Hearing research
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Neural activity of single eighth nerve fibers was recorded with glass microelectrodes in anesthetized normal-hearing rabbits. The units had a spontaneous rate ranging from 0 to approximately 120 spikes/s. In a large number of fibers this rate was below 2 spikes/s. ⋯ For the majority of fibers the dynamic range was 20-30 dB. Some fibers did not reach saturation within the stimulus intensity available. The tip-to-tail distance was 50 dB for high-frequency units at one octave below CF, a matter of potential interest for further studies of animals with inner ear lesions.
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The morphology of the basilar papilla of the bobtail lizard was investigated with standard light- and scanning-electron-microscopical methods. The papilla can be subdivided into two parts: a small apical segment which is rather uniform in structure and a long basal segment which displays various systematic changes along its length, for example in the density of the hair cells, the height and shape of the hair-cell stereovillar bundles, the number of stereovilli per bundle and the size of the tectorial structure. ⋯ Both tectorial structures are probably sensitive to changes in their ionic environment. The possible functional implications of the papillar morphology described here are discussed with respect to a model of frequency tuning in the bobtail lizard.
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Exposure of the vestibulo-cochlear anastomosis in the cat allows single-unit recording of afferent fibers originating from inner hair cells and olivocochlear efferent fibers to the outer hair cells. Comparison of data obtained from individual animals show similarities in absolute threshold and dynamic range between the efferent fibers and that subset of auditory-nerve afferents with low spontaneous rates and high thresholds [Liberman (1978) J. Acoust. ⋯ Am. 63, 442-455]. The afferent-efferent comparisons also show that interanimal variability in efferent excitability (thresholds and maximum discharge rates) is directly correlated with variation in the mean spontaneous discharge rate of the afferent fiber population. It is suggested that interanimal differences in efferent excitability may explain some of the interanimal variability in susceptibility to acoustic trauma.
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The origin of the frequency selectivity of neurons in the vertebrate auditory periphery is one of the most important questions in auditory research today. In an attempt to delineate the extent to which structures outside the sensory cells play a role in determining peripheral auditory responses, we measured the mechanical displacement of the basilar membrane and the selectivity of nerve fibres at the same location in the bobtail lizard. These data indicate a contribution to frequency selectivity, the tuning of which resembles a high-pass resonant filter characteristic, arising subsequent to the basilar membrane motion. A comparison of these data with the tuning of auditory-nerve fibres originating from papillar areas in other lizard species without a tectorial membrane, suggests that it is the involvement of the tectorial membrane in a mechanical resonance which increases the frequency selectivity.
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In previous studies describing the effects of electrically stimulating the olivocochlear bundle, it seems possible that both medial and lateral (MOC and LOC) efferents may have been stimulated. To selectively stimulate MOC efferents, we used an electrode placed at the origin of the MOC efferents in the brainstem (MOC stimulation). For comparison, a stimulating electrode was placed in the fourth ventricle at the decussation of the crossed olivocochlear bundle where both MOC and LOC efferents are present (midline-OCB stimulation). ⋯ These results, taken together with anatomical data in the literature, are consistent with the hypothesis that, in the cat, MOC and midline-OCB stimulation have their effect solely through synapses on outer hair cells. The data are consistent with the hypothesis that the level shifts are produced by MOC efferents acting on outer hair cells to reduce the mechanical stimulus to inner hair cells. It seems likely that some other mechanism is required to produce the plateau depressions, at least for auditory-nerve fibers with high spontaneous rates.