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1.  Inner-Ear Morphology of the New Zealand Kiwi (Apteryx mantelli) Suggests High-Frequency Specialization 
The sensory systems of the New Zealand kiwi appear to be uniquely adapted to occupy a nocturnal ground-dwelling niche. In addition to well-developed tactile and olfactory systems, the auditory system shows specializations of the ear, which are maintained along the central nervous system. Here, we provide a detailed description of the auditory nerve, hair cells, and stereovillar bundle orientation of the hair cells in the North Island brown kiwi. The auditory nerve of the kiwi contained about 8,000 fibers. Using the number of hair cells and innervating nerve fibers to calculate a ratio of average innervation density showed that the afferent innervation ratio in kiwi was denser than in most other birds examined. The average diameters of cochlear afferent axons in kiwi showed the typical gradient across the tonotopic axis. The kiwi basilar papilla showed a clear differentiation of tall and short hair cells. The proportion of short hair cells was higher than in the emu and likely reflects a bias towards higher frequencies represented on the kiwi basilar papilla. The orientation of the stereovillar bundles in the kiwi basilar papilla showed a pattern similar to that in most other birds but was most similar to that of the emu. Overall, many features of the auditory nerve, hair cells, and stereovilli bundle orientation in the kiwi are typical of most birds examined. Some features of the kiwi auditory system do, however, support a high-frequency specialization, specifically the innervation density and generally small size of hair-cell somata, whereas others showed the presumed ancestral condition similar to that found in the emu.
doi:10.1007/s10162-012-0341-4
PMCID: PMC3441955  PMID: 22772440
hair cell; basilar papilla; auditory nerve; Paleognathae
2.  Computational Diversity in the Cochlear Nucleus Angularis of the Barn Owl 
Journal of Neurophysiology  2002;89(4):2313-2329.
The cochlear nucleus angularis (NA) is widely assumed to form the starting point of a brain stem pathway for processing sound intensity in birds. Details of its function are unclear, however, and its evolutionary origin and relationship to the mammalian cochlear-nucleus complex are obscure. We have carried out extracellular single-unit recordings in the NA of ketamine-anesthetized barn owls. The aim was to re-evaluate the extent of heterogeneity in NA physiology because recent studies of cellular morphology had established several distinct types. Extensive characterization, using tuning curves, phase locking, peristimulus time histograms and rate-level functions for pure tones and noise, revealed five major response types. The most common one was a primary-like pattern that was distinguished from auditory-nerve fibers by showing lower vector strengths of phase locking and/or lower spontaneous rates. Two types of chopper responses were found (chopper-transient and a rare chopper-sustained), as well as onset units. Finally, we routinely encountered a complex response type with a pronounced inhibitory component, similar to the mammalian typeIV. Evidence is presented that this range of response types is representative for birds and that earlier conflicting reports may be due to methodological differences. All five response types defined were similar to well-known types in the mammalian cochlear nucleus. This suggests convergent evolution of neurons specialized for encoding different behaviorally relevant features of the auditory stimulus. It remains to be investigated whether the different response types correlate with morphological types and whether they establish different processing streams in the auditory brain stem of birds.
doi:10.1152/jn.00635.2002
PMCID: PMC3259745  PMID: 12612008
3.  Maps of interaural time difference in the chicken’s brainstem nucleus laminaris 
Biological cybernetics  2008;98(6):541-559.
Animals, including humans, use interaural time differences (ITDs) that arise from different sound path lengths to the two ears as a cue of horizontal sound source location. The nature of the neural code for ITD is still controversial. Current models differentiate between two population codes: either a map-like rate-place code of ITD along an array of neurons, consistent with a large body of data in the barn owl, or a population rate code, consistent with data from small mammals. Recently, it was proposed that these different codes reflect optimal coding strategies that depend on head size and sound frequency. The chicken makes an excellent test case of this proposal because its physical pre-requisites are similar to small mammals, yet it shares a more recent common ancestry with the owl. We show here that, like in the barn owl, the brainstem nucleus laminaris in mature chickens displayed the major features of a place code of ITD. ITD was topographically represented in the maximal responses of neurons along each isofrequency band, covering approximately the contralateral acoustic hemisphere. Furthermore, the represented ITD range appeared to change with frequency, consistent with a pressure gradient receiver mechanism in the avian middle ear. At very low frequencies, below400 Hz, maximal neural responses were symmetrically distributed around zero ITD and it remained unclear whether there was a topographic representation. These findings do not agree with the above predictions for optimal coding and thus revive the discussion as to what determines the neural coding strategies for ITDs.
doi:10.1007/s00422-008-0220-6
PMCID: PMC3170859  PMID: 18491165
Auditory; Hearing; Sound localization; Sensory
4.  Evidence for an Auditory Fovea in the New Zealand Kiwi (Apteryx mantelli) 
PLoS ONE  2011;6(8):e23771.
Kiwi are rare and strictly protected birds of iconic status in New Zealand. Yet, perhaps due to their unusual, nocturnal lifestyle, surprisingly little is known about their behaviour or physiology. In the present study, we exploited known correlations between morphology and physiology in the avian inner ear and brainstem to predict the frequency range of best hearing in the North Island brown kiwi. The mechanosensitive hair bundles of the sensory hair cells in the basilar papilla showed the typical change from tall bundles with few stereovilli to short bundles with many stereovilli along the apical-to-basal tonotopic axis. In contrast to most birds, however, the change was considerably less in the basal half of the epithelium. Dendritic lengths in the brainstem nucleus laminaris also showed the typical change along the tonotopic axis. However, as in the basilar papilla, the change was much less pronounced in the presumed high-frequency regions. Together, these morphological data suggest a fovea-like overrepresentation of a narrow high-frequency band in kiwi. Based on known correlations of hair-cell microanatomy and physiological responses in other birds, a specific prediction for the frequency representation along the basilar papilla of the kiwi was derived. The predicted overrepresentation of approximately 4-6 kHz matches potentially salient frequency bands of kiwi vocalisations and may thus be an adaptation to a nocturnal lifestyle in which auditory communication plays a dominant role.
doi:10.1371/journal.pone.0023771
PMCID: PMC3161079  PMID: 21887317

Results 1-5 (5)