hearing

The study of auditory illusions has been important in elucidating the rules of auditory grouping in humans, which adhere to principles first described by Gestalt psychologists to explain the formation of visual objects. For example, the human auditory system perceptually restores short, deleted segments of speech and other sounds (e.g., tones) when the resulting silent gaps are filled by a potential masking noise. This “temporal induction” of missing sounds accounts for phenomena such as “phonemic restoration” of speech and the so-called “continuity illusion”. Results from studies of such illusions suggest that the Gestalt principle of “continuity” allows humans to perceive complete auditory objects in the face of incomplete or degraded acoustic information in the presence of noise. Although similar findings have been reported in two nonhuman primates and one songbird, the general relevance of such illusory percepts in acoustic communication across a wide range of taxa remains unknown. The present study tested the hypothesis that female treefrogs experience the illusory perceptual restoration of discrete pulses in the male sexual advertisement signal when they are deleted and replaced by a potential masking noise. While added noise restored some attractiveness to degraded signals, there was no evidence that the frogs experienced the illusion of perceiving actual pulses that were missing. Instead, the added noise appeared to function as an acoustic ornament that made some signals more attractive than others as a result of an inherent sensory bias for greater sensory stimulation. Whether such sensory biases themselves adhere to Gestalt principles of auditory grouping remains to be demonstrated.

Hearing sensitivity and sound characteristics of poikilothermic animals are influenced by ambient temperature. The present study investigates the influence of temperature on the hearing ability and on sound characteristics in the neotropical Raphael catfish Platydoras costatus. Using the auditory evoked potential (AEP) recording technique, the hearing thresholds of eight specimens were tested at six different frequencies from 100 to 4000 Hz. The temporal resolution was determined by analyzing the minimum resolvable click period (0.3 - 5 ms). Hearing sensitivity and sound production were measured after acclimation to 22 °C followed by acclimation to 30 °C for at least three weeks each. Auditory sensitivity increased from 1 to 4 kHz with rising temperature, whereas the temporal resolution showed no change. Catfishes produce stridulation sound by rubbing the pectoral spines in the shoulder girdle during abduction and adduction of pectoral fins. Distress calls were recorded when fish were handheld. The stridulation sounds produced during pectoral fin spine abduction and adduction became shorter at the higher temperature. Maximum and minimum pulse period as well as pulse number did not change with temperature, whereas the dominant frequency of sounds was higher at the higher temperature. Our data indicate that constraints imposed on hearing sensitivity at different temperatures cannot be compensated even by longer acclimation. As sound characteristics also change with temperature, we suggest that the ambient temperature directly affects acoustic communication in the neotropical catfish P. costatus.

Batrachoidids, including the Lusitanian toadfish Halobatrachus didactylus, have become good models for studying acoustic communication as they are unusual strongly vocal species and rely on their hearing abilities to find mates during the breeding season. The major goals of this ongoing study are (1) to verify whether the hearing sensitivity of the Lusitanian toadfish is well adapted to detect its complex acoustic repertoire and (2) to investigate the encoding of conspecific sounds in the toadfish auditory system. Hearing sensitivity was measured based on potentials recorded from the inner ear saccule. Responses to conspecific sounds were evaluated with the auditory evoked potentials (AEP) recording technique. Both experiments were performed in reproductive males and females. Saccular potentials of the toadfish were evoked greatest at twice the stimulus frequency. Saccular hair cells of both males and females were most sensitive at frequencies between 15 Hz (lowest frequency measured) and 205 Hz, where thresholds were below 118 dB re. 1·µPa. Auditory thresholds increased towards higher frequencies. AEP recordings revealed that males and females are capable of resolving temporal patterns, amplitude fluctuations and frequency content of agonistic and mating calls. Our results suggest that the auditory system of the Lusitanian toadfish is well suited to detect fine features of vocalizations which might be important for acoustic communication and orientation, especially during mate attraction and territorial defence. This study was supported by MCTES, Portugal.

Current studies on the ontogenetic development of acoustic communication in the labyrinth fish Trichopsis vittata and the toadfish Halobatrachus didactylus show that the ability to detect conspecific sounds develops during growth. In order to investigate at which stage sound detection starts in an otophysine fish, we investigated the ontogenetic development of auditory sensitivity and vocalizations in the mochokid catfish Synodontis schoutedeni. Mochokids are also called squeakers because they produce broadband stridulation sounds during abduction and adduction of pectoral fin spines. Fish from five different size groups – from 22 mm standard length to 126 mm – were studied. Hearing thresholds were measured between 50 Hz and 6 kHz using the auditory evoked potentials (AEP) recording technique; stridulation sounds were recorded and their sound pressure levels determined. The smallest juveniles showed the poorest hearing abilities of all size groups between 50 Hz and 1000 Hz. At higher frequencies (5 and 6 kHz), hearing sensitivity was lowest in the largest groups. Duration of abduction and adduction sounds, sound pressure level and pulse period increased, while the dominant frequency of sounds decreased with size. Comparisons between audiograms and sound spectra revealed that the most sensitive frequencies correlate with the dominant frequencies of stridulation sounds in all Synodontis size groups and that all specimens are able to detect stridulation sounds. This study demonstrates that the hearing ability in the siluriform S. schoutedeni changes during ontogeny, whereas no changes were observed in two cypriniform species investigated earlier. Furthermore, S. schoutedeni is able to communicate acoustically at all stages of development, which contrasts to prior findings in teleosts.

Otophysine fishes (carps and minnows, catfishes, characins, knifefishes) comprise about 8000 species and are the most successful freshwater fish group. They have evolved mammal-like structures for hearing, namely a chain of tiny ossicles transmitting oscillations from the swimbladder, which acts as a tympanum or ear drum, to the inner ear. These Weberian ossicles enable otophysines to detect sound pressure and extend their hearing range to lower sound intensities and to sound frequencies of up to several kHz. They represent the largest group of hearing specialists. Hearing enhancement by the Weberian ossicles is frequency dependent. Removal of the tripus, the largest ossicle, in goldfish decreased hearing sensitivity as measured by the non-invasive auditory evoked potential (AEP) recording technique from 7 dB at 100 Hz up to 33 dB at 2 kHz. Despite their common trait, otophysines reveal a large diversity in hearing sensitivities both between and within orders. This is linked to swimbladder and ossicle morphology, which are in particular diverse in catfishes. Several families (doradids and ariids) possess large, free, unpaired swimbladders, while others (loricariids) have paired tiny swimbladders totally encapsulated in bone. Investigations of 11 species from 6 families showed that large bladders and ossicles as well as higher ossicle numbers improve hearing ability at higher frequencies. Which ecological or social constraints prompted the evolution of the Weberian apparatus in otophysine species? The similarity of auditory sensitivity between vocalizing and non-vocalizing species indicates that the Weberian apparatus evolved prior or mostly independently from the development of sound-generating mechanisms. Predator and/or prey detection in quiet freshwater habitats were probably the major selective pressures behind this hearing specialization.

Acoustic communication plays an important role in the social behaviours of vocal teleost fishes in the Family Batrachoididae (midshipman and toad fishes). The midshipman and toadfishes have become good models for investigating the neural and endocrine mechanisms of auditory perception and vocal production shared by all vertebrates, in part, because the reproductive success of these batrachoidid fishes is highly dependent on acoustic communication. The plainfin midshipman fish (Porichthys notatus), like other teleost fish, use the saccule as the main acoustic end organ for hearing to detect and locate vocalizing mates that produce multiharmonic advertisement calls during the breeding season. Recent work showed that the frequency sensitivity of the peripheral auditory system changed seasonally with female reproductive state such that reproductive females became better suited than non-reproductive females to encode the higher harmonics of the male’s advertisement call. Approximately one month before the breeding season, females showed peaks in circulating plasma levels of testosterone and 17beta-estradiol, which are now known to induce the female’s summer reproductive auditory phenotype and enhance auditory sensitivity to the dominant higher harmonic components of the male’s mate call. Furthermore, midshipman-specific oestrogen receptor alpha and androgen receptor have been identified in the saccule which now provides additional support for a direct steroid effect on the inner ear. Additional physiological evidence will be presented that suggests that the saccular hair cell receptors are the prime candidate sites for this novel form of steroid-dependent auditory plasticity. I will discuss why this auditory plasticity may represent an adaptation to enhance mate detection and localization during the breeding season.

Successful sound communication between animals located in vegetation requires that the sound signals penetrating the vegetation retain sufficient amplitude and structure to allow the receiving animals to extract the message. In many cases, the sounds should also provide information about the sender's position in space. One often finds that the messages have survived the sound degradation in the vegetation, whereas the information about position has been lost. However, the ability of animals to obtain directional information by listening to degraded sounds depends on the properties of their hearing systems. In many animals, the directional hearing is based only on differences of sound amplitude at the ears. Other animals also exploit differences in time-of-arrival (phase), and their directional hearing appears to be more immune to sound degradation. Different phase mechanisms (neural and acoustical) and strategies for sound communication will be discussed.

Little information exists about how noise exposure may modulate agonistic behaviour of some sonic fish in which perception of sounds produced by conspecifics is crucial in interpreting the message conveyed by the opponents. Recently, it has been demonstrated that temporary hearing impairment can be induced by exposing fish to certain periods of white noise. Experiments were designed to test the hypothesis that elevation of hearing thresholds my means of exposure to noise) of a sonic fish, the croaking gourami Trichopsis vittata, could alter the quality of sound produced because of the altered feedback route in sound perception and production loop. This experiment also tested the hypothesis that altered characteristics of the sound produced would subsequently modulate the outcome of the behavioural contest. Test subjects were exposed to 300 - 4000 Hz white noise (142 dB; re: 1 mPa) for 12 and 24 hours, respectively. The hearing threshold shifts were evaluated at the end of noise exposure as well as one day and five days after exposure with the use of the auditory brainstem response recording protocol. The frequency range tested (600 - 2500 Hz) corresponded to the fish best hearing range as well as to the range of the croaking sounds emitted by the fish. Hearing threshold was found significantly elevated after noise exposure. Recovery, however, appeared faster in 12-hr noise exposed fish than those from the 24-hr exposure group. The croaking sounds produced by noise-exposed fish were also recorded through staged contests. The results were used to compare with the sounds produced prior to the exposure. Details of differences in sound spectra as well as changes of agonistic behaviour are presented and discussed (supported by National Organization for Hearing Research, NIMH-58198, Institute of Museum and Library Service-LL90187).

We auditory evoked response (AER) technique an electrophysiological far-field recording method widely used in evaluating vertebrate hearing, was adopted to butterflies to overcome limitations of behavioural and invasive electrophysiological techniques. Response to tone bursts of different frequencies (250 Hz - 15 kHz) and amplitudes was recorded from the surface of the thorax of Erebia pronoe, E. aethiops and E. medusa (Lepidoptera: Satmidae). AER-waveforms of all three species consisted of 2-3 positive peaks. The polarity of these traces was not affected when switching the phase of the sound stimulus by 180°. Latencies of AERS increased with decreasing sound pressure level and frequencies. Perforation of the tympana at the base of the forewings totally eliminated AERS at all frequencies, demonstrating that the recorded potentials were generated solely within the auditory pathway in response to acoustical stimuli. Decapitation did not reduce the number of peaks and amplitude of AER traces, indicating that auditory evoked potentials are generated entirely within the thorax. AER audiograms revealed best auditory sensitivity between 2 and 4 kHz in E. pronoe and E. aethiops, with lowest thresholds at 36 and 31 dB SPL, respectively. These data demonstrate that the AER recording technique is a useful method for measuring hearing in insects when rapid measurements of a large number of species or when repeated testing of animals are required (e.g. during ontogenetic studies). This research was supported by the Austrian Science Fund (FWF grant No 12411 to F.L.) and by the Slovenian Science Foundation (SZF-fellowship to D.R.).

There is growing concern that aquatic vertebrates may be affected by the increasing noise of anthropogenic origin in their environment. Several studies have been conducted on the effects of noise on marine mammals, but only a few studies have dealt with fish. The aim of the present study is to measure and compare the immediate effects of intense noise exposure (160 dB re 1 µPa for 12 and 24 hours) on two otophysine fish species, the non-vocal cyprinid Carassius auratus and the catfish Pimelodus pictus, which produces low-frequency drumming and high-pitched stridulatory-sounds. The second aim of this study was to determine the effects of noise on the ability of P. pictus to communicate acoustically. Hearing sensitivity was determined utilising the auditory evoked potential (AEP) recording technique. Measurements were performed prior and directly after noise exposure as well as after several days of recovery. Threshold shifts immediately after noise exposure ranged from 13-22 dB in C. auratus and from 7-34 dB in P. pictus. In both species the greatest hearing loss occurred at their most sensitive frequencies (C. auratus: 500 - 1000 Hz; P. pictus: 500-4000 Hz). Sound energies in the pimeloid catfish were maximally 10 dB above hearing curves immediately after noise exposure. Differences in recovery were observed between species. Carassius auratus recovered completely after 3 days, whereas in Pimelodus pictus reduced auditory sensitivity of up to 15 dB was still observed after 3 days. Our results showed that these two hearing specialists are differently affected by noise exposure and that the threshold shifts are more persistent in P. pictus. The hearing loss in the vocalising pimelodid catfish indicates that sound communication is impaired in noisy environments. This research was supported by the Austrian Science Fund (FWF No 12411 to F.L.).