Biology subjects

Carney, L. H.

Publications and source records attributed to Carney, L. H..

17 recordsLinked to original sources

bioRxiv2026

Representations of Pitch and Timbre of Instrument Sounds in the Inferior Colliculus

PurposeThe neural representation of pitch and timbre in complex sounds has previously been studied using synthetic, controlled stimuli to investigate underlying encoding mechanisms. These studies provide information about how single attributes of sound are represented in the inferior colliculus (IC), a critical hub of the auditory pathway where neurons are sensitive to stimulus periodicity and spectral shape, giving rise to representations of pitch and timbre, respectively. However, there is a gap in understanding how natural sounds with both pitch and timbre attributes, such as instrument sounds, are represented in the IC. MethodsIn this study, extracellular recordings were made in the IC of awake rabbits in response to natural instrument stimuli varying in fundamental frequency (F0) to determine how instrument identity (timbre) and F0 (pitch) are represented in IC neurons. ResultsUsing decoding models for instrument identification, we found that instrument identity was redundantly encoded in a population of neurons with diverse rate and timing characteristics. F0 identification using decoding models trained on single-neuron rate responses was poor, but the population of rate responses contained enough information to identify F0 reliably. F0 information was also encoded in single-neuron temporal responses up to 196 Hz. F0 identification from a population of temporal responses was accurate up to approximately 900 Hz, but accuracy decreased at high F0s. For the task in which F0 was identified based on responses to both oboe and bassoon stimuli that had overlapping F0s, performance decreased compared to F0 identification based on responses to a single instrument. ConclusionThis result supports the hypothesis that pitch and timbre information are encoded jointly in the IC.

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bioRxiv2025

A computational model of the mammalian auditory periphery with a multichannel, energy-driven, medial olivocochlear reflex

The afferent auditory system, and how its specialized mechanisms and circuits support ecologically relevant auditory computations such as speech recognition, has received considerable attention in decades past. This work has culminated in accurate computational models of early afferent coding alongside a good understanding of how low-level mechanisms (e.g., peripheral tuning) impact auditory perception. In contrast, the auditory efferent system and its role in auditory perception is much less well understood. To address this gap in knowledge, we describe modifications to a model of the auditory periphery to include a medial olivocochlear efferent pathway that dynamically adjusts cochlear gain in response to sound via the classical medial olivocochlear reflex loop. We show that this model can simulate the effects of contralateral elicitors on auditory-nerve responses, including the effect of elicitors that are tonotopically distant from probes. Inclusion of across-frequency efferent effects necessitated a novel multichannel design.

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bioRxiv2025

Modelling Auditory Enhancement: Efferent Control of Cochlear Gain can Explain Level Dependence and Effects of Hearing Loss

The role of medial olivocochlear (MOC) efferent gain control in auditory enhancement (AE) was investigated using a subcortical auditory model. AE refers to the influence of a precursor on detectability of targets. The absence (or presence) of a precursor component at the target frequency enhances (or suppresses) detection under simultaneous masking conditions. Furthermore, the enhanced target under simultaneous masking acts as a stronger forward masker for a delayed probe tone, known as AE under forward masking. Psychoacoustic studies of AE report findings that challenge conventional expectations, and the underlying mechanisms remain unclear. For instance, listeners with hearing impairment have AE under simultaneous masking but not forward masking (Kreft et al., 2018; Kreft and Oxenham, 2019), whereas listeners with normal hearing have level-dependent AE under forward masking (Kreft and Oxenham, 2019). Our model with MOC efferent gain control successfully replicated these findings. In contrast, a model without efferent gain control failed to capture these effects, supporting the hypothesis that MOC-mediated cochlear gain modulation may play a role in AE and its alteration by hearing loss.

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bioRxiv2025

Chirp Sensitivity and Vowel Coding in the Inferior Colliculus

The inferior colliculus (IC) is an important brain region to understand neural encoding of complex sounds due to its diverse sound-feature sensitivities, including features that are affected by peripheral nonlinearities. Recent physiological studies in rabbit IC demonstrate that IC neurons are sensitive to chirp direction and velocity. Fast spectrotemporal changes, known as chirps, are contained within pitch-periods of natural vowels. Here, we use a combination of physiological and modeling strategies to assess the impact of chirp-sensitivity on vowel coding. Neural responses to vowel stimuli were recorded and vowel-token identification was evaluated based on average-rate and spike-timing metrics. Response timing was found to result in higher identification accuracy than rate. Additionally, rate bias towards low-velocity chirps, independent of chirp direction, was shown to correlate with higher vowel-identification accuracy based on timing. Also, direction bias in response to chirps of high velocity was shown to correlate with vowel-identification accuracy based on both rate and timing. Responses to natural-vowel tokens of individual neurons were simulated using an IC model with controllable chirp sensitivity. Responses of upward-biased, downward-biased, and non-selective model neurons were generated. Manipulating chirp sensitivity influenced response profiles across natural vowel tokens and vowel discrimination based on model-neuron responses. More work is needed to match all features of model responses to those of physiological recordings. HighlightsO_LIDue to phase differences between harmonics caused by vocal-tract resonances, spoken vowels contain fast (within-pitch-period) frequency sweeps, or chirps, to which neurons in the inferior colliculus (IC) are sensitive. C_LIO_LIBoth vowel responses and chirp-velocity sensitivity were recorded from single neurons in the IC. C_LIO_LIAn analysis of responses from a large number of IC neurons showed that features of chirp sensitivity were correlated to the accuracy of vowel identification based on average rate and/or timing of neural responses. C_LIO_LIAn IC model with chirp sensitivity was used to explore its impact on vowel responses. C_LI

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bioRxiv2025

Mechanisms of Tone-in-Noise Encoding in the Inferior Colliculus

Extracellular single-unit responses to tone-in-noise (TIN) stimuli were recorded in the inferior colliculus (IC) of awake female Dutch-belted rabbits. Stimuli consisted of wideband and narrowband tone-in-noise (TIN) with on-and off-characteristic frequency tones. Neural responses to wideband TIN showed a pattern of rates that increased when the tone matched CF and decreased (with respect to noise-alone responses) when the tone was above or below CF. This result differed from narrowband TIN IC responses that depended on envelope fluctuations in the stimulus, consistent with neural-fluctuation sensitivity. The WB-TIN responses could be fit with a difference-of-gaussians model that had narrow excitation and broad inhibition; responses to TIN could not be predicted by response-maps or spectrotemporal receptive fields. Responses to diotic and contralateral presentations of WB-TIN did not differ due to presentation ear. A single-CF computational model of the IC could not predict responses to wideband TIN. However, adding local off-CF inhibitory inputs to an on-CF IC model improved accuracy. These results suggest that broad inhibition could explain encoding of wideband TIN at suprathreshold signal-to-noise ratios, whereas neural fluctuation sensitivity is more important for narrowband sounds.

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bioRxiv2025

Forward masking in the Inferior Colliculus: Dynamics of Discharge-rate Recovery after Narrowband Noise Maskers

In forward masking the detection threshold for a target sound (probe) is elevated due to the presence of a preceding sound (masker). Although many factors are known to influence the probe response following a masker, the current work focused on the temporal separation (delay) between the masker and probe and the inter-trial interval (ITI). Human probe thresholds recover from forward masking within 150-300 ms, similar to neural threshold recovery in the IC within 300 ms after tone maskers. Our study focused on recovery of discharge rate of IC neurons in response to probe tones after narrowband gaussian noise (GN) forward maskers, with varying time delays. Additionally, we examined how prior masker trials influenced IC rates by varying ITI. Our findings showed that previous masker trials impacted probe-evoked discharge rates, with full recovery requiring ITIs over 1.5 s after 70 dB SPL narrowband GN maskers. Neural thresholds in the IC for probes preceded by noise maskers were in the range observed in psychoacoustical studies. Two proposed mechanisms for forward masking, persistence and efferent gain control, were tested using rate analyses or computational modeling. A physiological model with efferent feedback gain control had responses consistent with trends in the physiological recordings.

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bioRxiv2024

Otoacoustic emissions predict cochlear-nerve but not behavioral frequency tuning in an avian vocal-communication specialist

Frequency analysis by the cochlea forms a key foundation for all subsequent auditory processing. Stimulus-frequency otoacoustic emissions (SFOAEs) are a potentially powerful alternative to traditional behavioral experiments for estimating cochlear tuning without invasive testing, as is necessary in humans. Which methods accurately predict cochlear tuning remains controversial due to only a single animal study comparing SFOAE-based, behavioral, and cochlear frequency tuning in the same species. The budgerigar (Melopsittacus undulatus) is a parakeet species with human-like behavioral sensitivity to many sounds and the capacity to mimic speech. Intriguingly, previous studies of critical bands, psychophysical tuning curves, and critical ratios in budgerigars show that behavioral tuning sharpness increases dramatically with increasing frequency from 1-3.5 kHz, doubling once per octave with peak tuning sharpness from 3.5-4 kHz. The pattern contrasts with slower monotonic growth of behavioral tuning sharpness with increasing frequency in other animals, including most avian species, suggesting a possible auditory specialization in budgerigars. We measured SFOAE-based and cochlear-afferent tuning in budgerigars, for comparison to previously reported behavioral results. SFOAE-based and cochlear-afferent tuning sharpness both increased monotonically and relatively slowly for higher frequencies, in contrast to the behavioral pattern. SFOAE-based tuning in budgerigars accurately predicted cochlear frequency tuning, and both measures aligned with typical patterns of cochlear tuning in other species. Divergent behavioral tuning in budgerigars is unlikely attributable to the periphery and could reflect specializations for central processing of masked signals. Our findings highlight the value of SFOAEs for estimating cochlear tuning and caution against direct inference of peripheral tuning from behavioral critical bands, psychophysical tuning curves, and critical ratios.

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bioRxiv2024

Auditory Forward Masking Explained by a Subcortical Model with Efferent Control of Cochlear Gain

Previous physiological and psychophysical studies have explored whether feedback to the cochlea from the efferent system influences forward masking. The present work proposes that the limited growth-of-masking (GOM) observed in auditory-nerve (AN) fibers may have been misunderstood; namely, that this limitation may be due to the influence of anesthesia on the efferent system. Building on the premise that the unanesthetized AN may exhibit GOM similar to more central nuclei, the present computational modeling study demonstrates that feedback from the medial olivocochlear (MOC) efferents may account for GOM observed physiologically in onset-type neurons in both the cochlear nucleus and inferior colliculus (IC). Additionally, the computational model of MOC efferents used here generates a decrease in masking with longer masker-signal delays similar to that observed in IC physiology and in psychophysical studies. An advantage of this explanation over alternative physiological explanations (e.g., that forward masking requires inhibition from the superior paraolivary nucleus) is that this theory can explain forward masking observed in the brainstem, early in the ascending pathway. For explaining psychoacoustic results, one strength of this model is that it can account for the lack of elevation in thresholds observed when masker level is randomly varied from interval-to-interval, a result that is difficult to explain using the conventional temporal-window model of psychophysical forward masking. Future directions for evaluating the efferent mechanism as a contributing mechanism for psychoacoustical results are discussed. Significance StatementThe simulations presented here demonstrate that a recent computational model of the auditory subcortex including medial-olivocochlear efferents generates forward masking, an increase in detection threshold for a short probe tone following a preceding sound. This model explains results from physiological recordings and suggests potential connections to psychoacoustic experiments. The theory that efferent control of cochlear gain is a contributing mechanism for forward masking has several advantages. This theory can explain the strength of masking exhibited by cochlear nucleus neurons, a phenomenon not explained by current physiological theories in which the strength of forward-masking is not increased relative to the periphery until later in the ascending pathway. Additionally, this theory explains results for a psychoacoustic task with random variation in masker level, results not explained by the theory that persistent masker energy interferes with detection of the probe.

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bioRxiv2023

A fast and flexible approximation of power-law adaptation for auditory computational models

1.Power-law adaptation is a form of neural adaptation that has been shown to provide a better description of auditory-nerve adaptation dynamics as compared to simpler exponential-adaptation processes. However, the computational costs associated with power-law adaptation are high and, problematically, grow superlinearly with the number of samples in the simulation. This cost limits the applicability of power-law adaptation in simulations of responses to relatively long stimuli, such as speech, or in simulations for which high sampling rates are needed. Here, we present a simple approximation to power-law adaptation based on a parallel set of exponential-adaptation processes with different time constants, demonstrate that the approximation improves on an existing approximation provided in the literature, and provide updates to a popular phenomenological model of the auditory periphery that implements the new approximation.

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bioRxiv2023

Profile analysis in listeners with sensorineural hearing loss: behavioral data and computational models

Many sounds contain spectral modulations at multiple scales, but much is still unknown about how such spectral features are represented in the auditory system. One behavioral task that provides insight into this question is profile analysis. In a typical profile-analysis task, listeners are asked to discriminate between a complex tone with equal-amplitude components and a complex tone with a single incremented component. Because listeners can perform profile analysis even when the overall sound level of the stimuli is randomized from interval to interval, this task is thought to be a useful index of relative processing of spectral shape, rather than just sensitivity to absolute level changes. Here, we measured profile analysis across the frequency range in a group of listeners that varied widely in their hearing status. We then modeled the resulting behavioral data by decoding responses to the stimuli from computational models of the auditory nerve and inferior colliculus. We found that both hearing loss at the target frequency and increases in the target frequency were associated with poorer profile-analysis thresholds, and that these results could both be explained as the result of corresponding changes in sensitivity of temporal modulation-sensitive cells at the level of the inferior colliculus. These results suggest that key features of profile-analysis may reflect the limits of central neural tuning to temporal modulations.

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bioRxiv2023

Incorporating models of subcortical processing improves the ability to predict EEG responses to natural speech

The goal of describing how the human brain responds to complex acoustic stimuli has driven auditory neuroscience research for decades. Often, a systems-based approach has been taken, in which neurophysiological responses are modeled based on features of the presented stimulus. This includes a wealth of work modeling electroencephalogram (EEG) responses to complex acoustic stimuli such as speech. Examples of the acoustic features used in such modeling include the amplitude envelope and spectrogram of speech. These models implicitly assume a direct mapping from stimulus representation to cortical activity. However, in reality, the representation of sound is transformed as it passes through early stages of the auditory pathway, such that inputs to the cortex are fundamentally different from the raw audio signal that was presented. Thus, it could be valuable to account for the transformations taking place in lower-order auditory areas, such as the auditory nerve, cochlear nucleus, and inferior colliculus (IC) when predicting cortical responses to complex sounds. Specifically, because IC responses are more similar to cortical inputs than acoustic features derived directly from the audio signal, we hypothesized that linear mappings (temporal response functions; TRFs) fit to the outputs of an IC model would better predict EEG responses to speech stimuli. To this end, we modeled responses to the acoustic stimuli as they passed through the auditory nerve, cochlear nucleus, and inferior colliculus before fitting a TRF to the output of the modeled IC responses. Results showed that using model-IC responses in traditional systems analyses resulted in better predictions of EEG activity than using the envelope or spectrogram of a speech stimulus. Further, it was revealed that model-IC derived TRFs predict different aspects of the EEG than acoustic-feature TRFs, and combining both types of TRF models provides a more accurate prediction of the EEG response.x

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bioRxiv2022

Auditory-nerve Model including Efferent Dynamic Gain Control with Inputs from Cochlear Nucleus and Inferior Colliculus

We developed an auditory model with a time-varying, gain-control signal based on the physiology of the efferent system and the sub-cortical neural pathways. The medial olivocochlear (MOC) efferent stage of the model receives excitatory projections from both fluctuation-sensitive model neurons of the inferior colliculus (IC) and wide-dynamic-range model neurons of the cochlear nucleus. The response of the model MOC stage dynamically controls cochlear gain via simulated outer hair cells. In response to amplitude-modulated (AM) noise, firing rates of most IC neurons with band-enhanced modulation transfer functions in awake rabbits increase over a time course consistent with the dynamics of the MOC efferent feedback. These changes in the rates of IC neurons in awake rabbits were employed to adjust the parameters of the efferent stage of the proposed model. Responses of the proposed model to AM noise were able to simulate the increasing IC rate over time, while the model without the efferent system did not show this trend. The proposed model with efferent gain control provides a powerful tool for testing hypotheses, shedding insight on mechanisms in hearing, specifically those involving the efferent system.

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bioRxiv2022

Effects of sensorineural hearing loss on formant-frequency discrimination: Measurements and models

This study concerns the effect of hearing loss on discrimination of formant frequencies in vowels. In the response of the healthy ear to a harmonic sound, auditory-nerve (AN) rate functions fluctuate at the fundamental frequency, F0. Responses of inner-hair-cells (IHCs) tuned near spectral peaks are captured (or dominated) by a single harmonic, resulting in lower fluctuation depths than responses of IHCs tuned between spectral peaks. Therefore, the depth of neural fluctuations (NFs) varies along the tonotopic axis and encodes spectral peaks, including formant frequencies of vowels. This NF code is robust across a wide range of sound levels and in background noise. The NF profile is converted into a rate-place representation in the auditory midbrain, wherein neurons are sensitive to low-frequency fluctuations. The NF code is vulnerable to sensorineural hearing loss (SNHL) because capture depends upon saturation of IHCs, and thus the interaction of cochlear gain with IHC transduction. In this study, formant-frequency discrimination limens (DLFFs) were estimated for listeners with normal hearing or mild to moderate SNHL. The F0 was fixed at 100 Hz, and formant peaks were either aligned with harmonic frequencies or placed between harmonics. Formant peak frequencies were 600 and 2000 Hz, in the range of first and second formants of several vowels. The difficulty of the task was varied by changing formant bandwidth to modulate the contrast in the NF profile. Results were compared to predictions from model auditory-nerve and inferior colliculus (IC) neurons, with listeners audiograms used to individualize the AN model. Correlations between DLFFs, audiometric thresholds near the formant frequencies, age, and scores on the Quick speech-in-noise test are reported. SNHL had a strong effect on DLFF for the second formant frequency (F2), but relatively small effect on DLFF for the first formant (F1). The IC model appropriately predicted substantial threshold elevations for changes in F2 as a function of SNHL and little effect of SNHL on thresholds for changes in F1.

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bioRxiv2022

Distinguishing sensitivity to direction and velocity of fast frequency chirps from periodicity tuning in the inferior colliculus

Neurons in the mammalian inferior colliculus (IC) are sensitive to the velocity (speed and direction) of fast frequency chirps contained in Schroeder-phase harmonic complexes (SCHR). However, IC neurons are also sensitive to stimulus periodicity, a prominent feature of SCHR stimuli. Here, to disentangle velocity sensitivity from periodicity tuning, we introduced a novel stimulus consisting of aperiodic random chirps. Extracellular, single-unit recordings were made in the IC of Dutch-belted rabbits in response to both SCHR and aperiodic chirps. Rate-velocity functions were constructed from aperiodic-chirp responses and compared to SCHR rate profiles, revealing interactions between stimulus periodicity and neural velocity sensitivity. A generalized linear model analysis demonstrated that periodicity tuning influences SCHR response rates more strongly than velocity sensitivity. Principal component analysis of rate-velocity functions revealed that neurons were more often sensitive to the direction of lower-velocity chirps and were less often sensitive to the direction of higher-velocity chirps. Overall, these results demonstrate that sensitivity to chirp velocity is common in the IC. Harmonic sounds with complex phase spectra, such as speech and music, contain chirps, and velocity sensitivity would shape IC responses to these sounds. HighlightsIC neurons had diverse sensitivity to chirp velocity (speed and direction) of periodic and aperiodic-chirp stimuli. Both velocity and periodicity sensitivity were necessary to predict neural responses to Schroeder-phase harmonic complexes. Neurons were more commonly sensitive to the direction of chirps at lower-speeds (e.g., < 2 kHz/ms) than higher-speeds (e.g., > 2 kHz/ms) in the tested range. The chirp speeds for which IC neurons were most sensitive are present in common harmonic sounds with realistic phase spectra, such as speech and music.

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bioRxiv2022

Mechanisms of masking by Schroeder-phase harmonic tone complexes in the budgerigar (Melopsittacus undulatus)

Schroeder-phase harmonic tone complexes can have a flat temporal envelope and either rising or falling instantaneous-frequency sweeps within periods of the fundamental frequency (F0), depending on the phase-scaling parameter C. Human thresholds for tone detection in a concurrent Schroeder masker are 10-15 dB lower for positive C values (rising frequency sweeps) compared to negative (falling sweeps), potentially due to the impulse response of cochlear filtering, though this hypothesis remains controversial. Birds provide an interesting animal model for studies of Schroeder masking because prior reports suggest less behavioral threshold difference between maskers with opposite C values. However, most behavioral studies focused on relatively low masker F0s, and neurophysiological mechanisms in birds have not been explored. We performed behavioral Schroeder-masking experiments in budgerigars (Melopsittacus undulatus) using a wide range of masker F0 and C values. The signal frequency was 2800 Hz. Neural recordings at the midbrain processing level characterized encoding of behavioral stimuli in awake animals. Behavioral thresholds increased with increasing masker F0 and showed minimal difference between opposite C values, consistent with prior studies. Neural recordings showed prominent temporal and rate-based encoding of Schroeder F0, and in many neurons, marked response asymmetry between Schroeder stimuli with opposite C values. Neural thresholds for Schroeder-masked tone detection were (1) in most cases based on a response decrement compared to the masker alone, consistent with prominent modulation tuning in midbrain neurons, and (2) generally similar between opposite masker C values. These results highlight the likely importance of envelope cues in behavioral studies of Schroeder masking.

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bioRxiv2022

Representations of fricatives in sub-cortical model responses: comparisons with human perception

Fricatives are obstruent sound contrasts made by airflow constrictions in the vocal tract that produce turbulence across the constriction or at a site downstream from the constriction. Fricatives exhibit significant intra/inter-subject and contextual variability. Yet fricatives are perceived with high accuracy. The current study investigated modeled neural responses to fricatives in the auditory nerve (AN) and inferior colliculus (IC), with the hypothesis that response profiles across populations of neurons provide robust correlates to consonant perception. Stimuli were 270 intervocalic fricatives (10 speakers x 9 fricatives x 3 utterances). Computational model response profiles had characteristic frequencies that were log-spaced from 125 Hz to 8 or 20 kHz, to explore the impact of high-frequency responses. Confusion matrices generated by k-nearest-neighbor subspace classifiers were based on the profiles of average rates across characteristic frequencies as feature vectors. Model confusion matrices were compared them with published behavioral data. The modeled AN and IC neural responses provided better predictions of behavioral accuracy than the stimulus spectra, with IC showing better accuracy than AN. Behavioral fricative accuracy was explained by modeled neural response profiles, whereas confusions were only partially explained. Extended frequencies improved accuracy based on the model IC, corroborating the importance of extended high frequencies in speech perception.

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bioRxiv2022

Inherent envelope fluctuations in forward masking: Effects of age and hearing loss

Forward masking is generally greater for Gaussian noise (GN) than for low-fluctuation noise maskers, i.e., GN disruption. Because the minimal hearing loss that is associated with older age may affect GN disruption differently than more significant hearing loss, the current study explored the contribution of minimal hearing loss associated with older age to GN disruption. GN disruption was measured using three masker-signal delays (25, 75, and 150 ms) for three adult groups: younger participants with normal hearing, older participants with minimal hearing loss, and older participants with sensorineural hearing loss. The role of underlying mechanisms was tested using a computational model for midbrain neurons. The primary result suggests that older listeners with mild threshold elevations that typically occur with age may be more susceptible to the deleterious effects of masker-envelope fluctuations than younger listeners with normal hearing. Results from the computational model indicate that there may be a larger influence of efferent feedback and saturation of inner hair cells on forward masking and GN disruption than previously thought.

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