Hearing research
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The temporal pattern of the responses of neurons in the inferior colliculus of the anesthetized rat were studied using continuous tone or noise carrier signals, amplitude modulated by pseudorandom noise. Period histograms of the responses, cross-correlated with the pseudorandom noise, gave an estimate of the unit's impulse responses to modulation. The amplitude-modulation rate transfer function (MTF) was obtained by Fourier transforming the correlograms. ⋯ The MTFs for units responding to amplitude-modulated broadband noise were often flatter in the low frequency region than those generated with tone carriers at corresponding intensities. For some units addition of a broadband noise background to the modulated tone changed the response characteristic of the MTF from bandpass to lowpass and shifted the MTF peak to a lower modulation frequency. The results demonstrate that although neurons in the inferior colliculus are selectively sensitive to the modulation frequency of dynamic stimuli, the response characteristics are not invariant, but instead are closely dependent on the conditions under which the modulation is presented.
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Spatial maps of electrical excitation were constructed by comparing electrical threshold with acoustic CF for large populations of auditory nerve fibers in cats. Thresholds among fibers with the same CF varied by factors of 4 or more. ⋯ With longitudinally oriented pairs, the width, depth, and location of the minimum shifted with stimulus polarity; spread of excitation throughout the cochlea occurred with stimulus intensities 6.2 to 14 dB above the lowest threshold. With radially oriented pairs, minima were sharper and deeper; spread of excitation occurred at intensities 23.7 to 32.8 dB above the minimum threshold.
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Scanning electron microscopy was used to examine the basilar papilla of the granite spiny lizard. The papilla contains three distinct hair cell populations: an apical and a basal population with free-standing cilia, and a central population with a tectorial membrane. In the free-standing populations, stereocilium length decreases towards the ends of the papilla. ⋯ The low CF fibers are associated with the tectorial membrane hair cell population. Fiber CF correlated with hair cell cilium length, not position on basilar membrane, for hair cells with free-standing cilia. Tonotopic organization of high CF fibers could be predicted reasonably well from the histogram of fiber CFs.
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Damage to the tectorial membrane caused by acoustic trauma was examined with scanning and transmission electron microscopy immediately after exposure and at selected time points over a 10 day recovery period. At 0 h of recovery the structure of the tectorial membrane overlying the region of hair cell damage was severely disrupted and connections between the membrane and the basilar papilla were lost. By 24 h of recovery, regeneration of the tectorial membrane was evident in the secretion of new matrix materials by the supporting cells of the basilar papilla. ⋯ However, the regenerated segment included only the honeycomb-like structure of the lower layer of the normal tectorial membrane. The laterally-oriented fibers which form the upper layer of the membrane were not regenerated over the damaged region. These findings indicate that the tectorial membrane is regenerated in parallel with the hair cells during recovery from acoustic trauma but the full extent of this recovery and its effect on cochlear function are not yet clear.
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Examination of pure-tone acoustic damage in the chick cochlea revealed a significant amount of hair cell recovery over a 10 day period following the exposure. The recovery included both a regeneration of stereociliary bundles to replace those that were lost and a reshuffling of the mosaic pattern of the hair cell surfaces that survived. Ten-day-old chicks were exposed to a 1500 Hz pure tone at 120 dB SPL for 48 h and their cochleae were processed for scanning, transmission and light microscopy at 0 h, 24 h, 48 h, 4 d, 6 d and 10 d after exposure. ⋯ After 48 h of recovery, new hair cells were identifiable in the region of hair cell loss and with time they underwent a progressive maturation of their stereociliary bundles. The surviving hair cells showed a dramatic rearrangement and expansion of their surfaces but exhibited no repair of the damaged stereociliary bundles. These results suggest that the chick cochlea is capable of a significant amount of recovery and regeneration following acoustic trauma.