Biomedical subjects

G K Yates

Publications and source records attributed to G K Yates.

At least 19 recordsLinked to original sources

PubMed2001

In vivo evidence for a cochlear amplifier in the hair-cell bundle of lizards.

Vertebrate sensory hair cells achieve high sensitivity and frequency selectivity by adding self-generated mechanical energy to low-level signals. This allows them to detect signals that are smaller than thermal molecular motion and to achieve significant resonance amplitudes and frequency selectivity despite the viscosity of the surrounding fluid. In nonmammals, a great deal of in vitro evidence indicates that the active process responsible for this amplification is intimately associated with the hair cells' transduction channels in the stereovillar bundle. Here, we provide in vivo evidence of hair-cell bundle involvement in active processes. Electrical stimulation of the inner ear of a lizard at frequencies typical for this hearing organ induced low-level otoacoustic emissions that could be modulated by low-frequency sound. The unique modulation pattern permitted the tracing of the active process involved to the stereovillar bundles of the sensory hair cells. This supports the notion that, in nonmammals, the cochlear amplifier in the hair cells is driven by a bundle motor system.

Acoustic Stimulation↗

PubMed2000

Changes to low-frequency components of the TEOAE following acoustic trauma to the base of the cochlea.

Several studies have shown that acoustic trauma to the base of the cochlea can result in loss of transient-evoked otoacoustic emission (TEOAE) energy at frequencies much lower than those affected in the audiogram. We have extended these studies to show that the low-frequency emission energy was substantially affected if the transient stimulus included frequencies within the range affected by the trauma, otherwise the change observed was small. In keeping with the suggestion that TEOAEs are predominantly comprised of intermodulation distortion energy (Yates and Withnell, Hear. Res. 136 (1999) 49-64), trauma to the basal region of the cochlea was found to affect emission energy across a broad frequency range in response to a wide-band acoustic stimulus. Further, group delay measurements demonstrated that the dominant contribution to the TEOAE originated from the basal region of the cochlea.

Acoustic Stimulation↗

PubMed2000

Rate-intensity functions in the emu auditory nerve.

Rate-versus-intensity functions recorded from mammalian auditory-nerve fibers have been shown to form a continuum of shapes, ranging from saturating to straight and correlating well with spontaneous rate and sensitivity. These variations are believed to be a consequence of the interaction between the sensitivity of the hair-cell afferent synapse and the nonlinear, compressive growth of the cochlear amplifier that enhances mechanical vibrations on the basilar membrane. Little is known, however, about the cochlear amplifier in other vertebrate species. Rate-intensity functions were recorded from auditory-nerve fibers in chicks of the emu, a member of the Ratites, a primitive group of flightless birds that have poorly differentiated short and tall hair cells. Recorded data were found to be well fitted by analytical functions which have previously been shown to represent well the shapes of rate-intensity functions in guinea pigs. At the fibers' most sensitive frequencies, rate-intensity functions were almost exclusively of the sloping (80.9%) or straight (18.6%) type. Flat-saturating functions, the most common type in the mammal, represented only about 0.5% of the total in the emu. Below the best frequency of each fiber, the rate-intensity functions tended more towards the flat-saturating type, as is the case in mammals; a similar but weaker trend was seen above best frequency in most fibers, with only a small proportion (18%) showing the reverse trend. The emu rate-intensity functions were accepted as supporting previous evidence for the existence of a cochlear amplifier in birds, the conclusion was drawn further that the nonlinearity observed is probably due to saturation of the hair-cell transduction mechanism.

Animals↗

PubMed1999

The role of intermodulation distortion in transient-evoked otoacoustic emissions.

Transient-evoked otoacoustic emissions (TEOAEs) are low-intensity sounds recorded in the external ear canal immediately following stimulation by a transient stimulus, typically a click. While the details of their production is unknown, there is evidence to suggest that the amplitude of each component frequency reflects the physiological condition of the corresponding region of the cochlea. Certain observations are at variance with this assumption, however, suggesting that pathology at a basal site within the cochlea might affect the production of emissions at frequencies which are not characteristic for that site. We have recorded click-evoked emissions in guinea pigs using high-pass clicks and found emissions at frequencies which are not present in the stimulus and which could not, therefore, have originated from the characteristic place for those emission frequencies. These new frequencies are, by definition, intermodulation distortion frequencies and must have been generated from combinations of frequencies in the stimulus by non-linear processes within the cochlea. Further processing of the emissions by Kemp's technique of non-linear recovery showed that the magnitude of emissions at frequencies within the stimulus frequency pass-band was approximately the same as that of frequencies not present in the stimulus. We propose that, in guinea pigs at least, most of the click-evoked emission energy is generated as intermodulation distortion, produced by non-linear intermodulation between various frequency components of the stimulus. If this result is confirmed in humans, many of the anomalies in the literature may be resolved.

Acoustic Stimulation↗

PubMed1998

ATP in endolymph enhances electrically-evoked oto-acoustic emissions from the guinea pig cochlea.

ATP was iontophoresed into the scala media of the guinea pig cochlea. A reversible increase in the amplitude of electrically-evoked oto-acoustic emissions (EEOAEs), and reductions in the endocochlear potential (EP) and the cochlear microphonic (CM) were observed. These effects were consistent with an action of ATP on P2X receptors on outer hair cells (OHC). The results confirm that endogenous endolymphatic ATP, acting via P2X receptors on OHCs, may serve a regulatory function in the cochlea.

Adenosine Triphosphate↗

PubMed1998

Cochlear electrically evoked emissions modulated by mechanical transduction channels.

Cochlear outer hair cells are capable of both mechanical-to-electrical and electrical-to-mechanical transduction. Vibration of their stereocilia by sound is believed to stimulate somatic motility via a receptor potential developed across the basolateral membrane, thereby enhancing the mechanical vibration and increasing the sensitivity and frequency selectivity of the ear. Extrinsic electrical currents, applied at the tops of the cells, also appear to activate motility in vivo, presumably after entering the cell. Earlier experiments suggested such currents might enter through the transduction channels themselves, but an alternative shunt pathway through the membrane capacitance seems more likely on physical grounds. We therefore recorded electrically evoked oto-acoustic emissions while modulating the transduction channels by driving them with low-frequency sound. Recordings of the low-frequency cochlear microphonic provided a measure of the mean electrical conductance through the channels during sound stimulation. Emissions increased during displacement of the basilar membrane toward scala vestibuli, when the channels were biased open, and decreased on the opposite phase, and the modulation of the emission was in direct proportion to the cochlear microphonic. The results are the strongest evidence yet that electrically evoked emissions are generated directly by mechanisms related to cochlear transduction and lead to the surprising conclusion that, for frequencies up to at least 12 kHz, extrinsic electrical currents enter the hair cell predominantly by the resistive pathway through the transduction channels. Alternatively, the results might be consistent with direct modulation of a motility source driven by capacitive currents but whose output depends on the state of the channels.

Animals↗

PubMed1998

Onset of basilar membrane non-linearity reflected in cubic distortion tone input-output functions.

The basilar membrane (BM) input output (I/O) function is a non-linear compressive function over much of its operating range. A low level non-compressive region with a break-point or compression threshold between 20 and 40 dB SPL has been identified. To date, no similar compression threshold in cubic distortion tone otoacoustic emission (CDT) data, which would illustrate the dependence of the CDT on BM growth, has been demonstrated. A Taylor series expansion of the outer hair cell gating function yields an amplitude term for 2f1-f2 of p.A1(2).A2, where A1 and A2 are the displacement amplitudes of the BM for two pure tone input stimuli of levels L1 and L2, p a constant. By selectively varying either L1 or L2 with f2/f1 appropriately chosen to reduce suppression effects, the CDT I/O function can be examined for deviation from the power law. In particular, if the amplitude of the CDT were dependent on BM displacement amplitude, then it should be possible by an appropriate choice of parameters to measure compression threshold. We have examined CDT I/O functions for an f2 of 8 kHz in the guinea pig and found them to be consistent with the expected power law. With L1 held constant, L2 varied and f2/f1 = 1.6, a low level region with a slope of one and a compressive region with a slope of 0.14-0.27 corresponding to the analogous regions of the BM I/O function was identified, with a break-point or compression threshold of 22-33 dB SPL.

Acoustic Stimulation↗

PubMed1998

Enhancement of the transient-evoked otoacoustic emission produced by the addition of a pure tone in the guinea pig.

This study examined the transient-evoked otoacoustic emission obtained in response to a click stimulus presented in combination with a pure tone in the guinea pig. Low-pass filtered click waveforms were digitally generated using a sin(t)/t function windowed over 3 ms with an elevated cosine envelope. Transient-evoked otoacoustic emissions were obtained using the nonlinear derived response technique. Phase locked pure tones of various frequencies at approximately 70 dB SPL were electrically mixed with electrical clicks, with the pure tone present only for the three lower level stimuli in the train of four stimuli. Enhancement in the amplitude of the response spectrum at frequencies which corresponded to regions of the basilar membrane apical to the tone was observed with the addition of the tone. This finding is inconsistent with the transient-evoked otoacoustic emission being the result of independent generators. It suggests that intermodulation distortion energy may contribute to the transient-evoked otoacoustic emission, the enhancement in the emission response spectrum at frequencies below the pure tone being a result of a complex interaction on the basilar membrane of intermodulation distortion products.

Acoustic Stimulation↗

PubMed1998

Otoacoustic emissions measured with a physically open recording system.

Otoacoustic emissions have historically been measured with an acoustical probe assembly hermetically sealed in the ear canal, imposing in most cases a limited stimulus bandwidth. A physically open recording system should afford the possibility of a greater stimulus bandwidth but the change in acoustical load may affect the magnitude of otoacoustic emissions obtained. Here it is reported that the authors have measured in the guinea pig transient-evoked otoacoustic emissions extending in frequency to 20 kHz and cubic distortion tone otoacoustic emissions for f2 = 4737 and 8096 Hz with a physically open sound system. To address the effect of acoustical load provided by a physically open versus hermetically sealed system, the authors compared the amplitude of electrically evoked otoacoustic emissions recorded from a guinea pig in each case. The change in acoustical load in the ear canal introduced by the change in recording setup did not appear to make a substantial difference to the magnitude of otoacoustic emissions measured. A physically open recording system provides a good alternative to traditional acoustical probe assemblies sealed in the ear canal for the laboratory measurement of acoustically evoked otoacoustic emissions, with the advantage of permitting a greater stimulus bandwidth.

Acoustic Stimulation↗

PubMed1998

Enhancement of electrically evoked oto-acoustic emissions associated with low-frequency stimulus bias of the basilar membrane towards scala vestibuli.

Electrically evoked oto-acoustic emissions (EEOAEs) are sounds present in the ear canal when ac current is passed into the cochlea. EEOAEs are attributed to the activation of fast electromotile responses in outer hair cells (OHCs). An interesting property of EEOAEs is the phenomenon of "acoustic enhancement," where the emission amplitude is increased by moderate-level sound [D. C. Mountain and A. E. Hubbard, Hear. Res. 42, 195-202 (1989)]. In this report a form of enhancement is described which occurs with displacements of the basilar membrane toward scala vestibuli, during amplitude modulation of the EEOAE waveform by low-frequency tones. This "SV-bias enhancement" possibly consists of two components: (i) a low-level component induced by sound at levels which produce nonlinear growth of the cochlear microphonic and which may be equivalent to the "acoustic enhancement" described previously, and (ii) a high-level component which occurs at sound levels well above those which cause saturation of the cochlear microphonic. The low-level component could be explained by either an increased access of the extrinsically applied current to a membrane-based source of OHC motility, perhaps coupled with a reduction in negative feedback, or an increase in electromotile output during scala vestibuli displacements, but the origin of the high-level component is obscure.

Acoustic Stimulation↗

PubMed1997

Activity of primary auditory neurons in the cochlear ganglion of the emu Dromaius novaehollandiae: spontaneous discharge, frequency tuning, and phase locking.

The spontaneous and pure-tone sound-driven activity of primary auditory units was recorded in the cochlear ganglion of emu chicks aged between post-hatching days 1 and 14. Spontaneous activity tended to increase both with the chick's age and as a function of the unit's characteristic frequency (CF). The CF of 887 units ranged from 0.04 to 4 kHz, the thresholds down to 0 dB SPL. Although the CF range did not change, the thresholds improved with age during the first two weeks after hatching, by up to 18 dB at the highest frequencies. The threshold spread between units of similar CF in single animals was up to 60 dB. Rate-threshold tuning-curve symmetry varied with CF. In low-frequency units, the slope of the low-frequency tuning-curve flank was on average steeper than the high-frequency flank, whereas for high-frequency units, the reverse was true. Units of CF > 0.5 kHz generally showed low-frequency "tails" similar to those seen in mammalian primary auditory fibers. The mean tuning-curve frequency selectivity increased with CF. For units of CF > 0.2 kHz, thresholds were moderately correlated with tuning-curve frequency selectivity. Significant phase locking was observed up to about 4 kHz, the corner frequency of phase locking being at 1.15 kHz.

Animals↗

PubMed1996

Frequency tuning and acoustic enhancement of electrically evoked otoacoustic emissions in the guinea pig cochlea.

Electrically evoked otoacoustic emissions (EEOAEs) were generated by ac stimulation in scala media of turns 1, 2, and 3 in the guinea pig cochlea. In each turn EEOAEs were recorded at frequencies up to and slightly above the estimated characteristic frequency (CF) of the stimulation site. Acoustic enhancement of EEOAEs was present at all emission frequencies in turns 2 and 3 but could be demonstrated in turn 1 only at emission frequencies that fell within a notch in the EEOAE tuning function. There was no evidence, in any turn, of a transition from enhancement to suppression as the emission frequency approached the CF of the stimulation site. The results were not consistent with the hypothesis [D. C. Mountain and A. E. Hubbard, Hear, Res. 42, 195-202 (1989)] that acoustic enhancement results from a reduction in the effectiveness of forward transduction in a negative feedback loop.

Acoustic Stimulation↗

PubMed1994

Nonlinear input-output functions derived from the responses of guinea-pig cochlear nerve fibres: variations with characteristic frequency.

Rate-versus-level functions (RLFs) were recorded from individual cochlear nerve fibres in anaesthetised guinea-pigs. Variations in the shapes of these functions with frequency were used to derive input-output (IO) relationships for the mechanical preprocessing mechanisms in the cochlea. It was assumed that these preprocessing mechanisms operated linearly at frequencies well below each fibre's characteristic frequency (CF). The IO functions derived at each fibre's CF provided strong evidence of compressively nonlinear preprocessing in most regions of the cochlea. However, the apparent degree of compression depended on the fibre's CF, and hence on the presumed site of cochlear innervation. For fibres with CFs of between 1.5 and 3.6 kHz, the CF derived IO functions grew at rates of around 0.5 dB/dB. For fibres with CFs above 4 kHz, the IO functions were more compressive, with high-intensity asymptotic slopes of around 0.13 dB/dB. In the highest (> or = 10 kHz) CF fibres, the degree of compression depended on the physiological condition of the cochlea; the derived IO functions becoming more linear as the cochlea became less sensitive. The derived IO technique was not well suited to analyse responses evoked by very low frequency (e.g., < 500 Hz) tones. Nonetheless, the CF RLFs from fibres with CFs lower than approximately 1 kHz provided little evidence of mechanical nonlinearity near the apex of the cochlea. These findings imply a longitudinal variation in the mechanisms of cochlear preprocessing, and provide important new tests for functional models of the cochlea.

Acoustic Stimulation↗

PubMed1994

Evidence for electrically evoked travelling waves in the guinea pig cochlea.

Electrically evoked compound action potentials (EECAP) were produced by the injection of pulsed sinusoidal current through metal electrodes into the basal turn. Plots of current threshold against frequency closely resembled conventional compound action potentials (CAP) audiograms for frequencies represented apically of the electrode location. EECAPs were masked by sound and CAPs were masked by current in a manner consistent with the generation of a propagated travelling wave.

Acoustic Stimulation↗

PubMed1993

Cochlear nerve fiber responses to amplitude-modulated stimuli: variations with spontaneous rate and other response characteristics.

1. Single-fiber responses to sinusoidally amplitude-modulated (AM) tones were recorded from the cochlear nerves of anesthetized guinea pigs. Stimuli were presented at the fiber's characteristic frequency (CF) and covered the intensity range between the fiber's minimum rate threshold and 90-100 dB SPL in 5- or 6-dB steps. The amount of modulation in each fiber's response and the average rate of the responses were quantified. The observed response modulation was compared with the modulation to be expected on the assumption that the instantaneous discharge rates varied with intensity in the same way that the average rates did (i.e., as predicted from each fiber's average-rate vs. level function). 2. The difference between the observed and expected response modulation varied widely across fibers. In most fibers' the responses to a limited range of stimulus intensities (typically between 20 and 30 dB above the fiber's rate threshold) were modulated far more than expected on the basis of their average rates, with responses to stimuli either above or below this range differing progressively less from expectation. Little or no response modulation was observed above approximately 70 dB SPL in these fibers. Other fibers exhibited response modulation that exceeded the expected modulation by smaller amounts, but maintained this modulation to much higher sound pressure levels. 3. The discrepancy between the observed and expected responses to AM stimuli also varied with the frequency of modulation (fm) within individual fibers. The discrepancies were least pronounced at low fms (e.g., 10 Hz) but became progressively larger as fm was increased to between 50 and 320 Hz (subject to the inter-fiber variations described in 2, above). 4. The AM response characteristics varied systematically with the fiber's spontaneous rate and other response characteristics (e.g., rate threshold, CF rate vs. level function type, and rapid adaptation characteristics). In particular, the most sensitive, high spontaneous rate fibers had responses that adapted rapidly after the onset of a stimulus, and showed the greatest enhancement of AM-related information at low-to-moderate stimulus intensities. However, these fibers appeared incapable of encoding AM-related information at high intensities, since their response rates "saturated" and their AM response enhancements diminished around 30 dB above threshold. In contrast, the less sensitive (i.e., higher threshold), lower spontaneous rate fibers showed less evidence of rapid adaptation near the onsets of their response, and lesser enhancements of the modulated responses predicted from their average-rate versus level functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Mechanical preprocessing in the mammalian cochlea.

The mammalian cochlea responds with exquisite sensitivity to the small fluctuations in air pressure that make up the stimulus of sound. Moreover, it responds to pressure fluctuations that occur extremely rapidly and that vary over a wide range of intensities--in both cases, to an extent outside the capabilities of unaided nerve fibres. Research performed during the past decade has shown that these properties are dependent on a physiological source of mechanical energy that operates probably within the outer hair cells of the organ of Corti. These cells, which are anatomically and functionally similar to the primary receptor cells, the inner hair cells, are believed to function as a source of mechanical power to assist the mechanical sensitivity of the cochlea, by mechanisms that currently are not understood. Several possible mechanisms have been proposed, but each has limitations that may make it an unsuitable candidate. Recent work has also demonstrated the likely role of mechanoelectrical transduction in outer hair cells in controlling the power source and thereby influencing the sensitivity and amplitude range of the cochlea.

Animals↗

PubMed1991

Auditory-nerve spontaneous rates vary predictably with threshold.

The variation of spontaneous rate with auditory nerve thresholds is compared with predictions from a simple assumption: that spontaneous and driven activity are basically similar, both being evoked by inner hair cell transmembrane potential. Under this view, spontaneous activity is seen as a response to a standing current within the hair cell and should therefore vary with threshold in a manner predictable from measured rate-intensity functions. A method for comparing spontaneous rates of fibres with differing thresholds is developed and applied to previously-collected data. The results show that spontaneous rates are quite consistent with the hypothesis, indicating no need for more complicated theories of spontaneous activity.

Action Potentials↗

PubMed1991

Rate-versus-level functions of primary auditory nerve fibres: evidence for square law behaviour of all fibre categories in the guinea pig.

Detailed measurements of rate-versus-level (RI) functions close to threshold were made from single primary auditory nerve fibres in the guinea pig cochlea. For all fibres, a simple square law provided the best statistical fit to the data near threshold, regardless of spontaneous fibrin rate of the fibre. In no case was a better fit obtained with an exponent greater than 2. We conclude that a simple square law is an accurate description of the underlying synaptic drive to all primary auditory nerve fibres. For fibres with very low spontaneous firing rates the best square law fit near threshold frequently led to the formal mathematical estimate of a negative firing rate as the asymptotic value of the spontaneous firing rate. The 'negative spontaneous rate' of low spontaneous rate fibres derived from curve fitting can be conceptualized by postulating that for sound pressures well below threshold in these fibres the underlying synaptic drive lies below a threshold value at a site determining action potential generation.

Acoustic Stimulation↗