Automatic monitoring of mechano-electrical transduction in the guinea pig cochlea

Similarities and Differences Between Vestibular and Cochlear Systems - A Review of Clinical and Physiological Evidence

The evoked response to repeated brief stimuli, such as clicks or short tone bursts, is used for clinical evaluation of the function of both the auditory and vestibular systems. One auditory response is a neural potential - the Auditory Brainstem Response (ABR) - recorded by surface electrodes on the head. The clinical analogue for testing the otolithic response to abrupt sounds and vibration is the myogenic potential recorded from tensed muscles - the vestibular evoked myogenic potential (VEMP). VEMPs have provided clinicians with a long sought-after tool - a simple, clinically realistic indicator of the function of each of the 4 otolithic sensory regions. We review the basic neural evidence for VEMPs and discuss the similarities and differences between otolithic and cochlear receptors and afferents. VEMPs are probably initiated by sound or vibration selectively activating afferent neurons with irregular resting discharge originating from the unique type I receptors at a specialized region of the otolithic maculae (the striola). We review how changes in VEMP responses indicate the functional state of peripheral vestibular function and the likely transduction mechanisms allowing otolithic receptors and afferents to trigger such very short latency responses. In section "ELECTROPHYSIOLOGY" we show how cochlear and vestibular receptors and afferents have many similar electrophysiological characteristics [e.g., both generate microphonics, summating potentials, and compound action potentials (the vestibular evoked potential, VsEP)]. Recent electrophysiological evidence shows that the hydrodynamic changes in the labyrinth caused by increased fluid volume (endolymphatic hydrops), change the responses of utricular receptors and afferents in a way which mimics the changes in vestibular function attributed to endolymphatic hydrops in human patients. In section "MECHANICS OF OTOLITHS IN VEMPS TESTING" we show how the major VEMP results (latency and frequency response) follow from modeling the physical characteristics of the macula (dimensions, stiffness etc.). In particular, the structure and mechanical operation of the utricular macula explains the very fast response of the type I receptors and irregular afferents which is the very basis of VEMPs and these structural changes of the macula in Menière's Disease (MD) predict the upward shift of VEMP tuning in these patients.

Examining the Factors that Contribute to Non-Monotonic Growth of the [Formula: see text] Otoacoustic Emission in Humans

Cubic distortion product otoacoustic emission input-output functions in humans show a complex pattern of growth. To further investigate the growth of the [Formula: see text] otoacoustic emission, magnitude and phase input-output functions were obtained from human subjects using a range of stimulus levels, frequencies, and frequency ratios. Three factors related to cochlear nonlinearity may produce non-monotonic input-output functions: a two-component interaction, an operating point shift, and two-tone suppression. To complement data interpretation, a local model of distortion product otoacoustic emission generation was fit to the magnitude spectrum of the averaged ear canal sound pressure recording to quantify operating point shift. Results obtained are consistent with non-monotonic growth occurring primarily as a result of two-tone suppression and/or a two-component interaction. These two mechanisms are expected to operate at different stimulus levels, with different signature magnitude and phase patterns, and are unlikely to overlap in producing non-monotonic growth. An operating point shift was suggested in three cases. These results support multiple factors contributing to the complexity of growth of the [Formula: see text] otoacoustic emission in humans and highlight the importance of looking at phase in addition to magnitude when interpreting distortion product otoacoustic emission growth.

Utricular Sensitivity during Hydrodynamic Displacements of the Macula

To explore the effects of cochlear hair cell displacement, researchers have previously monitored functional and mechanical responses during low-frequency (LF) acoustic stimulation of the cochlea. The induced changes are believed to result from modulation of the conductance of mechano-electrical transduction (MET) channels on cochlear hair cells, along with receptor potential modulation. It is less clear how, or if, vestibular hair cell displacement affects vestibular function. Here, we have used LF (<20 Hz) hydrodynamic modulation of the utricular macula position, whilst recording functional and mechanical responses, to investigate the effects of utricular macula displacement. Measured responses included the Utricular Microphonic (UM), the vestibular short-latency evoked potential (VsEP), and laser Doppler vibrometry recordings of macular position. Over 1 cycle of the LF bias, the UM amplitude and waveform were cyclically modulated, with Boltzmann analysis suggesting a cyclic modulation of the vestibular MET gating. The VsEP amplitude was cyclically modulated throughout the LF bias, demonstrating a relative increase (~20-50 %; re baseline) and decrease (~10-20 %; re baseline), which is believed to be related to the MET conductance and vestibular hair cell sensitivity. The relationship between macular displacement and changes in UM and VsEP responses was consistent within and across animals. These results suggest that the sensory structures underlying the VsEP, often thought to be a cranial jerk-sensitive response, are at least partially sensitive to LF (and possibly static) pressures or motion. Furthermore, these results highlight the possibility that some of the vestibular dysfunction related to endolymphatic hydrops may be due to altered vestibular transduction following mechanical (or morphological) changes in the labyrinth.

Phase Locking of Auditory Nerve Fibers: The Role of Lowpass Filtering by Hair Cells

Phase locking of auditory-nerve-fiber (ANF) responses to the temporal fine structure of acoustic stimuli, a hallmark of the auditory system's temporal precision, is important for many aspects of hearing. Previous work has shown that phase-locked period histograms are often well described by exponential transfer functions relating instantaneous stimulus pressure to instantaneous spike rate, with no observed clipping of the histograms. The operating points and slopes of these functions change with stimulus level. The mechanism underlying this apparent gain control is unclear but is distinct from mechanical compression, is independent of refractoriness and spike-rate adaptation, and is apparently instantaneous. Here we show that these findings can be accounted for by a model consisting of a static Boltzmann transducer function yielding a clipped output, followed by a lowpass filter and a static exponential transfer function. Using responses to tones of ANFs from cats of both sexes, we show that, for a given ANF, the period histograms obtained at all stimulus levels for a given stimulus frequency can be described using one set of level-independent model parameters. The model also accounts for changes in the maximum and minimum instantaneous spike rates with changes in stimulus level. Notably, the estimated cutoff frequency is lower for low- than for high-spontaneous-rate ANFs, implying a synapse-specific contribution to lowpass filtering. These findings advance our understanding of ANF phase locking by highlighting the role of peripheral filtering mechanisms in shaping responses of individual ANFs.SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing. Period histograms typically retain an approximately sinusoidal shape across stimulus levels, with the peripheral auditory system operating as though its overall transfer function is an exponential function whose slope decreases with increasing stimulus level. This apparent gain control can be accounted for by a static saturating transducer function followed by a lowpass filter. In addition to attenuating the AC component, the filter approximately recovers the sinusoidal waveform of the stimulus. The estimated cutoff frequency varies with spontaneous rate, revealing a synaptic contribution to lowpass filtering. These findings highlight the significant impact of peripheral filtering mechanisms on phase locking.

Phase Locking of Auditory-Nerve Fibers Reveals Stereotyped Distortions and an Exponential Transfer Function with a Level-Dependent Slope

Phase locking of auditory-nerve-fiber (ANF) responses to the fine structure of acoustic stimuli is a hallmark of the auditory system's temporal precision and is important for many aspects of hearing. Period histograms from phase-locked ANF responses to low-frequency tones exhibit spike-rate and temporal asymmetries, but otherwise retain an approximately sinusoidal shape as stimulus level increases, even beyond the level at which the mean spike rate saturates. This is intriguing because apical cochlear mechanical vibrations show little compression, and mechanoelectrical transduction in the receptor cells is thought to obey a static sigmoidal nonlinearity, which might be expected to produce peak clipping at moderate and high stimulus levels. Here we analyze phase-locked responses of ANFs from cats of both sexes. We show that the lack of peak clipping is due neither to ANF refractoriness nor to spike-rate adaptation on time scales longer than the stimulus period. We demonstrate that the relationship between instantaneous pressure and instantaneous rate is well described by an exponential function whose slope decreases with increasing stimulus level. Relatively stereotyped harmonic distortions in the input to the exponential can account for the temporal asymmetry of the period histograms, including peak splitting. We show that the model accounts for published membrane-potential waveforms when assuming a power-of-three, but not a power-of-one, relationship to exocytosis. Finally, we demonstrate the relationship between the exponential transfer functions and the sigmoidal pseudotransducer functions obtained in the literature by plotting the maxima and minima of the voltage responses against the maxima and minima of the stimuli.SIGNIFICANCE STATEMENT Phase locking of auditory-nerve-fiber responses to the temporal fine structure of acoustic stimuli is important for many aspects of hearing, but the mechanisms underlying phase locking are not fully understood. Intriguingly, period histograms retain an approximately sinusoidal shape across sound levels, even when the mean rate has saturated. We find that neither refractoriness nor spike-rate adaptation is responsible for this behavior. Instead, the peripheral auditory system operates as though it contains an exponential transfer function whose slope changes with stimulus level. The underlying mechanism is distinct from the comparatively weak cochlear mechanical compression in the cochlear apex, and likely resides in the receptor cells.

Putative Auditory-Evoked Neurophonic Measurements Using a Novel Signal Processing Technique: A Pilot Case Study

With changes to cochlear implant candidacy and improvements in surgical technique, there is a need for accurate intraoperative assessment of low-frequency hearing thresholds during cochlear implantation. In electrocochleography, onset compound action potentials (CAPs) typically allow estimation of auditory threshold for frequencies above 1 kHz, but they are less accurate at lower frequencies. Auditory nerve neurophonic (ANN) waveforms, on the other hand, may overcome this limitation by allowing phase-locked neural activity to be tracked during a prolonged low-frequency stimulus rather than just at its onset (Henry, 1995). Lichtenhan et al. (2013) have used their auditory nerve overlapped waveform (ANOW) technique to measure these potentials from the round windows of cats and guinea pigs, and reported that in guinea pigs these potentials originate in the cochlear apex for stimuli below 70 dB SPL (Lichtenhan et al., 2014). Human intraoperative round window neurophonic measurements have been reported by Choudhury et al. (2012). We have done the same in hearing impaired awake participants, and present here the results of a pilot study in which we recorded responses evoked by 360, 525, and 725 Hz tone bursts from the cochlear promontory of one participant. We also present a modification to the existing measurement technique which halves recording time, extracting the auditory neurophonic by recording a single averaged waveform, and then subtracting from it a 180° group-delayed version of itself, rather than using alternating condensation and rarefaction sound stimuli. We cannot conclude that the waveforms we measured were purely neural responses originating from the apex of the cochlea: as with all neurophonic measurement procedures, the neural responses of interest cannot be separated from higher harmonics of the cochlear microphonic without forward masking, regardless of electrode location, stimuli or post-processing algorithm. In conclusion, the extraction of putative neurophonic waveforms can easily be incorporated into existing electrocochleographic measurement paradigms, but at this stage such measurements should be interpreted with caution.

In vivo recording of the vestibular microphonic in mammals

BACKGROUND

The Vestibular Microphonic (VM) has only featured in a handful of publications, mostly involving non-mammalian and ex vivo models. The VM is the extracellular analogue of the vestibular hair cell receptor current, and offers a tool to monitor vestibular hair cell activity in vivo.

OBJECTIVE

To characterise features of the VM measured in vivo in guinea pigs, using a relatively simple experimental setup.

METHODS

The VM, evoked by bone-conducted vibration (BCV), was recorded from the basal surface of either the utricular or saccular macula after surgical removal of the cochlea, in 27 guinea pigs.

RESULTS

The VM remained after vestibular nerve blockade, but was abolished following end-organ destruction or death. The VM reversed polarity as the recording electrode tracked across the utricular or saccular macula surface, or through the utricular macula. The VM could be evoked by BCV stimuli of frequencies between 100 Hz and 5 kHz, and was largest to vibrations between 600 Hz and 800 Hz. Experimental manipulations demonstrated a reduction in the VM amplitude with maculae displacement, or rupture of the utricular membrane.

CONCLUSIONS

Results mirror those obtained in previous ex vivo studies, and further demonstrate that vestibular hair cells are sensitive to vibrations of several kilohertz. Changes in the VM with maculae displacement or rupture suggest utricular hydrops may alter vestibular hair cell sensitivity due to either mechanical or ionic changes.

Predicting the location of missing outer hair cells using the electrical signal recorded at the round window

The electrical signal recorded at the round window was used to estimate the location of missing outer hair cells. The cochlear response was recorded to a low frequency tone embedded in high-pass filtered noise conditions. Cochlear damage was created by either overexposure to frequency-specific tones or laser light. In animals with continuous damage along the partition, the amplitude of the cochlear response increased as the high-pass cutoff frequency increased, eventually reaching a plateau. The cochlear distance at the onset of the plateau correlated with the anatomical onset of outer hair cell loss. A mathematical model replicated the physiologic data but was limited to cases with continuous hair cell loss in the middle and basal turns. The neural contribution to the cochlear response was determined by recording the response before and after application of Ouabain. Application of Ouabain eliminated or reduced auditory neural activity from approximately two turns of the cochlea. The amplitude of the cochlear response was reduced for moderate signal levels with a limited effect at higher levels, indicating that the cochlear response was dominated by outer hair cell currents at high signal levels and neural potentials at low to moderate signal levels.

Modelling cochlear mechanics

The cochlea plays a crucial role in mammal hearing. The basic function of the cochlea is to map sounds of different frequencies onto corresponding characteristic positions on the basilar membrane (BM). Sounds enter the fluid-filled cochlea and cause deflection of the BM due to pressure differences between the cochlear fluid chambers. These deflections travel along the cochlea, increasing in amplitude, until a frequency-dependent characteristic position and then decay away rapidly. The hair cells can detect these deflections and encode them as neural signals. Modelling the mechanics of the cochlea is of help in interpreting experimental observations and also can provide predictions of the results of experiments that cannot currently be performed due to technical limitations. This paper focuses on reviewing the numerical modelling of the mechanical and electrical processes in the cochlea, which include fluid coupling, micromechanics, the cochlear amplifier, nonlinearity, and electrical coupling.

Effect of metabolic presbyacusis on cochlear responses: a simulation approach using a physiologically-based model

In the presented model, electrical, acoustical, and mechanical elements of the cochlea are explicitly integrated into a signal transmission line where these elements convey physiological interpretations of the human cochlear structures. As a result, this physiologically-motivated model enables simulation of specific cochlear lesions such as presbyacusis. The hypothesis is that high-frequency hearing loss in older adults may be due to metabolic presbyacusis whereby age-related cellular/chemical degenerations in the lateral wall of the cochlea cause a reduction in the endocochlear potential. The simulations quantitatively confirm this hypothesis and emphasize that even if the outer and inner hair cells are totally active and intact, metabolic presbyacusis alone can significantly deteriorate the cochlear functionality. Specifically, in the model, as the endocochlear potential decreases, the transduction mechanism produces less receptor current such that there is a reduction in the battery of the somatic motor. This leads to a drastic decrease in cochlear amplification and frequency sensitivity, as well as changes in position-frequency map (tuning pattern) of the cochlea. In addition, the simulations show that the age-related reduction of the endocochlear potential significantly inhibits the firing rate of the auditory nerve which might contribute to the decline of temporal resolution in the aging auditory system.

Expression and dexamethasone-induced nuclear translocation of glucocorticoid and mineralocorticoid receptors in guinea pig cochlear cells

Glucocorticoids (GC) are powerful anti-inflammatory agents frequently used to protect the auditory organ against damage associated with a variety of conditions, including noise exposure and ototoxic drugs as well as bacterial and viral infections. In addition to glucocorticoid receptors (GC-R), natural and synthetic GC are known to bind mineralocorticoid receptors (MC-R) with great affinity. We used light and laser scanning confocal microscopy to investigate the expression of GC-R and MC-R in different cell populations of the guinea pig cochlea, and their translocation to different cell compartments after treatment with the synthetic GC dexamethasone. We found expression of both types of receptors in the cytoplasm and nucleus of sensory inner and outer hair cells as well as pillar, Hensen and Deiters cells in the organ of Corti, inner and outer sulcus cells, spiral ganglion neurons and several types of spiral ligament and spiral limbus cells; stria vascularis cells expressed mostly MC-R whereas fibrocytes type IV were positive for GC-R only. GC-R and MC-R were also localized at or near the plasma membrane of pillar cells and outer hair cells, whereas GC-R were found at or near the plasma membrane of Hensen cells only. We investigated the relative levels of receptor expression in the cytoplasm and the nucleus of Hensen cells treated with dexamethasone, and found they varied in a way suggestive of dose-induced translocation. These results suggest that the oto-protective effects of GC could be associated with the concerted activation of genomic and non-genomic, GC-R and MC-R mediated signaling pathways in different regions of the cochlea.

Large endolymphatic potentials from low-frequency and infrasonic tones in the guinea pig

Responses of the ear to low-frequency and infrasonic sounds have not been extensively studied. Understanding how the ear responds to low frequencies is increasingly important as environmental infrasounds are becoming more pervasive from sources such as wind turbines. This study shows endolymphatic potentials in the third cochlear turn from acoustic infrasound (5 Hz) are larger than from tones in the audible range (e.g., 50 and 500 Hz), in some cases with peak-to-peak amplitude greater than 20 mV. These large potentials were suppressed by higher-frequency tones and were rapidly abolished by perilymphatic injection of KCl at the cochlear apex, demonstrating their third-turn origins. Endolymphatic iso-potentials from 5 to 500 Hz were enhanced relative to perilymphatic potentials as frequency was lowered. Probe and infrasonic bias tones were used to study the origin of the enhanced potentials. Potentials were best explained as a saturating response summed with a sinusoidal voltage (Vo), that was phase delayed by an average of 60° relative to the biasing effects of the infrasound. Vo is thought to arise indirectly from hair cell activity, such as from strial potential changes caused by sustained current changes through the hair cells in each half cycle of the infrasound.

Changes in cochlear function during acute endolymphatic hydrops development in guinea pigs

Previous studies have injected artificial endolymph into scala media in anaesthetized guinea pigs as an acute model of endolymphatic hydrops. Here, we have injected artificial endolymph into scala media in guinea pigs at rates of 40-80 nl/min, whilst monitoring Compound Action Potential (CAP) thresholds, the Summating Potential (SP)/CAP ratio, Cochlear Microphonic (CM) distortion, low-frequency modulated Distortion Product Otoacoustic Emissions (DPOAEs), and the Endocochlear Potential (EP). We found that abrupt recovery of CAP thresholds, SP/CAP ratio, and CM and DPOAE asymmetric distortion could occur several times during a single injection of less than 3 μl, suggesting that endolymph pressure could periodically decrease while the injection was ongoing. Larger volumes are thought to produce a rupture of the membranous labyrinth, however, our results suggest that multiple injections, each larger than 3 μl and within 40 min of each other, cause multiple pressure-related changes, which are difficult to be explained on the basis of a simple labyrinth rupture. We have also examined the morphological changes of the temporal bones ex vivo using X-ray micro-tomography. Both the functional changes and the micro-CT images suggest ruptures of the membranous labyrinth may not always be responsible for abrupt changes in inner ear function. Our results provide a new insight into the changes in cochlear function occurring during acute hydrops development, which compares well to the clinical findings observed in Ménière's Disease. We suggest that hydrops development may be a continual process, yet cause discontinuous functional changes due to mechanisms other than a simple rupture of the membranous labyrinth.

Effects of low-frequency biasing on otoacoustic and neural measures suggest that stimulus-frequency otoacoustic emissions originate near the peak region of the traveling wave

Stimulus-frequency otoacoustic emissions (SFOAEs) have been used to study a variety of topics in cochlear mechanics, although a current topic of debate is where in the cochlea these emissions are generated. One hypothesis is that SFOAE generation is predominately near the peak region of the traveling wave. An opposing hypothesis is that SFOAE generation near the peak region is deemphasized compared to generation in the tail region of the traveling wave. A comparison was made between the effect of low-frequency biasing on both SFOAEs and a physiologic measure that arises from the peak region of the traveling wave--the compound action potential (CAP). SFOAE biasing was measured as the amplitude of spectral sidebands from varying bias tone levels. CAP biasing was measured as the suppression of CAP amplitude from varying bias tone levels. Measures of biasing effects were made throughout the cochlea. Results from cats show that the level of bias tone needed for maximum SFOAE sidebands and for 50% CAP reduction increased as probe frequency increased. Results from guinea pigs show an irregular bias effect as a function of probe frequency. In both species, there was a strong and positive relationship between the bias level needed for maximum SFOAE sidebands and for 50% CAP suppression. This relationship is consistent with the hypothesis that the majority of SFOAE is generated near the peak region of the traveling wave.

Behaviors of cubic distortion product otoacoustic emissions evoked by amplitude modulated tones

Distortion product otoacoustic emissions (DPOAEs) were measured using sinusoidal amplitude modulation (AM) tones. When one of the primary stimuli (f(1) or f(2), f(1) < f(2)) was amplitude modulated, a series of changes in the cubic difference tone (CDT) were observed. In the frequency domain, multiple sidebands were present around the CDT and their sizes grew with the modulation depth of the AM stimulus. In the time domain, the CDT showed different modulation patterns between two major signal conditions: the AM tone was used as the f(1) or the f(2). The CDT amplitude followed the AM tone when the f(1) was amplitude modulated. However, when the AM tone acted as the f(2), the CDT showed a more complex modulation pattern with a notch present at the AM tone peak. The relatively linear dependence of CDT on f(1) and the nonlinear relation with f(2) can be explained with a variable gain-control model representing hair cell functions at the DPOAE generation site. It is likely that processing of AM signals at a particular cochlear location depends on whether the hair cells are tuned to the frequency of the carrier. Nonlinear modulation is related to on-frequency carriers and off-frequency carriers are processed relatively linearly.

Using the cochlear microphonic as a tool to evaluate cochlear function in mouse models of hearing

The cochlear microphonic (CM) can be a useful analytical tool, but many investigators may not be fully familiar with its unique properties to interpret it accurately in mouse models of hearing. The purpose of this report is to develop a model for generation of the CM in wild-type (WT) and prestin knockout mice. Data and modeling results indicate that in the majority of cases, the CM is a passive response, and in the absence of outer hair cell (OHC) damage, mice lacking amplification are expected to generate WT levels of CM for inputs less than approximately 30 kHz. Hence, this cochlear potential is not a useful metric to estimate changes in amplifier gain. This modeling analysis may explain much of the paradoxical data in the literature. For example, various manipulations, including the application of salicylate and activation of the crossed olivocochlear bundle, reduce the compound action potential but increase or do not change the CM. Based on this current evaluation, CM measurements are consistent with early descriptions where this AC cochlear potential is dominated by basal OHCs, when recorded at the round window.

Responses of the ear to low frequency sounds, infrasound and wind turbines

Infrasonic sounds are generated internally in the body (by respiration, heartbeat, coughing, etc) and by external sources, such as air conditioning systems, inside vehicles, some industrial processes and, now becoming increasingly prevalent, wind turbines. It is widely assumed that infrasound presented at an amplitude below what is audible has no influence on the ear. In this review, we consider possible ways that low frequency sounds, at levels that may or may not be heard, could influence the function of the ear. The inner ear has elaborate mechanisms to attenuate low frequency sound components before they are transmitted to the brain. The auditory portion of the ear, the cochlea, has two types of sensory cells, inner hair cells (IHC) and outer hair cells (OHC), of which the IHC are coupled to the afferent fibers that transmit "hearing" to the brain. The sensory stereocilia ("hairs") on the IHC are "fluid coupled" to mechanical stimuli, so their responses depend on stimulus velocity and their sensitivity decreases as sound frequency is lowered. In contrast, the OHC are directly coupled to mechanical stimuli, so their input remains greater than for IHC at low frequencies. At very low frequencies the OHC are stimulated by sounds at levels below those that are heard. Although the hair cells in other sensory structures such as the saccule may be tuned to infrasonic frequencies, auditory stimulus coupling to these structures is inefficient so that they are unlikely to be influenced by airborne infrasound. Structures that are involved in endolymph volume regulation are also known to be influenced by infrasound, but their sensitivity is also thought to be low. There are, however, abnormal states in which the ear becomes hypersensitive to infrasound. In most cases, the inner ear's responses to infrasound can be considered normal, but they could be associated with unfamiliar sensations or subtle changes in physiology. This raises the possibility that exposure to the infrasound component of wind turbine noise could influence the physiology of the ear.

Deficient forward transduction and enhanced reverse transduction in the alpha tectorin C1509G human hearing loss mutation

Most forms of hearing loss are associated with loss of cochlear outer hair cells (OHCs). OHCs require the tectorial membrane (TM) for stereociliary bundle stimulation (forward transduction) and active feedback (reverse transduction). Alpha tectorin is a protein constituent of the TM and the C1509G mutation in alpha tectorin in humans results in autosomal dominant hearing loss. We engineered and validated this mutation in mice and found that the TM was shortened in heterozygous Tecta(C1509G/+) mice, reaching only the first row of OHCs. Thus, deficient forward transduction renders OHCs within the second and third rows non-functional, producing partial hearing loss. Surprisingly, both Tecta(C1509G/+) and Tecta(C1509G/C1509G) mice were found to have increased reverse transduction as assessed by sound- and electrically-evoked otoacoustic emissions. We show that an increase in prestin, a protein necessary for electromotility, in all three rows of OHCs underlies this phenomenon. This mouse model demonstrates a human hearing loss mutation in which OHC function is altered through a non-cell-autonomous variation in prestin.

Estimating the operating point of the cochlear transducer using low-frequency biased distortion products

Distortion products in the cochlear microphonic (CM) and in the ear canal in the form of distortion product otoacoustic emissions (DPOAEs) are generated by nonlinear transduction in the cochlea and are related to the resting position of the organ of Corti (OC). A 4.8 Hz acoustic bias tone was used to displace the OC, while the relative amplitude and phase of distortion products evoked by a single tone [most often 500 Hz, 90 dB SPL (sound pressure level)] or two simultaneously presented tones (most often 4 kHz and 4.8 kHz, 80 dB SPL) were monitored. Electrical responses recorded from the round window, scala tympani and scala media of the basal turn, and acoustic emissions in the ear canal were simultaneously measured and compared during the bias. Bias-induced changes in the distortion products were similar to those predicted from computer models of a saturating transducer with a first-order Boltzmann distribution. Our results suggest that biased DPOAEs can be used to non-invasively estimate the OC displacement, producing a measurement equivalent to the transducer operating point obtained via Boltzmann analysis of the basal turn CM. Low-frequency biased DPOAEs might provide a diagnostic tool to objectively diagnose abnormal displacements of the OC, as might occur with endolymphatic hydrops.

Displacements of the organ of Corti by gel injections into the cochlear apex

In order to transduce sounds efficiently, the stereocilia of hair cells in the organ of Corti must be positioned optimally. Mechanical displacements, such as pressure differentials across the organ caused by endolymphatic hydrops, may impair sensitivity. Studying this phenomenon has been limited by the technical difficulty of inducing sustained displacements of stereocilia in vivo. We have found that small injections (0.5-2 microL) of Healon gel into the cochlear apex of guinea pigs produced sustained changes of endocochlear potential (EP), summating potential (SP) and transducer operating point (OP) in a manner consistent with a mechanically-induced position change of the organ of Corti in the basal turn. Induced changes immediately recovered when injection ceased. In addition, effects of low-frequency bias tones on EP, SP and OP were enhanced during the injection of gel and remained hypersensitive after injection ceased. This is thought to result from the viscous gel mechanically limiting pressure shunting through the helicotrema. Cochlear microphonics measured as frequency was varied showed enhancement below 100 Hz but most notably in the sub-auditory range. Sensitivity to low-frequency biasing was also enhanced in animals with surgically-induced endolymphatic hydrops, suggesting that obstruction of the perilymphatic space by hydrops could contribute to the pathophysiology of this condition.

Comparing the optimal signal conditions for recording cubic and quadratic distortion product otoacoustic emissions

Odd- and even-order distortion products (DPs), evoked by two primary tones (f(1),f(2),f(1)<f(2)), represent different aspects of cochlear nonlinearity. The cubic and quadratic difference tones (CDT 2f(1)-f(2) and QDT f(2)-f(1)) are prominent representatives of the odd and even DPs. Distortion product otoacoustic emissions (DPOAEs) were measured within a primary level (L(1),L(2)) space over a wide range of f(2)f(1) ratios to compare the optimal signal conditions for these DPs. For CDT, the primary level difference decreased as L(1) increased with a rate proportional to the f(2)f(1) ratio. Moreover, the optimal ratio increased with L(1). A set of two formulas is proposed to describe the optimal signal conditions. However, for a given level of a primary, increasing the other tone level could maximize the QDT amplitude. The frequency ratio at the maximal QDT was about 1.3 and quite constant across different primary levels. A notch was found in the QDT amplitude at the f(2)f(1) ratio of about 1.22-1.25. These opposite behaviors suggest that the optimal recording conditions are different for CDT and QDT due to the different aspects in the cochlear nonlinearity. Optimizing the DPOAE recordings could improve the reliability in clinical or research practices.

Perilymph osmolality modulates cochlear function

OBJECTIVES/HYPOTHESIS

The cochlear amplifier is required for the exquisite sensitivity of mammalian hearing. Outer hair cells underlie the cochlear amplifier and they are unique in that they maintain an intracellular turgor pressure. Changing the turgor pressure of an isolated outer hair cells through osmotic challenge modulates its ability to produce electromotile force. We sought to determine the effect of osmotic challenge on cochlear function.

STUDY DESIGN

In vivo animal study.

METHODS

Hypotonic and hypertonic artificial perilymph was perfused through the scala tympani of anesthetized guinea pigs. Cochlear function was assessed by measuring the compound action potential, distortion product otoacoustic emissions, the cochlear microphonic, and the endocochlear potential.

RESULTS

Hypotonic perilymph decreased and hypertonic perilymph increased compound action potential and distortion product otoacoustic emission thresholds in a dose-dependent and reversible manner. The cochlear microphonic quadratic distortion product magnitude increased after hypotonic perfusion and decreased with hypertonic perfusion. There were no changes in the stimulus intensity growth curve of the low-frequency cochlear microphonic. The endocochlear potential was not affected by perilymph osmolality.

CONCLUSIONS

These data demonstrate that perilymph osmolality can modulate cochlear function and are consistent with what would be expected if outer hair cells turgor pressure changes the gain of the cochlear amplifier in vivo.

Altered traveling wave propagation and reduced endocochlear potential associated with cochlear dysplasia in the BETA2/NeuroD1 null mouse

The BETA2/NeuroD1 null mouse has cochlear dysplasia. Its cochlear duct is shorter than normal, there is a lack of spiral ganglion neurons, and there is hair cell disorganization. We measured vertical movements of the tectorial membrane at acoustic frequencies in excised cochleae in response to mechanical stimulation of the stapes using laser doppler vibrometry. While tuning curve sharpness was similar between wild-type, heterozygotes, and null mice in the base, null mutants had broader tuning in the apex. At both the base and the apex, null mice had less phase lag accumulation with increasing stimulus frequency than wild-type or heterozygote mice. In vivo studies demonstrated that the null mouse lacked distortion product otoacoustic emissions, and the cochlear microphonic and endocochlear potential were found to be severely reduced. Electrically evoked otoacoustic emissions could be elicited, although the amplitudes were lower than those of wild-type mice. Cochlear cross-sections revealed an incomplete partition malformation, with fenestrations within the modiolus that connected the cochlear turns. Outer hair cells from null mice demonstrated the normal pattern of prestin expression within their lateral walls and normal FM 1-43 dye entry. Overall, these data demonstrate that while tonotopicity can exist with cochlear dysplasia, traveling wave propagation is abnormally fast. Additionally, the presence of electrically evoked otoacoustic emissions suggests that outer hair cell reverse transduction is present, although the acoustic response is shaped by the alterations in cochlear mechanics.

Does BAPTA leave outer hair cell transduction channels closed?

The calcium chelator BAPTA was iontophoresed into the scala media of the second turn of the guinea pig cochlea. This produced a reduction in low frequency cochlear microphonic (CM) measured in scala media and an elevation of the cochlear action potential (CAP) threshold that lasted for the duration of the experiment. Using two pipettes, one filled with KCl and the other KCl and BAPTA (50, 20 and 5 mM) it was possible to observe the effect of passing current through one electrode while measuring the endolymphatic potential (EP) with the other. The results demonstrated that current passed via the BAPTA pipette caused a sustained increase in EP of 8.2, 12.9 and 7.8 mV in the three animals used. This increase coincided with the decrease in low frequency CM that indicated a causal connection between the two. In a second series of experiments, pipettes with larger tips were inserted into scala media in the first cochlear turn and BAPTA was allowed to diffuse from the pipette. The results confirmed the relationship between EP increase and the fall of scala media CM. One interpretation of these results is that lowering the Ca2+ concentration of endolymph with BAPTA inhibits mechano-electrical transduction in outer hair cells (OHCs) and leaves the hair cell transduction channels in a closed state, thus increasing the resistance across OHCs and increasing the EP. These findings are consistent with a model of hair cell transduction in which tension on stereo cilia opens the transduction channels.

Control of mammalian cochlear amplification by chloride anions

Chloride ions have been hypothesized to interact with the membrane outer hair cell (OHC) motor protein, prestin on its intracellular domain to confer voltage sensitivity (Oliver et al., 2001). Thus, we hypothesized previously that transmembrane chloride movements via the lateral membrane conductance of the cell, GmetL, could serve to underlie cochlear amplification in the mammal. Here, we report on experimental manipulations of chloride-dependent OHC motor activity in vitro and in vivo. In vitro, we focused on the signature electrical characteristic of the motor, the nonlinear capacitance of the cell. Using the well known ototoxicant, salicylate, which competes with the putative anion binding or interaction site of prestin to assess level-dependent interactions of chloride with prestin, we determined that the resting level of chloride in OHCs is near or below 10 mm, whereas perilymphatic levels are known to be approximately 140 mm. With this observation, we sought to determine the effects of perilymphatic chloride level manipulations of basilar membrane amplification in the living guinea pig. By either direct basolateral perfusion of the OHC with altered chloride content perilymphatic solutions or by the use of tributyltin, a chloride ionophore, we found alterations in OHC electromechanical activity and cochlear amplification, which are fully reversible. Because these anionic manipulations do not impact on the cation selective stereociliary process or the endolymphatic potential, our data lend additional support to the argument that prestin activity dominates the process of mammalian cochlear amplification.

ATP-gamma-S shifts the operating point of outer hair cell transduction towards scala tympani

ATP receptor agonists and antagonists alter cochlear mechanics as measured by changes in distortion product otoacoustic emissions (DPOAE). Some of the effects on DPOAEs are consistent with the hypothesis that ATP affects mechano-electrical transduction and the operating point of the outer hair cells (OHCs). This hypothesis was tested by monitoring the effect of ATP-gamma-S on the operating point of the OHCs. Guinea pigs anesthetized with urethane and with sectioned middle ear muscles were used. The cochlear microphonic (CM) was recorded differentially (scala vestibuli referenced to scala tympani) across the basal turn before and after perfusion (20 min) of the perilymph compartment with artificial perilymph (AP) and ATP-gamma-S dissolved in AP. The operating point was derived from the cochlear microphonics (CM) recorded in response low frequency (200 Hz) tones at high level (106, 112 and 118 dB SPL). The analysis procedure used a Boltzmann function to simulate the CM waveform and the Boltzmann parameters were adjusted to best-fit the calculated waveform to the CM. Compared to the initial perfusion with AP, ATP-gamma-S (333 microM) enhanced peak clipping of the positive peak of the CM (that occurs during organ of Corti displacements towards scala tympani), which was in keeping with ATP-induced displacement of the transducer towards scala tympani. CM waveform analysis quantified the degree of displacement and showed that the changes were consistent with the stimulus being centered on a different region of the transducer curve. The change of operating point meant that the stimulus was applied to a region of the transducer curve where there was greater saturation of the output on excursions towards scala tympani and less saturation towards scala vestibuli. A significant degree of recovery of the operating point was observed after washing with AP. Dose response curves generated by perfusing ATP-gamma-S (333 microM) in a cumulative manner yielded an EC(50) of 19.8 microM. The ATP antagonist PPADS (0.1 mM) failed to block the effect of ATP-gamma-S on operating point, suggesting the response was due to activation of metabotropic and not ionotropic ATP receptors. Multiple perfusions of AP had no significant effect (118 and 112 dB) or moved the operating point slightly (106 dB) in the direction opposite of ATP-gamma-S. Results are consistent with an ATP-gamma-S induced transducer change comparable to a static movement of the organ of Corti or reticular lamina towards scala tympani.

Cochlear transducer operating point adaptation

The operating point (OP) of outer hair cell (OHC) mechanotransduction can be defined as any shift away from the center position on the transduction function. It is a dc offset that can be described by percentage of the maximum transduction current or as an equivalent dc pressure in the ear canal. The change of OP can be determined from the changes of the second and third harmonics of the cochlear microphonic (CM) following a calibration of its initial value. We found that the initial OP was dependent on sound level and cochlear sensitivity. From CM generated by a lower sound level at 74 dB SPL to avoid saturation and suppression of basal turn cochlear amplification, the OHC OP was at constant 57% of the maximum transduction current (an ear canal pressure of -0.1 Pa). To perturb the OP, a constant force was applied to the bony shell of the cochlea at the 18 kHz best frequency location using a blunt probe. The force applied over the scala tympani induced an OP change as if the organ of Corti moved toward the scala vestibuli (SV) direction. During an application of the constant force, the second harmonic of the CM partially recovered toward the initial level, which could be described by two time constants. Removing the force induced recovery of the second harmonic to its normal level described by a single time constant. The force applied over the SV caused an opposite result. These data indicate an active mechanism for OHC transduction OP.

The effect of BAPTA and 4AP in scala media on transduction and cochlear gain

We have injected by iontophoresis 4-amino-pyridine, a K+ channel blocker and BAPTA, (a Ca++ chelator), into scala media of the first three turns of the guinea pig cochlea. We measured the reduction in outer hair cell (OHC) receptor current, as indicated by cochlear microphonic measured in scala media evoked by a 207 Hz tone, and compared this with the elevation of the cochlear action potential (CAP) threshold. We found that in the basal turn, for frequencies between 12 and 21 kHz, CAP threshold was elevated by about 30 dB, while in the second turn, at the 3 kHz place, the maximum elevation was 15 dB. In the third turn, iontophoresis of 4AP and BAPTA reduced CM by similar amounts to that in the basal and second turn, but caused negligible elevation of CAP threshold. We conclude that the gain of the cochlear amplifier is maximal for basal turn frequencies, is halved at 3 kHz, and is reduced to close to one for frequencies below 1 kHz (no active gain). The effect of 4AP and BAPTA on neural threshold and the receptor current represented by CM may be explained by their action on OHC transduction without the involvement of IHCs.

Neuromonitoring of cochlea and auditory nerve with multiple extracted parameters during induced hypoxia and nerve manipulation

A system capable of comprehensive and detailed monitoring of the cochlea and the auditory nerve during intraoperative surgery was developed. The cochlear blood flow (CBF) and the electrocochleogram (ECochGm) were recorded at the round window (RW) niche using a specially designed otic probe. The ECochGm was further processed to obtain cochlear microphonics (CM) and compound action potentials (CAP). The amplitude and phase of the CM were used to quantify the activity of outer hair cells (OHC); CAP amplitude and latency were used to describe the auditory nerve and the synaptic activity of the inner hair cells (IHC). In addition, concurrent monitoring with a second electrophysiological channel was achieved by recording compound nerve action potential (CNAP) obtained directly from the auditory nerve. Stimulation paradigms, instrumentation and signal processing methods were developed to extract and differentiate the activity of the OHC and the IHC in response to three different frequencies. Narrow band acoustical stimuli elicited CM signals indicating mainly nonlinear operation of the mechano-electrical transduction of the OHCs. Special envelope detectors were developed and applied to the ECochGm to extract the CM fundamental component and its harmonics in real time. The system was extensively validated in experimental animal surgeries by performing nerve compressions and manipulations.

Cochlear compression: effects of low-frequency biasing on quadratic distortion product otoacoustic emission

Distortion product otoacoustic emissions (DPOAEs) are generated from the nonlinear transduction n cochlear outer hair cells. The transducer function demonstrating a compressive nonlinearity can be estimated from low-frequency modulation of DPOAEs. Experimental results from the gerbils showed that the magnitude of quadratic difference tone (QDT, f2-f1) was either enhanced or suppressed depending on the phase of the low-frequency bias tone. Within one period of the bias tone, QDT magnitudes exhibited two similar modulation patterns, each resembling the absolute value of the second derivative of the transducer function. In the time domain, the center notches of the modulation patterns occurred around the zero crossings of the bias pressure, whereas peaks corresponded to the increase or decrease in bias pressure. Evaluated with respect to the bias pressure, modulated QDT magnitude displayed a double-modulation pattern marked by a separation of the center notches. Loading/unloading of the cochlear transducer or rise/fall in bias pressure shifted the center notch to positive or negative sound pressures, indicating a mechanical hysteresis. These results suggest that QDT arises from the compression that coexists with the active hysteresis in cochlear transduction. Modulation of QDT magnitude reflects the dynamic regulation of cochlear transducer gain and compression.

Constructing a cochlear transducer function from the summating potential using a low-frequency bias tone

A new method is developed to construct a cochlear transducer function using modulation of the summating potential (SP), a dc component of the electrical response of the cochlea to a sinusoid. It is mathematically shown that the magnitude of the SP is determined by the even-order terms of the power series representing a nonlinear function. The relationship between the SP magnitudes and the second derivative of the transducer function was determined by using a low-frequency bias tone to position a high-frequency probe tone at different places along the cochlear transducer function. Two probe tones (6 kHz and 12 kHz) ranging from 70 to 90 dB SPL and a 25-Hz bias tone at 130 dB SPL were simultaneously presented. Electric responses from the cochlea were recorded by an electrode placed at the round window to obtain the SP magnitudes. The experimental results from eight animals demonstrated that the SP magnitudes as a function of bias levels are essentially proportional to the second derivative of a sigmoidal Boltzmann function. This suggests that the low-frequency modulated SP amplitude can be used to construct a cochlear transducer function.

Cochlear hysteresis: observation with low-frequency modulated distortion product otoacoustic emissions

Low-frequency modulation of distortion product otoacoustic emissions (DPOAEs) can be used to estimate a nonlinear transducer function (fTr) of the cochlea. From gerbils, DPOAEs were measured while presenting a high-level bias tone. Within one period of the bias tone, the magnitudes of the cubic difference tone (CDT, 2f1 - f2) demonstrated two similar modulation patterns (MPs) each resembled the absolute value of the third derivative of the fTr. The center peaks of the MPs occurred at positive sound pressures for rising in bias pressure or loading of the cochlear transducer, and more negative pressures while decreasing bias amplitude or unloading. The corresponding fTr revealed a sigmoid-shaped hysteresis loop with counterclockwise traversal. Physiologic indices that characterized the double MP varied with primary level. A Boltzmann-function-based model with negative damping as a feedback component was proposed. The model was able to replicate the experimental results. Model parameters that fit to the CDT data indicated higher transducer gain and more prominent feedback role at lower primary levels. Both physiologic indices and model parameters suggest that the cochlear transducer dynamically changes its gain with input signal level and the nonlinear mechanism is a time-dependent feedback process.

Influence of hearing sensitivity on mechano-electric transduction

This study examined the relation between the extent of permanent hearing loss and the change in a third-order polynomial transducer function (PTF) representing mechano-electric transduction (MET). Mongolian gerbils were exposed to noise for 1 to 128 h. A control group received no exposure. The cochlear microphonic (CM) was recorded from a round-window electrode and stapes velocity was recorded with a laser Doppler vibrometer in response to Gaussian noise. A nonlinear systems identification procedure provided the frequency-domain coefficients of the PTF and their associated coherence functions. In the control group, the PTF in the high frequencies was dominated by linear and cubic terms. In noise-exposed animals, the magnitude of these terms decreased with increasing threshold, suggesting a progressive decrease in the receptor currents through basal hair cells. Moreover, the linear coherence increased and the cubic coherence decreased, indicating that MET in the cochlear base became linear. In the low frequencies, noise exposure altered the group delay of the CM, demonstrating a redistribution of hair-cell currents. The low-frequency PTF was characterized by an increase in the contribution in the quadratic term. With increasing threshold, the slope of the PTF decreased and the saturation for positive CM was eliminated.

Deriving a cochlear transducer function from low-frequency modulation of distortion product otoacoustic emissions

In this paper, a new method is introduced to derive a cochlear transducer function from measuring distortion product otoacoustic emissions (DPOAEs). It is shown that the cubic difference tone (CDT, 2f1-f2) is produced from the odd-order terms of a power series that approximates a nonlinear function characterizing cochlear transduction. Exploring the underlying mathematical formulation, it is found that the CDT is proportional to the third derivative of the transduction function when the primary levels are sufficiently small. DPOAEs were measured from nine gerbils in response to two-tone signals biased by a low-frequency tone with different amplitudes. The CDT magnitude was obtained at the peak regions of the bias tone. The results of the experiment demonstrated that the shape of the CDT magnitudes as a function of bias levels was similar to the absolute value of the third derivative of a sigmoidal function. A second-order Boltzmann function was derived from curve fitting the CDT data with an equation that represents the third derivative of the Boltzmann function. Both the CDT-bias function and the derived nonlinear transducer function showed effects of primary levels. The results of the study indicate that the low-frequency modulated DPOAEs can be used to estimate the cochlear transducer function.

Effects of 4-aminopyridine on electrically evoked cochlear emissions and mechano-transduction in guinea pig outer hair cells

Stimulation of the cochlea with alternating current produces sound in the ear canal. These electrically evoked oto-acoustic emissions (EEOAEs) are attributed to electro-motility of outer hair cells (OHCs). Earlier work suggested EEOAEs were sensitive to the open probability of OHC mechano-electrical transduction (MET) channels. They were attenuated by 4-aminopyridine (4-AP) and amplitude-modulated by low frequency sound, consistent with current gaining access to a motility source via the MET conductance. However, inconsistencies in the behaviour as well as physical considerations argued against this simple interpretation. In this study the behaviour of EEOAEs in the presence of 4-AP in scala media was examined along with OHC transfer functions derived from low frequency cochlear microphonic (CM) waveforms. Both the level and the modulation of the EEOAEs were reduced by 4-AP, but disproportionately more so than the 4-AP-induced loss of CM. In addition, the modulation as well as the level of the EEOAEs recovered more rapidly than the CM. Both these results indicated that 4-AP modified the process of EEOAE generation independently of its effect on the gross receptor current through the MET conductance. Changes in the derived OHC transfer functions, specifically shifts in the estimated operating bias of the MET channels, indicated the effects of 4-AP applied to the endolymphatic surface of OHCs were complex. It is suggested that both direct and indirect consequences of a 4-AP blockade may have contributed. 4-AP was ineffective when applied to scala tympani.

Distinguishing cochlear pathophysiology in 4-aminopyridine and furosemide treated ears using a nonlinear systems identification technique

To test the adequacy of physiologic indices derived from a third-order polynomial model quantifying cochlear mechano-electric transduction (MET), 24 Mongolian gerbils were exposed to either 250-mM glucose (control), 150-mM 4-aminopyridine (4-AP), or 30-mM furosemide solutions applied to the round window (RW) membrane. The cochlear microphonic (CM) was recorded from the RW in response to 68- and 88-dB SPL Gaussian noise. A nonlinear systems identification technique (NLID) provided the frequency-domain parameters and physiologic indices of the polynomial model of MET. The control group showed no change in both compound action potential (CAP) thresholds and CM. Exposure to 4-AP and furosemide resulted in a similar elevation in CAP thresholds and a reduction in CM. However, the polynomial model of MET showed different changes. The operating point, slope, and symmetry of the MET function, the polynomial model parameters, and related nonlinear coherences differed between the experimental groups. It is concluded that the NLID technique is sensitive and specific to alterations in the cochlear physiology.

Boltzmann analysis of CM waveforms using virtual instrument software

We describe a modification to our technique for the rapid analysis of low-frequency cochlear microphonic (CM) waveforms in the basal turn of the guinea pig cochlea (Patuzzi and Moleirinho, 1998). The transfer curve relating instantaneous sound pressure in the ear canal to instantaneous receptor current through the outer hair cells (OHCs) is determined from the distorted microphonic waveform generated in the extracellular fluid near the hair cells, assuming a first-order Boltzmann activation curve. Previously, the analysis was done in real time using custom-built electronic circuitry. Here, the same task is performed numerically using virtual instrument software (National Instruments LabVIEW 4.1) running on a personal computer. The assumed theoretical function describing the CM waveform is Vcm = Voff + Vsat/[1 + exp[(Eo+Z.Po.sin(2pi f + phi(tot)))/kT]], where the six parameters are (i) a DC offset voltage (Voff); (ii) the frequency of the sinusoidal stimulus (f); (iii) the phase of the sinusoidal stimulus (phi(tot)); (iv) the maximal amplitude of the distorted microphonic signal (Vsat); (v) the sensitivity of the transduction process (Z); and (vi) the operating point on the sigmoidal transfer curve (Eo). The software obtains the least-squares fit to the CM waveforms by continuously deriving the six parameters at a speed of about one determination per second. The independent fitting of the frequency and phase allows the data to be analysed off-line from data previously recorded to tape (i.e. the frequency and phase of the microphonic response need not be known accurately beforehand). We present here an outline of the software we have used, and give an example of the changes which can be monitored using the technique (transient asphyxia). The method's advantages and limitations have been discussed in our previous paper. The virtual instrument described here is available from the authors on request.