Coding of concurrent vocal signals by the auditory midbrain: effects of stimulus level and depth of modulation

Neural representations of concurrent sounds with overlapping spectra in rat inferior colliculus: Comparisons between temporal-fine structure and envelope

Perceptual segregation of multiple sounds, which overlap in both time and spectra, into individual auditory streams is critical for hearing in natural environments. Some cues such as interaural time disparities (ITDs) play an important role in the segregation, especially when sounds are separated in space. In this study, we investigated the neural representation of two uncorrelated narrowband noises that shared the identical spectrum in the rat inferior colliculus (IC) using frequency-following-response (FFR) recordings, when the ITD for each noise stimulus was manipulated. The results of this study showed that recorded FFRs exhibited two distinctive components: the fast-varying temporal fine structure (TFS) component (FFR_TFS) and the slow-varying envelope component (FFR_ENV). When a single narrowband noise was presented alone, the FFR_TFS, but not the FFR_ENV, was sensitive to ITDs. When two narrowband noises were presented simultaneously, the FFR_TFS took advantage of the ITD disparity that was associated with perceived spatial separation between the two concurrent sounds, and displayed a better linear synchronization to the sound with an ipsilateral-leading ITD. However, no effects of ITDs were found on the FFR_ENV. These results suggest that the FFR_TFS and FFR_ENV represent two distinct types of signal processing in the auditory brainstem and contribute differentially to sound segregation based on spatial cues: the FFR_TFS is more critical to spatial release from masking.

Exposure to advertisement calls of reproductive competitors activates vocal-acoustic and catecholaminergic neurons in the plainfin midshipman fish, Porichthys notatus

While the neural circuitry and physiology of the auditory system is well studied among vertebrates, far less is known about how the auditory system interacts with other neural substrates to mediate behavioral responses to social acoustic signals. One species that has been the subject of intensive neuroethological investigation with regard to the production and perception of social acoustic signals is the plainfin midshipman fish, Porichthys notatus, in part because acoustic communication is essential to their reproductive behavior. Nesting male midshipman vocally court females by producing a long duration advertisement call. Females localize males by their advertisement call, spawn and deposit all their eggs in their mate’s nest. As multiple courting males establish nests in close proximity to one another, the perception of another male’s call may modulate individual calling behavior in competition for females. We tested the hypothesis that nesting males exposed to advertisement calls of other males would show elevated neural activity in auditory and vocal-acoustic brain centers as well as differential activation of catecholaminergic neurons compared to males exposed only to ambient noise. Experimental brains were then double labeled by immunofluorescence (-ir) for tyrosine hydroxylase (TH), an enzyme necessary for catecholamine synthesis, and cFos, an immediate-early gene product used as a marker for neural activation. Males exposed to other advertisement calls showed a significantly greater percentage of TH-ir cells colocalized with cFos-ir in the noradrenergic locus coeruleus and the dopaminergic periventricular posterior tuberculum, as well as increased numbers of cFos-ir neurons in several levels of the auditory and vocal-acoustic pathway. Increased activation of catecholaminergic neurons may serve to coordinate appropriate behavioral responses to male competitors. Additionally, these results implicate a role for specific catecholaminergic neuronal groups in auditory-driven social behavior in fishes, consistent with a conserved function in social acoustic behavior across vertebrates.

Representation of complex vocalizations in the Lusitanian toadfish auditory system: evidence of fine temporal, frequency and amplitude discrimination

Many fishes rely on their auditory skills to interpret crucial information about predators and prey, and to communicate intraspecifically. Few studies, however, have examined how complex natural sounds are perceived in fishes. We investigated the representation of conspecific mating and agonistic calls in the auditory system of the Lusitanian toadfish Halobatrachus didactylus, and analysed auditory responses to heterospecific signals from ecologically relevant species: a sympatric vocal fish (meagre Argyrosomus regius) and a potential predator (dolphin Tursiops truncatus). Using auditory evoked potential (AEP) recordings, we showed that both sexes can resolve fine features of conspecific calls. The toadfish auditory system was most sensitive to frequencies well represented in the conspecific vocalizations (namely the mating boatwhistle), and revealed a fine representation of duration and pulsed structure of agonistic and mating calls. Stimuli and corresponding AEP amplitudes were highly correlated, indicating an accurate encoding of amplitude modulation. Moreover, Lusitanian toadfish were able to detect T. truncatus foraging sounds and A. regius calls, although at higher amplitudes. We provide strong evidence that the auditory system of a vocal fish, lacking accessory hearing structures, is capable of resolving fine features of complex vocalizations that are probably important for intraspecific communication and other relevant stimuli from the auditory scene.

Encoding properties of auditory neurons in the brain of a soniferous damselfish: response to simple tones and complex conspecific signals

The fish auditory system encodes important acoustic stimuli used in social communication, but few studies have examined response properties of central auditory neurons to natural signals. We determined the features and responses of single hindbrain and midbrain auditory neurons to tone bursts and playbacks of conspecific sounds in the soniferous damselfish, Abudefduf abdominalis. Most auditory neurons were either silent or had slow irregular resting discharge rates <20 spikes s(-1). Average best frequency for neurons to tone stimuli was approximately 130 Hz but ranged from 80 to 400 Hz with strong phase-locking. This low-frequency sensitivity matches the frequency band of natural sounds. Auditory neurons were also modulated by playbacks of conspecific sounds with thresholds similar to 100 Hz tones, but these thresholds were lower than that of tones at other test frequencies. Thresholds of neurons to natural sounds were lower in the midbrain than the hindbrain. This is the first study to compare response properties of auditory neurons to both simple tones and complex stimuli in the brain of a recently derived soniferous perciform that lacks accessory auditory structures. These data demonstrate that the auditory fish brain is most sensitive to the frequency and temporal components of natural pulsed sounds that provide important signals for conspecific communication.

Novel vocal repertoire and paired swimbladders of the three-spined toadfish, Batrachomoeus trispinosus: insights into the diversity of the Batrachoididae

Toadfishes (Teleostei: Batrachoididae) are one of the best-studied groups for understanding vocal communication in fishes. However, sounds have only been recorded from a low proportion of taxa within the family. Here, we used quantitative bioacoustic, morphological and phylogenetic methods to characterize vocal behavior and mechanisms in the three-spined toadfish, Batrachomoeus trispinosus. B. trispinosus produced two types of sound: long-duration 'hoots' and short-duration 'grunts' that were multiharmonic, amplitude and frequency modulated, with a dominant frequency below 1 kHz. Grunts and hoots formed four major classes of calls. Hoots were typically produced in succession as trains, while grunts occurred either singly or as grunt trains. Aside from hoot trains, grunts and grunt trains, a fourth class of calls consisted of single grunts with acoustic beats, apparently not previously reported for individuals from any teleost taxon. Beats typically had a predominant frequency around 2 kHz with a beat frequency around 300 Hz. Vocalizations also exhibited diel and lunar periodicities. Spectrographic cross-correlation and principal coordinates analysis of hoots from five other toadfish species revealed that B. trispinosus hoots were distinct. Unlike any other reported fish, B. trispinosus had a bilaterally divided swimbladder, forming two separate swimbladders. Phylogenetic analysis suggested B. trispinosus was a relatively basal batrachoidid, and the swimbladder and acoustic beats were independently derived. The swimbladder in B. trispinosus demonstrates that toadfishes have undergone a diversification of peripheral sonic mechanisms, which may be responsible for the concomitant innovations in vocal communication, namely the individual production of acoustic beats as reported in some tetrapods.

Neural mechanisms and behaviors for acoustic communication in teleost fish

Sound communication is not unique to humans but rather is a trait shared with most non-mammalian vertebrates. A practical way to address questions of vocal signal encoding has been to identify mechanisms in non-mammalian model systems that use acoustic communication signals in their social behavior. Teleost fishes, the largest group of living vertebrates, include both vocal and non-vocal species that exploit a wide range of acoustic niches. Here, we focus on those vocal species where combined behavioral and neurobiological studies have recently begun to elucidate a suite of adaptations for both the production and the perception of acoustic signals essential to their reproductive success and survival. Studies of these model systems show that teleost fish have the vocal-acoustic behaviors and neural systems both necessary and sufficient to solve acoustic problems common to all vertebrates. In particular, behavioral studies demonstrate that temporal features within a call, including pulse duration, rate and number, can all be important to a call's communicative value. Neurobiological studies have begun to show how these features are produced by a vocal motor system extending from forebrain to hindbrain levels and are encoded by peripheral and central auditory neurons. The abundance and variety of vocal fish present unique opportunities for parallel investigations of neural encoding, perception, and communication across a diversity of natural, acoustic habitats. As such, investigations in teleosts contribute to our delineating the evolution of the vocal and auditory systems of both non-mammalian and mammalian species, including humans.

Frequency response properties of lateral line superficial neuromasts in a vocal fish, with evidence for acoustic sensitivity

The mechanosensory lateral line of fish is a hair cell based sensory system that detects water motion using canal and superficial neuromasts. The trunk lateral line of the plainfin midshipman fish, Porichthys notatus, only has superficial neuromasts. The posterior lateral line nerve (PLLn) therefore innervates trunk superficial neuromasts exclusively and provides the opportunity to investigate the physiological responses of these receptors without the confounding influence of canal organs. We recorded single-unit activity from PLLn primary afferents in response to a vibrating sphere stimulus calibrated to produce an equal velocity across frequencies. Threshold tuning, isovelocity, and input/output curves were constructed using spike rate and vector strength, a measure of phase locking of spike times to the stimulus waveform. All units responded maximally to frequencies of 20-50 Hz. Units were classified as low-pass, band-pass, broadly tuned, or complex based on the shapes of tuning and isovelocity curves between 20 and 100 Hz. A 100 Hz stimulus caused an increase in spike rate in almost 50%, and significant synchronization in >80%, of all units. Midshipman vocalizations contain significant energy at and below 100 Hz, so these results demonstrate that the midshipman peripheral lateral line system can encode these acoustic signals. These results provide the first direct demonstration that units innervating superficial neuromasts in a teleost fish have heterogeneous frequency response properties, including an upper range of sensitivity that overlaps spectral peaks of behaviorally relevant acoustic stimuli.

Effects of temporal envelope modulation on acoustic signal recognition in a vocal fish, the plainfin midshipman

Amplitude modulation is an important parameter defining vertebrate acoustic communication signals. Nesting male plainfin midshipman fish, Porichthys notatus, emit simple, long duration hums in which modulation is strikingly absent. Envelope modulation is, however, introduced when the hums of adjacent males overlap to produce acoustic beats. Hums attract gravid females and can be mimicked with continuous tones at the fundamental frequency. While individual hums have flat envelopes, other midshipman signals are amplitude modulated. This study used one-choice playback tests with gravid females to examine the role of envelope modulation in hum recognition. Various pulse train and two-tone beat stimuli resembling natural communication signals were presented individually, and the responses compared to those for continuous pure tones. The effectiveness of pulse trains was graded and depended upon both pulse duration and the ratio of pulse to gap length. Midshipman were sensitive to beat modulations from 0.5 to 10 Hz, with fewer fish approaching the beat than the pure tone. Reducing the degree of modulation increased the effectiveness of beat stimuli. Hence, the lack of modulation in the midshipman's advertisement call corresponds to the importance of envelope modulation for the categorization of communication signals even in this relatively simple system.