Directional selectivity and frequency tuning of midbrain cells in the oyster toadfish, Opsanus tau.

Single-unit recordings were made from areas in the midbrain (torus semicircularis) of the oyster toadfish. We evaluated frequency tuning and directional responses using whole-body oscillation to simulate auditory stimulation by particle motion along axes in the horizontal and mid-sagittal planes. We also tested for bimodality in responses to auditory and hydrodynamic stimuli. One recording location in each animal was marked by a neurobiotin injection to confirm the recording site. Recordings were made in nucleus centralis, nucleus ventrolateralis, and the deep cell layer. Most units were frequency-selective with best frequencies between 50 and 141 Hz. Suppression of activity was apparent in 10% of the cells. Bimodality was common, including inhibition and suppression of background activity by auditory or hydrodynamic stimulation. The majority of the cells were directionally selective with directional response patterns that were sharpened compared with those of primary saccular afferents. The best directional axes were arrayed widely in spherical space, covering most azimuths and elevations. This representation is adequate for the computation of the motional axis of an auditory stimulus for sound source localization.

Directionality and frequency tuning of primary saccular afferents of a vocal fish, the plainfin midshipman (Porichthys notatus).

While particle motion is thought to directly stimulate the inner ear of most fish species, it is difficult to measure and might not be predictable from pressure measurements in a small tank. It is therefore important to replicate experiments conducted relative to pressure measurements using stimuli of known particle motion, to ensure that unmeasured components of the stimulus field do not produce misleading frequency response profiles. The frequency sensitivity of the inner ear of the plainfin midshipman fish, Porichthys notatus, in response to isopressure stimuli has been described. This study now examines the frequency and directional response properties of midshipman saccular afferents in response to whole-body displacements simulating acoustic particle motion. Best frequencies were distributed bimodally, with peaks at 50 Hz and 100 Hz. Most units had cosinusoidally shaped directional response profiles in the horizontal and vertical planes, though some units showed slight deviations from this pattern. A few units (probably saccular efferents) had omnidirectional directional response profiles and did not phase lock to the stimulus waveform. These results are consistent with responses of the midshipman saccular nerve to isopressure stimuli, and strengthen the hypothesis that the frequency sensitivity of the midshipman ear matches the frequency content of behaviorally relevant vocalizations.

Neural representations of the axis of acoustic particle motion in nucleus centralis of the torus semicircularis of the goldfish, Carassius auratus.

Experiments examined differential coding of acoustic particle motion axis in the auditory midbrain of goldfish. Animals were exposed to vibratory stimuli varying in axis orientation as action potentials were recorded from single units in the central neuropil of nucleus centralis in the torus semicircularis. Response magnitudes as a function of stimulation axis were visualized in three dimensional plots called directional response profiles. These are generally comparable to directional responses observed among primary saccular afferents in having substantially vertical orientations. Distortions in shape from the peripheral patterns indicate neural information processing. A three-dimensional model was used to evaluate the hypothesis that responses in the auditory midbrain reflect the convergence of excitatory and inhibitory primary afferent-like responses. Model afferent inputs were generated and combined arithmetically. This analysis gives insight into the mechanisms of information processing that appear to occur in brainstem nuclei. The lack of diversity in best axis directions suggests that this mechanism alone cannot account for directional hearing abilities in this species. The roles that this directional representation and processing may play in directional hearing and sound source localization are not yet clear. Implications of these data on current models of fish directional hearing are discussed.

Directional encoding by fish auditory systems.

This paper reviews and discusses several investigations of the peripheral neural code for the directional axis of acoustical particle motion in the saccule of two fishes: goldfish (Carassius auratus) and toadfish (Opsanus tau). Most saccular afferents are directional in the manner of hair cells, having a cosine-shaped directional response pattern. The saccular sensory epithelia are orientated almost vertically in a parasagittal plane. In the horizontal plane, these epithelia are orientated obliquely with respect to the midline. Hair-cell stereocilia project perpendicularly. Thus, directional response patterns of saccular afferents tend to be orientated in azimuth parallel to the orientation of the epithelia in the head. The oblique angle of the toadfish saccule is greater than that of the goldfish, and the range of best directions in the horizontal plane for each species reflects those differing orientations. The azimuth of acoustical particle motion could be computed by comparing the relative activation of the two saccules, as is the case for the ears of most terrestrial vertebrates. The spatial patterns of saccular hair-cell orientation of most fishes thus appear to have little function in azimuthal source location, but for toadfish are probably most important for determining the elevation of monopole sources.

Evolution of hearing in vertebrates: the inner ears and processing.

This paper considers aspects of the evolution of the vertebrate auditory system from an 'ichthyocentric' perspective. It is argued that all vertebrate auditory systems are required to do certain basic tasks including acoustic feature discrimination, sound source localization, frequency analysis, and auditory scene analysis, among others. These sorts of capabilities arose very early in the evolution of the vertebrates and have been modified by selection in different species. In some cases the same structures have been involved in detection and analysis throughout the vertebrates, while in other cases the mechanism by which the same type of analysis takes place may have changed.

Spectral contrasts underlying auditory stream segregation in goldfish (Carassius auratus).

This study investigates the effects of spectral separation of sounds on the ability of goldfish to acquire independent information about two simultaneous complex sources. Goldfish were conditioned to a complex sound made up of two sets of repeated acoustic pulses: a high-frequency pulse with a spectral envelope centered at 625 Hz, and a low-frequency pulse type centered at 240, 305, 390, or 500 Hz. The pulses were presented with each pulse type alternating with an overall pulse repetition rate of 40 pulses per second (pps), and a 20-pps rate between identical pulses. Two control groups were conditioned to the 625-Hz pulse alone, repeated at 40 and 20 pps, respectively. All groups were tested for generalization to the 625-Hz pulse repeated alone at several rates. If the two pulse types in the complex resulted in independent auditory streams, the animals were expected to generalize to the 625-Hz pulse trains as if they were repeated at 20 pps during conditioning. It was hypothesized that as the center frequency of the low-frequency pulse approached that of the 625-Hz pulse, the alternating trains would be perceived as a single auditory stream with a repetition rate of 40 pps. The group conditioned to alternating 625- and 240-Hz pulses generalized least, with maximum generalization at 20 Hz, suggesting that the animals formed at least one perceptual stream with a repetition rate of 20 pps. The other alternating pulse groups generalized to intermediate degrees. Goldfish can segregate at least one "auditory stream" from a complex mixture of sources. Segregation can be based on spectral envelope and grows more robust with growing spectral separation between the simultaneous sources. Auditory stream segregation and auditory scene analysis are shared among human listeners, European starlings, and goldfish, and may be primitive characteristics of the vertebrate sense of hearing.

Dendritic arbors and central projections of physiologically characterized auditory fibers from the saccule of the toadfish, Opsanus tau.

Neurobiotin was injected iontophoretically into saccular afferents of toadfish (Opsanus tau) after intracellular recording to examine dendritic arbors and central projections with respect to the physiological and directional response properties of the cells. Dendritic arbors of 36 afferents were examined in detail. Maximum diameter of the arbor and the number of terminal points were positively correlated with each other, but neither was predictive of spontaneous activity or sensitivity. Best azimuths were centered around 30 degrees -40 degrees, which corresponds to the angle of the saccule with respect to the fish's midline. In general, best elevations for afferents corresponded to hair cell orientations in the region innervated; unexpectedly low elevations obtained from afferents innervating the middle saccule may reflect curvature of the sensory epithelium against the otolith. Three efferent cells were filled partially. The location and large size of the efferent projections indicate that activity along the saccule could be modulated by a single efferent. All afferents projected to the dorsal zone of the descending octaval nucleus (dDON); many afferents bifurcated to terminate in the anterior octaval nucleus, and a few of those also had terminal fields in the medial zone of DON. All afferent projections into the dDON consisted of multiple axon collaterals projecting to numerous sites along the rostral-caudal extent of the nucleus. Variation in terminal field sites also was noted in the medial to lateral axis of the dDON; however, there were no consistent correlations between terminal field locations, physiology, and best directions of the saccular afferents.

Perception of two-tone complexes by the goldfish (Carassius auratus).

Previous experiments on the sense of hearing in goldfish have used a stimulus generalization paradigm to investigate the perceptual dimensions evoked by spectrally and temporally complex sounds. The present experiments investigated the effects on perception of the frequency separation between two tones. In the first set of experiments, six groups of goldfish were classically conditioned to a single tone and then tested for generalization to two-tone complexes having one frequency component equal to the conditioning tone, and the other differing by 2-256 Hz. Generalization declined with increasing frequency differences up to about 32 Hz, and then increased for wider frequency separations. These functions indicate that a restricted range of beat rates produces a perceptual quality that is quite unlike that of a single tone. The generalization function of frequency separation resembles the inverse of the 'fluctuation strength' and 'roughness' functions for human listeners. The second experiment investigated the effects of spectral location on the perception of a 32 Hz beat rate. Goldfish were conditioned to a two-tone complex (500 and 532 Hz) and then tested for generalization to single tones at various frequencies between 200 and 1200 Hz, and to two-tone complexes having a 32 Hz beat rate but with the lower tone component at various frequencies. For single-tone stimuli, generalization was relatively weak but showed a peak at 500 Hz. For the two-tone stimuli, generalization was more robust, but showed a similarly shaped gradient centered on 500 Hz. Thus, goldfish behaved as if they had acquired information about both temporal modulation and the frequency location of the tone components. These perceptual behaviors appear to be shared with humans and other vertebrates.

Auditory stream segregation in goldfish (Carassius auratus).

Goldfish were classically conditioned to a mixture of two pulse trains differing in both repetition rate and the spectral profile of the pulses. Animals were then tested for generalization to single pulse trains having one or the other spectral profile presented at a variety of repetition rates. Generalization functions of repetition rate were qualitatively similar to those obtained following conditioning to either of the pulse trains alone. Thus, the spectral profile of each pulse type was appropriately associated with the repetition rate at which that pulse type was presented during conditioning. These results indicate that the two concurrent pulse trains making up the conditioning stimuli were analyzed independently, forming two auditory streams. When either of the two pulse trains were presented with a 500 ms onset asynchrony, stream segregation was enhanced. These and other results suggest that many fundamental features of the human sense of hearing are widely shared among vertebrate animals, and may have developed first among fishes.

Diversity in frequency response properties of saccular afferents of the toadfish, Opsanus tau.

The frequency response of primary saccular afferents of toadfish (Opsanus tau) was studied in the time and frequency domains using the reverse correlation (revcor) method. Stimuli were noise bands with flat acceleration spectra delivered as whole-body motion. The recorded acceleration waveform was averaged over epochs preceding and following each spike. This average, termed the revcor, is an estimate of the response of an equivalent linear filter intervening between body motion and spike initiation. The spectrum of the revcor estimates the shape of the equivalent linear filter. Revcor responses were brief, damped oscillations indicative of relatively broadly tuned filters. Filter shapes were generally band-pass and differed in bandwidth, band edge slope, and characteristic frequency (74 Hz to 140 Hz). Filter shapes tend to be independent of stimulus level. Afferents can be placed into two groups with respect to characteristic frequency (74-88 Hz and 140 Hz). Some high-frequency afferents share a secondary peak at the characteristic frequency of low-frequency afferents, suggesting that an afferent may receive differently tuned peripheral inputs. For some afferents having similar filter shapes, revcor responses often differ only in polarity, probably reflecting inputs from hair cells oriented in opposite directions. The origin of frequency selectivity and its diversity among saccular afferents may arise from a combination of hair cell resonance and micromechanical processes. The resulting frequency analysis is the simplest yet observed among vertebrate animals. During courtship, male toadfish produce the 'boatwhistle' call, a periodic vocalization having several harmonics of a 130 Hz fundamental frequency. The saccule encodes the waveform of acoustic particle acceleration between < 50 and about 250 Hz. Thus, the fundamental frequency component of the boatwhistle is well encoded, but the successive higher harmonics are filtered out. The boatwhistle is thus encoded as a time-domain representation of its fundamental frequency or pulse repetition rate.

Directional response properties of saccular afferents of the toadfish, Opsanus tau.

The displacement sensitivity, frequency response, and directional response properties of primary saccular afferents of toadfish (Opsanus tau) were studied in response to a simulation of acoustic particle motion for which displacement magnitudes and directions were manipulated in azimuth and elevation. Stimuli were 50, 100, and 200 Hz sinusoidal, translatory oscillations of the animal at various axes in the horizontal and midsagittal planes. Thresholds in these planes defined a cell's characteristic axis (the axis having the lowest threshold) in spherical coordinates. Recordings were made from afferents in rostral, middle, and caudal bundles of the saccular nerve. The most sensitive saccular afferents responded with a phase-locked response to displacements as small as 0.1 nm. This sensitivity rivals that of the mammalian cochlea and is probably common to the sacculi and other otolith organs of most fishes. Most afferents showed lower thresholds at 100 Hz than at 50 or 200 Hz. Eighty percent of afferents have three-dimensional directional properties that would be expected if they innervated a group of hair cells having the same directional orientation on the saccular epithelium. Of the afferents that are not perfectly directional, most appear to innervate just two groups of hair cells having different orientations. The directional characteristics of afferents are qualitatively correlated with anatomically defined patterns of hair cell orientation on the saccule. In general, azimuths of best sensitivity tend to lie parallel to the plane of the otolith and sensory epithelium. Elevations of best sensitivity correspond well with hair cell orientation patterns in different regions of the saccular epithelium. Directional hearing in the horizontal plane probably depends upon the processing of interaural differences in overall response magnitude. These response differences arise from the gross orientations of the sacculi and are represented, in part, as time differences among nonspontaneous afferents that show level-dependent phase angles of synchronization. Directional hearing in the vertical plane may be derived from the processing of across-afferent profiles of activity within each saccule. Fishes were probably the first vertebrates to solve problems in sound source localization, and we suggest that their solutions formed a model for those of their terrestrial inheritors.

Evolution of the ear and hearing: issues and questions.

The ear appears to have arisen early in the evolution of the vertebrates. While there are significant interspecific differences in ear structure, it appears that receptor cell structure and the basic function of the ear and auditory system are similar among all vertebrate groups. In this paper we present the evolution of the sensory hair cells of the ear, the origins of the ear itself, and selected functions of the sense of hearing. We argue that there have been strong selective pressures in most vertebrate groups for the sorts of sound encoding and processing abilities that result in the efficient detection, localization, and identification of sound sources in noisy environments. Many of the encoding and processing strategies underlying these functions are shared as well.

Behavioral detection of acoustic particle motion by a teleost fish (Astronotus ocellatus): sensitivity and directionality.

Behavioral detection thresholds for oscars (Astronotus ocellatus) were measured in response to linear, oscillatory motion at 100 Hz along seven axes (-90 degrees, -60 degrees, -30 degrees, 0 degree, +30 degrees, and +60 degrees in azimuth, where 0 degree is the fish longitudinal axis; and the vertical axis) using a cardiac classical conditioning paradigm. Thresholds at all selected axes ranged from -58 to -56 dB re: 1 micron, which corresponds to 1.2 to 1.6 nm displacement (RMS). Thus, oscars appear to be equally sensitive to stimulation at all axes in three dimensional space. Behavioral thresholds of the oscar are close to the neural thresholds obtained from the most sensitive auditory nerve fibers in two other species, the goldfish (Carassius auratus) and the toadfish (Opsanus tau).

Perception of spectrally and temporally complex sounds by the goldfish (Carassius auratus).

Behavioral studies on complex sound perception in goldfish were carried out in order to help determine what, if any, differences exist between the sense of hearing of fishes and other vertebrates. A stimulus generalization paradigm was used with classical conditioning in three experiments to determine: (1) the perceptual relations between a pure tone and harmonic complexes having a fundamental frequency equal to that of the tone; (2) the combined effects on perception of pulse repetition rate and spectral envelope; and (3) whether goldfish can be shown to identify a complex source when presented simultaneously with another complex source. Experiment 1 showed that the perceptions of tones and harmonic complexes differ profoundly even for the cases in which they have common periodicities and frequency components. Experiment 2 demonstrated that pulse repetition rate and spectral location simultaneously control behavior, and that repetition rate exerts behavioral control independent of spectral location. Experiment 3 indicates that goldfish did not 'hear out' or analyze a complex target source within a mixture of complex sources. In general, goldfish appear to be aware of multiple acoustic dimensions of complex sounds, suggesting both pitch-like and timbre-like perceptual dimensions. These results do not permit a qualitative distinction between the sense of hearing of goldfish and that of other vertebrates.