Transient response of the basilar membrane measured in squirrel monkeys using the Mössbauer effect

Functional role of the olivo-cochlear bundle: a motor unit control system in the mammalian cochlea

A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von Békésy. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.

Evoked-response forward-masking functions in chinchillas with noise-induced permanent hearing loss

Evoked-response forward-masking functions were measured by chronic electrodes in the inferior colliculus of the chinchilla before and after exposure to an intense tone that produced a permanent hearing loss. Before exposure, the forward-masking time constants ranged from 50 to 90 ms. After exposure, the forward-masking time constants increased significantly in the region of hearing loss, but not in regions where hearing was normal. The effect of the hearing loss on the time course of forward masking was most pronounced once the hearing loss exceeded 20-25 dB. These physiological changes in the evoked-response forward-masking functions appear to parallel those observed psychophysically in human listeners.

Off-responses in the auditory system in relation to the signal end phase and neuronal characteristic frequency

Off-responses of single neurons from the inferior colliculus (IC) as well as summed off-responses (evoked potentials) from the IC, cochlear nucleus and auditory nerve were studied while varying the end phase of tonal signals. It was found that with tonal signals higher than the characteristic frequency, off-responses at all the auditory levels studied were greatest at the end phases near 0 and 180 degrees and were of minimal value at the end phases near 90 and 270 degrees. On the contrary, with tonal signals lower than the characteristic frequency, the greatest off-responses corresponded to the end phases near 90 and 270 degrees, and responses of the lowest value were registered at the end phases near 0 and 180 degrees. The observed phenomena did not result from transient 'off'-responses of the acoustic system. It is suggested that they reflect certain cochlear processes connected with different effectiveness of oscillation phases at frequencies below and above the resonance.

Covariation of latency and temporal resolution in the inferior colliculus of the cat

Onset latency of single-unit and multiunit responses was measured in the central nucleus of the inferior colliculus (ICC) of barbiturate anesthetized cats. The relationship of the units' onset latency with their characteristic frequency (CF), their best modulation frequency (BMF), and their location within the ICC was studied. Latencies were significantly correlated to BMF; they were only weakly correlated to CF. The contribution of CF to the delays can be attributed to the traveling wave mechanism; covariation of latency and temporal resolution (BMF) likely manifests neuronal mechanisms underlying periodicity coding.

Evidence for presynaptic facilitation in primary cochlear afferent neurons

Evidence for presynaptic facilitation was sought in the discharge patterns of single units in the chinchilla cochlear nerve. Pairs of acoustic clicks, separated by a variable interval, were delivered and spike discharge times stored for offline analysis. By choosing an appropriate binwidth (5 ms) and collecting data only from units with high characteristic frequencies (CF) the response to each click was contained in a single bin. The ratio of spike counts in the bins containing the responses to the two clicks was computed. For units with low spontaneous rates (SR) of discharge (SR less than 8/s), an enhancement of the response to the second click was seen for low stimulus levels. As the stimulus level was raised, the response to the second click became reduced, presumably because of adaptation to the first click. Units with high (SR greater than 15 spikes/s) seldom exhibited enhancement of the response to the second click. The results are explained with a conceptual model in which two processes, depletion and facilitation decay exponentially following a stimulus. Since the two processes have opposite influences on the rate of transmitter release, the magnitudes of both processes may be underestimated by observing their net effect.

Frequency and time domain comparison of low-frequency auditory fiber responses in two anuran amphibians

A comparative study of the phase-locked response of auditory nerve fibers was performed in two frog species, Eleutherodactylus coqui and Bombina orientalis. From the tuning characteristics and phase response of single auditory nerve fibers to low frequency tones (0.08-1.0 kHz) we attempt to deduce the mechanics of the auditory organ responsible for low-frequency hearing in the frog, the amphibian papilla (a.p.). The phase-locked responses of auditory nerve fibers in B. orientalis were essentially identical to those from cells with similar CFs in E. coqui, despite the presence of a conspicuous caudal extension of the a.p. in E. coqui (an apparently derived morphology), a feature completely absent in B. orientalis. The fine structure of the frequency-dependent phase behavior was examined in both species with a residual phase analysis. The most significant non-linear phase behavior was confined to neurons with CFs less than 0.3 kHz. The intensity dependence of the phase response in E. coqui revealed that the preferred firing phase of an auditory nerve fiber depends upon the relation of test frequency (TF) and CF of the neuron examined. For TFs greater than CF there was a progressive phase lag as stimulus level was increased; the inverse was true for TFs less than CF. Click latencies measured in E. coqui were inversely related to CF and were similar though systematically shorter than the response latencies estimated from the phase-frequency functions. The click response was similar to that documented in other species, showing a significant level dependence and the presence of multiple peaks, with the time between peaks related to the period of the neuron's CF. A 'neurogram' was compiled for a.p. fiber responses in both species in response to several pure tones. Based on the known tonotopy of the a.p. this measure reflects the phase response of the a.p. over the extent of its length. The population phase response in anurans is quite similar to that obtained from mammalian auditory nerve fibers for the same range of test frequencies (0.08-1.0 kHz). The similarity between the responses of auditory fibers in these two anuran species suggests the micromechanics of the a.p. rostral to the tectorial curtain is similar in both species and that it is the likely site for the origin of the CF-dependent time delays.

Cochlear emissions simulated using one-dimensional model of cochlear hydrodynamics

Stapes velocity was computed using a nonlinear, one-dimensional model of cochlear hydromechanics. The model's compliances and damping coefficients were mechanically nonlinear and instantaneously varying in proportion to simulated current injected into the cochlea. Experimental data showing the spectral content of the pressure waveform near the eardrum during the delivery of sound and current to the cochlea were compared with model results.

Synchronized responses of primary auditory fibre-populations in Caiman crocodilus (L.) to single tones and clicks

Measurements of the responses to tones and clicks were made from single primary auditory fibres of the caiman. The distribution of the amplitude and phase of the fundamental component of the response rate modulation over the best frequencies of the fibres is comparable to that reported in the cat, despite the fact that the basilar membrane in caiman is only 4.5 mm long. However, much higher intensities are needed in the caiman (75-85 dB SPL) than reported in the cat (20 dB SPL) to obtain systematic distributions of the phase of the responses, probably due to the larger scatter of the phase responses in the caiman. The slopes of the phase distributions are very similar to those in cat. Single unit phase responses as a function of stimulus frequency at 85 dB SPL can be approximated by one, or in fibres with low best frequency, two straight lines. At lower intensities the deviation of the phase-frequency responses from a straight line increases as the group delay at the best frequency becomes larger. The shortest latencies of click responses are obtained with rarefaction clicks. Group delay estimates obtained from the responses to clicks and from the straight line approximations of the phase-frequency responses are related in a way expected for linear filter systems and accurately predict the measured distributions of the phase of the responses over the neural best frequency. The obtained group delays and click latencies in the caiman are very similar to those reported by other workers in the cat, the squirrel monkey and the treefrog, despite large morphological and probably functional differences of their inner ears. The click latencies are also very similar to those in the pigeon. The results are consistent with the existence of a mechanical travelling wave reported previously on the basilar membrane of the caiman, but at the same stimulus level the phase characteristic of the present single unit responses is steeper and the wave length estimates from the neural population phase distributions are shorter than those observed directly in the motion of the basilar membrane. Since the neural responses are an indirect estimate of the basilar membrane motion it cannot be decided whether the difference between neural and mechanical data is due to deterioration of the basilar membrane responses during the direct measurements or whether the basilar membrane response is sharpened by additional tuning mechanisms.

A mechanism for stimulated acoustic emissions in the cochlea

Stimulated acoustic emissions in the cochlea are explained in terms of its hydraulic properties. The mathematical model predicts that these 'echoes' are caused by reflections which result from a discontinuity in the resistive and reactive components of the impedance which occurs at resonance. This discontinuity is a direct result of the wavelength-dependent nature of the fluid inertance and occurs without the consideration of non-linearities. Calculations of the time delay of tone bursts to and from the places of reflection by determination of the group velocity agree with observations of the response latencies for the frequencies concerned.

Dose rate to the inner ear during Mössbauer experiments

The most widely used technique for studying vibrations of the inner ear utilises the Mössbauer effect; this requires placement of a radioactive source on the basilar membrane. This source, although small in size and less than 37 MBq (1 mCi) in strength, is placed in close proximity to sensitive receptor cells. Using a series solution for the radiation field of a rectangular source the absorbed dose rate delivered to receptor cells at various depths and at points off-axis from the centre of the source is calculated. It is concluded that the dose delivered during the course of a Mössbauer experiment may well be sufficient to damage receptor cells and cause a loss of response.

Efferent neural control of cochlear mechanics? Olivocochlear bundle stimulation affects cochlear biomechanical nonlinearity

We confirm the report of Mountain (Mountain, D.C. (1980): Science 210, 77-72) that stimulating the crossed olivocochlear bundle (COCB) can change the magnitude of the distortion product (f2-f1) in the ear-canal sound pressure. Our results are extended to include (2f1-f2) as well as (f2-f1) from anesthetized chinchillas with both middle-ear muscles sectioned. In contrast to Mountain's report, the polarity of the change can be either positive, negative or absent, depending on the choice of two-tone stimulus frequencies. The influence of two-tone stimulus level is also complex, but we have not yet seen the polarity of the COCB effect change with stimulus level. The magnitude and polarity of the change in (2f1-f2) are not simply related to those for (f2-f1). The effect of COCB stimulation is abolished when scala tympani is perfused with artificial perilymph containing 10(-5) M d-tubocurarine. These results demonstrate that the COCB effect is postsynaptic, probably mediated by outer hair cells. We suggest that the normal cochlea contains an active biomechanical mechanism which reduces the damping of the cochlear-partition motion and is modulated by activating the efferents. It is thus possible that the central nervous system may be able to control the dynamics of the motion of the cochlear partition.

VIII nerve response to click stimuli in normal and pathological cochleas

A flat 30--50 dB hearing loss was established in chinchillas following a 5 day exposure to an octave band of noise (354--708 Hz, 95 dB SPL). After exposure, single auditory nerve fiber recordings were obtained using click and tone burst stimuli. The thresholds of units from the noise-treated animals were elevated 30--70 dB and the tuning curves were abnormally broad. At the threshold for click stimulation, the fiber latencies were shorter in the noise-treated animals than those in normal animals. However, the latencies for the two groups were similar when stimulated at the same intensities. As indicated by the number of peaks in the PST histograms obtained with clicks, the units from the noise-treated animals showed considerably more damping in the neural response than those from normal units. The temporal spacing between the peaks in the histograms for units of similar CF was the same in the normal and noise-treated groups, although this cannot be taken to infer that an individual unit PST histogram would remain the same after noise exposure as before. These limited neural data therefore show changes in the same direction as those in the transient mechanical response of the basilar membrane reported by Robles et al.

Some ECMR properties in relation to other signals from the auditory periphery

Some basic characteristics of monkey and human acoustic emissions are reviewed and some new data presented. These characteristics are discussed in relation to the sensorineural output of the cochlea. Input--output functions and the frequency dispersal in the waveform of acoustic emissions (AE) from monkeys are described. New findings include changes in the human AE during trains of clicks and sustained suppression of the AE after short bursts of noise of moderate intensity. The similarly of click-evoked AE latencies and whole-nerve action potential (AP) latencies for low stimulus level toneburst stimuli, and the apparent discrepancy between these latencies and those of single cochlear nerve-fibre action potentials and derived impulse responses is discussed. It is argued that at low stimulus levels AE are generated either coincident with the primary cochlear impulse or very soon afterwards. It is proposed that the AE generator is peripheral to the afferent synapse of the inner hair cells because of the lack of adaptation effects in AE. However, attention is drawn to changes in the waveform of the click-evoked AE with increasing stimulus intensity and with diuretic intoxication, that qualitatively parallel known changes in single nerve-fibre firing properties. These observations are consistent with the concept that AE are a product of a sharply tuned and vulnerable cochlear filter mechanism.

Frequency selectivity of the peripheral auditory analyzer studied using broad band noise

The sharpness of the frequency tuning of single auditory nerve fibers was studied in the rat on the basis of responses to broad band noise. The cross-spectra between the sound stimulus and the sound-to-stimulus locked discharge rate were used as estimates of the transfer functions of the peripheral auditory analyzer. The sharpness of the tuning obtained in that way was measured as Q3dB and Q10dB. It was found that these Q-values decreased with increasing stimulus intensity but that the decrease was different when it was based on measurements at 3 dB points of the obtained transfer functions compared with measuring it at 10 dB points. The change in width was less for fibers with low CF. In all the fibers studied the frequency of maximal response (CF) decreased with increasing sound intensity. The implication of these findings for pitch perception and for noise induced hearing loss is discussed.