Neurons In Primary Auditory Cortex Research Articles

Auditory cortex neuronal tuning is sensitive to statistical structure of early acoustic environment

Event Abstract Back to Event Auditory cortex neuronal tuning is sensitive to statistical structure of early acoustic environment Hania Koever1*, Yi-Ting L. Tseng1, Kirt Gill1 and Shaowen Bao1 1 University of California at Berkeley, United States Whereas sensory stimuli vary along a continuum, behavioral responses to these stimuli are discrete. The perceptual system can help translate sensory inputs into behaviors by parsing continuous stimuli into discrete perceptual categories. Some forms of categorical perception are innate and may reflect intrinsic characteristics of sensory processing; others, however, depend on learning and experience. In humans, for example, native English speakers perceive a distinct difference between the speech sounds /la/ and /ra/, whereas native Japanese speakers do not. Likewise, some species of songbirds categorically perceive note duration in a manner that depends on experience with their specific native song dialect. Despite evidence for experience-dependent categorical sound perception in a range of species from humans to rodents, it is not clear what rules neurons use to segment acoustic inputs into categories. Evidence from linguistics suggests that the statistical structure of acoustic inputs during an early stage of development affects categorical perception. For example, the distribution of speech sounds plays a role, with bimodal, but not unimodal distributions of speech sounds leading to categorical perception in infants. Additionally, infants are sensitive to the transitional probabilities between speech sounds, and perceptually group sounds that repeatedly occur together. To investigate the effect of the statistical structure of early acoustic inputs on neural tuning in auditory cortex, we exposed young rats (postnatal day (p)9 to p45) to sequences of pure tones that were grouped into two categories based on transitional probabilities alone. Pure tones were organized into one-second long sequences of six tones, in which all tones within a sequence were drawn from either a low frequency or high frequency category of tones. In this design, tones within a category had a high probability of occurring successively within a sequence (high transitional probability), whereas tones from different categories never occurred within a sequence (low transitional probability). We used extracellular recording in anaesthetized rats to map primary auditory cortex, and compared neural tuning in exposed rats to that of naïve rats, as well as rats exposed to control tone sequences with homogeneous statistics. Neurons in primary auditory cortex of the categorical sequence group differed from those in the naïve and control groups in terms of the shape of their frequency response tuning curves. In particular, neurons tuned to frequencies near category boundaries had steeper slopes at the category boundary. In addition, receptive fields exhibited a tendency to code for either the low or high category, rather than straddling two categories. In conclusion, neuronal tuning in primary auditory cortex is sensitive to the transitional probabilities of sounds encountered during an early period of development. We are currently performing behavioral experiments to determine whether the altered neuronal tuning has a perceptual correlate. Conference: Computational and Systems Neuroscience 2010, Salt Lake City, UT, United States, 25 Feb - 2 Mar, 2010. Presentation Type: Poster Presentation Topic: Poster session III Citation: Koever H, Tseng YL, Gill K and Bao S (2010). Auditory cortex neuronal tuning is sensitive to statistical structure of early acoustic environment. Front. Neurosci. Conference Abstract: Computational and Systems Neuroscience 2010. doi: 10.3389/conf.fnins.2010.03.00268 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: 05 Mar 2010; Published Online: 05 Mar 2010. * Correspondence: Hania Koever, University of California at Berkeley, Berkeley, United States, haniakoever@berkeley.edu 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 Hania Koever Yi-Ting L Tseng Kirt Gill Shaowen Bao Google Hania Koever Yi-Ting L Tseng Kirt Gill Shaowen Bao Google Scholar Hania Koever Yi-Ting L Tseng Kirt Gill Shaowen Bao PubMed Hania Koever Yi-Ting L Tseng Kirt Gill Shaowen Bao Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Identification of excitation and inhibition in the auditory cortex using nonlinear modeling

Identification of excitation and inhibition in the auditory cortex using nonlinear modeling

Auditory Cortex: Representation through Sparsification?

The recent discovery of combination-sensitive neurons in the primary auditory cortex of awake marmosets may reconcile previous, apparently contradictory, findings that cortical neurons produce strong, sustained responses, but also represent stimuli sparsely.

Encoding of Sound Spatial Information by Neurons in The Rat Primary Auditory Cortex*

Many previous studies have shown that auditory cortical neurons are sensitive to sound spatial information, however, the mechanism of sound spatial coding is still not fully understood. Until now, detail studies on sound spatial coding have not been reported in the rat primary auditory cortex. Using electrophysiological technique, spatial response areas of 151 neurons in the rat primary auditory cortex were investigated. The relationships between spike counts and average first-spike latencies in the spatial response areas were analyzed. The results showed that, the majority (52.32%) of cortical neurons exhibited contralateral preference in the frontal auditory space whereas other neurons exhibited ipsilateral preference (18.54%) and midline preference (18.54%), and only a few neurons were included in the category of omnidirection (3.31%) and complex (7.28%). For the majority of cortical neurons, the arithmetic center of the preferred spatial area were distributed in the up and middle portion of the contralateral space relative to the recording side. Most neurons responded strongly to stimuli from their preferred space with shorter average first-spike latencies, and responded weakly to stimuli from non-preferred space with longer average latencies. In the spatial response area, the spike counts were negatively correlated with average first-spike latencies. The auditory cortex might use the information of both spike counts and average response time to code sound spatial information.

Effects of adaptation on spectrotemporal receptive fields in primary auditory cortex

Spectrotemporal receptive fields of nonlinear neurons in primary auditory cortex are stimulus dependent or context dependent. Here we show that a variant of stimulus-specific adaptation also contributes to this context dependence. Responses to sound stimulus frequencies close to the neuron's best frequency adapt with an average time constant of approximately 7 s. In contrast, responses away from the best frequency do not adapt, but in fact slightly increase over our 30-s observation window. Such stimulus-specific adaptation could function in enhancing stimulus discrimination and in maximizing neural information transmission by reducing redundancy. It also needs to be taken into account when comparing spectrotemporal receptive fields measured under adapted and nonadapted conditions.

Current Source Density Profiles of Stimulus-Specific Adaptation in Rat Auditory Cortex

Neurons in primary auditory cortex (A1) are known to exhibit a phenomenon known as stimulus-specific adaptation (SSA), which means that, when tested with pure tones, they will respond more strongly to a particular frequency if it is presented as a rare, unexpected "oddball" stimulus than when the same stimulus forms part of a series of common, "standard" stimuli. Although SSA has occasionally been observed in midbrain neurons that form part of the paraleminscal auditory pathway, it is thought to be weak, rare, or nonexistent among neurons of the leminscal pathway that provide the main afferent input to A1, so that SSA seen in A1 is likely generated within A1 by local mechanisms. To study the contributions that neural processing within the different cytoarchitectonic layers of A1 may make to SSA, we recorded local field potentials in A1 of the rat in response to standard and oddball tones and subjected these to current source density analysis. Although our results show that SSA can be observed throughout all layers of A1, right from the earliest part of the response, there are nevertheless significant differences between layers, with SSA becoming significantly stronger as stimulus-related activity passes from the main thalamorecipient layers III and IV to layer V.

Responses of neurons in the cat primary auditory cortex to sequential sounds

In the natural acoustic environment sounds frequently arrive at the two ears in quick succession. The responses of a cortical neuron to acoustic stimuli can be dramatically altered, usually suppressed, by a preceding sound. The purpose of this study was to determine if the binaural interaction evoked by a preceding sound is involved in subsequent suppressive interactions observed in auditory cortex neurons. Responses of neurons in the primary auditory cortex (AI) exhibiting binaural suppressive interactions (EO/I) were studied in barbiturate-anesthetized cats. For the majority (72.5%) of EO/I neurons studied, the response to a monaural contralateral stimulus was suppressed by a preceding monaural contralateral stimulus, but was not changed by a preceding monaural ipsilateral stimulus. For this subset of EO/I neurons, when a monaural contralateral stimulus was preceded by a binaural stimulus, the level of both the ipsilateral and the contralateral component of the binaural stimulus influenced the response to the subsequent monaural contralateral stimulus. When the contralateral level of the binaural stimulus was constant, increasing its ipsilateral level decreased the suppression of the response to the subsequent monaural contralateral stimulus. When the ipsilateral level of the binaural stimulus was constant, increasing its contralateral level increased the suppression of the response to the subsequent monaural contralateral stimulus. These results demonstrate that the sequential inhibition of responses of AI neurons is a function of the product of a preceding binaural interaction. The magnitude of the response to the contralateral stimulus is related to, but not determined by the magnitude of the response to the preceding binaural stimulus. Possible mechanisms of this sequential interaction are discussed.

Wiener-Volterra Characterization of Neurons in Primary Auditory Cortex Using Poisson-Distributed Impulse Train Inputs

An extension of the Wiener-Volterra theory to a Poisson-distributed impulse train input was used to characterize the temporal response properties of neurons in primary auditory cortex (AI) of the ketamine-anesthetized cat. Both first- and second-order "Poisson-Wiener" (PW) models were tested on their predictions of temporal modulation transfer functions (tMTFs), which were derived from extracellular spike responses to periodic click trains with click repetition rates of 2-64 Hz. Second-order (i.e., nonlinear) PW fits to the measured tMTFs could be described as very good in a majority of cases (e.g., predictability >or=80%) and were almost always superior to first-order (i.e., linear) fits. In all sampled neurons, second-order PW kernels showed strong compressive nonlinearities (i.e., a depression of the impulse response) but never expansive nonlinearities (i.e., a facilitation of the impulse response). In neurons with low-pass tMTFs, the depression decayed exponentially with the interstimulus lag, whereas in neurons with band-pass tMTFs, the depression was typically double-peaked, and the second peak occurred at a lag that correlated with the neuron's best modulation frequency. It appears that modulation-tuning in AI arises in part from an interplay of two nonlinear processes with distinct time courses.

Rapid Synaptic Depression Explains Nonlinear Modulation of Spectro-Temporal Tuning in Primary Auditory Cortex by Natural Stimuli

In this study, we explored ways to account more accurately for responses of neurons in primary auditory cortex (A1) to natural sounds. The auditory cortex has evolved to extract behaviorally relevant information from complex natural sounds, but most of our understanding of its function is derived from experiments using simple synthetic stimuli. Previous neurophysiological studies have found that existing models, such as the linear spectro-temporal receptive field (STRF), fail to capture the entire functional relationship between natural stimuli and neural responses. To study this problem, we compared STRFs for A1 neurons estimated using a natural stimulus, continuous speech, with STRFs estimated using synthetic ripple noise. For about one-third of the neurons, we found significant differences between STRFs, usually in the temporal dynamics of inhibition and/or overall gain. This shift in tuning resulted primarily from differences in the coarse temporal structure of the speech and noise stimuli. Using simulations, we found that the stimulus dependence of spectro-temporal tuning can be explained by a model in which synaptic inputs to A1 neurons are susceptible to rapid nonlinear depression. This dynamic reshaping of spectro-temporal tuning suggests that synaptic depression may enable efficient encoding of natural auditory stimuli.

Differential representation of spectral and temporal information by primary auditory cortex neurons in awake cats: Relevance to auditory scene analysis

We investigated how the primary auditory cortex (AI) neurons encode the two major requisites for auditory scene analysis, i.e., spectral and temporal information. Single-unit activities in awake cats AI were studied by presenting 0.5-s-long tone bursts and click trains. First of all, the neurons ( n = 92) were classified into 3 types based on the time-course of excitatory responses to tone bursts: 1) phasic cells (P-cells; 26%), giving only transient responses; 2) tonic cells (T-cells; 34%), giving sustained responses with little or no adaptation; and 3) phasic-tonic cells (PT-cells; 40%), giving sustained responses with some tendency of adaptation. Other tone–response variables differed among cell types. For example, P-cells showed the shortest latency and smallest spiking jitter while T-cells had the sharpest frequency tuning. PT-cells generally fell in the intermediate between the two extremes. Click trains also revealed between-neuron-type differences for the emergent probability of excitatory responses (P-cells > PT-cells > T-cells) and their temporal features. For example, a substantial fraction of P-cells conducted stimulus-locking responses, but none of the T-cells did. f r -dependency characteristics of the stimulus locking resembled that reported for “comodulation masking release,” a behavioral model of auditory scene analysis. Each type neurons were omnipresent throughout the AI and none of them showed intrinsic oscillation. These findings suggest that: 1) T-cells preferentially encode spectral information with a rate-place code and 2) P-cells preferentially encode acoustic transients with a temporal code whereby rate-place coded information is potentially bound for scene analysis.

Temporal Coherence in the Perceptual Organization and Cortical Representation of Auditory Scenes

Just as the visual system parses complex scenes into identifiable objects, the auditory system must organize sound elements scattered in frequency and time into coherent "streams." Current neurocomputational theories of auditory streaming rely on tonotopic organization of the auditory system to explain the observation that sequential spectrally distant sound elements tend to form separate perceptual streams. Here, we show that spectral components that are well separated in frequency are no longer heard as separate streams if presented synchronously rather than consecutively. In contrast, responses from neurons in primary auditory cortex of ferrets show that both synchronous and asynchronous tone sequences produce comparably segregated responses along the tonotopic axis. The results argue against tonotopic separation per se as a neural correlate of stream segregation. Instead we propose a computational model of stream segregation that can account for the data by using temporal coherence as the primary criterion for predicting stream formation.

Sensory Responses during Sleep in Primate Primary and Secondary Auditory Cortex

Most sensory stimuli do not reach conscious perception during sleep. It has been thought that the thalamus prevents the relay of sensory information to cortex during sleep, but the consequences for cortical responses to sensory signals in this physiological state remain unclear. We recorded from two auditory cortical areas downstream of the thalamus in naturally sleeping marmoset monkeys. Single neurons in primary auditory cortex either increased or decreased their responses during sleep compared with wakefulness. In lateral belt, a secondary auditory cortical area, the response modulation was also bidirectional and showed no clear systematic depressive effect of sleep. When averaged across neurons, sound-evoked activity in these two auditory cortical areas was surprisingly well preserved during sleep. Neural responses to acoustic stimulation were present during both slow-wave and rapid-eye movement sleep, were repeatedly observed over multiple sleep cycles, and demonstrated similar discharge patterns to the responses recorded during wakefulness in the same neuron. Our results suggest that the thalamus is not as effective a gate for the flow of sensory information as previously thought. At the cortical stage, a novel pattern of activation/deactivation appears across neurons. Because the neural signal reaches as far as secondary auditory cortex, this leaves open the possibility of altered sensory processing of auditory information during sleep.

Neural Mechanisms of Selective Auditory Attention in Rats (Dissertation)

AbstractHow does attention modulate sensory representations? In order to probe the underlying neural mechanisms, we established a simple rodent model of modality-specific attention. Rats were trained to perform distinct auditory two-tone discrimination and olfactory odor discrimination in a two alternative choice (2AC) paradigm. To determine auditory cortex’s role in this frequency discrimination task, we used GABA-A receptor agonist muscimol to transiently and reversibly inactivate auditory cortexes bilaterally in rats performing simple interleaved auditory and olfactory discrimination. With olfactory discrimination performance serving as internal control for motivation and decision making capability, we found only auditory two-tone discrimination was selectively impaired in these rats. This shows the auditory cortex is involved in this two-tone discrimination task.To investigate the neural correlate of modality-specific attention in the auditory cortex, we trained rats to perform interleaved auditory and olfactory blocks (of 50~70 trials each) in a single session. In auditory blocks, pure tones were either presented with or without a neutral odor (caproic acid, n=2 and 3 respectively), and subjects were rewarded for discriminating auditory stimuli. In olfactory blocks, both task odors and pure tones were presented simultaneously, and subjects were rewarded for discriminating olfactory stimuli. We recorded neural responses in primary auditory cortex (area A1) in freely moving rats while subjects performed this behavior. Single unit responses to tones were heterogeneous, and included transient, sustained, and suppressed. We found 205 of 802 units recorded responsive to the stimuli we used. Of these 205 units, 18.5% showed modality-specific attentional modulation of the anticipatory activity before tone onset. In addition, we also observed in smaller proportion of units (11.2%) modality-specific attentional modulation of the tone-evoked responses; in most cases, the responses to a particular auditory stimulus was enhanced in the auditory block (or, equivalently, suppressed in the olfactory block). Attention increased choice probability of the population in the auditory block. We have also observed significant behavior choice probability in small proportions of units. Our results suggest that shifting attention between audition to olfaction tasks can modulate the activity of single neurons in primary auditory cortex.

Modulation of auditory responses by modality-specific attention in rat primary auditory cortex

AbstractHow does attention modulate sensory representations? In order to probe the underlying neural mechanisms, we established a simple rodent model of modality-specific attention. Here we describe results of experiments in freely moving rats in which we have used tetrodes to record neural responses in primary auditory cortex (area A1) while subjects performed this behavior.Subjects were first trained to perform distinct auditory and olfactory two alternative forced-choice (2AFC) tasks. Training and testing were conducted in a custom three-poke computer-controlled behavioral apparatus. Subjects initiated trials with a center-poke, which triggered presentation of a tone (either 5 or 15 Hz), an odor (either R(-)-2-Octanol or S(+)-2-Octanol), or both. Subjects responded moving to the left or right poke. Correct responses were rewarded with water. Auditory and olfactory blocks (of 50 trials each) were interleaved in a single session. In auditory blocks, pure tones were either presented with or without a null odor (caproic acid, n=2 and 3 respectively), and subjects were cued to perform the task based on auditory stimuli. In olfactory blocks, both odors and pure tones were presented simultaneously, and subjects were cued to perform the task based on olfactory stimuli.After subjects reached consistent performance on the interleaved blocks, tetrode drives were implanted in primary auditory cortex of the left hemisphere. Single unit responses to tones were heterogeneous, and included transient, sustained, and suppressed. Among 304 responsive units recorded, 19% (58 units) showed modality-specific attentional modulation of at least one of the tone-evoked responses; in most cases, the responses to a particular auditory stimulus was enhanced in the auditory block (or, equivalently, suppressed in the olfactory block). In addition, we also observed modality-specific attentional modulation of the spontaneous activity in similar proportion of units (61 units). Our results suggest that shifting attention from audition to olfaction and back can modulate the activity of single neurons in primary auditory cortex.

A functional model of the slope sensitivity of the primary auditory cortex neurons in awake cats.

Our measurement of single‐spike responses to tone‐burst stimuli showed that there are heterogenic response types of the primary auditory cortex (A1) neurons in awake cats: phasic neurons sensitive to stimulus amplitude‐rise‐slope at the onset of stimuli, sustained cells sensitive to the stimulus slope, and the steady state level at later time‐phase of stimuli. In this study, in order to explain the behavior of the slope‐sensitive phasic cells, an auditory model has been developed. The model consists of six stages: the inner hair cell (IHC), the primary auditory nerve (AN), the cochlear nucleus (CN), the inferior colliculus (IC), the medial geniculate body (MGB), and the A1 neuron. The model was implemented as a MATLAB program on a personal computer using Meddis’s model for IHC [R. Meddis, J. Acoust. Soc. Am. 79, 702‐711 (1986)] and Maki’s model for AN, CN, and IC [Maki et al., J. Acoust. Soc. Jpn. 60, 304‐313 (2000)], while the models of MGB and A1 neuron were newly developed. The model output successfully replicated the physiological data in terms of response rise‐time, maximum driven rate, and onset‐latency corresponding to the slopes of input stimuli. [Work supported by KAKENHI 20300076.]

Linking the Response Properties of Cells in Auditory Cortex with Network Architecture: Cotuning versus Lateral Inhibition

The frequency-intensity receptive fields (RF) of neurons in primary auditory cortex (AI) are heterogeneous. Some neurons have V-shaped RFs, whereas others have enclosed ovoid RFs. Moreover, there is a wide range of temporal response profiles ranging from phasic to tonic firing. The mechanisms underlying this diversity of receptive field properties are yet unknown. Here we study the characteristics of thalamocortical (TC) and intracortical connectivity that give rise to the individual cell responses. Using a mouse auditory TC slice preparation, we found that the amplitude of synaptic responses in AI varies non-monotonically with the intensity of the stimulation in the medial geniculate nucleus (MGv). We constructed a network model of MGv and AI that was simulated using either rate model cells or in vitro neurons through an iterative procedure that used the recorded neural responses to reconstruct network activity. We compared the receptive fields and firing profiles obtained with networks configured to have either cotuned excitatory and inhibitory inputs or relatively broad, lateral inhibitory inputs. Each of these networks yielded distinct response properties consistent with those documented in vivo with natural stimuli. The cotuned network produced V-shaped RFs, phasic-tonic firing profiles, and predominantly monotonic rate-level functions. The lateral inhibitory network produced enclosed RFs with narrow frequency tuning, a variety of firing profiles, and robust non-monotonic rate-level functions. We conclude that both types of circuits must be present to account for the wide variety of responses observed in vivo.

Perception and cortical neural coding of harmonic fusion in ferrets.

This study examined the perception and cortical representation of harmonic complex tones, from the perspective of the spectral fusion evoked by such sounds. Experiment 1 tested whether ferrets spontaneously distinguish harmonic from inharmonic tones. In baseline sessions, ferrets detected a pure tone terminating a sequence of inharmonic tones. After they reached proficiency, a small fraction of the inharmonic tones were replaced with harmonic tones. Some of the animals confused the harmonic tones with the pure tones at twice the false-alarm rate. Experiment 2 sought correlates of harmonic fusion in single neurons of primary auditory cortex and anterior auditory field, by comparing responses to harmonic tones with those to inharmonic tones in the awake alert ferret. The effects of spectro-temporal filtering were accounted for by using the measured spectrotemporal receptive field to predict responses and by seeking correlates of fusion in the predictability of responses. Only 12% of units sampled distinguished harmonic tones from inharmonic tones, a small percentage that is consistent with the relatively weak ability of the ferrets to spontaneously discriminate harmonic tones from inharmonic tones in Experiment 1.

Cortical activity patterns predict speech discrimination ability

Neural activity in the cerebral cortex can explain many aspects of sensory perception. Extensive psychophysical and neurophysiological studies of visual motion and vibrotactile processing show that the firing rate of cortical neurons averaged across 50-500 ms is well correlated with discrimination ability. In this study, we tested the hypothesis that primary auditory cortex (A1) neurons use temporal precision on the order of 1-10 ms to represent speech sounds shifted into the rat hearing range. Neural discrimination was highly correlated with behavioral performance on 11 consonant-discrimination tasks when spike timing was preserved and was not correlated when spike timing was eliminated. This result suggests that spike timing contributes to the auditory cortex representation of consonant sounds.

Lateral Sharpening of Cortical Frequency Tuning by Approximately Balanced Inhibition

Cortical inhibition plays an important role in shaping neuronal processing. The underlying synaptic mechanisms remain controversial. Here, in vivo whole-cell recordings from neurons in the rat primary auditory cortex revealed that the frequency tuning curve of inhibitory input was broader than that of excitatory input. This results in relatively stronger inhibition in frequency domains flanking the preferred frequencies of the cell and a significant sharpening of the frequency tuning of membrane responses. The less selective inhibition can be attributed to a broader bandwidth and lower threshold of spike tonal receptive field of fast-spike inhibitory neurons than nearby excitatory neurons, although both types of neurons receive similar ranges of excitatory input and are organized into the same tonotopic map. Thus, the balance between excitation and inhibition is only approximate, and intracortical inhibition with high sensitivity and low selectivity can laterally sharpen the frequency tuning of neurons, ensuring their highly selective representation.

Level invariant representation of sounds by populations of neurons in primary auditory cortex.

A fundamental feature of auditory perception is the constancy of sound recognition over a large range of intensities. Although this invariance has been described in behavioral studies, the underlying neural mechanism is essentially unknown. Here we show a putative level-invariant representation of sounds by populations of neurons in primary auditory cortex (A1) that may provide a neural basis for the behavioral observations. Previous studies reported that pure-tone frequency tuning of most A1 neurons widens with increasing sound level. In sharp contrast, we found that a large proportion of neurons in A1 of awake marmosets were narrowly and separably tuned to both frequency and sound level. Tuning characteristics and firing rates of the neural population were preserved across all tested sound levels. These response properties lead to a level-invariant representation of sounds over the population of A1 neurons. Such a representation is an important step for robust feature recognition in natural environments.