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Results for “Cochlear Microphonic Potentials”

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At least 811 records · Page 45Linked to original sources
PubMed1982

Directionality of sound pressure transformation at the cat's pinna.

The directionality of the cat's pinna was studied by using the amplitude of the cochlear microphonic (CM) as a quantitative indicator of tympanic sound pressure level (SPL). It was found that tympanic SPL varied with the location of a free field stimulator in anechoic space. For high (tonal) frequencies, there was a circumscribed optimal area for tympanic SPL in the frontal ipsilateral sound field, in confirmation of previous findings with other techniques that the pinna has an acoustical axis. The directionality of the pinna, determined from the solid angle enclosed by the 5 dB isointensity-decrement line with respect to the optimal position, increased with frequency. For low tonal frequencies, no circumscribed optimal area in the frontal sound field could be distinguished, and tympanic SPL fell by only 10-12 dB for displacements of 90 degrees into the contralateral sound field. Excision of the pinna abolished the circumscribed optimal areas for tympanic SPL and revealed the pinna produces up to 28 dB amplification of acoustic signals delivered 'on-axis'.

Animals↗
PubMed1982

Two-tone interactions in the cochlear microphonic.

Two-tone interactions are explored for the cochlear microphonic (CM) in the guinea pig. Recordings are made from turns one and three using differential electrodes in the perilymphatic space or pipettes placed in scala media through a fenestra over the stria vascularis. We focus on magnitude changes associated with the introduction of appropriate interference tones and on various types of phase shift concomitant with these magnitude variations that have not received documentation in the literature. Based on extensive parametric data, it is suggested that some features of the gross interference phenomenon may be a consequence of the vectorial summation of outputs from contributing hair cell generators. These spatial effects appear to determine phase behavior and the influence of probe frequency on the frequency of maximal interference. In addition, the apparent interval between out defined best frequency (CF) and the frequency of maximal interference is most likely due to an underestimation of CF resulting from phase cancellation between CM-producing hair cell populations. However, after compensating for these spatial effects, several aspects of the CM interference phenomenon seem to be analogous to two-tone suppression in auditory nerve fibers. A direct one-to-one relationship is not implied since the latter reflect the outputs of inner hair cells while CM interference most likely reflects outer hair cell behavior. As a result, the association between suppression and interference must be sought in the process by which outer hair cell influence inner hair cell transduction.

Acoustic Stimulation↗
PubMed1982

Reverse correlation study of cochlear filtering in normal and pathological guinea pig ears.

The filtering properties of single cochlear fibres have been determined in normal and kanamycin-treated guinea pigs using the reverse correlation technique. This method allows investigation of filtering over a wide dynamic range. For normal guinea pig fibres, the near threshold filter functions obtained with this method correspond to the tone derived frequency threshold curves ((FTCs). The 10 dB bandwidth of the filter functions increased monotonically with increasing noise levels above threshold. Thus with noise levels at approximately 50 dB above threshold, the 10 dB bandwidth had increased by a factor of 1.3--3. The changes in 3 dB bandwidth with increasing levels were, for some fibres, different from those of the 10 dB bandwidths. For the pathological fibres, the derived filter functions corresponded to their tone determined FTCs, and were therefore comparatively broadly tuned. Their tuning (Q10dB) approximated to those of normal fibres when the latter were measured 60 dB or more above threshold (i.e., at similar levels of stimulus), and did not increase further with increase in level. The findings in the normal guinea pig are consistent with those obtained by others in rodents, but are not consistent with those from the cat, where normal filtering is more robust to high levels of stimulus noise.

Acoustic Stimulation↗
PubMed1982

Cochlear transduction: an integrative model and review.

A model for cochlear transduction is presented that is based on considerations of the cell biology of its receptor cells, particularly the mechanisms of transmitter release at recepto-neural synapses. Two new interrelated hypotheses on the functional organization of the organ of Corti result from these considerations, one dealing with the possibility of electrotonic interaction between inner and outer hair cells and the other with a possible contributing source to acoustic emissions of cochlear origin that results from vesicular membrane turnover.

Acoustic Stimulation↗
PubMed1983

Effects on guinea pig cochlea from exposure to moderately intense broad-band noise.

Guinea pigs, which were either anesthetized (A) or conscious (U), were exposed to four 2 h sessions of broad-band noise of 96 dB SPL. Cochlear microphonics and N1 thresholds were measured prior to killing from 1 to 13 days later. The cochleas were examined by SEM and by section. The U series suffered less N1 threshold loss and recovered within 10-13 days, while the large initial loss in the A series did not completely reverse within the period of study. Initially, the IHC stereocilia in the basal half of the cochlea showed marked bending, the affected area being somewhat more extensive in the A group. These hairs gradually recovered, although not progressively. On the other hand, the disturbance to OHC stereocilia, which appeared to be less pronounced early on and was more apically centered, developed over time into marked permanent damage. The outermost row consistently showed the greatest effect with hairs becoming elongated or fused, and occasionally lost altogether. Susceptibility to noise varied between individuals of both groups. The recovery of N1 threshold was concomitant with the recovery of the erectness of the IHC hairs.

Animals↗
PubMed1983

Spatial receptive fields in the cat inferior colliculus.

Auditory spatial receptive fields of 122 single units in the inferior colliculus of 8 anesthetised cats were studied with free-field acoustic stimuli presented in the frontal hemisphere. The best frequency and best frequency threshold were determined for each unit with the speaker located in a position where the unit responded strongly. The intensity was then raised to 10 dB above threshold at the best frequency and the boundaries of the spatial receptive field were determined. For sounds of low intensity, receptive field size appeared to be a continuum with respect to best frequency. Units of high best frequency had small circumscribed fields located in the contralateral frontal hemifield. The boundaries of receptive fields for units of progressively lower best frequency expanded in all directions. Thus for intermediate frequencies, fields typically filled the contralateral hemifield whereas for low frequencies, units could be activated by stimulation from any position tested. At higher intensities, the boundaries of the receptive fields of units expanded. Circumscribed receptive field centres lay on a line corresponding to the acoustical axis of the contralateral pinna. For these units with small receptive fields, the free-field response to low intensity sounds appeared to be attributable more to the directional properties of the contralateral pinna than to significant binaural interaction.

Acoustic Stimulation↗
PubMed1983

Delayed evoked oto-acoustic emissions and their suppression by Gaussian-shaped pressure impulses.

The sound pressure of delayed evoked oto-acoustic emissions was measured as a function of the sound pressure of the stimulating sound impulse. They are directly proportional for sensation levels of the stimuli lower than about 20 dB; above that level, the emission saturates. Spontaneous emissions lying in the same frequency range as evoked emissions seem to influence this simple relation. Within the linear range, delayed emissions superimpose linearly even throughout long lasting delayed emissions. Short test tone bursts were used as test sound, to produce masking-period patterns, and as stimulus, to produce suppression-period patterns, respectively, while low-frequency Gaussian-shaped pressure impulses served as masker and as suppressor. The very close relation of the two patterns is indicated by their mirrored forms. This is considered directly relevant to the phenomenon of masking.

Acoustic Stimulation↗
PubMed1983

Spontaneous and impulsively evoked otoacoustic emissions: indicators of cochlear pathology?

The first author's right ear produces a spontaneous otoacoustic emission (SOAE) at 7529 Hz and 16 dB SPL. An external continuous tone is able to suppress the SOAE. The 3 dB iso-suppression curve is broadly tuned and displaced, relative to the SOAE, toward higher frequencies. An audiogram notch exists at frequencies just below that of the SOAE. We explain the occurrence of both spontaneous and impulsively evoked OAEs in terms of disruption of active feedback mechanisms of the OHCs upon basilar membrane vibration. According to this hypothesis, each segment of the organ of Corti feeds back positively upon its segment of basilar membrane and negatively upon adjacent segments. If a patch of OHC loss exists, adjacent segments of the basilar membrane are released from the negative feedback and respond to an impulsive stimulus with exaggerated oscillations at their resonance frequencies, thus producing OAEs. At particularly sharp transitions between normal and abnormal regions of the organ of Corti SOAEs may be generated.

Audiometry↗
PubMed1983

Responses of gerbil and guinea pig auditory nerve fibers to low-frequency sinusoids.

The characteristics of time-locked auditory nerve fiber responses to 50 Hz acoustic sinusoids were studied in gerbils and guinea pigs. Whereas the time-locked responses of all guinea pig fibers produced single-peaked period histograms, those of the gerbil produced distorted, multiple-peaked response histograms, especially fibers with characteristic frequencies (CFs) between 2 and 10 kHz. Although the shapes of the period histograms vary with stimulus intensity, the phases of the fundamental components are essentially invariant over the range of stimulus intensities used. In contrast to the phase of the cochlear microphonic produced by the 50 Hz stimulus, which was constant along the length of the cochlea in both species, the phase of the neural responses depends on the fiber CF in each of the two species. In guinea pigs, the phase of the neural responses relative to the acoustic stimulus decreases with the fiber CF from a phase lead of 90 degrees for fibers with CFs below 300 Hz to a phase lag of nearly 60 degrees for fibers with CFs greater than 3 kHz. In gerbils, the response phase also decreases with increasing CF below 2 kHz and above 10 kHz but undergoes an abrupt 160 degrees phase increase between those frequencies.

Acoustic Stimulation↗
PubMed1983

Variability of noise-induced damage in the guinea pig cochlea: electrophysiological and morphological correlates after strictly controlled exposures.

Anaesthetized guinea pigs were exposed to loud tones (1 h, 10 kHz, 112-118 dB SPL) with continuous control of the sound pressure at the tympanic membrane. N1 electrocochleograms were used to measure functional damage immediately and 21 days after the exposure. Damage to the organ of Corti was assessed by scanning electron microscopy and light and transmission electron microscopy. Principal findings were: (1) Functional impairment after 21 days showed large inter-animal variation which was not the result of changes in the effective damaging energy. (2) Structural damage to the stereocilia was also variable and did not always correlate with functional impairment, although when N1 thresholds were elevated damage to the stereocilia was always present. (3) Unknown factors within the cochlea must be responsible for variations in individual susceptibility to permanent noise-induced hearing loss.

Animals↗
PubMed1983

An active process in cochlear mechanics.

A model for cochlear mechanics is proposed to take account of its two systems, one passive and one active. The classical passive system stimulates the inner hair cells directly at levels above about 40 dB SL. At intensities below about 60 dB an active process, the 'cochlear amplifier' (CA), somehow provides additional energy that enhances the vibration of a narrow segment of the basilar membrane near the apical foot of the familiar, traveling wave envelope. The outer hair cells are essential for CA. The active system acts like a high-Q acoustic resonator, and it accounts for the great sensitivity and sharp tuning expressed by the 'tips' of neural tuning curves. The tips are selectively vulnerable to anoxia, noise exposure and other trauma. The CA model explains the detection of small differences in time as well as in frequency, the dual character of the electrocochleogram, recruitment of loudness in cochlear hearing impairment, the long latency of normal neural responses near threshold, acoustic emissions (both stimulated and spontaneous) and the locus of TTS in the frequency range above the exposure tone. Both the classical high-intensity system and the active low-level CA system are highly nonlinear and they combine to compress the great dynamic range of hearing into a much narrower range of mechanical movement of the cilia of the inner hair cells. The mechanism of CA is unknown, and the problem remains of how its action can be triggered by submolecular movements near threshold.

Auditory Threshold↗
PubMed1984

Single-neuron labeling and chronic cochlear pathology. IV. Stereocilia damage and alterations in rate- and phase-level functions.

The rate and phase of auditory-nerve response to tone bursts were studied as a function of stimulus level in normal and acoustically traumatized animals. The rate- and phase-level functions of normal auditory-nerve fibers are often separable into a low-intensity component (component I) and high-intensity component (component II), as defined by a dip in the rate function and a simultaneous abrupt shift in the phase function at stimulus levels near 90 dB SPL [10,12,9]. Baseline data are established by defining the relation between stimulus frequency and the characteristic frequency and spontaneous discharge rate of a fiber normally required for the appearance of these two components in the response. Abnormalities of the level functions are shown to occur in acoustically traumatized ears. Noise-induced threshold shift is often characterized by selective attenuation of component I. In some instances, it appears that component I has been eliminated, leaving a response which is identical in threshold, phase and maximum discharge rate to a normal component II. Results of single-unit labeling in such a case suggest that the selective attenuation of component I is associated with selective loss of the tallest row of stereocilia on the inner hair cells (IHCs). It is suggested that component I is normally generated through an interaction between the outer hair cells and the tall row of IHC stereocilia, while component II requires only the shorter row of IHC stereocilia.

Animals↗
PubMed1984

The auditory neurophonic: basic properties.

In anesthetized cats an AC signal or neurophonic can be recorded from the auditory nerve and from the scalp when the cochlea is stimulated with low frequency tones. This study examines some of the basic properties of the auditory neurophonics. The auditory nerve signal, termed the auditory nerve neurophonic (ANN), was differentially recorded with a pair of platinum-iridium ball electrodes placed on either side of the auditory nerve as it exits the internal meatus. The signal recorded from the scalp, termed the frequency following response (FFR), was recorded with silver wire. For purposes of comparison the round window-recorded cochlear microphonic was also examined under identical stimulus conditions. Several measures of the response to acoustic stimulation were taken for each recording configuration. Among these were total response amplitude as a function of stimulus level, spectral component amplitude and phase as a function of stimulus level, fundamental component amplitude as a function of stimulus frequency, response amplitude as a function of time after stimulus onset, response amplitude as a function of forward masker intensity. By all these measures the neurophonic responses are signals that are distinct from the CM and share many of the properties of single units in the auditory nerve. In addition, micro-injections of kainic acid into the cochlear nucleus leave these responses largely unaffected, while tetrodotoxin injections into the cochlea greatly diminish both neurophonic responses, while leaving the CM largely intact. From these results, we conclude that at stimulus levels below 90 dB SPL the ANN is almost entirely neural in origin, while the FFR is certainly largely neural, that is, that both responses are quite distinct from the CM. We also conclude that they represent a spatial summation of neural activity in the auditory nerve, probably arising from the phase-locked response of single units to low frequency stimuli. In addition to demonstrating that the neurophonics are neural responses, we have begun the process of relating their properties to the distributed phase-locked activity in the auditory nerve.

Acoustic Stimulation↗
PubMed1984

Frequency dependence of directional amplification at the cat's pinna.

We examined in detail the effects of changing stimulus frequency upon the inclination of the acoustical axis of the pinna and upon the solid angle (area) subtended by isoamplification contours. We measured the relative sound pressure level difference between points on a 1 m radius, coordinate sphere using the cochlear microphonic as an indicator of tympanic sound pressure. The inclination of the acoustical axis for a given frequency was found to vary with the posture of the pinna, and with the pinna in a drooped position (following midline incision) there was a frequency spreading of axial positions such that high frequency axes were inclined progressively more laterally. However, with the pinna in an upright posture the axes for all frequencies tested were relatively tightly clustered. Alternative models for sound localization can be formulated to suit either situation, but it seems likely that the cat can use the frequency spreading effect of its pinna sound transformation as a cue to location. The pinna becomes more directional at higher frequencies, and this is clearly shown when the solid angle of isoamplification contours is plotted against frequency. The inverse relationship formed was shown to be closely matched by a model based upon diffraction by the outer dimension of the pinna.

Acoustics↗
PubMed1984

Modifications of the nonlinearity of the cochlear microphonic responses produced by noise exposure in the guinea pig.

Cochlear microphonics (CM) were recorded with differential electrodes from several locations in the guinea pig cochlea. Input-output curves and amplitude-frequency curves were plotted before and after short exposures to intense noise. In addition to amplitude losses, important changes in the intensity functions resulted in a decrease of the nonlinearity and in a modification of the frequency response. The mechanisms producing these alterations are discussed.

Animals↗
PubMed1984

Ear canal acoustic and round window electrical correlates of 2f1-f2 distortion generated in the cochlea.

Close parallels have been found between the behaviour of the distortion product 2f1-f2 measured acoustically in the meatus (ACDP) and the 2f1-f2 component (CMDP) measured in the gross cochlear electrical response from the round window during two-tone stimulation. The two response modes show comparable growth with increasing stimulus intensity. Their group latencies and their frequency-selective susceptibility to suppression by a third tone are very similar. The common origin of these two responses is discussed.

Acoustic Stimulation↗
PubMed1984

Properties of spatial receptive fields in the central nucleus of the cat inferior colliculus. I. Responses to tones of low intensity.

Single neurones in the central nucleus of the inferior colliculus (ICC) of barbiturate-anesthetized cats were examined using free-field, pure-tone stimuli of low intensity at the neurones' best frequency. Receptive field size was inversely correlated with best frequency. Almost all neurones were maximally excited by stimulus positions in the hemifield contralateral to the recording electrode, irrespective of their best frequency. Simultaneous cochlear microphonic recording revealed that the neurones' best excitatory area was also the spatial region associated with maximum amplification by the contralateral outer ear. This amplification resulted in extremely low (less than -20 dB SPL in some neurones) best frequency thresholds. Response patterns were found not to vary markedly with speaker position. The results suggest that most ICC neurones are more sensitive to stimulation of the contralateral ear than to stimulation of the ipsilateral ear.

Animals↗