Inferior Colliculus Neurons Research Articles

Stimulus-Specific Adaptation in the Inferior Colliculus of the Anesthetized Rat

To identify sounds as novel, there must be some neural representation of commonly occurring sounds. Stimulus-specific adaptation (SSA) is a reduction in neural response to a repeated sound. Previous studies using an oddball stimulus paradigm have shown that SSA occurs at the cortex, but this study demonstrates that neurons in the inferior colliculus (IC) also show strong SSA using this paradigm. The majority (66%) of IC neurons showed some degree of SSA. Approximately 18% of neurons showed near-complete SSA. Neurons with SSA were found throughout the IC. Responses of IC neurons were reduced mainly during the onset component of the response, and latency was shorter in response to the oddball stimulus than to the standard. Neurons with near-complete SSA were broadly tuned to frequency, suggesting a high degree of convergence. Thus, some of the mechanisms that may underlie novelty detection and behavioral habituation to common sounds are already well developed at the midbrain.

Temporal Features of Spectral Integration in the Inferior Colliculus: Effects of Stimulus Duration and Rise Time

This report examines temporal features of facilitation and suppression that underlie spectrally integrative responses to complex vocal signals. Auditory responses were recorded from 160 neurons in the inferior colliculus (IC) of awake mustached bats. Sixty-two neurons showed combination-sensitive facilitation: responses to best frequency (BF) signals were facilitated by well-timed signals at least an octave lower in frequency, in the range 16-31 kHz. Temporal features and strength of facilitation were generally unaffected by changes in duration of facilitating signals from 4 to 31 ms. Changes in stimulus rise time from 0.5 to 5.0 ms had little effect on facilitatory strength. These results suggest that low frequency facilitating inputs to high BF neurons have phasic-on temporal patterns and are responsive to stimulus rise times over the tested range. We also recorded from 98 neurons showing low-frequency (11-32 kHz) suppression of higher BF responses. Effects of changing duration were related to the frequency of suppressive signals. Signals<23 kHz usually evoked suppression sustained throughout signal duration. This and other features of such suppression are consistent with a cochlear origin that results in masking of responses to higher, near-BF signal frequencies. Signals in the 23- to 30-kHz range-frequencies in the first sonar harmonic-generally evoked phasic suppression of BF responses. This may result from neural inhibitory interactions within and below IC. In many neurons, we observed two or more forms of the spectral interactions described here. Thus IC neurons display temporally and spectrally complex responses to sound that result from multiple spectral interactions at different levels of the ascending auditory pathway.

Accurate Sound Localization in Reverberant Environments Is Mediated by Robust Encoding of Spatial Cues in the Auditory Midbrain

In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener's ears, distorting the spatial cues for sound localization. Yet, human listeners have little difficulty localizing sounds in most settings. Because reverberant energy builds up over time, the source location is represented relatively faithfully during the early portion of a sound, but this representation becomes increasingly degraded later in the stimulus. We show that the directional sensitivity of single neurons in the auditory midbrain of anesthetized cats follows a similar time course, although onset dominance in temporal response patterns results in more robust directional sensitivity than expected, suggesting a simple mechanism for improving directional sensitivity in reverberation. In parallel behavioral experiments, we demonstrate that human lateralization judgments are consistent with predictions from a population rate model decoding the observed midbrain responses, suggesting a subcortical origin for robust sound localization in reverberant environments.

Protein expression of small conductance calcium-activated potassium channels is altered in inferior colliculus neurons of the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) exhibits inherited predisposition to sound stimuli-induced generalized tonic-clonic seizures (audiogenic reflex seizures) and is a valid model to study the physiopathology of epilepsy. In this model, the inferior colliculus (IC) exhibits enhanced neuronal firing that is critical in the initiation of reflex audiogenic seizures. The mechanisms underlying IC neuronal hyperexcitability that leads to seizure susceptibility are not as yet fully understood. The present report shows that the levels of protein expression of SK1 and SK3 subtypes of the small conductance Ca 2+-activated K + channels were significantly decreased, while SK2 channel proteins were increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3), as compared to control Sprague-Dawley rats. No significant change was found in the expression of BK channel proteins in IC neurons of SN-GEPR-3s. Single episode of reflex audiogenic seizures in the GEPR-3s did not significantly alter the protein expression of SK1–3 and BK channels in IC neurons compared to SN-GEPR-3s. Thus, downregulation of SK1 and SK3 channels and upregulation of SK2 channels provide direct evidence that these Ca 2+-activated K + channels play important roles in IC neuronal hyperexcitability that leads to inherited seizure susceptibility in the GEPR.

Wide-dynamic-range forward suppression in marmoset inferior colliculus neurons is generated centrally and accounts for perceptual masking.

An organism's ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the input's response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20 ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker sound pressure levels (SPLs) and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or nonmonotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide-dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.

Transient gain adjustment in the inferior colliculus is serotonin- and calcium-dependent

In the inferior colliculus (IC), a brief period of acoustic conditioning can transiently enhance evoked discharge rate. The cellular basis of this phenomenon was assessed with whole cell current-clamp recordings in a gerbil IC brain slice preparation. The current needed to elicit a single action potential was first established for each neuron. A 5s synaptic stimulus train was delivered to the lateral lemniscus (LL), and followed immediately by the initial current pulse to assess a change in postsynaptic gain. The majority of IC neurons (66%) displayed an increase in current-evoked action potentials (Positive Gain). Despite the blockade of ionotropic glutamate receptors, this effect was correlated with membrane depolarization that occurred during the synaptic train. The postsynaptic mechanism for positive gain was examined by selective blockade of specific neurotransmitter receptors. Gain in action potentials was enhanced by antagonists of metabotropic glutamate, acetylcholine, GABA A and glycine receptors. In contrast, the gain was blocked or reduced by an antagonist to ionotropic serotonin receptors (5-HT 3R). Blocking voltage-activated calcium channels with verapamil also reduced the effect. These results suggest that 5-HT 3R activation, coupled with increased intracellular calcium, can transiently alter postsynaptic excitability in IC neurons.

The physiological role of pre- and postsynaptic GABA B receptors in membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus

In the inferior colliculus (IC), GABAergic inhibition mediated by GABA A receptors has been shown to play a significant role in regulating physiological responses, but little is known about the physiological role of GABA B receptors in IC neurons. In the present study, we used whole-cell patch clamp recording in vitro to investigate the effects of activation of GABA B receptors on membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus (ICD). Repetitive stimulation of GABAergic inputs to ICD neurons at high frequencies could elicit a slow and long-lasting postsynaptic response, which was reversibly abolished by the GABA B receptor antagonist, CGP 35348. The results suggest that postsynaptic GABA B receptors can directly mediate inhibitory synaptic transmission in ICD. The role of postsynaptic GABA B receptors in regulation of membrane excitability was further investigated by application of the GABA B receptor agonist, baclofen. Baclofen hyperpolarized the cell, reduced the membrane input resistance and firing rate, increased the threshold for generating action potentials (APs), and decreased the amplitude of the AP and its associated after-hyperpolarization. The Ca 2+-mediated rebound depolarization following hyperpolarization and the depolarization hump at the beginning of membrane depolarization were also suppressed by baclofen. In voltage clamp experiments, baclofen induced inward rectifying K + current and reduced low- and high-threshold Ca 2+ currents, which may account for the suppression of membrane excitability by postsynaptic GABA B receptors. Application of baclofen also reduced excitatory synaptic responses mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and inhibitory synaptic responses mediated by GABA A receptors. Baclofen increased the ratios of 2nd/1st excitatory and inhibitory postsynaptic currents to paired-pulse stimulation of the synaptic inputs. These results suggest that fast glutamatergic and GABAergic synaptic transmission in ICD can be modulated by presynaptic GABA B receptors.

Involvement of GABA-Mediated Inhibition in Shaping the Frequency Selectivity of Neurons in the Inferior Colliculus of the Big Brown Bat, Eptesicus fuscus

In central auditory signal processing, neural inhibition plays an important role in sharpening the selectivity of auditory neurons. The present study examines the involvement of GABA-mediated inhibition in shaping the frequency selectivity of neurons in the bat inferior colliculus (IC) using forward masking paradigm and bicuculline application. At each study session, we recorded two IC neurons with a pair of electrodes and reciprocally studied whether a sound that served as a probe to elicit response of one neuron might serve as a masker to affect the frequency tuning curve (FTC) of the other paired neuron. Among the 33 pairs of IC neurons recorded, this forward masking paradigm produces sharpening of the FTC in 29 (88%) pairs of IC neurons and broadening of the FTC in 4 (12%) pairs of IC neurons. The degree of sharpening of FTC decreases with recording depth as well as with the difference in the best frequency and recording depth between each pair of IC neurons. Although bicuculline application broadens the FTC of all IC neurons, forward masking still produces sharpening of the FTC in most IC neurons. These data suggest that population of IC neurons are highly correlated during frequency analysis such that frequency selectivity of some groups of IC neurons is improved through inhibition while the spectrum of frequency sensitivity of other groups of IC neurons is enhanced through excitation. Biological significance of these data relevant to acoustic signal processing is discussed.

Changes in neuronal activity and gene expression in guinea-pig auditory brainstem after unilateral partial hearing loss

Spontaneous neural hyperactivity in the central auditory pathway is often associated with deafness, the most common form of which is partial hearing loss. We quantified both peripheral hearing loss and spontaneous activity in single neurons of the contralateral inferior colliculus in a guinea-pig model 1 week after a unilateral partial deafness induced by cochlear mechanical lesion. We also measured mRNA levels of candidate genes in the same animals using quantitative real-time PCR. Spontaneous hyperactivity was most marked in the frequency region of the peripheral hearing loss. Expression of glutamate decarboxylase 1 (GAD1), GABA-A receptor subunit alpha-1 (GABRA1), and potassium channel subfamily K member 15 (KCNK15) was decreased ipsilaterally in the cochlear nucleus and bilaterally in the inferior colliculus. A member of RAB family of small GTPase (RAB3A) was decreased in both ipsilateral cochlear nucleus and contralateral inferior colliculus. RAB3 GTPase activating protein subunit 1 (RAB3GAP1) and glycine receptor subunit alpha-1 (GLRA1) were reduced ipsilaterally in the cochlear nucleus only. These results suggest that a decrease in inhibitory neurotransmission and an increase in membrane excitability may contribute to elevated neuronal spontaneous activity in the auditory brainstem following unilateral partial hearing loss.

The interaction of excitatory and inhibitory synaptic inputs determines the sound response properties of the neurons of inferior colliculus

The interaction of excitatory and inhibitory synaptic inputs determines the sound response properties of the neurons of inferior colliculus

Off-channel effect of high-frequency overstimulation on duration tuning of low-frequency inferior colliculus neurons in guinea pigs

Conclusion. High-frequency overstimulation can cause the loss of duration selectivity in low-frequency inferior colliculus neurons in guinea pigs. Objective. To investigate the effect of high-frequency overstimulation on duration tuning in low-frequency inferior colliculus neurons in guinea pigs. Materials and methods. Duration tuning pattern was recorded by measuring the spikes of single neurons in response to the best frequency (BF) of different durations. The effect of high-frequency overstimulation was verified by comparing the responses before and after the tone exposure. Results. In total, 40 duration-tuned neurons were successfully recorded before and after the tone exposure. After the high-frequency tone trauma, a total of 29 neurons (72.5%) became non-duration-tuned.

Off-channel effect of high-frequency overstimulation on duration tuning of low-frequency inferior colliculus neurons in guinea pigs

Off-channel effect of high-frequency overstimulation on duration tuning of low-frequency inferior colliculus neurons in guinea pigs

Stimulus-specific adaptation in subcortical regions of the central auditory system

Event Abstract Back to Event Stimulus-specific adaptation in subcortical regions of the central auditory system Manuel S. Malmierca1*, F M Antunes1, D. Pérez-González1, 2 and E. Covey1, 2 1 Institute for Neuroscience of Castilla y Leon, University of Salamanca, Spain 2 Department of Psychology, University of Washington, United States Mismatch negativity (MMN) studies have undeniably had a profound impact on our understanding of many cognitive phenomena and are a powerful tool when dealing with various clinical populations. However, because these studies rely on scalp-recorded gross potentials they do not reveal much about processing at subcortical levels. This has led to the assumption that adaptation to common sounds and detection of novel sounds is a function of the cortex with little or no contribution of subcortical processing. The pioneering electrophysiological studies in cat by Ulanovsky and colleagues (2004) came to the same conclusion. These authors used an oddball stimulus paradigm similar to that used in MMN studies, to demonstrate that neurons in the auditory cortex (AC) show a reduced response to a repeated stimulus, but briefly resume firing if a novel stimulus is presented. This phenomenon is called stimulus-specific adaptation (SSA) This same study failed to demonstrate SSA in the medial geniculate body (MGB). More recently, however, SSA has been observed in the inferior colliculus (IC) of the rat (Perez-Gonzalez et al., 2005; Malmierca et al. 2009) indicating that SSA is not an exclusive feature of the cortex. Since the MGB receives its main inputs from the IC and AC, both of which show SSA, we reexamined the issue of whether neurons in the rat MGB show SSA. Our data demonstrate that SSA is indeed present throughout the MGB, being more prominent in the dorsal and medial subdivisions. This finding raises the question of whether SSA in the IC is created there of whether it is inherited from the cortex via descending projections. To address the issue of whether synaptic inhibition is involved in creating SSA in the IC, we recorded responses of rat IC neurons using an oddball stimulation protocol similar to that of Ulanovsky et al. (2004). while applying microiontophoretic injections of bicuculline, a GABAA receptor blocker. Our preliminary results indicate that SSA is at least partially created in the IC through the action of GABAergic inhibition. These results taken together indicate that SSA and enhanced responses to novel stimuli are present at every level from the IC on, and that SSA is not simply a property inherited through excitatory descending projections from the AC. Acknowledgments: Supported by Spanish MEC (BFU2006-00572) and JCYL (GR221) to MSM, NSF grant (IOS-0719295) to EC and MEC fellowship (BES-2007-15642) to FMA. S. Cristaudo participated on the preliminary microiontophoretic experiments. Conference: MMN 09 Fifth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications, Budapest, Hungary, 4 Apr - 7 Apr, 2009. Presentation Type: Oral Presentation Topic: Abstracts Citation: Malmierca MS, Antunes F, Pérez-González D and Covey E (2009). Stimulus-specific adaptation in subcortical regions of the central auditory system. Conference Abstract: MMN 09 Fifth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications. doi: 10.3389/conf.neuro.09.2009.05.042 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 23 Mar 2009; Published Online: 23 Mar 2009. * Correspondence: Manuel S Malmierca, Institute for Neuroscience of Castilla y Leon, University of Salamanca, Salamanca, Spain, msm@usal.es Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Manuel S Malmierca F M Antunes D. Pérez-González E. Covey Google Manuel S Malmierca F M Antunes D. Pérez-González E. Covey Google Scholar Manuel S Malmierca F M Antunes D. Pérez-González E. 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Calcium channel dysfunction in inferior colliculus neurons of the genetically epilepsy-prone rat

Voltage-gated calcium (Ca 2+) channels are thought to play an important role in epileptogenesis and seizure generation. Here, using the whole cell configuration of patch-clamp techniques, we report on the modifications of biophysical and pharmacological properties of high threshold voltage-activated Ca 2+ channel currents in inferior colliculus (IC) neurons of the genetically epilepsy-prone rats (GEPR-3s). Ca 2+ channel currents were measured by depolarizing pulses from a holding potential of −80 mV using barium (Ba 2+) as the charge carrier. We found that the current density of high threshold voltage-activated Ca 2+ channels was significantly larger in IC neurons of seizure-naive GEPR-3s compared to control Sprague–Dawley rats, and that seizure episodes further enhanced the current density in the GEPR-3s. The increased current density was reflected by both a −20 mV shifts in channel activation and a 25% increase in the non-inactivating fraction of channels in seizure-naive GEPR-3s. Such changes were reduced by seizure episodes in the GEPR-3s. Pharmacological analysis of the current density suggests that upregulation of L-, N- and R-type of Ca 2+ channels may contribute to IC neuronal hyperexcitability that leads to seizure susceptibility in the GEPR-3s.

Modulation of major voltage- and ligand-gated ion channels in cultured neurons of the rat inferior colliculus by lidocaine

The purpose of the present study was to explore how lidocaine as a therapeutic drug for tinnitus targets voltage- and ligand-gated ion channels and changes the excitability of central auditory neurons. Membrane currents mediated by major voltage- and ligand-gated channels were recorded from primary cultured neurons of the inferior colliculus (IC) in rats with whole-cell patch-clamp techniques in the presence and absence of lidocaine. The effects of lidocaine on the current-evoked firing of action potentials were also examined. Lidocaine at 100 micromol/L significantly suppressed voltage-gated sodium currents, transient outward potassium currents, and the glycine-induced chloride currents to 87.66%+/-2.12%, 96.33%+/-0.35%, and 91.46%+/-2.69% of that of the control level, respectively. At 1 mmol/L, lidocaine further suppressed the 3 currents to 70.26%+/-4.69%, 62.80%+/-2.61%, and 89.11%+/-3.17% of that of the control level, respectively. However, lidocaine at concentrations lower than 1 mmol/L did not significantly affect GABA- or aspartate-induced currents. At a higher concentration (3 mmol/L), lidocaine slightly depressed the GABA-induced current to 87.70%+/-1.87% of that of the control level. Finally, lidocaine at 100 mumol/L was shown to significantly suppress the current-evoked firing of IC neurons to 58.62%+/-11.22% of that of the control level, indicating that lidocaine decreases neuronal excitability. Although the action of lidocaine on the ion channels and receptors is complex and non-specific, it has an overall inhibitory effect on IC neurons at a clinically-relevant concentration, suggesting a central mechanism for lidocaine to suppress tinnitus.

Sensitive Response to Low-Frequency Cochlear Distortion Products in the Auditory Midbrain

During auditory stimulation with several frequency components, distortion products (DPs) are generated as byproduct of nonlinear cochlear amplification. After generated, DP energy is reemitted into the ear channel where it can be measured as DP otoacoustic emission (DPOAE), and it also induces an excitatory response at cochlear places related to the DP frequencies. We measured responses of 91 inferior colliculus (IC) neurons in the gerbil during two-tone stimulation with frequencies well above the unit's receptive field but adequate to generate a distinct distortion product (f2-f1 or 2f1-f2) at the unit's characteristic frequency (CF). Neuronal responses to DPs could be accounted for by the simultaneously measured DPOAEs for DP frequencies >1.3 kHz. For DP frequencies <1.3 kHz (n = 25), there was a discrepancy between intracochlear DP magnitude and DPOAE level, and most neurons responded as if the intracochlear DP level was significantly higher than the DPOAE level in the ear channel. In 12% of those low-frequency neurons, responses to the DPs could be elicited even if the stimulus tone levels were below the threshold level of the neuron at CF. High intracochlear f2-f1 and 2f1-f2 DP-levels were verified by cancellation of the neuronal DP response with a third phase-adjusted tone stimulus at the DP frequency. A frequency-specific reduction of middle ear gain at low frequencies is possibly involved in the reduction of DPOAE level. The results indicate that pitch-related properties of complex stimuli may be produced partially by high intracochlear f2-f1 distortion levels.

Presentation rate of stimulus modulates neuronal responses to frequency modulated sweeps in the mouse inferior colliculus

Presentation rate (PR) represents the number of sound stimuli within the unit time. It is one of the essential indexes reflecting acoustical energy and information received by auditory system in a given time. Our recent investigations have shown that the temporal properties of inferior collicular (IC) neurons responding to the pure tone bursts are closely correlated to PR. The frequency modulated sweeps (FMs) share major dynamic parts in both human languages and animal communicating sounds. The modulation range (MR) and sweep directions of FMs, which are important temporal features of FMs, play key roles in speech distinguish in human. Although neuronal selectivity of MRs and sweep directions has been intensively studied in echolocating bats, whether the PR could modulate the neuronal selectivity of the FM remains unclear. In present study we addressed it in the mouse inferior colliculus (IC). 20 mice ( Mus musculus KM) were used in the experiments. Under the condition of free field stimulation and by using normal electrophysiological techniques, we recorded the responses of 90 IC neurons to FMs with different modulation ranges (2, 5, 10, 15, 20 and 25 kHz; the center frequency was always set at the CF of each recorded) at different presentation rates (0.5, 1.0, 5.0, 10.0 or 20.0 Hz). The main results were as follows: 1) When the PR was fixed at a mediate value (2.0 Hz), the majority of neurons responded best to narrower MRs (narrow-pass, up-sweep: 63.33%; down-sweep: 60.00%). Other types such as band-pass (up-sweep, 17/90, 18.89%; down-sweep, 12/90, 13.33%), all-pass (up-sweep, 15/90, 16.67%; down-sweep, 17/90, 18.89%) and wide-pass (up-sweep, 4/90, 4.44%; down-sweep, 4/90, 4.44%) also be found; 2) When different PRs were used, from 0.2 Hz to 10 Hz, the percentage of the narrow-pass neurons decreased, while the percentage of band-pass and all-pass neurons increased slightly and that of wide-pass neurons changed little; 3) As the PR increased, the recorded neuron's firing rate decreased, the latency delayed and the firing duration shortened; 4) The directional selectivity of the IC neurons was modulated by the MR and this influence was PR-dependant. Also, the PRs could affect the directional selectivity of the neurons alone and displayed MR-dependant. Our data demonstrated that PR could modulate the neural coding of MR and directional selectivity of the IC neurons responding to FMs stimuli, and the PR and MR have the co-modulated effect. It reveals that the temporal representation of dynamic sounds in the central auditory system is correlated with the repetition rate of naturally occurring sound.

Age-related alteration in temporal representation induced by the increment of sound load in the mouse inferior collicular neurons

Age-related alteration in temporal representation induced by the increment of sound load in the mouse inferior collicular neurons

Temporal integration of inferior collicular neurons responding to sequentially combined CF-FM stimuli in the least horseshoe bat, Rhinolophus pusillus ( Microchiroptera: rhinolophidae)

Temporal integration of inferior collicular neurons responding to sequentially combined CF-FM stimuli in the least horseshoe bat, Rhinolophus pusillus ( Microchiroptera: rhinolophidae)

Retuning of inferior colliculus neurons following spiral ganglion lesions: a single-neuron model of converging inputs.

Lesions of spiral ganglion cells, representing a restricted sector of the auditory nerve array, produce immediate changes in the frequency tuning of inferior colliculus (IC) neurons. There is a loss of excitation at the lesion frequencies, yet responses to adjacent frequencies remain intact and new regions of activity appear. This leads to immediate changes in tuning and in tonotopic progression. Similar effects are seen after different methods of peripheral damage and in auditory neurons in other nuclei. The mechanisms that underlie these postlesion changes are unknown, but the acute effects seen in IC strongly suggest the “unmasking” of latent inputs by the removal of inhibition. In this study, we explore computational models of single neurons with a convergence of excitatory and inhibitory inputs from a range of characteristic frequencies (CFs), which can simulate the narrow prelesion tuning of IC neurons, and account for the changes in CF tuning after a lesion. The models can reproduce the data if inputs are aligned relative to one another in a precise order along the dendrites of model IC neurons. Frequency tuning in these neurons approximates that seen physiologically. Removal of inputs representing a narrow range of frequencies leads to unmasking of previously subthreshold excitatory inputs, which causes changes in CF. Conversely, if all of the inputs converge at the same point on the cell body, receptive fields are broad and unmasking rarely results in CF changes. However, if the inhibition is tonic with no stimulus-driven component, then unmasking can still produce changes in CF.