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SUMMARY Acipenser fulvescens, the lake sturgeon, belongs to one of the few extant non-teleost ray-finned (bony) fishes. The sturgeons (family Acipenseridae) have a phylogenetic history that dates back about 250 million years. The study reported here is the first investigation of peripheral coding strategies for spectral analysis in the auditory system in a non-teleost bony fish. We used a shaker system to simulate the particle motion component of sound during electrophysiological recordings of isolated single units from the eighth nerve innervating the saccule and lagena. Background activity and response characteristics of saccular and lagenar afferents (such as thresholds, response–level functions and temporal firing) resembled the ones found in teleosts. The distribution of best frequencies also resembled data in teleosts (except for Carassius auratus, goldfish) tested with the same stimulation method. The saccule and lagena in A. fulvescens contain otoconia, in contrast to the solid otoliths found in teleosts, however, this difference in otolith structure did not appear to affect threshold, frequency tuning, intensity- or temporal responses of auditory afferents. In general, the physiological characteristics common to A. fulvescens, teleosts and land vertebrates reflect important functions of the auditory system that may have been conserved throughout the evolution of vertebrates.

SUMMARYComparisons of left and right auditory input are required for sound source localization in most terrestrial vertebrates. Previous physiological and neuroanatomical studies have indicated that binaural convergence is present in the ascending auditory system of the toadfish. In this study, we introduce a new technique, otolith tipping, to reversibly alter directional auditory input to the central nervous system of a fish. The normal directional response pattern (DRP) was recorded extracellularly for auditory cells in the first-order descending octaval nucleus (DON) or the midbrain torus semicircularis (TS) using particle motion stimuli in the horizontal and mid-sagittal planes. The same stimuli were used during tipping of the saccular otolith to evaluate changes in the DRPs. Post-tipping DRPs were generated and compared with the pre-tipping DRPs to ensure that the data had been collected consistently from the same unit. In the DON, ipsilateral or contralateral tipping most often eliminated spike activity, but changes in spike rate(±) and DRP shape were also documented. In the TS, tipping most often caused a change in spike rate (±) and altered the shape or best axis of the DRP. The data indicate that there are complex interactions of excitatory and inhibitory inputs in the DON and TS resulting from the convergence of binaural inputs. As in anurans, but unlike other terrestrial vertebrates,binaural processing associated with encoding the direction of a sound source begins in the first-order auditory nucleus of this teleost.

SUMMARY Zebrafish (Danio rerio) were placed in small wells that could be driven vertically with a series of calibrated sinusoids. Video images of the fish were obtained and analyzed to determine the levels and frequencies at which the fish responded to the stimulus tones. It was found that fish 4 days post fertilization (dpf) did not respond to the stimulus tones, whereas fish 5 dpf to adult did respond. It was further found that the stimulus thresholds and frequency bandwidth to which the fish responded did not change from 5 dpf to adult; indicating that the otolithic organ adaptations for high-frequency hearing are already present in larval fish. Deflating the swimbladders in adult fish eliminated their response, which is consistent with sensing sound pressure. Deflating the swimbladder in larval fish did not affect their thresholds, which is consistent with sensing the particle motion of the fluid directly. Because adult fish with Weberian ossicles have a greater input to the inner ear for a given sound pressure level (SPL), the finding that the adult and larval fish respond at the same SPL with intact swimbladders suggests that the acoustic startle response threshold is adjusted as the fish develop in order to maintain appropriate reactions to relevant stimuli.

We investigated the roles of the swim bladder and the lateral line system in sound localization behavior by the plainfin midshipman fish (Porichthys notatus). Reproductive female midshipman underwent either surgical deflation of the swim bladder or cryoablation of the lateral line and were then tested in a monopolar sound source localization task. Fish with nominally "deflated" swim bladders performed similar to sham-deflated controls; however, post-experiment evaluation of swim bladder deflation revealed that a majority of "deflated" fish (88%, 7 of the 8 fish) that exhibited positive phonotaxis had partially inflated swim bladders. In total, 95% (21/22) of fish that localized the source had at least partially inflated swim-bladders, indicating that pressure reception is likely required for sound source localization. In lateral line experiments, no difference was observed in the proportion of females exhibiting positive phonotaxis with ablated- (37%) versus sham-ablated (47%) lateral line systems. These data suggest that the lateral line system is likely not required for sound source localization, although this system may be important for fine-tuning the approach to the sound source. We found that midshipman can solve the 180° ambiguity of source direction in the shallow water of our test tank, which is similar to their nesting environment. We also found that the potential directional cues (phase relationship between pressure and particle motion) in shallow water differs from a theoretical free-field. Therefore, the general question of how fish use acoustic pressure cues to solve the 180° ambiguity of source direction from the particle motion vector remains unresolved.

The masking effects of tones on the detection auditory signals were studied in goldfish using the psychophysical tuning-curve paradigm. For signals below 350 Hz, masking is an inverse function of the frequency separation between masker and signal; a finding consistent with previous masking studies on fishes, birds and mammals. For signals above 350 Hz, masking peaks occur both in the 350 Hz region and at the frequency of the signal. Quantitative comparisons with recent neural tuning curves for goldfish saccular neurones suggest that the filtering observed may be determined by mechanical frequency selectivity below 350 Hz, but by a neural analysis of temporal patterns above this range.

Microphonic potentials were recorded from the ears of the goldfish during acoustic stimulation in a situation where sound pressure and particle displacement could be varied. Microphonic potentials from fishes with the swim bladder intact were proportional to sound pressure. After removal of the swim bladder, sound pressure sensitivity declined by 20-35 dB and the response was generated in proportion to particle displacement. The ear's sensitivity to direct vibration of the head increases at between -3 and -6 dB/octave between 70 and 1500 Hz and is not affected by the removal of the swim bladder. It is concluded that the peripheral auditory system of the goldfish may function as a pressure detector or as a displacement detector, depending upon the impedance of the applied signal.

Comparable auditory sound pressure level (SPL) and sound intensity level (SIL) threshold curves were determined in air and under water in Rana catesbeiana. Threshold curves were determined using chronic metal electrode implants which detected multi-unit responses of the torus semicircularis to incident sound. In terms of SPL, hearing thresholds in water and air are similar below 0·2 kHz. Above 0·2 kHz, the sensitivity under water falls off at about 16 dB/octave to reach an average loss of about 30 dB above 0·4 kHz. In terms of SIL, the organism is about 30 dB more sensitive under water than in air below 0·2 kHz and equally sensitive in air and water above 0·4 kHz. The relative merits of the two measures are discussed and an attempt is made to relate the results to morphology of the middle and inner ears. This report is the first to compare aerial and underwater hearing abilities in any organism using electrode implants.