Hearing research
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Electroneural response patterns of single auditory-nerve neurons were studied in aminoglycoside-deafened squirrel monkeys. The electrical stimuli were delivered through bipolar electrodes implanted in the scala tympani. The effects of pulse width, shape, frequency, and intensity on neural adaptation, phase locking, and spectral content were evaluated. ⋯ This may account for the proportional relationship between pitch and stimulus intensity seen in some cochlear implant patients. Our study demonstrates that auditory-nerve neurons comply with basic neurophysiological principles in their responses to electrical stimulation. These principles should be incorporated into the cochlear prosthesis stimulator if more normal neural response patterns are desired in the cochlear prosthesis patient.
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Despite the independent evolution of birds and mammals, a number of structural similarities of their hearing organs have developed in parallel. By tracing the peripheral origin of functionally-characterized primary neurons, the present study demonstrates functional similarities between the respective hair cell populations of the hearing organs of birds and mammals. ⋯ The finding that (with the exception of a specialized area near the apical end) only tall hair cells situated on the neural limbus receive active afferent innervation is a functional parallel to the mammalian inner hair cells. The thresholds of afferents increase with distance of the related hair cells from the neural side of the papilla and cover a range of more than 50 dB within the area of tall hair cells.
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The response properties of single auditory-nerve fibers in barbiturate-anaesthetized cats were recorded with and without simultaneous presentation of sound to the contralateral ear. The tendons to the middle ear muscles on both sides were cut before all experiments, and contralateral stimuli were restricted to levels below the threshold for crosstalk to the ipsilateral ear. Contralateral tones and broad-band noise were found to suppress the responses of auditory-nerve afferents to ipsilateral tones at their characteristic frequency (CF), but not to tones off CF. ⋯ The suppressive effect of contralateral sound completely disappeared immediately after severing the entire olivocochlear bundle (OCB) within the internal auditory meatus. the completeness of the OCB cuts was assessed histologically. Most of the suppressive effect remained after lesions to the OCB in the floor of the IVth ventricle which eliminated the crossed olivocochlear projection but spared most of the uncrossed projection. It is argued that this suppressive effect of contralateral sound is mediated by the uncrossed olivocochlear efferents to the outer hair cells.
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The suppression by moderate-level contralateral sound of auditory-nerve-fiber responses to ipsilateral stimuli at the characteristic frequency (CF) was studied in barbiturate-anesthetized cats. The dependence of suppression strength on ipsilateral and contralateral stimulus variables, including level, frequency, bandwidth, and timing relationships, was investigated. The principal findings were: (1) Contralateral-sound suppression is greatest when the ipsilateral stimulus level is within the dynamic range of the unit. (2) When the contralateral stimuli are tones, suppression is greatest when the contralateral tone frequency is at or near CF. (3) Units with CFs above 3-4 kHz are only weakly suppressed by contralateral CF tones but more strongly suppressed by contralateral broad-band noise. (4) Continuous contralateral stimuli are significantly more effective suppressors than are gated stimuli. The characteristics of contralateral-sound suppression are compared with the physiology and anatomy of the uncrossed medial olivocochlear efferents, the subset of efferents which are the primary mediators of the effect.
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In order to study the effects of efferent activity, olivocochlear efferents were stimulated with an electrode in the fourth ventricle at the decussation of the crossed olivocochlear bundle (midline-OCB stimulation) or with an electrode at the brainstem origin of medial efferents (MOC stimulation). Tuning curves, or similar measures of threshold, were obtained from auditory-nerve fibers in the presence or absence of efferent stimulation. Efferent stimulation raised the thresholds of fibers for tones at the characteristic frequency (CF) by an amount which varied with the spontaneous rate (SR) of the auditory-nerve fiber. ⋯ This can be understood in terms of efferent activity depressing basilar membrane motion and affecting regions at, and apical to, the activated efferent synapses. To explain the low-SR threshold shifts, an additional way in which efferent activity inhibits responses appears to be required. The data are consistent with one function of the medial efferents being to raise the thresholds of auditory-nerve fibers and thereby adjust the effective range of the auditory system.