Action Potential Tuning Curves Research Articles

Tuning of SFOAEs Evoked by Low-Frequency Tones Is Not Compatible with Localized Emission Generation.

Stimulus-frequency otoacoustic emissions (SFOAEs) appear to be well suited for assessing frequency selectivity because, at least on theoretical grounds, they originate over a restricted region of the cochlea near the characteristic place of the evoking tone. In support of this view, we previously found good agreement between SFOAE suppression tuning curves (SF-STCs) and a control measure of frequency selectivity (compound action potential suppression tuning curves (CAP-STC)) for frequencies above 3 kHz in chinchillas. For lower frequencies, however, SF-STCs and were over five times broader than the CAP-STCs and demonstrated more high-pass rather than narrow band-pass filter characteristics. Here, we test the hypothesis that the broad tuning of low-frequency SF-STCs is because emissions originate over a broad region of the cochlea extending basal to the characteristic place of the evoking tone. We removed contributions of the hypothesized basally located SFOAE sources by either pre-suppressing them with a high-frequency interference tone (IT; 4.2, 6.2, or 9.2 kHz at 75 dB sound pressure level (SPL)) or by inducing acoustic trauma at high frequencies (exposures to 8, 5, and lastly 3-kHz tones at 110-115 dB SPL). The 1-kHz SF-STCs and CAP-STCs were measured for baseline, IT present and following the acoustic trauma conditions in anesthetized chinchillas. The IT and acoustic trauma affected SF-STCs in an almost indistinguishable way. The SF-STCs changed progressively from a broad high-pass to narrow band-pass shape as the frequency of the IT was lowered and for subsequent exposures to lower-frequency tones. Both results were in agreement with the "basal sources" hypothesis. In contrast, CAP-STCs were not changed by either manipulation, indicating that neither the IT nor acoustic trauma affected the 1-kHz characteristic place. Thus, unlike CAPs, SFOAEs cannot be considered as a place-specific measure of cochlear function at low frequencies, at least in chinchillas.

Estimating cochlear frequency selectivity with stimulus-frequency otoacoustic emissions in chinchillas.

It has been suggested that the tuning of the cochlear filters can be derived from measures of otoacoustic emissions (OAEs). Two approaches have been proposed to estimate cochlear frequency selectivity using OAEs evoked with a single tone (stimulus-frequency (SF)) OAEs: based on SFOAE group delays (SF-GDs) and on SFOAE suppression tuning curves (SF-STCs). The aim of this study was to evaluate whether either SF-GDs or SF-STCs obtained with low probe levels (30 dB SPL) correlate with more direct measures of cochlear tuning (compound action potential suppression tuning curves (CAP-STCs)) in chinchillas. The SFOAE-based estimates of tuning covaried with CAP-STCs tuning for >3 kHz probe frequencies, indicating that these measures are related to cochlear frequency selectivity. However, the relationship may be too weak to predict tuning with either SFOAE method in an individual. The SF-GD prediction of tuning was sharper than CAP-STC tuning. On the other hand, SF-STCs were consistently broader than CAP-STCs implying that SFOAEs may have less restricted region of generation in the cochlea than CAPs. Inclusion of <3 kHz data in a statistical model resulted in no significant or borderline significant covariation among the three methods: neither SFOAE test appears to reliably estimate an individual's CAP-STC tuning at low-frequencies. At the group level, SF-GDs and CAP-STCs showed similar tuning at low frequencies, while SF-STCs were over five times broader than the CAP-STCs indicating that low-frequency SFOAE may originate over a very broad region of the cochlea extending ≥5 mm basal to the tonotopic place of the probe.

A mouse model for human deafness DFNB22 reveals that hearing impairment is due to a loss of inner hair cell stimulation

The gene causative for the human nonsyndromic recessive form of deafness DFNB22 encodes otoancorin, a 120-kDa inner ear-specific protein that is expressed on the surface of the spiral limbus in the cochlea. Gene targeting in ES cells was used to create an EGFP knock-in, otoancorin KO (Otoa(EGFP/EGFP)) mouse. In the Otoa(EGFP/EGFP) mouse, the tectorial membrane (TM), a ribbon-like strip of ECM that is normally anchored by one edge to the spiral limbus and lies over the organ of Corti, retains its general form, and remains in close proximity to the organ of Corti, but is detached from the limbal surface. Measurements of cochlear microphonic potentials, distortion product otoacoustic emissions, and basilar membrane motion indicate that the TM remains functionally attached to the electromotile, sensorimotor outer hair cells of the organ of Corti, and that the amplification and frequency tuning of the basilar membrane responses to sounds are almost normal. The compound action potential masker tuning curves, a measure of the tuning of the sensory inner hair cells, are also sharply tuned, but the thresholds of the compound action potentials, a measure of inner hair cell sensitivity, are significantly elevated. These results indicate that the hearing loss in patients with Otoa mutations is caused by a defect in inner hair cell stimulation, and reveal the limbal attachment of the TM plays a critical role in this process.

Auditory steady-state evoked potentials vs. compound action potentials for the measurement of suppression tuning curves in the sedated dog puppy

Auditory steady-state evoked potential (ASSEP) tuning curves were compared to compound action potential (CAP) tuning curves, both measured at 2 Hz, using sedated beagle puppies. The effect of two types of masker (narrowband noise and sinusoidal) on the tuning curve parameters was assessed. Whatever the masker type, CAP tuning curve parameters were qualitatively and quantitatively similar to the ASSEP ones, with a similar inter-subject variability, but with a greater incidence of upward tip displacement. Whatever the procedure, sinusoidal maskers produced sharper tuning curves than narrow-band maskers. Although these differences are not likely to have significant implications for clinical work, from a fundamental point of view, their origin requires further investigations. The same amount of time was needed to record a CAP and an ASSEP 13-point tuning curve. The data further validate the ASSEP technique, which has the advantages of having a smaller tendency to produce upward tip shifts than the CAP technique. Moreover, being non invasive, ASSEP tuning curves can be easily repeated over time in the same subject for clinical and research purposes.SumarioSe compararon las curvas de afinamiento de los potenciales evocados auditivos de estado estable (ASSEP) con las curvas de afinamiento de los potenciales compuestos de acción (CAD), ambas medidas a 2 Hz, utilizando cachorros de Beagle sedados. Se evaluó el efecto de dos tipos de enmascarador (ruido de banda estrecha y ruido sinusoidal) sobre los parámetros de la curva de afinamiento. Sin importar el tipo de enmascarador, los parámetros de la curva de afinamiento de los CAP fueron cuantitativa y cualitativamente similares a los de los ASSEP, con una variabilidad inter-sujeto similar, pero con una incidencia mayor de desplazamiento de la punta ascendente. Sin importar el procedimiento, los enmascaradores sinusoidales produjeron curvas de afinamiento más agudas que los enmascaradores de banda estrecha. Aunque estas diferencias no parece que puedan tener implicaciones significativas para el trabajo clínico, desde un punto de vista fundamental, su origen requiere de mayores investigaciones. Se requirió la misma cantidad de tiempo para registrar una curva de afinamiento de 13 puntos de CAP y de ASSEP. La información validó más aún la técnica de los ASSEP, que tiene la ventaja de mostrar una menor tendencia a producir desplazamientos de punta ascendente que la técnica de CAP. Más aún, por ser no invasivas, las curvas de afinamiento de los ASSEP pueden ser repetidas fácilmente en el tiempo, en el mismo sujeto, para propósitos clínicos y de investigación.

The Motor Protein Prestin Is a Bullet-shaped Molecule with Inner Cavities

Prestin is a transmembrane motor protein localized at the outer hair cells (OHCs) of the mammalian inner ear. Voltage-dependent conformational changes in prestin generate changes in the length of OHCs. A loss of prestin function is reported to induce severe auditory deficiencies, suggesting prestin-dependent changes of OHC length may be at least a part of cochlear amplification. Here we expressed the recombinant FLAG-fused prestin proteins in Sf9 cells and purified to particles of a uniform size in EM. The square-shaped top view of purified prestin, the binding of multiple anti-FLAG antibodies to each prestin particle, the native-PAGE analysis, and the much larger molecular weight obtained from size exclusion chromatography than the estimation for the monomer all support that prestin is a tetramer (Zheng, J., Du, G. G., Anderson, C. T., Keller, J. P., Orem, A., Dallos, P., and Cheatham, M. (2006) J. Biol. Chem. 281, 19916-19924). From negatively stained prestin particles, the three-dimensional structure was reconstructed at 2 nm resolution assuming 4-fold symmetry. Prestin is shown to be a bullet-shaped particle with a large cytoplasmic domain. The surface representation demonstrates indentations on the molecule, and the slice images indicate the inner cavities of sparse densities. The dimensions, 77 x 77 x 115 A, are consistent with the previously reported sizes of motor proteins on the surface of OHCs.

Unexceptional sharpness of frequency tuning in the human cochlea

The responses to sound of auditory-nerve fibers are well known in many animals but are topics of conjecture for humans. Some investigators have claimed that the auditory-nerve fibers of humans are more sharply tuned than are those of various experimental animals. Here we invalidate such claims. First, we show that forward-masking psychophysical tuning curves, which were used as the principal support for those claims, greatly overestimate the sharpness of cochlear tuning in experimental animals and, hence, also probably in humans. Second, we calibrate compound action potential tuning curves against the tuning of auditory-nerve fibers in experimental animals and use compound action potential tuning curves recorded in humans to show that the sharpness of tuning in human cochleae is not exceptional and that it is actually similar to tuning in all mammals and birds for which comparisons are possible. Third, we note that the similarity of frequency of tuning across species with widely diverse cochlear lengths and auditory bandwidths implies that for any given stimulus frequency the "cochlear amplifier" is confined to a highly localized region of the cochlea.

Cochlear function in mice with only one copy of the prestin gene

Targeted deletion of the prestin gene reduces cochlear sensitivity and eliminates both frequency selectivity and outer hair cell (OHC) somatic electromotility. In addition, it has been reported by Liberman and colleagues that F2 generation heterozygotes exhibit a 6 dB reduction in sensitivity, as well as a decrease in protein and electromotility. Considering that the active process is non-linear, a halving of somatic electromotility would be expected to produce a much larger change in sensitivity. We therefore re-evaluated comparisons between heterozygotes and wildtype mice using both in vivo and in vitro electrophysiology, as well as molecular biology. Data reported here for F3-F5 generation mice indicate that compound action potential thresholds and tuning curves, as well as the cochlear microphonic, are similar in heterozygotes and wildtype controls. Measurements of non-linear capacitance in isolated OHCs demonstrate that charge density, as well as the voltage dependence and sensitivity of motor function, is indistinguishable in the two genotypes, as is somatic electromotility. In addition, both immunocytochemistry and western blot analysis in young adult mice suggest that prestin protein in heterozygotes is near normal. In contrast, prestin mRNA is always less than in wildtype mice at all ages tested. Results from F3-F5 generation mice suggest that one copy of the prestin gene is capable of compensating for the deleted copy and that heterozygous mice do not suffer peripheral hearing impairment.

Reduction in sharpness of frequency tuning but not endocochlear potential in aging and noise-exposed BALB/cJ mice.

Schuknecht proposed categories for human age-related hearing loss (ARHL) based upon whether the primary degeneration involves the organ of Corti (sensory ARHL), spiral ganglion cells (neural), stria vascularis (strial), or a combination of these (mixed). Genetically standardized mouse ARHL models can help validate Schuknecht's framework and clarify the underlying cellular processes. Much recent work has focused on the mouse Ahl locus, which promotes both ARHL and noise-induced hearing loss. On the C57BL/6 inbred background, Ahl has been associated with degeneration of organ of Corti, afferent neurons, and stria vascularis/spiral ligament, suggesting that it promotes mixed (sensory/neural/strial) ARHL. Some cochlear degeneration in C57BL/6 mice could be caused by genes other than Ahl, however. The question of what constitutes Ahl-related pathology can be addressed by comparing C57BL/6 mice with other strains that carry the same allele, including BALB/c substrains. We examined the effects of aging and broadband noise exposure in inbred BALB/cJ mice (1.5-13.0 mos) using measures of frequency tuning (compound action potential tuning curves) (CAPTCs), strial function (endocochlear potential recording, EP), and light microscopy. Aging and noise led to generally similar physiological and anatomical changes. Reductions in sensitivity and sharpness of frequency tuning were not consistently linked to hair cell loss, reduction in the EP, or changes in the lateral wall. Instead they appeared best explained by alterations in supporting cells in the basal half of the cochlear and in the spiral limbus in the apex. These results emphasize the importance of cell types other than hair cells in cochlear pathology. They also indicate that Ahl does not necessarily promote a strial form of ARHL.

Electrocochleographic changes in guinea pigs immunized with type II collagen

Electrocochleograms (ECoGs) were assessed in guinea pigs with cochlear impairment induced by immunization with type II collagen. Action potential (AP) tuning curves (TCs) were studied using a forward masking technique. Not only AP TCs, but the intensity function of APs and summating potentials (SPs) were assessed. The results were as follows: 1) The TCs displayed the normal pattern in all animals, and the Q10dB values of the curves were within the normal range. 2) Abnormal changes in AP, such as threshold elevation, decrease in amplitude and prolongation of latency were observed. 3) Decreases in +SP were observed in eight of ten animals, but no negative SPs were observed. 4) All animals exhibited degeneration of spiral ganglion cells on light microscopy, despite the preservation of normal hair cells. 5) Levels of antibody against type II collagen were much higher than in controls. This suggests that the normal pattern of TCs may have been the result of only minor hair cell damage. The cause of the decrease in +SP, however, is unknown. Further investigation should be performed to resolve this question. In addition, there may be differences in immune reaction from animal to animal.

Latency and Amplitude Compound Action Potential Tuning Curves for Tonal Stimuli with Nontraditional Envelopes

To evaluate the influence of the acoustic context on the latency and amplitude tuning curves (TCs) of cochlear nerve compound action potentials (CAPs), tonal stimuli were generated with a variety of amplitude modulation envelopes. CAPs were produced by intensity increases (onsets) and decreases (offsets) from the low ambient sound level and by intensity changes from a preexisting tonal level. Onset CAPs from ambient levels generated V-shaped TCs. However, when simultaneous masking was used with onset CAPs which were produced by a 5- to 12-dB increase from preexisting levels of approximately 65 dB SPL, TCs were W-shaped and similar in appearance to those produced by simultaneous masking of offset CAPs. The forward masking of this same CAP resulted in a very sharp V-shaped TC. These data suggest that the preadaptation to 10 ms of a moderate level of a tonal stimulus can increase the tuning of ensembles of cochlear neurons to subsequent transient amplitude changes.

Hypothermia Differentially Affects Tuning Curves Generated by Forward and by Simultaneous Masking

In the gerbil maintained at euthermic (37.5 degrees C) conditions, forward masking produces a compound action potential tuning curve (CAP TC) which is less sensitive but more sharply tuned than that which is generated by simultaneous masking. These differences between forward- and simultaneously-masked CAP TCs are minimized at hypothermic (30 degrees C) conditions. The unmasking effect occurs at both temperatures, suggesting that hypothermia does not exert these changes by eliminating two-tone suppression.

Cochlear action potential tuning curves recorded with a derived response technique

Previous action potential (AP) tuning curve methods have used a reduction in amplitude of the probe-elicited AP as an indication of tone-induced masking. The reduction criterion used in different studies has varied from 25% to 100%. For low level probe stimuli, which elicit a low-amplitude AP, this is a sensitive indicator. In contrast, for high-amplitude AP responses elicited by high-level stimuli, the required reduction in absolute terms is large, making it an insensitive indicator. AP tuning curves have been recorded using a sensitive method for detecting masker/probe interaction with a fixed criterion, unrelated to the unmasked AP amplitude. For each masking condition, a derived response was obtained by digitally subtracting the tone-masked AP waveform from the unmasked response. Derived responses are generated if there are ANY changes in the AP waveform induced by the masker, including amplitude changes, latency changes, or even changes in AP morphology not necessarily associated with the major peaks. A fixed criterion (10 microV) of tone-derived (TD) response was used as an indication of interaction of the responses to the masker and probe. Tuning curves generated by this method were compared with those generated by conventional amplitude reduction (AR) methods. TD tuning curves show different characteristics, especially with respect to increasing probe levels. They appear to give a good representation of the array of afferent fibers responding to a probe stimulus. In addition, frequency regions making minor contributions to the AP are better represented in TD tuning curves.

Absence of tonic activity of the crossed olivocochlear bundle in determining compound action potential thresholds, amplitudes and masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities

In Nembutal- or Urethane-anaesthetised guinea pigs A N 1 audiograms and N 1 input-output functions were measured as were compound action potential (CAP) tuning curves under forward masking and simultaneous masking conditions. Then the crossed olivocochlear bundle was lesioned at the floor of the fourth ventricle and the cochlear responses were re-measured. There were never any changes in the N 1 audiograms, input-output functions, or the CAP tuning curves. Thus, the crossed efferent pathways do not appear to play any tonic role in determining cochlear threshold sensitivities, selectivities or masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.

Latency and Amplitude Tuning Curves of the N1 and N2 Components of the Cochlear Nerve Compound Action Potential

Compound action potential tuning curves (CAP TCs) generated by masking the N1 component of the CAP provide a means of assessing the ability of the cochlea to selectively tune to certain stimuli. This paper examines some of the factors which can influence this TC when a moderately intense (i.e. 40-80 dB SPL) probe stimulus is used. At these levels, each of the four corners of the trapezoidal stimulus envelope is capable of generating a CAP. Also, short stimulus rise times can merge the CAPs produced by the first two corners, but this does not appear to have a major effect on the CAP TC. It was shown that the N2 component of the CAP for the first corner of the stimulus is equally capable of producing a well-tuned TC. Another study has shown that, in addition to amplitude decrements, one can use latency increases as a criterion for CAP TCs. We have demonstrated that latency TCs are more finely tuned than amplitude TCs at high levels, especially when the stimulus rise time is short.

Action potential tuning curves recorded with a derived response technique

A number of laboratories use tonal masking methods in conjunction with the cochlear action potential (AP) to generate AP tuning curves. In these studies, the amplitude of AP elicited by a probe stimulus is monitored as the level of masker is varied. An amplitude reduction (AR) by a specified amount (25%–100%) is used to indicate masking of the response. An alternative method was investigated in which interaction of the probe and masker is monitored by digital subtraction of the unmasked and masked responses, generating a tone‐derived (TD) response. An amplitude criterion of 10‐μV TD response is used to indicate masker/probe interaction. Tuning curves were recorded from the round window of anesthetized guinea pigs using AR and TD methods. With near threshold probe stimuli, similar tuning curves were obtained. With higher levels however, markedly different results were observed. AR tuning curves became elevated and sharper while TD curves became lower and broader. The TD method appears to better represent the frequency spread of responses with increasing stimulus level as demonstrated at the single unit level. In addition, TD tuning curves were able to detect second harmonic distortions of the probe stimulus, while AR tuning curves did not. [Work supported by Program project grant P01 NS24372 and Training Grant NS 07278.]

Recoveries of whole-nerve AP thresholds, amplitudes and tuning curves in gerbils following noise exposure

The recoveries of whole-nerve action potential (AP) thresholds, AP amplitudes and AP tuning curves in gerbils were monitored following two weeks of exposure to band-pass noise at 85 dBA. Recordings were made by means of electrodes chronically implanted in the subjects' bullas. The noise exposure caused threshold elevations in all of our subjects, with the greatest shifts occurring an octave or more above the 2 kHz upper cutoff frequency of the noise. The magnitudes of the shifts varied greatly (up to 26 dB) across subjects. Thresholds of animals with the smallest initial loss of sensitivity returned to pre-exposure values within 16 days, while those of animals with greater initial losses remained elevated beyond 16 days. AP amplitudes and AP tuning curves (APTCs) were most affected at frequencies where the initial threshold shifts were greatest. At these frequencies AP amplitudes were reduced, and APTCs showed broadened tips, reduced tip-to-tail ratios, and in some cases a shift in the frequency of the tip. These effects did not necessarily reverse with threshold recovery, suggesting that AP amplitudes and AP tuning curves are more sensitive indices of acoustic injury than are AP thresholds.

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables.

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.

Correlative development of cochlear action potential sensitivity, latency, and frequency selectivity

The present study was undertaken in order to study the rate of maturation of the cochlear action potential sensitivity, latency, and tuning curves for various stimulus frequencies and to correlate the development of latency and frequency selectivity with threshold development. This study showed a two-stage model of maturation in which thresholds and latencies declined rapidly and linearly during the first stage, and slowly and exponentially during the last stage. These results suggest that latencies and thresholds are influenced by similar maturational processes. The correlative study of frequency selectivity and thresholds showed a linear relationship during the period of rapid maturation, and conversely, a dissociation between the development of tuning and threshold during the late period of slow acquisition of adult sensitivity. This dissociation was particularly marked at 8 kHz where Q 10 values continued to increase even when thresholds were adultlike. These results were discussed in relation to anatomical and functional data available from the rat cochlea.

AP and ABR tuning curves: A comparative study

Whole‐nerve action potential (AP) and auditory brain‐stem response (ABR) were recorded from anesthetized eats. Response amplitude and latency to a probe stimulus were measured and tuning curves were constructed using three masking paradigms; forward, simultaneous (a tone burst masker overlapping the probe), and continuous (a continuous masker tone). AP and ABR tuning curves were generated on the basis of criterion changes in both amplitude and latency of the response. Within each of the masking techniques, tuning curves developed from the amplitude and latency criterion changes of the AP and the ABR showed no significant differences. The only significant differences were measured across masking conditions. Forward masking tuning curves were found to have steeper high‐frequency slopes, higher Q 10 values, and lower sensitivity indicies (defined as the difference between probe level and the level at the tip of the tuning curve). Low‐frequency slopes and the tip‐to‐tail ratios showed no significant differences. [Work supported partially by the Deafness Research Foundation.]

Effect of Signal Temporal Shaping on the Frequency Specificity of the Action Potential in Gerbils: Effet de la forme temporelle du signal sur la spécificité fréquentielle du potentiel global chez la gerboise

We examined the effects of temporal shaping of a tone burst on the frequency specificity of evoked responses in gerbils. Frequency specificity was assessed by means of whole-nerve action potential (AP) tuning curves (APTCs). The envelopes of the onset and offset portions of the AP-eliciting tone burst (or probe) were either a linear or nonlinear (1/4 sine) function of time. At probe frequencies of 1, 4 and 8 kHz, APTCs in the two envelope conditions were similar in shape and had equally narrow tips. This was true at both low (40-50 dB SPL) and high (70-75 dB SPL) probe levels. It suggests that tone bursts in the two envelope conditions evoke AP responses that are equally frequency specific. Despite these similarities, a notable difference in the two conditions was found at high probe levels. Specifically, raising the level of an 8-kHz probe from 40 to 70 dB SPL resulted in a downward shift of the APTC tip frequency, and this shift was greater for probes with linear on-off envelopes than for those with non...