide as the frequency range that evoked discharges even after inhibition was blocked. In most cells, tones evoked complex synaptic response configurations that varied with frequency, suggesting that these cells were not innervated by congruent excitatory and inhibitory projections. Synaptic tuning was not only wide but was also diverse, in which some cells were dominated by excitation (n = 20), others were dominated by excitation with sideband inhibition (n = 21), but most were dominated by inhibition with little evidence of excitation (n = 31). Another unexpected finding was that some cells responded with inhibition to the onset and offset of tones over a wide frequency range, in which the patterns of synaptic responses changed markedly with frequency. These cells never fired to tones at 50 dB sound pressure level but fired to frequency-modulated sweeps at that intensity and were directionally selective. Thus, the features revealed by whole-cell recordings show that the processing in many IC cells results from inputs spectrally broader and more complex than previously believed.

First-spike latency information in single neurons increases when referenced to population onset

It is well known that many stimulus parameters, such as sound location in the auditory system or contrast in the visual system, can modulate the timing of the first spike in sensory neurons. Could first-spike latency be a candidate neural code? Most studies measuring first-spike latency information assume that the brain has an independent reference for stimulus onset from which to extract latency. This assumption creates an obvious confound that casts doubt on the feasibility of first-spike latency codes. If latency is measured relative to an internal reference of stimulus onset calculated from the responses of the neural population, the information conveyed by the latency of single neurons might decrease because of correlated changes in latency across the population. Here we assess the effects of a realistic model of stimulus onset detection on the first-spike latency information conveyed by single neurons in the auditory system. Contrary to expectation, we find that on average, the information contained in single neurons does not decrease; in fact, the majority of neurons show a slight increase in the information conveyed by latency referenced to a population onset. Our results show that first-spike latency codes are a feasible mechanism for information transfer even when biologically plausible estimates of stimulus onset are taken into account.

Intracellular responses and morphology of rat ventral complex of the lateral lemniscus neurons in vivo

The function of the ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL), collectively termed ventral complex of the lateral lemniscus (VCLL), is unclear. Several studies have suggested that it plays a role in coding the temporal aspects of sound. In our study, a sample (n = 161) of intracellular responses to dichotically presented noise or tone bursts was collected from the VCLL of urethane-anesthetized rats in vivo. Intracellular recordings revealed six distinct response types to tones, distinguished by their synaptic and membrane characteristics as well as firing pattern. Three of these response types were correlated with distinct cellular morphologies revealed by intracellular injection of neurobiotin. 3D reconstructions of recorded neurons within the VCLL showed the spatial distribution of various response properties, including response type, laterality, characteristic frequency (CF), and binaural influences. Cells that responded to monaural (55%) or binaural (45%) stimulation were distributed throughout the VCLL. Almost all VCLL units were responsive to contralateral stimulation (97%). Most neurons were excited by contralateral stimulation (83%), many exclusively (43%), and some in conjunction with ipsilateral inhibition (28%) or excitation (12%). The INLL contained mostly binaural neurons (65%), typically with ipsilateral inhibition and contralateral excitation. These results indicate that the VCLL is not a monaural structure and there is a dorsal-ventral segregation of binaural and monaural cells. 3D reconstructions of intracellular CFs did not reveal the presence of any tonotopic arrangement within the VCLL. Presumably, the proposed timing role of this structure does not require a systematic representation of tonal frequency.

Responses of Neurons in the Rat's Ventral Nucleus of the Lateral Lemniscus to Amplitude-Modulated Tones

Recordings were made from single neurons in the rat's ventral nucleus of the lateral lemniscus (VNLL) to determine responses to amplitude-modulated (AM) tones. The neurons were first characterized on the basis of their response to tone bursts presented to the contralateral ear and a distinction was made between those with transient onset responses and those with sustained responses. Sinusoidal AM tones were then presented to the contralateral ear with a carrier that matched the neuron's characteristic frequency (CF). Modulation transfer functions were generated on the basis of firing rate (MTFFR) and vector strength (MTFVS). Ninety-two percent of onset neurons that responded continuously to AM tones had band-pass MTFFRs with best modulation frequencies from 10 to 300 Hz. Fifty-four percent of sustained neurons had band-pass MTFFRs with best modulation frequencies from 10 to 500 Hz; other neurons had band-suppressed, all-pass, low-pass, or high-pass functions. Most neurons showed either band-pass or low-pass MTFVS. Responses were well synchronized to the modulation cycle with maximum vector strengths ranging from 0.37 to 0.98 for sustained neurons and 0.78 to 0.99 for onset neurons. The upper frequency limit for response synchrony was higher than that reported for inferior colliculus, but lower than that seen in more peripheral structures. Results suggest that VNLL neurons, especially those with onset responses to tone bursts, are sensitive to temporal features of sounds and narrowly tuned to different modulation rates. However, there was no evidence of a topographic relation between dorsoventral position along the length of VNLL and best modulation frequency as determined by either firing rate or vector strength.

Intracellular recording reveals temporal integration in inferior colliculus neurons of awake bats

The central nucleus of the inferior colliculus (IC) is a major integrative center in the central auditory system. It receives information from both the ascending and descending auditory pathways. To determine how single IC neurons integrate information over a wide range of sound frequencies and sound levels, we examined their intracellular responses to frequency-modulated (FM) sounds in awake little brown bats (Myotis lucifugus). Postsynaptic potentials were recorded in response to downward FM sweeps of the range typical for little brown bats (80-20 kHz) and to three FM subcomponents (80-60, 60-40, and 40-20 kHz). The majority of recorded neurons responded to the 80- to 20-kHz downward FM sweep with a complex response. In this response an initial hyperpolarization was followed by depolarization with or without spike followed by hyperpolarization. Intracellular recordings in response to three FM subcomponents revealed that these neurons receive excitatory and inhibitory inputs from a wide range of sound frequencies. One third of IC neurons performed nearly linear temporal summation across a wide range of sound frequencies, whereas two thirds of IC neurons exhibited nonlinear summation with different degrees of nonlinearity. Some IC neurons showed different latencies of postsynaptic potentials in response to different FM subcomponents. Often responses to the later FM subcomponent occurred before responses to the earlier ones. This phenomenon may be responsible for response selectivity of IC neurons to FM sweeps.

Responses of neurons in the rat's ventral nucleus of the lateral lemniscus to monaural and binaural tone bursts

Responses to monaural and binaural tone bursts were recorded from neurons in the rat's ventral nucleus of the lateral lemniscus (VNLL). Most of the neurons (55%) had V- or U-shaped frequency-tuning curves with a single clearly defined characteristic frequency (CF). However, many neurons had more complex, multipeaked tuning curves (37%), or other patterns (8%). Temporal firing patterns included both onset and sustained responses to contralateral tone bursts. Onset and sustained responses were distributed along the dorsoventral length of VNLL with no indication of segregation into different regions. Onset neurons had shorter average first-spike latencies than neurons with sustained responses (means, 8.3 vs. 14.8 ms). They also had less jitter, as reflected in the SD of first-spike latencies, than neurons with sustained responses (means, 0.59 and 4.2 ms, respectively). The extent of jitter decreased with an increase in stimulus intensity for neurons with sustained responses, but remained unchanged for onset neurons tested over the same range. Many neurons had binaural responses, primarily of the excitatory/inhibitory (EI) type, widely distributed along the dorsoventral extent of VNLL. Local application of the AMPA receptor antagonist NBQX reduced excitatory responses, indicating that responses were dependent on synaptic activity and not recorded from passing fibers. The results show that many neurons in VNLL have a precision of timing that is well suited for processing auditory temporal information. In the rat, these neurons are intermingled among cells with less precise temporal response features and include cells with binaural as well as monaural responses.