Biomedical subjects

G K Yates

Publications and source records attributed to G K Yates.

At least 37 records · Page 2Linked to original sources

PubMed1990

Basilar membrane nonlinearity and its influence on auditory nerve rate-intensity functions.

Previous papers have shown that the shapes of rate-intensity functions of auditory nerve fibres vary with spontaneous rate (Sachs and Abbas 1974; Sachs et al. 1989; Winter et al. 1990; Yates et al. 1990), and that the variation is due to the nonlinear properties of the basilar membrane. This paper examines the basilar membrane nonlinearity and provides a semi-quantitative explanation for it in terms of previous models (Zwicker 1979; Patuzzi et al. 1989) and an analogue model. It thereby provides explanations for the shapes of the basilar membrane input-output curves and for the way in which they vary with trauma. The shapes of the neural rate-intensity functions are quantified and shown to be consistent with the low-threshold data of Geisler et al. (1985). Several nonlinear properties of the cochlea, such as recruitment, are also interpreted.

Animals↗

PubMed1990

Saturation of outer hair cell receptor currents causes two-tone suppression.

Zwicker [Biol. Cybern. 35, 243-250, (1979); J. Acoust. Soc. Am. 80, 163-176 (1986)] has previously proposed that many nonlinear phenomena in the mammalian cochlea can be explained by saturation of a positive feedback process which enhances mechanical sensitivity, although the site of the nonlinearity producing this saturation has so far remained obscure. In this paper we present evidence suggesting that the nonlinearity of mechano-electrical transduction in the outer hair cells is the dominant nonlinearity producing two-tone suppression in the mammalian cochlea. In particular, we show that: (i) suppression of the extracellular summating potential (SP), recorded from a particular place within the organ of Corti, has characteristics similar to the suppression of activity in the auditory-nerve; (ii) that SP suppression occurs at approximately constant basilar membrane displacement, inferred from the SP iso-response contours; and that (iii) the onset of SP suppression with suppressor tones on the tail of the frequency tuning curve closely parallels the onset of nonlinearity in the local cochlear microphonic. Since previous studies (Patuzzi et al., 1989) have demonstrated that the vibration of the basilar membrane at its characteristic frequency is very sensitive to changes in outer hair cell receptor current, we consider that interference in outer hair cell currents caused by nonlinearity in mechano-electrical transduction is an adequate explanation of two-tone suppression. This requires that outer hair cell receptor currents deviate from linearity at a suppressor tone level below that required to produce a significant DC receptor potential within the inner hair cells, and that the active process within the cochlea is distributed along a local region of the cochlea, basal of the vibration peak.

Acoustic Stimulation↗

PubMed1990

Diversity of characteristic frequency rate-intensity functions in guinea pig auditory nerve fibres.

Rate-intensity functions at characteristic frequency (CF) were recorded from single fibres in the auditory nerve of anaesthetised guinea pigs. Within the same animal, CF rate-intensity functions, although probably forming a continuum, could be conveniently divided into three groups; (1) Saturating; reach maximum discharge rate within 30 dB of threshold, (2) Sloping-saturation; initially rapid growth in discharge rate leading to a slower growth in discharge rate but not saturating and (3) Straight; approximately constant increase in firing rate per decibel increase in sound pressure up to the maximum sound pressures used. Thresholds for individual fibres were plotted relative to compound action potential thresholds at the appropriate frequency. Fibres with straight CF rate-intensity functions had the highest thresholds. Fibres of the saturating CF sloping-saturation CF rate-intensity type had thresholds intermediate between saturating and straight. There was a close relationship between the type of CF rate-intensity function exhibited by a fibre and its spontaneous discharge rate. Fibres with saturating CF rate-intensity functions generally had high spontaneous discharge rates (greater than 18/s), whereas those with straight CF rate-intensity functions generally had low spontaneous discharge rates (less than 0.5/s). The majority of fibres with sloping-saturation CF rate-intensity functions had spontaneous rates between 0.5/s and 18/s. There was a negative correlation (r = -0.59) between the logarithm of the spontaneous discharge rate and relative threshold at CF with the lowest spontaneous rate fibres having the highest thresholds and vice-versa. This diversity of CF rate-intensity functions has functional implications for both frequency and intensity coding at high sound pressures in the mammalian auditory system.

Action Potentials↗

PubMed1990

Basilar membrane nonlinearity determines auditory nerve rate-intensity functions and cochlear dynamic range.

In a previous paper (Winter et al., 1990) we demonstrated the existence of a new type of auditory-nerve rate-intensity function, the straight type, as well as a correlation between rate-level type, threshold and spontaneous rate. In this paper we now show that the variation in rate-intensity functions has its origin in the basilar membrane nonlinearity. Comparison of rate-intensity functions at characteristic frequency and at a tail-frequency show that the rate-intensity functions are identical at low firing rates and that the sloping-saturation and straight types deviate from the standard function only at higher firing rates. The frequencies at which the deviations occur, and the change from saturating to sloping-saturation or straight, are closely correlated with the characteristic frequency of the fibre. Using the tail-frequency rate-intensity function as a calibration, it is possible to derive the basilar membrane input-output function at characteristic frequency from the characteristic frequency rate-intensity function. The resulting derived basilar membrane input-output functions are of a simple form and agree well with published direct measurements of basilar membrane motion. They show that the wide dynamic range to which the cochlea responds, about 120 decibels, is compressed by the basilar membrane nonlinearity into a much smaller range of about 30-35 decibels. General characteristics of the derived basilar membrane input-output curves show features which agree well with psychoacoustic studies of loudness estimation.

Acoustic Stimulation↗

PubMed1989

The origin of the low-frequency microphonic in the first cochlear turn of guinea-pig.

Low-frequency microphonic potentials (100 Hz to 2000 Hz) have been measured in the first turn of the guinea pig cochlea before and after a variety of manipulations of the cochlea. These included ablation of the apical turns, iontophoresis of streptomycin, dc current injection into the first turn, acoustic trauma and two-tone interference with pure tones. These manipulations indicate that the low-frequency microphonic measured in the first turn and at the round window is generated predominantly by the hair cells of this region. It is a convenient and relatively uncomplicated indicator of the integrity of the mechano-electrical transduction process of these cells.

Acoustic Stimulation↗

PubMed1989

Changes in cochlear microphonic and neural sensitivity produced by acoustic trauma.

The low-frequency (200 Hz) microphonic potentials at the round window and in the organ of Corti of the first turn of the guinea pig cochlea have been measured before and after acoustic overstimulation. Reductions in the amplitude of this microphonic after loud sound are highly correlated with neural threshold elevation in this region. The fall in the microphonic amplitude appears due to an inactivation of mechano-electrical transduction channels at the apex of the outer hair cells into a closed state. These results are consistent with the idea that the current through the outer hair cells controls the mechanical sensitivity of the organ of Corti, and that the temporary loss of mechanical and neural sensitivity following loud sound is due to a simple inactivation of the mechano-electrical transduction channels.

Action Potentials↗

PubMed1989

Outer hair cell receptor current and sensorineural hearing loss.

It is argued in this paper that many nonlinear phenomena in audition and many types of sensorineural hearing loss can be explained by a disruption of the mechano-electrical transduction process at the apex of the outer hair cells. This is done using experimental data and a simple model of the active role of outer hair cells in cochlear mechanics based on our previous experiments with acoustic trauma. The causes of sensorineural loss addressed include acoustic trauma, aminoglycoside ototoxicity, intoxication with loop diuretics, hypoxia and Meniere's disease. The nonlinear phenomena discussed include loudness compression, two-tone suppression and modulation of cochlear sensitivity by very low-frequency tones. In every case considered the reduction in neural sensitivity was related to the reduction in outer hair cell receptor current in a quantitatively similar way. We conclude that the link is causal.

Animals↗

PubMed1988

Auditory peripheral tuning: evidence for a simple resonance phenomenon in the lizard Tiliqua.

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.

Acoustic Stimulation↗

PubMed1988

Modulation transfer function of efferent neurones in the guinea pig cochlea.

The dynamic properties of single olivocochlear efferent neurones in the guinea pig cochlea were examined using sinusoidally amplitude modulated (AM) pure tones. The neural discharge, when displayed as a cyclic histogram, clearly followed the rapid fluctuations in the continuous input sound. Modulation transfer functions (MTFs) were constructed and in most cases showed a peak in the modulation response (MR) at a modulation frequency (MF) of 100 Hz. At this frequency a gain of as much as 12 dB was evident relative to the 30% modulated input signal. In 24% of neurones however, a large MR was present even at low MFs. This plurality of MTFs may be the result of recorded neurones emanating from a variety of cell bodies of origin. Efferent group delays (mean of 8.2 +/- 1.0 ms) were shorter and more tightly distributed than the minimum onset latency measurements (mean of 24.2 +/- 12.5) made on the same neurones. It seems evident that a post-synaptic potential build is required from the onset of a stimulus to the first spike discharge. This may occur within a single afferent-interneurone(s)-efferent reflex arc. Among a variety of alternative explanations, the observation is consistent with the notion that the olivocochlear neurones receive facilitatory input from higher centres, which is suppressed under barbiturate anaesthesia. Continuous AM signals may allow post-synaptic build up and eliminate the dependence on this higher input and hence yield a short group delay.

Acoustic Stimulation↗

PubMed1987

Dynamic effects in the input/output relationship of auditory nerve.

Cyclic histograms of the responses of single auditory ganglion cells in the guinea pig were recorded during stimulation with amplitude modulated tones. Modulation frequencies ranged from 10 Hz to 800 Hz. The response histograms, phase-locked to the modulation signals, were analysed for mean action potential rate and for the amplitude of the fundamental component at the modulation frequency. Expected values for the amplitude of the modulation responses were calculated using the variation of mean firing rate with intensity. The observed responses differed from the expected responses in several ways. First, the amplitudes of the modulation responses were larger than expected. Second, the stimulus intensities at which the observed modulation responses peaked was greater by about 7-10 dB than the expected intensity for maximum response. Third, both the magnitude of the response at a given intensity and the intensity at which the response peaked increased with modulation frequency. Fourth, the responses extended to higher stimulus intensities than expected. The observed modulation responses were compared with predictions from the Schroeder and Hall model of adaptation and were found to agree with good quantitative precision. These results suggest that the observed modulation responses are another manifestation of the very rapid (less than 20 ms) adaptation seen in the onset responses of nerve fibres [(1985) Hear. Res. 17, 1-12]. It is concluded that the static input-output responses of auditory nerve are not a good predictor of the dynamic responses to fluctuating stimuli.

Adaptation, Physiological↗

PubMed1987

Longitudinal flow of endolymph measured by distribution of tetraethylammonium and choline in scala media.

Longitudinal endolymph flow rate in the guinea pig cochlea was measured by determining the rate of migration of extrinsic ions, tetraethylammonium chloride (TEA) or choline, with a potassium sensitive ion-selective microelectrode (ISM). Low concentrations of iontophoretically injected TEA were detected with the ISM at various distances from the injection electrode. The results were variable when the ISM was used to record spread of TEA from turn II to turn I and vice versa. However, consistent data were obtained when the TEA spread was measured at different electrode separations (0.2, 0.5, 0.7 mm) within turn II. Electrode locations were systematically exchanged without changing their distance, i.e. the ISM electrode was placed basally or apically with respect to the TEA electrode. Comparison of data with a model, which combines the bulk diffusion of TEA and the flow of endolymph, is consistent with a rate of endolymph flow in turn II of about 0.2 mm/min, apex to base. A similar value was also obtained with the iontophoretic injection of choline. The endolymph flow rate may be different in turn I as indicated by measurements of compound action potential (CAP) changes. However, the results of experiments when TEA spread is measured at large distances must be interpreted cautiously because TEA may enter cellular walls of the cochlear duct and alternative routes of transport may be involved.

Action Potentials↗

PubMed1987

The low-frequency response of inner hair cells in the guinea pig cochlea: implications for fluid coupling and resonance of the stereocilia.

AC receptor potentials within the inner hair cells of the basal turn of the guinea pig cochlea have been recorded for stimuli in the frequency range 20 Hz to 3200 Hz. Comparison of these potentials with potentials recorded in scala media suggests that the stereocilia of many inner hair cells are stimulated by the transverse velocity of the cochlear partition for very low frequency, but above a transition frequency in the range 400 Hz to 1000 Hz they become entrained with partition displacement. It is suggested that such a transition is probably a simple consequence of the fluid coupling that drives these cells, and that mechanical resonance of the free-standing stereocilia of the inner hair cells does not occur in the basal turn of the guinea pig. These results do not, however, preclude the possibility of mechanical resonance involving the stereocilia of the outer hair cells. The results also indicate that the bodies of these cells low-pass filter the intracellular receptor potential, with a cutoff frequency of approximately 1000 Hz.

Acoustic Stimulation↗

PubMed1986

Basilar membrane measurements and the travelling wave.

From the original measurements of G. von Békésy (1942) until a few years ago, the basilar membrane was considered to undergo simple passive linear vibration. Recent measurements have completely altered this notion. It is now known that the BM is highly non linear and very sharply tuned. Indeed, BM can now account for most of the properties of the eighth nerve response to sound. The non linearity can be approximated by a hyperbolic function and appears to be part of an active process in the outer hair cell. At the characteristic frequency, CAP threshold (10 dB SPL) corresponds to 0.3 nm motion and the non linearity shows half saturation at 10 nm. The sigmoid shape of the full range BM input-output curve is due to the combination of a less sensitive linear passive component with the added sensitivity of the active non linear function. A hyperbolic input-output function is also present in the cochlear microphonics, and at low frequencies the half saturation value again corresponds to 10 nm BM displacement. With induced threshold loss (e.g. noise trauma) the nonlinearity disappears from the BM, but is still present in the CM. This suggests that the pathology is in the active mechanical feedback process, rather than in the receptor system. It appears that BM mechanics at low amplitudes near the resonant frequency is controlled by a nonlinear mechano-electrical transducer followed by a vulnerable, linear, active mechanism (electro-mechanical?) feeding back in positive phase onto BM vibration.

Animals↗

PubMed1986

Velocity and displacement coupling of mammalian inner hair cells and the mechanical resonance of the free-standing stereocilia.

Some controversy still exists as to whether the inner hair cells of the mammalian cochlea respond to the velocity or displacement of the basilar membrane or to a combination of these over their operating frequency range. A comparison between the nonlinear properties of the receptor potentials within inner hair cells of the basal turn of the guinea pig cochlea and the potentials recorded within the scala media of the same animals for stimulus frequencies between 200 and 3,200 Hz provides evidence that these inner hair cells change from velocity sensitivity to displacement sensitivity at about 1,000 Hz. We infer from this that viscosity within the subtectorial space is high enough to preclude mechanical resonance of the freestanding stereocilia of these cells as a frequency-selective mechanism within the mammalian cochlea.

Acoustic Stimulation↗

PubMed1985

Very rapid adaptation in the guinea pig auditory nerve.

Guinea pig auditory ganglion cell responses to 100-ms duration tone bursts were recorded over a range of stimulus intensities. The responses, recorded in the form of peristimulus/poststimulus time histograms, were analysed by reduction into two phases. The first phase was a rapid exponential adaptation from an initial onset response; the second was a more gradual reduction in the firing rate which, over the 100 ms duration of the stimulus, appeared to be a linear function of time. The first, rapid, phase was nonlinear in its response to changes in stimulus intensity, exhibiting a change in amplitude and having a time constant which decreased with increasing intensity. Individual units were consistent in the magnitude and time course of this phase. The second phase was also nonlinear with intensity, and was far more variable from unit to unit. With the recording parameters employed it was not possible to determine whether the effect of intensity on the second phase was an effect on the magnitude or time course, or both. Stimulus termination responses were also analysed, and typically were of one of two forms. If, at any particular stimulus intensity, the unit under study showed little sign of the slower adaptation then the termination response was a simple depression of activity (perhaps to zero) which recovered with an exponential time constant of about 25 ms, independent of intensity. If, however, the peristimulus responses showed a significant amount of the slow adaptation then the termination responses also exhibited a second, slower, phase of recovery. This was modelled over the recording epoch as a linear function of time. The magnitude of the slow offset response also increased with intensity faster than did the average firing rate.

Acoustic Stimulation↗

PubMed1983

A method for monitoring end-tidal CO2 during neurophysiological experiments on small laboratory animals.

The Hewlett Packard Model 47210A Capnometer, intended for monitoring expired CO2 in human patients, can be modified for use in acute neurophysiological studies on small animals. The airway adaptor which would normally be attached to an endotracheal tube is modified and its internal volume is reduced to 0.2 ml, and is incorporated into a guinea pig head-holder. We have verified that the modification does not alter the Capnometer's accuracy. There is no significant increase in resistance in the airway when the adaptor is in place, and the waveform of instantaneous CO2 shows rapid changes on inspiration and expiration with well-resolved peaks and troughs. Measurement of arterial pCO2 suggests that the Capnometer and modified airway adaptor form an accurate system for obtaining a continuous record of end-tidal CO2 in small guinea pigs.

Airway Resistance↗

PubMed1983

Recovery of eighth nerve action potential thresholds after exposure to short, intense pure tones: similarities with temporary threshold shift.

Exposure of guinea pig cochleas to short (25, 250 and 1000 ms), intense (90 or 100 dB SPL) 10 kHz pure tones reduced cochlear sensitivity to 14 kHz test tones presented at intervals varying from 5 ms to 70 s after the exposure tone. The recovery of cochlear sensitivity, determined as the SPL required to evoke a 20 microV compound action potential (CAP), depended on both the intensity and the duration of the exposure tone and appeared to take place in two or more phases. After a 25 ms, 100 dB SPL exposure, CAP threshold increased by up to 13.5 dB and generally recovered very rapidly (25 ms), although some loss persisted for as long as 400 ms. A similar, but greater and longer, elevation of threshold was seen after long exposure tones. Lower exposure tone intensities (90 dB SPL) produced threshold elevations which generally lasted for only short durations (25-35 ms). The rapid recovery is consistent with the time course of recovery from rapid adaptation, while the slow recovery component is similar to that seen in the cochlea after much longer exposures and may be related to the phenomenon of temporary threshold shift. Long and/or loud exposures also frequently resulted in a third form of threshold elevation, identified in only a few animals, which recovered with a time constant in excess of 25 s. A fourth component occasionally persisted for the duration of the experiment.

Acoustic Stimulation↗

PubMed1983

The influence of Mossbauer source size and position on phase and amplitude measurements of the guinea pig basilar membrane.

Phase and amplitude measurements were made from the incus and basilar membrane in guinea pig using the Mossbauer technique. The basilar membrane/incus ratio had a maximum of about 60 dB and a phase accumulation of between 9 and 12 radians to CF. Two source sizes were used (60 X 85 and 20 X 60 microns) and the source was placed either on the modiolar edge of the basilar membrane or in the middle. Notches in plots of the basilar membrane/incus ratio occur at stimulus frequencies that appear to be associated with source size rather than position, suggesting that artefacts could be produced by the presence of the Mossbauer source.

Animals↗