Comodulation masking release determined in the mouse (Mus musculus) using a flanking-band paradigm

Behind the mask(ing): how frogs cope with noise

Albert Feng was a pioneer in the field of auditory neuroethology who used frogs to investigate the neural basis of spectral and temporal processing and directional hearing. Among his many contributions was connecting neural mechanisms for sound pattern recognition and localization to the problems of auditory masking that frogs encounter when communicating in noisy, real-world environments. Feng's neurophysiological studies of auditory processing foreshadowed and inspired subsequent behavioral investigations of auditory masking in frogs. For frogs, vocal communication frequently occurs in breeding choruses, where males form dense aggregations and produce loud species-specific advertisement calls to attract potential mates and repel competitive rivals. In this review, we aim to highlight how Feng's research advanced our understanding of how frogs cope with noise. We structure our narrative around three themes woven throughout Feng's research-spectral, temporal, and directional processing-to illustrate how frogs can mitigate problems of auditory masking by exploiting frequency separation between signals and noise, temporal fluctuations in noise amplitude, and spatial separation between signals and noise. We conclude by proposing future research that would build on Feng's considerable legacy to advance our understanding of hearing and sound communication in frogs and other vertebrates.

Comodulation masking release with random variations of flanking-band center frequencies

Comodulation masking release (CMR) is an effect that is associated with auditory sensitivity to coherent amplitude modulations in different frequency regions. The present study investigated if this comodulation is detected by a direct comparison of auditory filter outputs, or if common masker fluctuations are first extracted by a broadly tuned stage that integrates information across a large spectral range. To this end, a modified flanking-band experiment with a narrowband noise masker at the signal frequency (on-frequency masker), and two flanking bands (FBs), one centered below and one above the signal frequency, were used. The center frequencies (CFs) of FBs changed whenever the masker had a local envelope minimum. The center frequencies were randomly chosen from a range of frequencies around the average CF of each FB. A CMR was measured even for large CF variations of FBs, where the envelopes at the off-frequency auditory filters were no longer the same as the masker envelope at the on-frequency auditory filter. This supports the hypothesis of a broadly tuned stage to determine masker comodulation. For two experimental settings, CMR deteriorated for very large variations of CFs of FBs, suggesting a spectral weighting of the off-frequency auditory filters in this broadly tuned stage.

Wide sensory filters underlie performance in memory-based discrimination and generalization

The way animals respond to a stimulus depends largely on an internal comparison between the current sensation and the memory of previous stimuli and outcomes. We know little about the accuracy with which the physical properties of the stimuli influence this type of memory-based discriminative decisions. Research has focused largely on discriminations between stimuli presented in quick succession, where animals can make relative inferences (same or different; higher or lower) from trial to trial. In the current study we used a memory-based task to explore how the stimulus’ physical properties, in this case tone frequency, affect auditory discrimination and generalization in mice. Mice performed ad libitum while living in groups in their home quarters. We found that the frequency distance between safe and conditioned sounds had a constraining effect on discrimination. As the safe-to-conditioned distance decreased across groups, performance deteriorated rapidly, even for frequency differences significantly larger than reported discrimination thresholds. Generalization width was influenced both by the physical distance and the previous experience of the mice, and was not accompanied by a decrease in sensory acuity. In conclusion, memory-based discriminations along a single stimulus dimension are inherently hard, reflecting a high overlap between the memory traces of the relevant stimuli. Memory-based discriminations rely therefore on wide sensory filters.

Masking release in temporally fluctuating noise depends on comodulation and overall level in Cope's gray treefrog

Many animals communicate acoustically in large social aggregations. Among the best studied are frogs, in which males form large breeding choruses where they produce loud vocalizations to attract mates. Although chorus noise poses significant challenges to communication, it also possesses features, such as comodulation in amplitude fluctuations, that listeners may be evolutionarily adapted to exploit in order to achieve release from masking. This study investigated the extent to which the benefits of comodulation masking release (CMR) depend on overall noise level in Cope's gray treefrog (Hyla chrysoscelis). Masked signal recognition thresholds were measured in response to vocalizations in the presence of chorus-shaped noise presented at two levels. The noises were either unmodulated or modulated with an envelope that was correlated (comodulated) or uncorrelated (deviant) across the frequency spectrum. Signal-to-noise ratios (SNRs) were lower at the higher noise level, and this effect was driven by relatively lower SNRs in modulated conditions, especially the comodulated condition. These results, which confirm that frogs benefit from CMR in a level-dependent manner, are discussed in relation to previous studies of CMR in humans and animals and in light of implications of the unique amphibian inner ear for considerations of within-channel versus across-channel mechanisms.

Detection of Tones Masked by Fluctuating Noise in Rat Auditory Cortex

Sounds in natural settings always appear over a noisy background. The masked threshold of a pure tone in white noise (the lowest sound level at which the tone can be detected in the presence of masking noise) is largely determined by energy masking in the peripheral auditory system: when the signal-to-noise ratio within a frequency band centered at the target tone frequency is large enough, the tone can be detected. However, when additional information is supplied to the auditory system, for example in the presence of slow and coherent modulations of a broadband masker (often found in natural sounds), masked thresholds can be reduced substantially below the values expected from pure energy masking. Here, we used intracellular recordings in vivo in rat auditory cortex in order to study neuronal responses to pure tones masked by broadband maskers and amplitude-modulated broadband maskers. When tones were embedded in amplitude-modulated noise, detection thresholds were substantially lower than when embedded in unmodulated noise. The main cue for tone detection in modulated noise consisted of the suppression of the locking of the neuronal responses to the amplitude modulation of the noise by low-level tones.

Comodulation Enhances Signal Detection via Priming of Auditory Cortical Circuits

Acoustic environments are composed of complex overlapping sounds that the auditory system is required to segregate into discrete perceptual objects. The functions of distinct auditory processing stations in this challenging task are poorly understood. Here we show a direct role for mouse auditory cortex in detection and segregation of acoustic information. We measured the sensitivity of auditory cortical neurons to brief tones embedded in masking noise. By altering spectrotemporal characteristics of the masker, we reveal that sensitivity to pure tone stimuli is strongly enhanced in coherently modulated broadband noise, corresponding to the psychoacoustic phenomenon comodulation masking release. Improvements in detection were largest following priming periods of noise alone, indicating that cortical segregation is enhanced over time. Transient opsin-mediated silencing of auditory cortex during the priming period almost completely abolished these improvements, suggesting that cortical processing may play a direct and significant role in detection of quiet sounds in noisy environments.

SIGNIFICANCE STATEMENT Auditory systems are adept at detecting and segregating competing sound sources, but there is little direct evidence of how this process occurs in the mammalian auditory pathway. We demonstrate that coherent broadband noise enhances signal representation in auditory cortex, and that prolonged exposure to noise is necessary to produce this enhancement. Using optogenetic perturbation to selectively silence auditory cortex during early noise processing, we show that cortical processing plays a crucial role in the segregation of competing sounds.

Comodulation masking release in the inferior colliculus by combined signal enhancement and masker reduction

Auditory signals that contain coherent level fluctuations of a masker in different frequency regions enhance the detectability of an embedded sinusoidal target signal, an effect commonly known as comodulation masking release (CMR). Neural correlates have been proposed at different stages of the auditory system. While later stages seem to suppress the response to the masker, earlier stages are more likely to enhance their response to the signal when the masker is comodulated. Using a flanking band masking paradigm, the present study investigates how CMR is represented at the level of the inferior colliculus of the Mongolian gerbil. The responses to a target signal at various sound pressure levels in three different masking conditions were compared. In one condition the masker was a 10-Hz amplitude modulated sinusoid centered at the signal frequency while in the other two conditions six off-frequency carriers (flanking bands) were added. From 81 units 26 showed a change that enhanced the detectability of the signal if the temporal modulation of the added flanking bands was identical to that of the masker at the signal frequency compared to the other two masking conditions. This study shows that the response characteristics of these neurons represent an intermediate stage between the representation in the cochlear nucleus and the auditory cortex. This means that the response is increased during the signal intervals but is also decreased for the following masker portions.

Communication masking in marine mammals: A review and research strategy

Underwater noise, whether of natural or anthropogenic origin, has the ability to interfere with the way in which marine mammals receive acoustic signals (i.e., for communication, social interaction, foraging, navigation, etc.). This phenomenon, termed auditory masking, has been well studied in humans and terrestrial vertebrates (in particular birds), but less so in marine mammals. Anthropogenic underwater noise seems to be increasing in parts of the world's oceans and concerns about associated bioacoustic effects, including masking, are growing. In this article, we review our understanding of masking in marine mammals, summarise data on marine mammal hearing as they relate to masking (including audiograms, critical ratios, critical bandwidths, and auditory integration times), discuss masking release processes of receivers (including comodulation masking release and spatial release from masking) and anti-masking strategies of signalers (e.g. Lombard effect), and set a research framework for improved assessment of potential masking in marine mammals.

Auditory sensitivity of seals and sea lions in complex listening scenarios

Standard audiometric data, such as audiograms and critical ratios, are often used to inform marine mammal noise-exposure criteria. However, these measurements are obtained using simple, artificial stimuli-i.e., pure tones and flat-spectrum noise-while natural sounds typically have more complex structure. In this study, detection thresholds for complex signals were measured in (I) quiet and (II) masked conditions for one California sea lion (Zalophus californianus) and one harbor seal (Phoca vitulina). In Experiment I, detection thresholds in quiet conditions were obtained for complex signals designed to isolate three common features of natural sounds: Frequency modulation, amplitude modulation, and harmonic structure. In Experiment II, detection thresholds were obtained for the same complex signals embedded in two types of masking noise: Synthetic flat-spectrum noise and recorded shipping noise. To evaluate how accurately standard hearing data predict detection of complex sounds, the results of Experiments I and II were compared to predictions based on subject audiograms and critical ratios combined with a basic hearing model. Both subjects exhibited greater-than-predicted sensitivity to harmonic signals in quiet and masked conditions, as well as to frequency-modulated signals in masked conditions. These differences indicate that the complex features of naturally occurring sounds enhance detectability relative to simple stimuli.

Frequency tuning in the behaving mouse: different bandwidths for discrimination and generalization

When faced with sensory stimuli, an organism may be required to detect very small differences in a physical parameter (discrimination), while in other situations it may have to generalize over many possible values of the same physical parameter. This decision may be based both on learned information and on sensory aspects of perception. In the present study we describe frequency processing in the behaving mouse using both discrimination and generalization as two key aspects of behaviour. We used a novel naturalistic behavioural apparatus designed for mice, the Audiobox, and paradigm contingencies that were identical for both auditory discrimination and generalization, the latter measured using latent inhibition. Mice learned to discriminate between frequencies that were an octave apart in a single trial. They showed significant discrimination between tone frequencies that were as close as 4–7%, and had d' of about 1 for ΔF of around 10%. In contrast, pre-exposure frequencies that were half an octave or less below the conditioned tone elicited latent inhibition, showing a generalization bandwidth of at least half an octave. Thus, in the same apparatus and using the same general memory paradigm, mice showed generalization gradients that were considerably wider than their discrimination threshold, indicating that environmental requirements and previous experience can determine whether the same two frequencies will be considered same or different. Remarkably, generalization gradients paralleled the typical bandwidths established in the auditory periphery and midbrain, suggesting that frequencies may be considered similar when falling within the same critical band.

Target-specific IPSC kinetics promote temporal processing in auditory parallel pathways

The acoustic environment contains biologically relevant information on timescales from microseconds to tens of seconds. The auditory brainstem nuclei process this temporal information through parallel pathways that originate in the cochlear nucleus from different classes of cells. Although the roles of ion channels and excitatory synapses in temporal processing have been well studied, the contribution of inhibition is less well understood. Here, we show in CBA/CaJ mice that the two major projection neurons of the ventral cochlear nucleus, the bushy and T-stellate cells, receive glycinergic inhibition with different synaptic conductance time courses. Bushy cells, which provide precisely timed spike trains used in sound localization and pitch identification, receive slow inhibitory inputs. In contrast, T-stellate cells, which encode slower envelope information, receive inhibition that is eightfold faster. Both types of inhibition improved the precision of spike timing but engage different cellular mechanisms and operate on different timescales. Computer models reveal that slow IPSCs in bushy cells can improve spike timing on the scale of tens of microseconds. Although fast and slow IPSCs in T-stellate cells improve spike timing on the scale of milliseconds, only fast IPSCs can enhance the detection of narrowband acoustic signals in a complex background. Our results suggest that target-specific IPSC kinetics are critical for the segregated parallel processing of temporal information from the sensory environment.

Masking release for sweeping masker components with correlated envelopes

To separate sounds from different sound sources, common properties of natural sounds are used by the auditory system, such as coherent temporal envelope fluctuations and correlated changes of frequency in different frequency regions. The present study investigates how the auditory system processes a combination of these cues using a generalized comodulation masking release (CMR) paradigm. CMR is the effect of a better signal detectability in the presence of comodulated maskers than in the presence of maskers with uncorrelated envelope fluctuations across frequencies. Using a flanking-band paradigm, the results of the first experiment of the present study show that CMR is still observed for the masker and the signal coherently sweeping up or down in frequency over time, up to a sweep rate of six octaves per second. Motivated by the successful modeling of CMR using filters sensitive to temporal modulations and recent physiological evidence of spectro-temporal modulation filters, the second experiment investigates whether CMR is also observed for spectro-temporal masker modulations generated using time-shifted versions of the masker envelope for each component. The thresholds increase as soon as the temporally coherent masker modulation is changed to a spectro-temporal masker modulation, indicating that spectro-temporal modulation filters are presumably not required in CMR models.

Effects of sequential streaming on auditory masking using psychoacoustics and auditory evoked potentials

The present study was aimed at investigating the relationship between the mismatch negativity (MMN) and psychoacoustical effects of sequential streaming on comodulation masking release (CMR). The influence of sequential streaming on CMR was investigated using a psychoacoustical alternative forced-choice procedure and electroencephalography (EEG) for the same group of subjects. The psychoacoustical data showed, that adding precursors comprising of only off-signal-frequency maskers abolished the CMR. Complementary EEG data showed an MMN irrespective of the masker envelope correlation across frequency when only the off-signal-frequency masker components were present. The addition of such precursors promotes a separation of the on- and off-frequency masker components into distinct auditory objects preventing the auditory system from using comodulation as an additional cue. A frequency-specific adaptation changing the representation of the flanking bands in the streaming conditions may also contribute to the reduction of CMR in the stream conditions, however, it is unlikely that adaptation is the primary reason for the streaming effect. A neurophysiological correlate of sequential streaming was found in EEG data using MMN, but the magnitude of the MMN was not correlated with the audibility of the signal in CMR experiments. Dipole source analysis indicated different cortical regions involved in processing auditory streaming and modulation detection. In particular, neural sources for processing auditory streaming include cortical regions involved in decision-making.