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

Auditory streaming and rhythmic masking release in Cope's gray treefrog

Auditory streaming involves perceptually assigning overlapping sound sequences to their respective sources. Although critical for acoustic communication, few studies have investigated the role of auditory streaming in nonhuman animals. This study used the rhythmic masking release paradigm to investigate auditory streaming in Cope's gray treefrog (Hyla chrysoscelis). In this paradigm, the temporal rhythm of a Target sequence is masked in the presence of a Distractor sequence. A release from masking can be induced by adding a Captor sequence that perceptually "captures" the Distractor into an auditory stream segregated from the Target. Here, the Target was a sequence of repeated pulses mimicking the rhythm of the species' advertisement call. Gravid females exhibited robust phonotaxis to the Target alone, but responses declined significantly when Target pulses were interleaved with those of a Distractor at the same frequency, indicating the Target's attractive temporal rhythm was masked. However, addition of a remote-frequency Captor resulted in a significant increase in responses to the Target, suggesting the Target could be segregated from a separate stream consisting of integrated Distractor and Captor sequences. This result sheds light on how auditory streaming may facilitate acoustic communication in frogs and other animals.

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.

Neuroethology of sound localization in anurans

Albert Feng pioneered the study of neuroethology of sound localization in anurans by combining behavioral experiments on phonotaxis with detailed investigations of neural processing of sound direction from the periphery to the central nervous system. The main advantage of these studies is that many species of female frogs readily perform phonotaxis towards loudspeakers emitting the species-specific advertisement call. Behavioral studies using synthetic calls can identify which parameters are important for phonotaxis and also quantify localization accuracy. Feng was the first to investigate binaural processing using single-unit recordings in the first two auditory nuclei in the central auditory pathway and later investigated the directional properties of auditory nerve fibers with free-field stimulation. These studies showed not only that the frog ear is inherently directional by virtue of acoustical coupling or crosstalk between the two eardrums, but also confirmed that there are extratympanic pathways that affect directionality in the low-frequency region of the frog's hearing range. Feng's recordings in the midbrain also showed that directional information is enhanced by cross-midline inhibition. An important contribution toward the end of his career involved his participation in neuroethological research with a team of scientists working with frogs that produce ultrasonic calls.

Noise affects mate choice based on visual information via cross-sensory interference

Animal communication is often hampered by noise interference. Noise masking has primarily been studied in terms of its unimodal effect on sound information provision and use, while little is known about its cross-modal effect and how animals weigh unimodal and multimodal courtship cues in noisy environments. Here, we examined the cross-modal effects of background noise on female visual perception of mate choice and female preference for multimodal displays (sound + vocal sac) in a species of treefrog. We tested female mate choices using audio/video playbacks in the presence and absence of noise (white noise band-filtered to match or mismatch female sensitive hearing range, heterospecific chorus). Surprisingly, multimodal displays do not improve receiver performance in noise. The heterospecific chorus and white noise band-filtered to match female sensitive hearing ranges, significantly reduced female responses to the attractive visual stimuli in addition to directly impairing auditory information use. Meanwhile, the cross-modal impacts of background noise are influenced to some extent by whether the noise band matches female sensitive hearing range and the difficulty of distinguishing tasks. Our results add to the evidence for cross-modal effects of noise and are the first to demonstrate that background noise can disrupt female responses to visual information related to mate choice, which may reduce the communication efficiency of audiovisual signals in noisy environments and impose fitness consequences. This study has key ecological and evolutionary implications because it illustrates how noise influences mate choice in wildlife via cross-sensory interference, which is crucial in revealing the function and evolution of multimodal signals in noisy environments as well as informing evidence-based conservation strategies for forecasting and mitigating the multimodal impacts of noise interference on wildlife.

Spatial release from masking in crocodilians

Ambient noise is a major constraint on acoustic communication in both animals and humans. One mechanism to overcome this problem is Spatial Release from Masking (SRM), the ability to distinguish a target sound signal from masking noise when both sources are spatially separated. SRM is well described in humans but has been poorly explored in animals. Although laboratory tests with trained individuals have suggested that SRM may be a widespread ability in vertebrates, it may play a limited role in natural environments. Here we combine field experiments with investigations in captivity to test whether crocodilians experience SRM. We show that 2 species of crocodilians are able to use SRM in their natural habitat and that it quickly becomes effective for small angles between the target signal source and the noise source, becoming maximal when the angle exceeds 15^∘. Crocodiles can therefore take advantage of SRM to improve sound scene analysis and the detection of biologically relevant signals.

The ability to separate target sound signals from masking noise is identified in wild and captive crocodilian species.

Customizable Recorder of Animal Kinesis (CRoAK): A multi-axis instrumented enclosure for measuring animal movements

Accurately quantifying animal activity and movements is of fundamental importance in a broad range of disciplines, from biomedical research to behavioral ecology. In many instances, it is desirable to measure natural movements in controlled sensory environments in which the animals are not physically or chemically restrained, but their movements are nevertheless constrained to occur within a fixed volume. Here, we describe a novel device to quantify the movements of small animals in response to sensory stimulation. The device consists of an Arduino controlled inertial measurement unit that senses angular velocity (along three axes) of a suspended mesh enclosure that temporarily houses the animal subject. We validated the device by measuring the phonotaxis behavior of gravid female frogs in response to acoustic broadcasts of male mating calls. The system, as designed, proved effective at measuring natural movements made in response to acoustic stimulation.

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.