A psychophysical approach to measuring the threshold for acoustic stimulation in zebrafish (Danio rerio)

Zebrafish are a popular vertebrate animal model for biomedical research including investigations of the auditory system. Responses to acoustic stimulation have been a challenge to carefully measure in zebrafish. Here, the authors have developed a procedure for measuring hearing sensitivity in adult zebrafish using an appetitive automated Go/No Go task. In this task, a trial is initiated when a fish passes through an observing gate. In a sound trial, the fish is reinforced by an automated food delivery system when it enters the reinforcement compartment. If the fish enters the reinforcement compartment during a no-sound trial, a timeout is implemented. Zebrafish successfully learned this task in a median of about ten days of daily training. Zebrafish were most sensitive at a frequency of 800 Hz, which corresponds well with sensitivity reported from physiological methods. As far as the authors know, the present study is the first to provide hearing thresholds for zebrafish using a conventional combination of operant conditioning and psychophysical procedures. This could open the door to other kinds of tests using acoustic stimuli as are commonly conducted in many other laboratory animals.

Control of vocal production in budgerigars (Melopsittacus undulatus): selective reinforcement, call differentiation, and stimulus control

Budgerigars were trained to make a specific call using a real-time automated call recognition system and food reward. Calls produced by the bird were followed by food only if they were similar enough to a template call. The selective reinforcement of a particular call type increased the similarity of the bird's call to the template and decreased overall call variation, including call duration. After the birds' performance reached asymptote (i.e. the calls became matched to the template with no further increase in similarity), a call differentiation procedure was introduced. This procedure consisted of both matching-to-template and non-matching-to-template trials. In order to receive food in non-matching-to-template trials, the birds had to produce a call that was sufficiently different from the template call. This procedure resulted in a `new' call emerging from the first template call which occurred gradually rather than abruptly. In the third procedure, called two-template matching training, the birds had to match their calls to the first template call (signaled by illuminating the left LED) and to the second template call (signaled by illuminating the right LED). The calls produced in both first and second template call trials were well controlled by the position of the LED. These results extend the effects of selective reinforcement, differential reinforcement, and stimulus control on response topographies to the domain of vocalizations in budgerigars.

Differential reinforcement of an approach response in zebrafish (Danio rerio)

Five zebrafish were trained to approach a target using a fully automated training procedure. During a training session, if the distance between the fish and the target was closer than an arbitrarily set distance, the approach response was reinforced by food. The fish continued to respond under this reinforcement contingency and the distance criterion could be shortened up to eighty times within a 1h session. The initial distance limit was then shortened for the next test training session. Once the initial distance criterion was reduced to a final minimum distance, the distance criterion was fixed at this value for the next nine successive sessions. In a second experiment using different fish, we manipulated approach distances in three conditions. The first condition was identical to the changing criterion training as in Experiment 1. In the second condition, only response distances under a distance criterion were reinforced. And in the last condition, only response distances over the distance criterion were reinforced. Results show that zebrafish can control the distance between themselves and a target. In other words, zebrafish are sensitive to the spatial consequences of their behavior. The present results show that a differential reinforcement paradigm can be successfully applied to zebrafish which therefore enhances their value as a vertebrate model for studies of complex behavior including visuomotor learning.

Amphibious auditory responses of the American alligator (Alligator mississipiensis)

Animals that thrive both on land and underwater are faced with the task of interpreting stimuli in different media. This becomes a challenge to the sensory receptors in that stimuli (e.g., sound, motion) may convey the same type of information but are transmitted with different physical characteristics. We used auditory brainstem responses to examine hearing abilities of a species that makes full use of these two environments, the American alligator (Alligator mississipiensis). In water, alligators responded to tones from 100 Hz to 2,000 Hz, with peak sensitivity at 800 Hz. In air, they responded to tones from 100 Hz to 8,000 Hz, with peak sensitivity around 1,000 Hz. We also examined the contribution to hearing of an air bubble that becomes trapped in the middle ear as the animal submerges. This bubble has been previously implicated in underwater hearing. Our studies show that the trapped air bubble has no affect on auditory thresholds, suggesting the bubble is not an important adaptation for underwater hearing in this species.

Hair cell death in a hearing-deficient canary

Cell death has been documented in bird auditory inner ear epithelia after induced damage. This cell death is quickly followed by an increase in supporting cell division and regeneration of the epithelium, thereby suggesting a possible relationship between these two processes. However, aspects of this relationship still need to be better understood. The Belgian Waterslager (BWS) canary is an ideal system in which to study cell death and subsequent cell division. In contrast to mixed breed (MB) canaries, cell division normally occurs in the auditory end organ of the BWS without any external manipulation. In addition, some of the cells in the auditory epithelium may be dying through an apoptotic-like process. In the present study two methods were used to quantify dying cells in the BWS and MB canary auditory epithelia: morphological criteria and TUNEL. Results confirm that some of the abnormal hair cells in the BWS auditory epithelium are apoptotic-like. The presence of both cell death and cell division indicates that these processes act concurrently in the adult end organ. Future studies are needed to determine if cell death is a stimulus for the observed cell division.

Masking by harmonic complexes in birds: behavioral thresholds and cochlear responses

Thresholds for pure tones embedded in harmonic complexes were measured behaviorally and physiologically for three species of birds, and physiologically in gerbils. The harmonic maskers were generated using the Schroeder-phase algorithm, characterized by monotonically increasing or decreasing phase across frequency. Previous work has shown that these stimuli produce large differences in masking in humans but not budgerigars. In this study, we show that for two additional species of birds, the patterns of masking were similar to those shown for budgerigars, with masking differing only slightly for the two Schroeder-phase waveforms, and in the opposite direction from that demonstrated in humans. Amounts of masking among species corresponded qualitatively to differences in their critical ratios. Evoked potential measurements in birds and gerbils indicated responses that were consistent with the behaviorally measured thresholds in birds and humans. Results are interpreted in light of differences in frequency selectivity and cochlear temporal processing across species.

A quantitative analysis of the nerve fibers in the VIIIth nerve of Belgian Waterslager canaries with a hereditary sensorineural hearing loss

The number of auditory nerve fibers was determined for non-Belgian Waterslager canaries (non-BWS) and Belgian Waterslager canaries (BWS) that are affected by a sensorineural high frequency hearing loss and a 30% reduction in the number of auditory hair cells. Counts were obtained from semithin cross sections of the Durcupan-embedded auditory nerve at the level of the internal auditory meatus. In addition, the number of lagenar fibers was determined from cross sections near the apical end of the cochlear duct in order to separate them from the total number of auditory nerve fibers. The mean number of auditory nerve fibers was 6076 in non-BWS and 5363 in BWS canaries, representing a 12% reduction in BWS. This small reduction in the number of auditory nerve fibers, as compared to the larger reduction in hair cell number, might be explained by a predominant loss of abneural hair cells in BWS, since it has been shown for other species that a large proportion of abneural hair cells are devoid of afferent innervation. In addition, we observed that despite the prominent hair cell pathologies documented for BWS canaries, the mean diameter of auditory nerve fibers from non-BWS canaries (2.22+/-0.81 microm) did not differ from those of BWS canaries (2.21+/-0.96 microm).

Frequency discrimination in budgerigars (Melopsittacus undulatus): effects of tone duration and tonal context

Studies of frequency resolving power in budgerigars (Melopsittacus undulatus) have shown that this species has excellent discrimination abilities for both simple and complex sounds falling in the region of 2 to 4 kHz--the frequency range of their contact call. In four experiments, frequency discrimination by budgerigars of short tones similar to elements found in the contact call was examined. Frequency difference limens (FDLs) for simple pure tones at 2.86 kHz were constant for tone durations above 20 ms but higher for shorter tones. Budgerigars generally showed larger FDLs for shorter duration 1-, 2-, and 4-kHz pure tones. FDLs in budgerigars for 20-ms tones embedded in a sequence of six other tones were similar to FDLs measured for tones of the same frequency presented in isolation. Moreover, there was no effect of introducing trial-by-trial variation in the location of the frequency change in the seven-tone complexes for budgerigars, a condition for which humans showed a large decrement in performance. Taken together, these results suggest budgerigars possess enhanced spectral resolving power for short duration pure tones when they are embedded in contact call-like tonal patterns.

Neither endocochlear potential nor tegmentum vasculosum are affected in hearing impaired belgian waterslager canaries

We previously showed that the Belgian Waterslager canary strain is affected by a hereditary hearing loss that is associated with a reduced number of hair cells and hair cell pathologies in the basilar papilla. Since hair cell pathologies were also present in the sacculus, Weisleder et al. (1994) suggested that these birds are afflicted by Scheibe's like dysplasia, a cochleo-saccular defect. In mammals, cochleo-saccular defects are characterized primarily by the lack of an endocochlear potential and abnormalities in the Stria vascularis which only secondarily lead to hair cell loss (Steel and Bock, 1983; Steel, 1994; 1995). Here we report the endocochlear potential of six ears from three non-Belgian Waterslager canaries and three ears of two Belgian Waterslager canaries to decide if Waterslager canaries are affected by a cochleo-saccular or by a neuroepithelial defect. The mean endocochlear potential was 17.6+/-2. 5 mV in the non-Waterslager canaries and 20.3+/-0.6 mV in Waterslager canaries. In addition, and consistent with the presence of a normal endocochlear potential, light microscopy of the tegmentum vasculosum provided no evidence for pathology. These data show that Belgian Waterslager canaries are affected by a neuroepithelial rather than a cochleo-saccular inner ear defect.

Masking by harmonic complexes in budgerigars (Melopsittacus undulatus)

In humans, masking by harmonic complexes is dependent not only on the frequency content of the masker, but also its phase spectrum. Complexes that have highly modulated temporal waveforms due to the selection of their component phases usually provide less masking than those with flatter temporal envelopes. Moreover, harmonic complexes that are created with negative Schroeder phases (component phases monotonically decreasing with increasing harmonic frequency) may provide more masking than those created with positive Schroeder phases (monotonically increasing phase), even though both temporal envelopes are equally flat. To date, there has been little comparative work on the masking effectiveness of harmonic complexes. Using operant conditioning and the method of constant stimuli, masking of pure tones by harmonic complexes was examined in budgerigars at several different masker levels for complexes constructed with two different fundamental frequencies. In contrast to humans, thresholds in budgerigars differed very little for the two Schroeder-phase waveforms. Moreover, when there was a difference in masking by these two waveforms, the positive Schroeder was the more effective masker--the reverse of that described for humans. Control experiments showed that phase selection was relevant to the masking ability of harmonic complexes in budgerigars. Release from masking occurred when the components were in coherent phase, compared with a complex with random phases selected for each component. It is suggested that these psychoacoustic differences may emerge from structural and functional differences between the avian and mammalian peripheral auditory systems involving traveling wave mechanics and spectral tuning characteristics.

Avian species differences in susceptibility to noise exposure

Previous studies of hair cell regeneration and hearing recovery in birds after acoustic overstimulation have involved relatively few species. Studies of the effects of acoustic overexposure typically report high variability. Though it is impossible to tell, the data so far also suggest there may be considerable species differences in the degree of damage and the time course and extent of recovery. To examine this issue, we exposed four species of birds (quail, budgerigars, canaries, and zebra finches) to identical conditions of acoustic overstimulation and systematically analyzed changes in hearing sensitivity, basilar papilla morphology, and hair cell number. Quail and budgerigars showed the greatest susceptibility to threshold shift and hair cell loss after overstimulation with either pure tone or bandpass noise, while identical types of overstimulation in canaries and zebra finches resulted in much less of a threshold shift and a smaller, more diffuse hair cell loss. All four species showed some recovery of threshold sensitivity and hair cell number over time. Canary and zebra finch hearing and hair cell number recovered to within normal limits while quail and budgerigars continued to have an approximately 20 dB threshold shift and incomplete recovery of hair cell number. In a final experiment, birds were exposed to identical wide-band noise overstimulation under conditions of artificial middle ear ventilation. Hair cell loss was substantially increased in both budgerigars and canaries suggesting that middle ear air pressure regulation and correlated changes in middle ear transfer function are one factor influencing susceptibility to acoustic overstimulation in small birds.

Detection of modulation in spectral envelopes and linear-rippled noises by budgerigars (Melopsittacus undulatus)

Budgerigars were trained to discriminate complex sounds with two different types of spectral profiles from flat-spectrum, wideband noise. In one case, complex sounds with a sinusoidal ripple in (log) amplitude across (log) frequency bandwidth were generated by combining 201 logarithmically spaced tones covering the frequency region from 500 Hz to 10 kHz. A second type of rippled stimulus was generated by delaying broadband noise and adding it to the original noise in an iterative fashion. In each case, thresholds for modulation depth (i.e., peak-to-valley in dB) were measured at several different ripple frequencies (i.e., cycles/octave for logarithmic profiles) or different repetition pitches (i.e., delay for ripple noises). Budgerigars were similar to humans in detecting ripple at low spatial frequencies, but were considerably more sensitive than humans in detecting ripples in log ripple spectra at high spatial frequencies. Budgerigars were also similar to humans in detecting linear ripple in broadband noise over a wide range of repetition pitches. Taken together, these data show that the avian auditory system is at least as good, if not better, than the human auditory system at detecting spectral ripples in noise despite gross anatomical differences in both the peripheral and central auditory nervous systems.

Effects of deafening on the calls and warble song of adult budgerigars (Melopsittacus undulatus)

Budgerigars are small Australian parrots that learn new vocalizations throughout adulthood. Earlier work has shown that an external acoustic model and auditory feedback are necessary for the development of normal contact calls in this species. Here, the role of auditory feedback in the maintenance of species-typical contact calls and warble song in adult budgerigars is documented. Deafened adult birds (five male, one female) vocalized less frequently and showed both suprasegmental and segmental changes in their contact calls and warble song. Contact calls of all adult-deafened budgerigars showed abnormalities in acoustic structure within days to a few weeks following surgery. Within 6 months of surgery, nearly all contact calls produced by deafened birds were strikingly abnormal, showing highly variable patterns of frequency modulation and duration. The warble song of deafened male budgerigars also differed significantly from that of normal budgerigars on several acoustic measures. These results show that auditory feedback is necessary for the maintenance of a normal, species-typical vocal repertoire in budgerigars.

Hearing and vocalizations of wild-caught Australian budgerigars (Melopsittacus undulatus)

This study examined the hearing and contact calls of wild-caught Australian budgerigars (Melopsittacus undulatus) and compared these data to hearing and vocalizations in the much more extensively studied domesticated budgerigar. The spectral energy in the contact calls of both wild-caught and domesticated budgerigars falls almost exclusively in the frequency of 2-4 kHz. Absolute and masked thresholds were similar in both groups of birds. Similar to the results found in domesticated birds, critical ratio functions for the wild-caught budgerigars decreased at frequencies of 1.0 kHz-2.86 kHz and then increased again dramatically at frequencies above 2.86 kHz.

Detection of changes in timbre and harmonicity in complex sounds by zebra finches (Taeniopygia guttata) and budgerigars (Melopsittacus undulatus)

Thresholds for detecting alterations in the timbre and harmonicity of complex harmonic signals were measured in zebra finches, budgerigars, and humans. The stimuli used in this experiment were designed to have particular salience for zebra finches by modeling them after natural zebra finch calls. All 3 species showed similar abilities for detecting an amplitude decrement in a single component of a harmonic complex. However, zebra finches and budgerigars were extraordinarily sensitive to the mistunings of single harmonics and exhibited significantly lower thresholds compared with humans at 2 different fundamental frequencies, 570 Hz and 285 Hz. Randomizing relative phases of components in a harmonic stimulus resulted in a significant increase in threshold for detecting mistunings in zebra finches but not in humans. Decreasing the duration of mistuned harmonic stimuli resulted in higher thresholds for both birds and humans. The overall superiority of birds in discriminating inharmonicity suggests that birds and mammals may use different strategies in processing these complex harmonic sounds.

Control of vocal intensity in budgerigars (Melopsittacus undulatus): differential reinforcement of vocal intensity and the Lombard effect

Call production in budgerigars was studied using operant conditioning. In several experiments, budgerigars were reinforced with food for producing calls that were above or below a criterion level of intensity. This differential reinforcement procedure was successful in controlling vocal intensity in both directions showing that the intensity with which budgerigars produce vocalizations is under voluntary control. In additional experiments, call intensity maintained by food reinforcement was measured both in the quiet and in the presence of various levels of broadband noise. Call intensity in budgerigars increased significantly in noise, paralleling the well-known Lombard effect in humans which is the reflexive increase in speech intensity during communication in noise. Call intensity was measured in broadband noise and in a notched noise (no energy between 1.5 and 4.5 kHz) with the same overall level. Results show that noise in the spectral region of contact calls is most effective in causing an increase in vocal intensity. In aggregate, these experiments show that budgerigars have voluntary control over the intensive aspect of their vocalizations, that they normally monitor their vocal output though external auditory feedback, and, like humans, they exhibit the Lombard effect.

Recovery of hearing and vocal behavior after hair-cell regeneration

Postmitotic hair-cell regeneration in the inner ear of birds provides an opportunity to study the effect of renewed auditory input on auditory perception, vocal production, and vocal learning in a vertebrate. We used behavioral conditioning to test both perception and vocal production in a small Australian parrot, the budgerigar. Results show that both auditory perception and vocal production are disrupted when hair cells are damaged or lost but that these behaviors return to near normal over time. Precision in vocal production completely recovers well before recovery of full auditory function. These results may have particular relevance for understanding the relation between hearing loss and human speech production especially where there is consideration of an auditory prosthetic device. The present results show, at least for a bird, that even limited recovery of auditory input soon after deafening can support full recovery of vocal precision.

Perception of synthetic /ba/-/wa/ speech continuum by budgerigars (Melopsittacus undulatus)

Other than humans, extensive vocal learning has only been widely demonstrated in birds. Moreover, there are only a handful of avian species that are known to be good mimics of human speech. One such species is the budgerigar (Melopsittacus undulatus), which is a popular mimic of human speech and learns new vocalizations throughout adult life. Using operant conditioning procedures with a repeating background task, we tested budgerigars on the discrimination of tokens from two synthetic /ba/-/wa/ speech continua that differed in syllable, but not transition, duration. Budgerigars showed a significant improvement in discrimination performance on both continua near the phonetic boundary for humans. Budgerigars also showed a shift in the location of the phonetic boundary with a change in syllable length, similar to what has been described for humans and other primates. These results on a nonmammalian species provide support for the operation of a general, nonphonetic, auditory process as one mechanism which can lead to the well-known stimulus-length effect in humans.

Vocal development in budgerigars (Melopsittacus undulatus): contact calls

Budgerigars have a complex vocal repertoire, some of which develops through learning. The authors examined the course of vocal development in budgerigars from hatching to about 4 weeks postfledging (approximately 85 days old). Food-begging calls showed changes in duration, peak frequency, bandwidth, and frequency modulation with age. Within a week of fledging, each bird produced a contact call bearing a strong resemblance to a shortened version of its patterned food-begging call. By 4 weeks postfledging, budgerigar contact call repertoires often contained more than one call type, and there was clear evidence of sharing and imitation among the calls of parents, fledglings, and other social companions. Perceptual testing showed that whereas acoustic variation in the structure of developing calls decreased both within and between nestling birds, the discrimination of these calls was easier for adult birds as young birds matured. These results suggest parallels with certain aspects of language development in humans.

Evidence for supporting cell proliferation and hair cell differentiation in the basilar papilla of adult Belgian Waterslager canaries (Serinus canarius)

We used the bromodeoxyuridine technique to study the proliferative activity in the basilar papilla of normal and Belgian Waterslager canaries with and without preceding sound trauma. Without sound trauma, there were, on average, six supporting cell divisions per day in the basilar papilla of Waterslager canaries. This rate of supporting cell proliferation corresponds well with estimates of the rate of hair cell differentiation derived from counts of immature-appearing hair cells obtained by using scanning electron microscopy of the Waterslager basilar papilla. Thus, supporting cell division appeared correlated with hair cell differentiation in Waterslager canaries. Bromodeoxyuridine labeling of cells in undamaged non-Waterslager canaries also indicated a very low rate of supporting cell division. In contrast with Waterslager canaries, this low rate of proliferation was not associated with a measurable rate of hair cell differentiation. In both normal and Waterslager canaries, exposure to traumatizing sound induced a dramatic increase in the rate of cell proliferation. These data show that a very low rate of supporting cell proliferation is normally present in birds, but it is not associated with a corresponding rate differentiation of hair cells. Only an increase above this low ambient rate of supporting cell proliferation, such as that following loss of hair cells, induces the differentiation of new hair cells in birds. The reason why Waterslager canaries do not completely compensate for their inherited hair cell deficit of 30% is not clear, when they can clearly respond to additional cochlear trauma from noise exposure with an increase in proliferation rate.

Mechanisms of vocal production in budgerigars (Melopsittacus undulatus)

Songbirds vocalizing in helium show a change in the spectral quality of their vocalizations. This effect is due to an increase in the speed of sound in helium that in turn alters the resonance properties of the vocal tract. Here, this approach is extended to a psittacine, the budgerigar (Melopsittacus undulatus), whose syringeal anatomy and innervation differ from that of a songbird. Contact calls from birds vocalizing in heliox (70/30 helium/oxygen environment) showed an overall increase in the amount of energy at frequencies above the fundamental, slight changes in the frequency of the fundamental and harmonics, and some change in the level of harmonics. Calls produced by a syringeally denervated bird showed more dramatic changes. Recordings from live birds were compared with sounds produced by various simple "artificial" tracheal and syringeal models. Results suggest that budgerigars produce contact calls using the syringeal membranes as a unitary sound source which produces acoustic energy in a narrow frequency band whose fundamental frequency is matched to the resonant frequency of the trachea. The syrinx is not normally coupled to the tracheal resonator, and resonances probably play only a minor role in shaping the spectrum of contact calls.

[Continuous proliferation of supporting cells and indications for hair cell differentiation in the inner ear of adult song birds with genetic cochlear hearing loss]

Our previous investigations demonstrated that the Belgian Waterslagers (BWS) canary (Serinus canarius) was affected by an inherited sensorineural hearing loss. Compared to normal canaries of others strains, hair cell numbers in these birds were reduced on average by 30%. Since other birds are able to replace similar hair cell numbers after cochlear trauma, we investigated if BWS have the potential for supporting cell proliferation with subsequent hair cell differentiation or if they lack the repair mechanisms known to operate in other birds. In the present study the S-phase marker bromodeoxyuridine (BrdU) was used to demonstrate DNA synthesis and thus cell proliferation. We found on average six labelled nuclei per basilar papilla in BWS. This number of proliferating cells was in accordance with previous estimates of newly generated hair cells as based on the frequency of immature-appearing hair cells observed by scanning electron microscopy. We conclude that the division of supporting cells in BWS precedes the differentiation of hair cells. In contrast to BWS we found on average only one supporting cell division per day in normal canaries of other strains. However, this supporting cell proliferation in normal birds is probably not related to a loss of hair cells and does not lead to the differentiation of new hair cells. Our data indicate that differentiation of hair cells after supporting cell division occurs only if the rate of supporting cell proliferation is increased above the normal low level (probably by the loss of hair cells). Since BWS do not repair their basilar papilla despite a 30% hair cell loss (as compared to normal canaries) although they continuously produce new hair cells, we suggest that the regulation of the regeneration process is abnormal.

Testosterone induction of male-like vocalizations in female budgerigars (Melopsittacus undulatus)

Budgerigars possess a complex learned vocal repertoire that includes a long rambling song called warble. While both males and females can warble, there are marked sex differences in the amount of song produced. Males generally warble daily at a high rate, while females rarely sing. Both the quantity and the quality of male warble are influenced by circulating levels of testosterone. Males have higher circulating levels of testosterone than females. In this study we asked whether administering testosterone to adult female budgerigars would activate male-like vocal behavior. Four females were implanted with 8-mm Silastic implants filled with testosterone and recorded before and after implantation. Within 10-14 days, all four birds showed: (1) changes in there color to the male-like condition (blue), (2) male-typical patterns of precopulatory behavior, and (3) an increase in their rate of warbling to male-like levels. Sound analysis revealed that the acoustic structure of the testosterone-induced female warble is similar to the male warble. These data indicate that sex differences in budgerigar vocal behavior are not based on early organizational effects of steroid hormones, but rather are probably the result of adult sex differences in circulating testosterone levels.

[Indications of continuous hair cell regeneration in a song bird with genetically-induced cochlear hearing loss]

In recent years evidence has accumulated that birds in contrast to mammals have a great capacity to replace lost hair cells after cochlear trauma. Despite this capacity for cochlear repair, a hereditary hearing deficit for frequencies above 2 kHz has been described in a peculiar strain of canaries (Belgian Water-slagers). Because previous thresholds were determined by psychophysical methods, the origin of the hearing loss could not be identified. In order to determine if this loss originated in the cochlea and if these birds lack the potential for hair cell regeneration, we carried out physiological and morphological analyses of the hearing organ. Our results showed that most of the hair cells displayed severe pathologies. Also, found were small, microvilli-covered cells that resembled forms described during normal hair cell development. Small microvilli-covered cells with small sterovillar bundles have been described as regenerating hair cells in other birds after severe cochlear insults. These observations indicate that adult Belgian Waterslager canaries continuously produce new cochlear hair cells. They do not, however, succeed in reforming a normal basilar papilla. We believe that these birds are a promising model for future studies of cochlear hair cell repair mechanisms.

Critical bands and critical-ratio bandwidth in the European starling

Critical bands (CB) and critical-ratio (CR) bandwidth were determined in five European starlings (Sturnus vulgaris) using a GO/NOGO procedure and the method of constant stimuli. Test-tone frequencies were 1, 2, 4, and 6.3 kHz. Critical ratios were independent of the level of the white noise masker. The lowest CR of 21.8 dB was found at 1 kHz, and the CR monotonically increased on average by 2.3 dB per octave. CR-bandwidths at a masker spectrum level of 41 dB were 151, 191, 437, and 501 Hz at 1, 2, 4, and 6.3 kHz, respectively. With the exception of the test-tone frequency of 6.3 kHz, the size of the critical bands measured with a band-narrowing procedure was similar to that of the CR-bandwidth. CBs were 135, 233, 345, and 1156 Hz at 1, 2, 4, and 6.3 kHz, respectively. A repeat measurement at 6.3 kHz with another speaker position yielded a CB of 860 Hz. The results of this psychoacoustic study in the starling are discussed with respect to comparative data from other vertebrates and to neurophysiological bandwidth measurements of tuning curves of auditory-nerve fibres.

Discrimination of synthetic full-formant and sinewave/ra-la/continua by budgerigars (Melopsittacus undulatus) and zebra finches (Taeniopygia guttata)

Discrimination of three synthetic versions of a/ra-la/ speech continuum was studied in two species of birds. The stimuli used in these experiments were identical to those used in a previous study of speech perception by humans [Best et al., Percept. Psychophys. 45, 237-250 (1989)]. Budgerigars and zebra finches were trained using operant conditioning and tested on three different series of acoustic stimuli: three-formant synthetic speech, sinewave versions of those tokens, and isolated F3 tones from the sinewave speech. Both species showed enhanced discrimination performance near the /l/-/r/ boundary in the full-formant speech continuum, whereas for the F3 continuum, neither species showed a peak near this boundary. These results are similar to human discrimination of the same continua. Budgerigars also showed a peak in discrimination of the sinewave analog continuum paralleling that for full-formant syllables, similar to humans who are induced to perceive sinewave speech as speech. Zebra finches, by contrast, showed a relatively flat function mirroring their performance for F3 sinewaves, similar to humans who are induced to perceive sinewave speech as nonspeech. These data provide new evidence of species similarities and differences in the discrimination of speech and speechlike sounds. These data also strengthen and refine previous findings on the sensitivities of the vertebrate auditory system to the acoustic distinctions between speech sound categories.

Peripheral basis for the auditory deficit in Belgian Waterslager canaries (Serinus canarius)

Recently, behavioural thresholds obtained in canaries of the Belgian Waterslager strain showed that these birds have an inherited auditory deficit. Canaries of this strain have absolute auditory thresholds at frequencies above 2.0 kHz that are as much as 40 dB above the threshold of canaries of other strains. We obtained audiograms from cochlear microphonics and from compound action potentials from the 8th nerve of Waterslager and non-Waterslager canaries and compare these results to previous behavioural data on hearing in this species. We also examined the growth of evoked potential amplitude-intensity functions in Waterslager and non-Waterslager canaries. Together with reflectance measurements of middle-ear function from both Waterslager and non-Waterslager canaries, we conclude that the origin of auditory deficit in Waterslager canaries lies in the cochlea.

Inner-ear abnormalities and their functional consequences in Belgian Waterslager canaries (Serinus canarius)

Recent reports of elevated auditory thresholds in canaries of the Belgian Waterslager strain have shown that this strain has an inherited auditory deficit in which absolute auditory thresholds at high frequencies (i.e. above 2.0 kHz) are as much as 40 dB less sensitive than the thresholds of mixed-breed canaries and those of other strains. The measurement of CAP audiograms showed that the hearing deficit is already present at the level of the auditory nerve (Gleich and Dooling, 1992). Here we show gross abnormalities in the anatomy of the basilar papilla of Belgian Waterslager canaries at the level of the hair cell. The extent of these abnormalities was correlated with the amount of hearing deficit as measured behaviorally.

Vocal plasticity in budgerigars (Melopsittacus undulatus): evidence for social factors in the learning of contact calls

Distance or contact calls of 6 unrelated adult male budgerigars (Melopsittacus undulatus) were recorded before and during 8 weeks of social contact. The 6 birds were housed in 2 separate groups of 3 each in adjoining cages. Birds in each cage could hear but not see the birds in the neighboring cage. At the beginning of the study, none of the birds shared any contact call types. The first appearance of 1 bird's imitation of a cagemate's contact call type occurred after 1 week. Call type repertoires continued to change; some call types dropped out of the repertoires, and others were modified over time. Birds in the same cage shared the same dominant call type 8 weeks later, and the dominant call types differed between the 2 cages. Thus, budgerigars can learn calls as adults, and call type convergence is achieved through mutual vocal imitation of social companions. In the absence of social but not aural contact, vocal imitation was greatly reduced.

Perception of conspecific faces by budgerigars (Melopsittacus undulatus): II. Synthetic models

Perception of faces by budgerigars (Melopsittacus undulatus) was studied with computerized images modeled after natural faces. Individual facial characteristics were varied with all others held constant; then relative importance among several features was determined by varying each within a single experiment. Characteristics with the potential to signal important biological information (e.g., age or sex) were perceptually salient, whereas characteristics that vary among faces but have limited potential to signal important information were not. Model faces were also presented in a normal or an altered configuration. Birds discriminated among faces in a normal configuration more easily than among models with an altered configuration even when the facial features on which the discrimination was based differed in the same way; this suggests that configurational cues play an important role in face perception by budgerigars.

Perception of conspecific faces by budgerigars (Melopsittacus undulatus): I. Natural faces

Perception of faces by 4 budgerigars (Melopsittacus undulatus), a species of small parrot, was studied with a same-different discrimination task. Reaction times were taken as a measure of the similarity between pairs of faces and analyzed with multidimensional scaling to reveal patterns of similarity among the faces. The perception of natural faces was tested to determine which characteristics were perceptually salient. Color, patterns of markings, darkness of the iris, and size of the pupil corresponded to the observed patterns of similarity among the faces. Differences among budgerigar faces were more salient than differences among zebra finch faces, and budgerigar faces were perceptually distinct from the faces of other avian species. The results from these experiments provide a basis for understanding the ways in which these signals function in the coordination of social behaviors.

Auditory perception of conspecific and heterospecific vocalizations in birds: evidence for special processes

Budgerigars (Melopsittacus undulatus), canaries (Serinus canaria), and zebra finches (Poephila guttata castanotis) were tested for their ability to discriminate among distance calls of each species. For comparison, starlings (Sturnus vulgaris) were tested on the same sounds. Response latencies to detect a change in a repeating background of sound were taken as a measure of the perceptual similarity among calls. All 4 species showed clear evidence of 3 perceptual categories corresponding to the calls of the 3 species. Also, budgerigars, canaries, and zebra finches showed an enhanced ability to discriminate among calls of their own species over the calls of the others. Starlings discriminated more efficiently among canary calls than among budgerigar or zebra finch calls. The results show species differences in discrimination of species-specific acoustic communication signals and provide insight into the nature of specialized perceptual processes.

Sound localization in small birds: absolute localization in azimuth

Nine small birds of 3 species (Melopsittacus undulatus, Serinus canarius, and Poephila guttata) were trained in an operant procedure to fly to sound sources for food reward. The angle between the 2 sound sources was varied on a session-by-session basis, and threshold (i.e., minimum resolvable angle) was taken as the angle that corresponded to a performance level of 75% correct. In all, thresholds were calculated for pure tones of 5 different frequencies, noise bands of 3 different spectral compositions, and species-specific contact or distance calls recorded from each of the 3 species. Thresholds for both simple and complex stimuli were larger than 25 degrees. There were statistically significant species differences for each stimulus set, but these differences were not correlated with species differences in head size. Birds with 1 ear plugged performed as well as binaural birds in this task. Birds deafened in 1 ear, however, performed at chance.

Perception of distance calls by budgerigars (Melopsittacus undulatus) and zebra finches (Poephila guttata): assessing species-specific advantages

Budgerigars (Melopsittacus undulatus) and zebra finches (Poephila guttata) share a common functional class of vocalizations called distance calls. The perception of species-specific distance calls by both species was measured with a habituation-dishabituation operant paradigm. Changes in discrimination performance were noted as listening conditions were modified or stimulus properties were altered. Both species showed better performance for calls of their own species. For zebra finches this tendency increased slightly when a background noise was added to the testing environment. Shifting the frequency region of the calls affected the discrimination performance of male budgerigars but not females or zebra finches. Reversing the temporal order of the calls affected the perceptual advantage for conspecific vocalizations in zebra finches but not in budgerigars. These results highlight species differences in perceiving acoustic communication signals.

Detection of gaps in noise by budgerigars (Melopsittacus undulatus) and zebra finches (Poephila guttata)

Temporal gap detection thresholds were obtained for two species of birds, budgerigars and zebra finches, which are known to have different auditory filter bandwidths. Both species showed gap detection thresholds of about 2.5 msec for broadband noise stimuli. Comparing octave bands of noise centered at 1, 3, and 5 kHz, zebra finches showed the smallest gap thresholds for the noise band centered at 5 kHz whereas budgerigars showed the smallest gap detection thresholds for the noise band centered at 3 kHz. The results from zebra finches are generally consistent with filter theories of auditory spectro-temporal perception whereas the result from budgerigars are not. In aggregate, these comparative data suggest the relation between spectral and temporal resolving power in these two species may involve different mechanisms.

Detection of auditory sinusoids of fixed and uncertain frequency by budgerigars (Melopsittacus undulatus) and zebra finches (Poephila guttata)

Three budgerigars and three zebra finches were tested for their ability to detect sinusoidal stimuli in the presence of broadband noise. Masked thresholds for 1, 2, and 4 KHz pure tones were measured with a fixed frequency condition, in which only one test frequency was presented in a session, and with an uncertain frequency condition, in which three signal frequencies were presented in random order in one session. The critical signal/noise ratios obtained in the fixed frequency condition were similar to those reported in a previous study (Okanoya and Dooling, 1987) for both species. When tested in the uncertain frequency condition, critical ratios for zebra finches increased for 1.5 dB at 1 KHz signal but remained unchanged 2 and 4 kHz. The critical ratios for budgerigars showed no difference at any frequency in the uncertain frequency condition. These results suggest that (1) budgerigars and zebra finches are similar in the degree to which attention factors are involved in the detection of signals in noise, and (2) the unusual shape of the budgerigar critical ratio function is not the result of central attentional processes.

Hearing and vocalizations in hybrid Waterslager-Roller canaries (Serinus canarius)

A male canary from the Belgian Waterslager strain, which is known for its elevated high-frequency thresholds and a female of German Roller canary with normal high-frequency thresholds were bred. Operant techniques and a psychophysical tracking procedure were used to measure auditory sensitivity of six F1 hybrid canaries from this cross. Three patterns of auditory sensitivity were observed in the six hybrid birds. Four birds showed elevated high-frequency hearing characteristic of the Belgian Waterslager strain; one bird showed normal hearing, and one bird showed an intermediate pattern of auditory sensitivity. A spectral analysis of contact calls recorded from these F1 hybrid canaries showed a spectral distribution of energy which was intermediate between that of Belgian Waterslager strain and the German Roller strain. These results suggest there may be a relation between hearing sensitivity and the spectral distribution of energy in the calls of domestic canaries. Such hybrids may prove useful for examining the genetic basis of more complex behaviors such as vocal learning.

Speech perception by budgerigars (Melopsittacus undulatus): spoken vowels

Discrimination of natural, sustained vowels was studied in 5 budgerigars. The birds were trained using operant conditioning procedures on a same-different task, which was structured so that response latencies would provide a measure of stimulus similarity. These response latencies were used to construct similarity matrices, which were then analyzed by multidimensional scaling (MDS) procedures. MDS produced spatial maps of these speech sounds where perceptual similarity was represented by spatial proximity. The results of the three experiments suggest that budgerigars perceive natural, spoken vowels according to phonetic categories, find the acoustic differences among different talkers less salient than the acoustic differences among vowel categories, and use formant frequencies in making these complex discriminations.

Speech perception by budgerigars (Melopsittacus undulatus): the voiced-voiceless distinction

Discrimination of synthetic speech sounds from the bilabial, alveolar, and velar voice onset time (VOT) series was studied in 5 budgerigars. The birds were trained, using operant conditioning procedures, to detect changes in a repeating background of sound consisting of a synthetic speech token. Response latencies for detection were measured and were used to construct similarity matrices. Multidimensional scaling procedures were then used to produce spatial maps of these speech sounds, in which perceptual similarity was represented by spatial proximity. The results of these experiments suggest that budgerigars discriminate among synthetic speech sounds from these three VOT continua, especially between those from the bilabial and alveolar series, in a categorical fashion.

Strain differences in auditory thresholds in the canary (Serinus canarius)

Canaries (Serinus canarius) of the Belgian waterslager strain from two different colonies were trained with operant techniques for audiometric testing. For both young and old birds, absolute thresholds in the middle- to high-frequency region of the audiogram were between 30 and 40 dB (SPL) higher than those of other song birds, including canaries of other strains. Thus the Belgian waterslager canary, selectively bred for loud, low-pitched song, has also developed poor high-frequency hearing.

Hearing in passerine and psittacine birds: a comparative study of absolute and masked auditory thresholds

Operant conditioning and a psychophysical tracking procedure were used to measure auditory thresholds for pure tones in quiet and in noise for seven species of small birds--the budgerigar, canary, cockatiel, European starling, song sparrow, swamp sparrow, and the zebra finch. Audibility curves are roughly similar among the seven birds, with the maximum sensitivity between 2 and 5 kHz and poorer sensitivity outside this narrow region. Critical ratios (signal-to-noise ratio at masked threshold) were calculated from pure-tone thresholds in noise. Except for the budgerigar, the critical ratio functions of all birds increase at the rate of 3 dB/octave. This pattern is typical of that observed in most vertebrates. Critical ratios in the budgerigar, on the other hand, decrease gradually from 0.5 kHz to 2.8 kHz and increase dramatically above 2.8 kHz. The present research demonstrates that the critical ratio function for the budgerigar is not only different from other vertebrates but also different from other birds.

Auditory pathways in the budgerigar. I. Thalamo-telencephalic projections

Thalamo-telencephalic auditory pathways in the budgerigar (Melopsittacus undulatus) were studied using horseradish peroxidase (HRP) histochemistry and amino acids autoradiography. The results indicate that in this species the thalamic auditory relay nucleus, n. ovoidalis, projects upon a circumscribed region of the caudal and caudomedial neostriatum including field 'L' and immediately adjacent portions of the neostriatum intermedium, pars dorsolateralis (NIDL). This region of NIDL also receives inputs from another thalamic nucleus, n. dorsolateralis posterior (DLP). In the DLP is in receipt of tectal inputs. Projections of DLP upon NIDL were confirmed with amino acids autoradiography. The results of the HRP experiments indicate that different portions of n. ovoidalis project upon different portions of field 'L' and NIDL. Neurons in the dorsal and lateral portions of the n. ovoidalis project upon more medial portions of field 'L'. Neurons located centrally in the n. ovoidalis project upon central and lateral portions of field 'L'. Neurons in the ventromedial portion of the n. ovoidalis are labeled in all cases in which HRP is placed in either field 'L' or in the DLP projection field immediately adjacent to field 'L' proper. HRP injections placed in NIDL lateral to the projection fields of the n. ovoidalis and DLP label neurons within other diencephalic nuclei including the n. subrotundus. The caudal and intermediate levels of the neostriatum intermedium apparently serve as a complex processing area for many thalamic inputs in this species. The existence of multiple ascending thalamo-telencephalic projections from portions of the thalamus receiving inputs from both the visual (i.e., tectal) and auditory (i.e., n. mesencephalicus lateralis pars dorsalis) portions of the midbrain roof (i.e., from DLP and from n. ovoidalis) suggests the possibility that intermodal associations may take place in these telencephalic fields. Such partially converging pathways may provide a basics for intermodal associations which are important in individual recognition and social signalling systems in this species.

Perception of species-specific contact calls by budgerigars (Melopsittacus undulatus)

Three budgerigars were trained with operant techniques to discriminate examples of species-specific contact calls. Budgerigars were also tested on vocalizations from another avian species--the canary (Serinus canarius). Budgerigars showed an equivalent ability to discriminate and remember both budgerigar and canary calls. Additional tests showed that both temporal and spectral cues were important in the discrimination of species-specific calls by the budgerigar. However, spectral cues occurring in the region of 2.0-4.0 kHz appear to be critical for the discrimination. These experiments support the notion of a generalized, but highly sophisticated, perceptual learning system in the budgerigar for the processing of vocal signals.

Nonsimultaneous auditory masking in the budgerigar (Melopsittacus undulatus)

The frequency resolving power of budgerigars and that of humans were compared on several nonsimultaneous masking procedures in which one pure tone is used to mask another. Similar patterns of frequency selectivity were found for all three masking procedures (forward, backward, and combined forward-backward) for both species. Budgerigars showed considerably greater frequency resolving power on all three procedures than humans. Budgerigars also showed differences in frequency resolving power across masking conditions, but human listeners did not. These results indicate that the budgerigar auditory system may be even more highly tuned than was previously thought and suggest fundamental differences between the mechanisms of frequency selectivity of bird and those of humans.

Forward and backward auditory masking in the parakeet (Melopsittacus undulatus)

Backward, foward and simultaneous masking of brief tone pulses was examined in four parakeets. The pattern of non-simultaneous masking in parakeets is quite similar to that observed in humans with rates of threshold recovery faster for backward compared to forward masking conditions. These results may have relevance for the design of vocal signals and the coding of species-specific information in parakeets.

Auditory sensitivity, equal loudness, temporal resolving power, and vocalizations in the house finch (Carpodacus mexicanus)

Absolute thresholds for pure tones were measured in four house finches by use of avoidance conditioning and a modified method of limits. Response reaction time to each tone presentation served as a data base for generating a family of "equal loudness" contours. Temporal resolving power was measured in two additional birds and compared with similar measures in man. The results are discussed in relation to previous studies of vocalizations in the house finch, and the average power spectra of selected individual vocalizations are presented.

Hearing in the parakeet (Melopsittacus undulatus): absolute thresholds, critical ratios, frequency difference limens, and vocalizations

Avoidance conditioning and the method of limits were used to measure absolute auditory thresholds, masked thresholds, and critical ratios in 4 parakeets. The same procedure was then used to study frequency difference limens in 6 additional animals. The power spectrum and "constancy of intonation" of the parakeet call were also measured and related to the absolute and differential frequency sensitivity. The mechanism of frequency analysis in the parakeet ear was considered in relation to the present results and to the anatomical and functional differences between the avian and mammalian auditory systems.