Reduced processing efficiency impacts auditory detection of amplitude modulation in children: Evidence from an experimental and modeling study.

Auditory detection of the Amplitude Modulation (AM) of sounds, crucial for speech perception, improves until 10 years of age. This protracted development may not only be explained by sensory maturation, but also by improvements in processing efficiency: the ability to make efficient use of available sensory information. This hypothesis was tested behaviorally on 86 6-to-9-year-olds and 15 adults using AM-detection tasks assessing absolute sensitivity, masking, and response consistency in the AM domain. Absolute sensitivity was estimated by the detection thresholds of a sinusoidal AM applied to a pure-tone carrier; AM masking was estimated as the elevation of AM-detection thresholds produced when replacing the pure-tone carrier by a narrowband noise; response consistency was estimated using a double-pass paradigm where the same set of stimuli was presented twice. Results showed that AM sensitivity improved from childhood to adulthood, but did not change between 6 and 9 years. AM masking did not change with age, suggesting that the selectivity of perceptual AM filters was adult-like by 6 years. However, response consistency increased developmentally, supporting the hypothesis of reduced processing efficiency in early childhood. At the group level, double-pass data of children and adults were well simulated by a model of the human auditory system assuming a higher level of internal noise for children. At the individual level, for both children and adults, double-pass data were better simulated when assuming a sub-optimal decision strategy in addition to differences in internal noise. In conclusion, processing efficiency for AM detection is reduced in childhood. Moreover, worse AM detection was linked to both systematic and stochastic inefficiencies, in both children and adults.

Sensorineural hearing loss alters auditory discrimination of natural soundscapes.

OBJECTIVE: The ability to discriminate natural soundscapes recorded in a temperate terrestrial biome was measured in 15 hearing-impaired (HI) listeners with bilateral, mild to severe sensorineural hearing loss and 15 normal-hearing (NH) controls. DESIGN: Soundscape discrimination was measured using a three-interval oddity paradigm and the method of constant stimuli. On each trial, sequences of 2-second recordings varying the habitat, season and period of the day were presented diotically at a nominal SPL of 60 or 80 dB. RESULTS: Discrimination scores were above chance level for both groups, but they were poorer for HI than NH listeners. On average, the scores of HI listeners were relatively well accounted for by those of NH listeners tested with stimuli spectrally-shaped to match the frequency-dependent reduction in audibility of individual HI listeners. However, the scores of HI listeners were not significantly correlated with pure-tone audiometric thresholds and age. CONCLUSIONS: These results indicate that the ability to discriminate natural soundscapes associated with changes in habitat, season and period of the day is disrupted but it is not abolished. The deficits of the HI listeners are partly accounted for by reduced audibility. Supra-threshold auditory deficits and individual listening strategies may also explain differences between NH and HI listeners.

Human Auditory Ecology: Extending Hearing Research to the Perception of Natural Soundscapes by Humans in Rapidly Changing Environments.

Research in hearing sciences has provided extensive knowledge about how the human auditory system processes speech and assists communication. In contrast, little is known about how this system processes "natural soundscapes," that is the complex arrangements of biological and geophysical sounds shaped by sound propagation through non-anthropogenic habitats [Grinfeder et al. (2022). Frontiers in Ecology and Evolution. 10: 894232]. This is surprising given that, for many species, the capacity to process natural soundscapes determines survival and reproduction through the ability to represent and monitor the immediate environment. Here we propose a framework to encourage research programmes in the field of "human auditory ecology," focusing on the study of human auditory perception of ecological processes at work in natural habitats. Based on large acoustic databases with high ecological validity, these programmes should investigate the extent to which this presumably ancestral monitoring function of the human auditory system is adapted to specific information conveyed by natural soundscapes, whether it operate throughout the life span or whether it emerges through individual learning or cultural transmission. Beyond fundamental knowledge of human hearing, these programmes should yield a better understanding of how normal-hearing and hearing-impaired listeners monitor rural and city green and blue spaces and benefit from them, and whether rehabilitation devices (hearing aids and cochlear implants) restore natural soundscape perception and emotional responses back to normal. Importantly, they should also reveal whether and how humans hear the rapid changes in the environment brought about by human activity.

Auditory discrimination of natural soundscapes.

A previous modelling study reported that spectro-temporal cues perceptually relevant to humans provide enough information to accurately classify "natural soundscapes" recorded in four distinct temperate habitats of a biosphere reserve [Thoret, Varnet, Boubenec, Ferriere, Le Tourneau, Krause, and Lorenzi (2020). J. Acoust. Soc. Am. 147, 3260]. The goal of the present study was to assess this prediction for humans using 2 s samples taken from the same soundscape recordings. Thirty-one listeners were asked to discriminate these recordings based on differences in habitat, season, or period of the day using an oddity task. Listeners' performance was well above chance, demonstrating effective processing of these differences and suggesting a general high sensitivity for natural soundscape discrimination. This performance did not improve with training up to 10 h. Additional results obtained for habitat discrimination indicate that temporal cues play only a minor role; instead, listeners appear to base their decisions primarily on gross spectral cues related to biological sound sources and habitat acoustics. Convolutional neural networks were trained to perform a similar task using spectro-temporal cues extracted by an auditory model as input. The results are consistent with the idea that humans exclude the available temporal information when discriminating short samples of habitats, implying a form of a sub-optimality.

Probing temporal modulation detection in white noise using intrinsic envelope fluctuations: A reverse-correlation study.

Part of the detrimental effect caused by a stationary noise on sound perception results from the masking of relevant amplitude modulations (AM) in the signal by random intrinsic envelope fluctuations arising from the filtering of noise by cochlear channels. This study capitalizes on this phenomenon to probe AM detection strategies for human listeners using a reverse correlation analysis. Eight normal-hearing listeners were asked to detect the presence of a 4-Hz sinusoidal AM target applied to a 1-kHz tone carrier using a yes-no task with 3000 trials/participant. All stimuli were embedded in a white-noise masker. A reverse-correlation analysis was then carried on the data to compute "psychophysical kernels" showing which aspects of the stimulus' temporal envelope influenced the listener's responses. These results were compared to data simulated with different implementations of a modulation-filterbank model. Psychophysical kernels revealed that human listeners were able to track the position of AM peaks in the target, similar to the models. However, they also showed a marked temporal decay and a consistent phase shift compared to the ideal template. In light of the simulated data, this was interpreted as an evidence for the presence of phase uncertainty in the processing of intrinsic envelope fluctuations.

Temporal integration for amplitude modulation in childhood: Interaction between internal noise and memory.

It is still unclear whether the gradual improvement in amplitude-modulation (AM) sensitivity typically found in children up to 10 years of age reflects an improvement in "processing efficiency" (the central ability to use information extracted by sensory mechanisms). This hypothesis was tested by evaluating temporal integration for AM, a capacity relying on memory and decision factors. This was achieved by measuring the effect of increasing the number of AM cycles (2 vs 8) on AM-detection thresholds for three groups of children aged from 5 to 11 years and a group of young adults. AM-detection thresholds were measured using a forced-choice procedure and sinusoidal AM (4 or 32 Hz rate) applied to a 1024-Hz pure-tone carrier. All age groups demonstrated temporal integration for AM at both rates; that is, significant improvements in AM sensitivity with a higher number of AM cycles. However, an effect of age is observed as both 5-6 year olds and adults exhibited more temporal integration compared to 7-8 and 10-11 year olds at both rates. This difference is due to: (i) the 5-6 year olds displaying the worst thresholds with 2 AM cycles, but similar thresholds with 8 cycles compared to the 7-8 and 10-11 year olds, and, (ii) adults showing the best thresholds with 8 AM cycles but similar thresholds with 2 cycles compared to the 7-8 and 10-11 year olds. Computational modelling indicated that higher levels of internal noise combined with poorer short-term memory capacities in children accounted for the developmental trends. Improvement in processing efficiency may therefore account for the development of AM detection in childhood.

Double-pass consistency for amplitude- and frequency-modulation detection in normal-hearing listeners.

Amplitude modulation (AM) and frequency modulation (FM) provide crucial auditory information. If FM is encoded as AM, it should be possible to give a unified account of AM and FM perception both in terms of response consistency and performance. These two aspects of behavior were estimated for normal-hearing participants using a constant-stimuli, forced-choice detection task repeated twice with the same stimuli (double pass). Sinusoidal AM or FM with rates of 2 or 20 Hz were applied to a 500-Hz pure-tone carrier and presented at detection threshold. All stimuli were masked by a modulation noise. Percent agreement of responses across passes and percent-correct detection for the two passes were used to estimate consistency and performance, respectively. These data were simulated using a model implementing peripheral processes, a central modulation filterbank, an additive internal noise, and a template-matching device. Different levels of internal noise were required to reproduce AM and FM data, but a single level could account for the 2- and 20-Hz AM data. As for FM, two levels of internal noise were needed to account for detection at slow and fast rates. Finally, the level of internal noise yielding best predictions increased with the level of the modulation-noise masker. Overall, these results suggest that different sources of internal variability are involved for AM and FM detection at low audio frequencies.

Contributions of Age-Related and Audibility-Related Deficits to Aided Consonant Identification in Presbycusis: A Causal-Inference Analysis.

The decline of speech intelligibility in presbycusis can be regarded as resulting from the combined contribution of two main groups of factors: (1) audibility-related factors and (2) age-related factors. In particular, there is now an abundant scientific literature on the crucial role of suprathreshold auditory abilities and cognitive functions, which have been found to decline with age even in the absence of audiometric hearing loss. However, researchers investigating the direct effect of aging in presbycusis have to deal with the methodological issue that age and peripheral hearing loss covary to a large extent. In the present study, we analyzed a dataset of consonant-identification scores measured in quiet and in noise for a large cohort (n = 459, age = 42-92) of hearing-impaired (HI) and normal-hearing (NH) listeners. HI listeners were provided with a frequency-dependent amplification adjusted to their audiometric profile. Their scores in the two conditions were predicted from their pure-tone average (PTA) and age, as well as from their Extended Speech Intelligibility Index (ESII), a measure of the impact of audibility loss on speech intelligibility. We relied on a causal-inference approach combined with Bayesian modeling to disentangle the direct causal effects of age and audibility on intelligibility from the indirect effect of age on hearing loss. The analysis revealed that the direct effect of PTA on HI intelligibility scores was 5 times higher than the effect of age. This overwhelming effect of PTA was not due to a residual audibility loss despite amplification, as confirmed by a ESII-based model. More plausibly, the marginal role of age could be a consequence of the relatively little cognitively-demanding task used in this study. Furthermore, the amount of variance in intelligibility scores was smaller for NH than HI listeners, even after accounting for age and audibility, reflecting the presence of additional suprathreshold deficits in the latter group. Although the non-sense-syllable materials and the particular amplification settings used in this study potentially restrict the generalization of the findings, we think that these promising results call for a wider use of causal-inference analysis in audiology, e.g., as a way to disentangle the influence of the various cognitive factors and suprathreshold deficits associated to presbycusis.

Mechanisms of Spectrotemporal Modulation Detection for Normal- and Hearing-Impaired Listeners.

Spectrotemporal modulations (STM) are essential features of speech signals that make them intelligible. While their encoding has been widely investigated in neurophysiology, we still lack a full understanding of how STMs are processed at the behavioral level and how cochlear hearing loss impacts this processing. Here, we introduce a novel methodological framework based on psychophysical reverse correlation deployed in the modulation space to characterize the mechanisms underlying STM detection in noise. We derive perceptual filters for young normal-hearing and older hearing-impaired individuals performing a detection task of an elementary target STM (a given product of temporal and spectral modulations) embedded in other masking STMs. Analyzed with computational tools, our data show that both groups rely on a comparable linear (band-pass)-nonlinear processing cascade, which can be well accounted for by a temporal modulation filter bank model combined with cross-correlation against the target representation. Our results also suggest that the modulation mistuning observed for the hearing-impaired group results primarily from broader cochlear filters. Yet, we find idiosyncratic behaviors that cannot be captured by cochlear tuning alone, highlighting the need to consider variability originating from additional mechanisms. Overall, this integrated experimental-computational approach offers a principled way to assess suprathreshold processing distortions in each individual and could thus be used to further investigate interindividual differences in speech intelligibility.

Characterizing amplitude and frequency modulation cues in natural soundscapes: A pilot study on four habitats of a biosphere reserve.

Natural soundscapes correspond to the acoustical patterns produced by biological and geophysical sound sources at different spatial and temporal scales for a given habitat. This pilot study aims to characterize the temporal-modulation information available to humans when perceiving variations in soundscapes within and across natural habitats. This is addressed by processing soundscapes from a previous study [Krause, Gage, and Joo. (2011). Landscape Ecol. 26, 1247] via models of human auditory processing extracting modulation at the output of cochlear filters. The soundscapes represent combinations of elevation, animal, and vegetation diversity in four habitats of the biosphere reserve in the Sequoia National Park (Sierra Nevada, USA). Bayesian statistical analysis and support vector machine classifiers indicate that: (i) amplitude-modulation (AM) and frequency-modulation (FM) spectra distinguish the soundscapes associated with each habitat; and (ii) for each habitat, diurnal and seasonal variations are associated with salient changes in AM and FM cues at rates between about 1 and 100 Hz in the low (<0.5 kHz) and high (>1-3 kHz) audio-frequency range. Support vector machine classifications further indicate that soundscape variations can be classified accurately based on these perceptually inspired representations.

Noise-Sensitive But More Precise Subcortical Representations Coexist with Robust Cortical Encoding of Natural Vocalizations.

Humans and animals maintain accurate sound discrimination in the presence of loud sources of background noise. It is commonly assumed that this ability relies on the robustness of auditory cortex responses. However, only a few attempts have been made to characterize neural discrimination of communication sounds masked by noise at each stage of the auditory system and to quantify the noise effects on the neuronal discrimination in terms of alterations in amplitude modulations. Here, we measured neural discrimination between communication sounds masked by a vocalization-shaped stationary noise from multiunit responses recorded in the cochlear nucleus, inferior colliculus, auditory thalamus, and primary and secondary auditory cortex at several signal-to-noise ratios (SNRs) in anesthetized male or female guinea pigs. Masking noise decreased sound discrimination of neuronal populations in each auditory structure, but collicular and thalamic populations showed better performance than cortical populations at each SNR. In contrast, in each auditory structure, discrimination by neuronal populations was slightly decreased when tone-vocoded vocalizations were tested. These results shed new light on the specific contributions of subcortical structures to robust sound encoding, and suggest that the distortion of slow amplitude modulation cues conveyed by communication sounds is one of the factors constraining the neuronal discrimination in subcortical and cortical levels.SIGNIFICANCE STATEMENT Dissecting how auditory neurons discriminate communication sounds in noise is a major goal in auditory neuroscience. Robust sound coding in noise is often viewed as a specific property of cortical networks, although this remains to be demonstrated. Here, we tested the discrimination performance of neuronal populations at five levels of the auditory system in response to conspecific vocalizations masked by noise. In each acoustic condition, subcortical neurons better discriminated target vocalizations than cortical ones and in each structure, the reduction in discrimination performance was related to the reduction in slow amplitude modulation cues.

A two-path model of auditory modulation detection using temporal fine structure and envelope cues.

A model using temporal-envelope cues was previously developed to explain perceptual interference effects between amplitude modulation and frequency modulation (FM). As that model could not accurately predict FM sensitivity and the interference effects, temporal fine structure (TFS) cues were added to the model. Thus, following the initial stage of the model consisting of a linear filter bank simulating cochlear filtering, processing was split into an 'envelope path' based on envelope power cues and a 'TFS path' based on a measure of the distribution of time intervals between successive zero-crossings. This yielded independent detectability indices for envelope and TFS cues, which were optimally combined to produce a single decision statistic. Independent internal noises in the envelope and TFS paths were adjusted to match the data. Simulations indicate that TFS cues are required to account for FM data for young normal-hearing listeners and that TFS processing is impaired for both older normal-hearing and hearing-impaired listeners. The role of TFS was further assessed by relating the monaural FM sensitivity to measures of interaural phase difference, commonly assumed to rely on binaural TFS sensitivity. The model demonstrates that binaural TFS sensitivity is considerably lower than monaural TFS sensitivity. Similar to FM thresholds, interaural phase difference sensitivity declined with age and hearing loss, although higher degradations were observed in binaural temporal processing compared to monaural processing. Overall, this model provides a novel tool to explore the mechanisms involved in FM processing in the normal auditory system and the degradations in FM sensitivity with ageing and hearing loss.

Development of temporal auditory processing in childhood: Changes in efficiency rather than temporal-modulation selectivity.

The ability to detect amplitude modulation (AM) is essential to distinguish the spectro-temporal features of speech from those of a competing masker. Previous work shows that AM sensitivity improves until 10 years of age. This may relate to the development of sensory factors (tuning of AM filters, susceptibility to AM masking) or to changes in processing efficiency (reduction in internal noise, optimization of decision strategies). To disentangle these hypotheses, three groups of children (5-11 years) and one of young adults completed psychophysical tasks measuring thresholds for detecting sinusoidal AM (with a rate of 4, 8, or 32 Hz) applied to carriers whose inherent modulations exerted different amounts of AM masking. Results showed that between 5 and 11 years, AM detection thresholds improved and that susceptibility to AM masking slightly increased. However, the effects of AM rate and carrier were not associated with age, suggesting that sensory factors are mature by 5 years. Subsequent modelling indicated that reducing internal noise by a factor 10 accounted for the observed developmental trends. Finally, children's consonant identification thresholds in noise related to some extent to AM sensitivity. Increased efficiency in AM detection may support better use of temporal information in speech during childhood.

High-Frequency Sensorineural Hearing Loss Alters Cue-Weighting Strategies for Discriminating Stop Consonants in Noise.

There is increasing evidence that hearing-impaired (HI) individuals do not use the same listening strategies as normal-hearing (NH) individuals, even when wearing optimally fitted hearing aids. In this perspective, better characterization of individual perceptual strategies is an important step toward designing more effective speech-processing algorithms. Here, we describe two complementary approaches for (a) revealing the acoustic cues used by a participant in a /d/-/g/ categorization task in noise and (b) measuring the relative contributions of these cues to decision. These two approaches involve natural speech recordings altered by the addition of a "bump noise." The bumps were narrowband bursts of noise localized on the spectrotemporal locations of the acoustic cues, allowing the experimenter to manipulate the consonant percept. The cue-weighting strategies were estimated for three groups of participants: 17 NH listeners, 18 HI listeners with high-frequency loss, and 15 HI listeners with flat loss. HI participants were provided with individual frequency-dependent amplification to compensate for their hearing loss. Although all listeners relied more heavily on the high-frequency cue than on the low-frequency cue, an important variability was observed in the individual weights, mostly explained by differences in internal noise. Individuals with high-frequency loss relied slightly less heavily on the high-frequency cue relative to the low-frequency cue, compared with NH individuals, suggesting a possible influence of supra-threshold deficits on cue-weighting strategies. Altogether, these results suggest a need for individually tailored speech-in-noise processing in hearing aids, if more effective speech discriminability in noise is to be achieved.

Accounting for masking of frequency modulation by amplitude modulation with the modulation filter-bank concept.

Frequency modulation (FM) is assumed to be detected through amplitude modulation (AM) created by cochlear filtering for modulation rates above 10 Hz and carrier frequencies (fc) above 4 kHz. If this is the case, a model of modulation perception based on the concept of AM filters should predict masking effects between AM and FM. To test this, masking effects of sinusoidal AM on sinusoidal FM detection thresholds were assessed on normal-hearing listeners as a function of FM rate, fc, duration, AM rate, AM depth, and phase difference between FM and AM. The data were compared to predictions of a computational model implementing an AM filter-bank. Consistent with model predictions, AM masked FM with some AM-masking-AM features (broad tuning and effect of AM-masker depth). Similar masking was predicted and observed at fc = 0.5 and 5 kHz for a 2 Hz AM masker, inconsistent with the notion that additional (e.g., temporal fine-structure) cues drive slow-rate FM detection at low fc. However, masking was lower than predicted and, unlike model predictions, did not show beating or phase effects. Broadly, the modulation filter-bank concept successfully explained some AM-masking-FM effects, but could not give a complete account of both AM and FM detection.

Amplitude modulation detection and modulation masking in school-age children and adults.

Two experiments were performed to better understand on- and off-frequency modulation masking in normal-hearing school-age children and adults. Experiment 1 estimated thresholds for detecting 16-, 64- or 256-Hz sinusoidal amplitude modulation (AM) imposed on a 4300-Hz pure tone. Thresholds tended to improve with age, with larger developmental effects for 64- and 256-Hz AM than 16-Hz AM. Detection of 16-Hz AM was also measured with a 1000-Hz off-frequency masker tone carrying 16-Hz AM. Off-frequency modulation masking was larger for younger than older children and adults when the masker was gated with the target, but not when the masker was continuous. Experiment 2 measured detection of 16- or 64-Hz sinusoidal AM carried on a bandpass noise with and without additional on-frequency masker AM. Children and adults demonstrated modulation masking with similar tuning to modulation rate. Rate-dependent age effects for AM detection on a pure-tone carrier are consistent with maturation of temporal resolution, an effect that may be obscured by modulation masking for noise carriers. Children were more susceptible than adults to off-frequency modulation masking for gated stimuli, consistent with maturation in the ability to listen selectively in frequency, but the children were not more susceptible to on-frequency modulation masking than adults.

Sensorineural hearing loss impairs sensitivity but spares temporal integration for detection of frequency modulation.

The effect of the number of modulation cycles (N) on frequency-modulation (FM) detection thresholds (FMDTs) was measured with and without interfering amplitude modulation (AM) for hearing-impaired (HI) listeners, using a 500-Hz sinusoidal carrier and FM rates of 2 and 20 Hz. The data were compared with FMDTs for normal-hearing (NH) listeners and AM detection thresholds (AMDTs) for NH and HI listeners [Wallaert, Moore, and Lorenzi (2016). J. Acoust. Soc. 139, 3088-3096; Wallaert, Moore, Ewert, and Lorenzi (2017). J. Acoust. Soc. 141, 971-980]. FMDTs were higher for HI than for NH listeners, but the effect of increasing N was similar across groups. In contrast, AMDTs were lower and the effect of increasing N was greater for HI listeners than for NH listeners. A model of temporal-envelope processing based on a modulation filter-bank and a template-matching decision strategy accounted better for the FMDTs at 20 Hz than at 2 Hz for young NH listeners and predicted greater temporal integration of FM than observed for all groups. These results suggest that different mechanisms underlie AM and FM detection at low rates and that hearing loss impairs FM-detection mechanisms, but preserves the memory and decision processes responsible for temporal integration of FM.

Speech recognition for school-age children and adults tested in multi-tone vs multi-noise-band maskers.

The present study set out to test whether greater susceptibility to modulation masking could be responsible for immature recognition of speech in noise for school-age children. Listeners were normal-hearing four- to ten-year-olds and adults. Target sentences were filtered into 28 adjacent narrow bands (100-7800 Hz), and the masker was either spectrally matched noise bands or tones centered on each of the speech bands. In experiment 1, odd- and even-numbered bands of target-plus-masker were presented to opposite ears. Performance improved with child age in all conditions, but this improvement was larger for the multi-tone than the multi-noise-band masker. This outcome is contrary to the expectation that children are more susceptible than adults to masking produced by inherent modulation of the noise masker. In experiment 2, odd-numbered bands were presented to both ears, with the masker diotic and the target either diotic or binaurally out of phase. The binaural difference cue was particularly beneficial for young children tested in the multi-tone masker, suggesting that development of auditory stream segregation may play a role in the child-adult difference for this condition. Overall, results provide no evidence of greater susceptibility to modulation masking in children than adults.

Dual Coding of Frequency Modulation in the Ventral Cochlear Nucleus.

Frequency modulation (FM) is a common acoustic feature of natural sounds and is known to play a role in robust sound source recognition. Auditory neurons show precise stimulus-synchronized discharge patterns that may be used for the representation of low-rate FM. However, it remains unclear whether this representation is based on synchronization to slow temporal envelope (ENV) cues resulting from cochlear filtering or phase locking to faster temporal fine structure (TFS) cues. To investigate the plausibility of those encoding schemes, single units of the ventral cochlear nucleus of guinea pigs of either sex were recorded in response to sine FM tones centered at the unit's best frequency (BF). The results show that, in contrast to high-BF units, for modulation depths within the receptive field, low-BF units (<4 kHz) demonstrate good phase locking to TFS. For modulation depths extending beyond the receptive field, the discharge patterns follow the ENV and fluctuate at the modulation rate. The receptive field proved to be a good predictor of the ENV responses for most primary-like and chopper units. The current in vivo data also reveal a high level of diversity in responses across unit types. TFS cues are mainly conveyed by low-frequency and primary-like units and ENV cues by chopper and onset units. The diversity of responses exhibited by cochlear nucleus neurons provides a neural basis for a dual-coding scheme of FM in the brainstem based on both ENV and TFS cues.SIGNIFICANCE STATEMENT Natural sounds, including speech, convey informative temporal modulations in frequency. Understanding how the auditory system represents those frequency modulations (FM) has important implications as robust sound source recognition depends crucially on the reception of low-rate FM cues. Here, we recorded 115 single-unit responses from the ventral cochlear nucleus in response to FM and provide the first physiological evidence of a dual-coding mechanism of FM via synchronization to temporal envelope cues and phase locking to temporal fine structure cues. We also demonstrate a diversity of neural responses with different coding specializations. These results support the dual-coding scheme proposed by psychophysicists to account for FM sensitivity in humans and provide new insights on how this might be implemented in the early stages of the auditory pathway.