Auditory capabilities of birds in relation to the structural diversity of the basilar papilla.

The basilar papilla length increases systematically with body mass for 41 species from more than 10 avian orders and this relation does not differ between phylogenetic groups. Audiograms of 25 non-strigiform and 12 owl species, normalized relative to best frequency and best threshold, were used to compare audiogram shapes. The analysis revealed that the high frequency flank of the audiogram was remarkably similar across non-strigiform species. The high-frequency limit was on average 1.1 octaves above the best frequency, the low-frequency flank was less steep and showed much more species dependent variability. Audiogram shape in owls was much more variable. Morphological gradients along the basilar papilla revealed a small species dependent variability for the basal region of the basilar papilla and an increasing degree of variability towards the apex. In non-strigiform species, frequency selectivity for 2 and 4 kHz varied systematically with the space on the basilar papilla devoted to processing the corresponding frequency range. Space on the papilla did not vary systematically with frequency selectivity at 1 kHz. This difference between test frequencies might be related to the transition from electrical hair-cell tuning, that dominates below 1-2 kHz, to micromechanical tuning at higher frequencies.

[Endolymph homeostasis and Menière's disease: fundamentals, pathological changes, aminoglycosides]. / Endolymph-Homöostase und M. Menière : Grundlagen, pathologische Veränderungen, Aminoglykoside.

Although low dose intratympanal gentamicin has empirically been very effective in treating Menière's disease, the mechanisms of elimination or amelioration of vertigo are still insufficiently understood. Most animal studies investigating the effect of aminoglycosides used high doses that damage or kill hair cells and many other cell types of the inner ear. Additional studies are needed to investigate the effects of low dose gentamicin to elucidate the mechanisms affecting vertigo. In this article it will be explained how disturbances of endolymph homeostasis lead to endolymphatic hydrops and finally to leakage of K(+) from the endolymph into the perilymphatic space. This can lead to a non-physiological activation of vestibular nerve fibres thus causing vertigo.

Safety of gold in stapes surgery.

Gold prostheses in middle ear surgery were found to have a higher extrusion rate than prostheses made from titanium. Incidences of deafness after insertion of a gold piston into the vestibule during stapes surgery have been observed. The aim of this study was to find out to what degree gold cations can diffuse from the prosthesis piston into the perilymph. For this, gold prostheses were incubated in artificial perilymph for four months, after which the gold content of the perilymph was analysed. As gold exhibits a special behaviour in complex fluids like the perilymph, a new analysing method was developed. The results show that gold does leak out of the pistons, that it can be reliably measured and that the amount of gold reaching the perilymph depends on the contact area. As the amount of gold measured in the perilymph stays far below the toxic level, it is very unlikely that the gold cations diffusing from a stapes prosthesis into the perilymph have a toxic effect on the inner ear hair cells. Inflammatory or allergic reactions to gold induced by direct tissue contact, however, cannot be excluded.

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).

Domestication differentially affects cochlear nucleus subdivisions in the gerbil.

We analyzed the effects of domestication on the subdivisions of the cochlear nucleus in the gerbil (Meriones unguiculatus) by comparing their volumes and rostrocaudal extents in laboratory gerbils and in age-matched F1 offspring of gerbils caught in the wild. In addition, soma size was systematically analyzed in the anteroventral cochlear nucleus of both groups. Total cochlear nucleus volume and rostrocaudal extent were not significantly different between groups either for young (postnatal day 9) animals before the onset of hearing or for young 4-month-old animals. However, the dorsal cochlear nucleus was significantly larger and the anteroventral cochlear nucleus was significantly smaller in young adults of the wild strain. Thus the relative proportions of the cochlear nucleus subdivisions differed between the groups. In addition, soma size was significantly larger in the low-frequency portion of the anterovental cochlear nucleus in domesticated gerbils compared to wild gerbils. To our knowledge, this is the first reported instance of a well-defined brain structure (e.g., the antreovental cochlear nucleus) being larger in the domesticated than in the wild form.

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.

A quantitative study of cochlear afferent axons in birds.

This paper is a comparative study of auditory-nerve morphology in birds. The chicken (Gallus gallus), the emu (Dromaius novaehollandiae) and the starling (Sturnus vulgaris) were chosen as unspecialised birds that have already been used in auditory research. The data are discussed in comparison to a similar earlier study on the barn owl, a bird with highly specialised hearing, in an attempt to separate general avian patterns from species specialisations. Average numbers of afferent fibres from 8775 (starling) to 12¿ omitted¿406 (chicken) were counted, excluding fibres to the lagenar macula. The number of fibres representing different frequency ranges showed broad maxima in the chicken and emu, corresponding to hearing ranges of best sensitivity and/or particular behavioural relevance. Mean axon diameters were around 2 microm in the chicken and starling, and around 3 microm in the emu. Virtually all auditory afferents were myelinated. The mean thickness of the myelin sheaths was between 0.33 microm (starling) and 0.4 microm (emu). There was a consistent pattern in the diameters of axons deriving from different regions. Axons from very basal, i.e. highest-frequency, parts of the basilar papilla were always the smallest. In the emu and the chicken, axons from the middle papillar regions were, in addition, larger than axons innervating apical regions.

Postnatal development of GABA- and glycine-like immunoreactivity in the cochlear nucleus of the Mongolian gerbil (Meriones unguiculatus).

The maturation of the morphological substrate for inhibitory interactions was investigated in the cochlear nucleus of the gerbil with immunocytochemistry for gamma aminobutyric acid (GABA) and glycine on alternating vibratome sections. The patterns of immunostaining obtained with both antibodies in the adult closely conformed to the general mammalian scheme. Qualitative analyses revealed an age-related increase in staining intensity and in the relative numbers of immunolabelled cells after birth up to the age of 3-4 weeks. As early as birth and in all subdivisions of the cochlear nucleus, a few labelled cells and puncta in the sections were stained either with the GABA or the glycine antibody. Immunoreactive puncta and cells were, however, far less abundant than in the adult, and the staining intensity of cells was only weak. The most strikingly GABA-immunolabelled cells at birth were the Golgi cells of the granule-cell domains. The numbers of weakly GABA- and glycine-immunostained cells of the dorsal cochlear nucleus clearly increased between birth and the third postnatal week. At approximately the onset of hearing (postnatal day 12-14), some cells of the dorsal cochlear nucleus and small cells of the ventral cochlear nucleus gained adult-like GABA-staining properties. Almost adult-like labelling intensity was observed in glycine-immunoreactive cells of the deep dorsal cochlear nucleus and in some small cells of the ventral cochlear nucleus. Puncta staining to both antibodies appeared adult-like throughout the cochlear nucleus. About 2 weeks after the onset of hearing (at the latest), adult-like staining of all subsets of immunoreactive cells occurred throughout the cochlear nucleus in all specimens.

The postnatal growth of cochlear nucleus subdivisions and neuronal somata of the anteroventral cochlear nucleus in the Mongolian gerbil (Meriones unguiculatus).

We have analysed the postnatal volume increase of the cochlear nucleus subdivisions from birth to senescence in the Mongolian gerbil. All subdivisions showed the most prominent growth in the 2nd week of life before the onset of hearing on day 12. After the onset of hearing, the dorsal and the postero-ventral portion of the cochlear nucleus showed only a moderate degree of further growth. The antero-ventral cochlear nucleus, however, almost doubled in volume after the onset of hearing, reaching a maximum in 4-month-old animals. In ageing gerbils, we observed a small but systematic reduction of the cochlear nucleus volume. To further assess which elements contribute to the growth of the antero-ventral cochlear nucleus after the onset of hearing, we analysed the somal size of the neurones. Before the onset of hearing, somal and volume growth were highly correlated. However, somata had reached an adult size by the onset of hearing. We conclude that the growth of the antero-ventral cochlear nucleus after the onset of hearing is due to changes in the neuropil.

Fine structure of the basilar papilla of the emu: implications for the evolution of avian hair-cell types.

The morphology of the basilar papilla of the emu was investigated quantitatively with light and scanning electron microscopical techniques. The emu is a member of the Paleognathae, a group of flightless birds that represent the most primitive living avian species. The comparison of the emu papilla with that of other, more advanced birds provides insights into the evolution of the avian papilla. The morphology of the emu papilla is that of an unspecialised bird, but shows the full range of features previously shown to be typical for the avian basilar papilla. For example, the orientation of the hair cells' sensitive axes varied in characteristic fashion both along and across the papilla. Many of the quantitative details correlate well with the representation of predominantly low frequencies along the papilla. The most distinctive features were an unusually high density of hair cells and an unusual tallness of the hair-cell bodies. This suggests that the evolution of morphologically very short hair cells, which are a hallmark of avian papillae, is a recent development in evolution. The small degree of differentiation in hair-cell size contrasts with the observation that a significant number of hair cells in the emu lack afferent innervation. It is therefore suggested that the development of functionally different hair-cell types in birds preceded the differentiation into morphologically tall and short hair cells.

Age-dependent effects of the onset of a conductive hearing loss on the volume of the cochlear nucleus subdivisions and the expression of c-fos in the mongolian gerbil (Meriones unguiculatus).

A monaural conductive hearing loss was induced by interrupting the chain of the middle ear ossicles on the right side in gerbils of four different age groups (P12-14, P20-21, P42 and P84). The volumes of the cochlear nucleus subdivisions and the number of cells that expressed immunoreactivity for c-fos after noise stimulation were determined on the left and right side in the deprived animals, and in undeprived control animals when they reached the age of 6 months. The anteroventral cochlear nucleus on the deprived side was reduced in volume when the deprivation started before the age of 3 months. The other cochlear nucleus subdivisions showed no systematic age-dependent reductions. The expression of c-fos in the dorsal cochlear nucleus appeared more resistant to a hearing loss, with deprivation being more effective in younger animals. c-fos expression was also dramatically reduced in the ventral cochlear nucleus, regardless of age at the onset of hearing loss.

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.

[Continuous proliferation of supporting cells and indications for hair cell differentiation in the inner ear of adult song birds with genetic cochlear hearing loss]. / Kontinuierliche Proliferation von Stützzellen und Hinweise für Haarzelldifferenzierung im Hörorgan von adulten Singvögeln mit genetischer cochleärer Hörstörung.

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.

Surface morphology of basilar papilla of the tufted duck Aythya fuligula, and domestic chicken Gallus gallus domesticus.

Quantitative details of the surface morphology of the hearing organ, the Papilla basilaris, as seen in the scanning electron microscope are described for the tufted duck Aythya fuligula and for comparison for the domestic chicken Gallus gallus domesticus, for which some published information is already available. As in the other avian species investigated to date, each papilla shows a unique constellation of features. The papilla of the tufted duck is 3.5 mm long in the unfixed state and contains 8,200 sensory hair cells. It shows systematic changes in its surface features along the length and across the width of the sensory epithelium. In general, its features and those of the chicken Papilla basilaris can be described as relatively primitive in comparison with other species. The tufted duck papilla does, however, show one feature that has so far been found to be well developed only in advanced papillae; the number of stereovilli per hair cell bundle is generally much higher on hair cells of the neural than those on the abneural side. This difference is only weakly developed in the chicken. It is clear that features considered to be evolutionarily advanced were acquired independently of one another during evolution and that each bird species can show a mosaic of primitive and advanced features.

Sound induced expression of c-Fos in GABA positive neurones of the gerbil cochlear nucleus.

To characterize c-Fos-expressing neurones in more detail, a double labelling technique was developed to simultaneously demonstrate the presence of GABA-like and c-Fos-like immunoreactivity in cryostat sections of the gerbil cochlear nucleus. The animals were exposed to a wide-band noise stimulus for the induction of c-Fos expression. The distribution of c-Fos-positive cells and GABA-positive somata was consistent with previous reports. Our double labelling approach revealed the presence of GABA-positive cells that showed c-Fos expression after noise stimulation. One interpretation of this finding is that a prolonged acoustic exposure induces long term changes in inhibitory interneurones to adjust the output of the cochlear nucleus.

An excitation-pattern model for the starling (Sturnus vulgaris).

This paper develops and tests an excitation-pattern model for the starling. Like excitation-pattern models for humans [e.g., Zwicker, Acustica 6, 365-381 (1956); Florentine and Buus, J. Acoust. Soc. Am. 70, 1646-1654 (1981)], the model for starlings provides a unified account of a large body of data. The foundation of the model is a critical-band scale, which is derived as an equal-distance scale according to a cochlear-map function. The cochlear-map function is determined as a best-fitting function to physiological data relating characteristic frequency (CF) of auditory-nerve fibers to their place of innervation on the basilar papilla. Excitation patterns are derived from auditory-nerve measurements of levels at CF necessary to produce firing rates equal to those evoked by a test tone. The shape of these excitation patterns is independent of level and frequency when plotted on a cochlear-distance scale. The resulting model indicates that 10-dB bandwidths of auditory-nerve tuning curves and frequency DLs can be approximated as equal distances along the basilar papilla. Predictions of level discrimination are in good agreement with the data, except below 20 dB SL. Overall, the present work indicates that excitation-pattern models account for a wide range of auditory phenomena in both humans and starlings, when the models take into account differences in critical-band scales, absolute thresholds, excitation-pattern slopes, and growth of excitation, which is linear in starlings, but nonlinear in humans.

[Indications of continuous hair cell regeneration in a song bird with genetically-induced cochlear hearing loss]. / Hinweise für eine kontinuierliche Haarzellregeneration bei einem Singvogel mit erblich bedingter cochleärer Hörstörung.

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.

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.

Temporal modulation transfer functions in the European Starling (Sturnus vulgaris): II. Responses of auditory-nerve fibres.

The temporal resolution of cochlear-nerve fibres in the European starling was determined with sinusoidally amplitude-modulated noise stimuli similar to those previously used in a psychoacoustic study in this species (Klump and Okanoya, 1991). Temporal modulation transfer curves (TMTFs) were constructed for cochlear afferents allowing a direct comparison with the starling's behavioural performance. On average, the neuron's detection of modulation was less sensitive than that obtained in the behavioural experiments, although the most sensitive cells approached the values determined psychophysically. The shapes of the neural TMTFs generally resembled low-pass or band-pass filter functions, and the shapes of the averaged neural functions were very similar to those obtained in the behavioural study for two different types of stimuli (gated and continuous carrier). Minimum integration times calculated from the upper cut-off frequency of the neural TMTFs had a median of 0.97 ms with a range of 0.25 to 15.9 ms. The relations between the minimum integration times and the tuning characteristics of the cells (tuning curve bandwidth, Q10 dB-value, high- and low-frequency slopes of the tuning curves) are discussed. Finally, we compare the TMTF data recorded in the starling auditory nerve with data from neurophysiological and behavioural observations on temporal resolution using other experimental paradigms in this and other vertebrate species.