Comparison of low- and high-side two-tone suppression in inner hair cell and organ of Corti responses

Change in cochlear response in an animal model of otitis media with effusion

Our previous studies confirm that middle ear mobility is reduced in the presence of otitis media with effusion (OME). Variations in middle ear function may result in changes in cochlear response in OME ears. With the long-term goal of evaluating cochlear function in OME ears, the aim of this study was to measure the displacement of the basilar membrane (BM) in guinea pig ears with OME. Vibrations of the BM at the apex and basal turn were measured in an in vitro preparation extracted 3 and 14 days after injection of lipopolysaccharide in the middle ear of guinea pigs. The results show that the displacement sensitivity of the BM at the apex and the basal turn to sound pressure in the ear canal was reduced up to 25 dB at their characteristic frequencies, respectively. Cochlear gain with respect to umbo movement was also changed in ears with OME in both groups. This study provides data for analysis of the change of BM vibration in a guinea pig OME model.

The effects of low- and high-frequency suppressors on psychophysical estimates of basilar-membrane compression and gain

Physiological studies suggest that the increase in suppression as a function of suppressor level is greater for a suppressor below than above the signal frequency. This study investigated the pattern of gain reduction underlying this increase in suppression. Temporal masking curves (TMCs) were obtained by measuring the level of a 2.2-kHz sinusoidal off-frequency masker or 4-kHz on-frequency sinusoidal masker required to mask a brief 4-kHz sinusoidal signal at 10 dB SL, for masker-signal intervals of 20-100 ms. TMCs were also obtained in the presence of a 3- or 4.75-kHz sinusoidal suppressor gated with the 4-kHz masker, for suppressor levels of 40-70 dB SPL. The decrease in gain (increase in suppression) as a function of suppressor level was greater with a 3-kHz suppressor than with a 4.75-kHz suppressor, in line with previous findings. Basilar membrane input-output (I/O) functions derived from the TMCs showed a shift to higher input (4-kHz masker) levels of the low-level (linear) portion of the I/O function with the addition of a suppressor, with partial linearization of the function, but no reduction in maximum compression.

Ensemble spontaneous activity in the guinea-pig cochlear nerve

Spectral analysis of electrical noise recorded from the round window (RW) of the cochlea is referred to as the ensemble spontaneous activity (ESA) of the cochlear nerve. The ESA is considered to represent the summed spontaneous activity of single fibers of the auditory nerve and changes in the spectral characteristics of the ESA have been observed in humans with tinnitus. Experiments were undertaken to determine the relationship of the ESA to auditory neurotransmission. The ESA consisted of energy centered at approximately 900 Hz, similar to the spectral peak of single auditory neuron discharges. The amplitude of the ESA was correlated with good auditory sensitivity in the 12-30 kHz region of the cochlea. Constant pure tones of 12-22 kHz suppressed the ESA reducing its amplitude in a frequency and intensity dependent manner implying that the ESA recorded at the RW is generated or dominated by neurons in the basal region of the cochlea. The ESA was significantly suppressed by round window perfusion of the P2X receptor agonist adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) (10 mM) the glutamate receptor antagonist 6-7-dinitroquinoxaline-2,3-dione (DNQX) (1 mM), and the sodium channel antagonist tetrodotoxin (TTX) (20 microM). Following intravenous furosemide injection (40 mg/kg) reduction and recovery of the ESA correlated with similar changes in the endocochlear potential (EP). Following DNQX and ATPgammaS an additional spectral peak at 200 Hz was often observed. This peak has been postulated to be a correlate of tinnitus in humans but had not previously been observed in a guinea-pig model of tinnitus. These data confirm the spectral characteristics of the ESA in guinea-pigs and show it is dependent on the sensitivity of the auditory nerve and intact auditory neurotransmission. In addition these experiments support the view that the ESA represents summed spontaneous neural activity in the cochlea and provide a platform for studies of the influence of ototoxic compounds on the spontaneous neural outflow of the cochlea as a model of tinnitus.

Psychophysical suppression measured with bandlimited noise extended below and/or above the signal: effects of age and hearing loss

The objectives of this study were to measure suppression with bandlimited noise extended below and above the signal, at lower and higher signal frequencies, between younger and older subjects, and between subjects with normal hearing and cochlear hearing loss. Psychophysical suppression was assessed by measuring forward-masked thresholds at 0.8 and 2.0 kHz in bandlimited maskers as a function of masker bandwidth. Bandpass-masker bandwidth was increased by introducing noise components below and above the signal frequency while keeping the noise centered on the signal frequency, and also by adding noise below the signal only, and above the signal only. Subjects were younger and older adults with normal hearing and older adults with cochlear hearing loss. For all subjects, suppression was larger when noise was added below the signal than when noise was added above the signal, consistent with some physiological evidence of stronger suppression below a fiber's characteristic frequency than above. For subjects with normal hearing, suppression was greater at higher than at lower frequencies. For older subjects with hearing loss, suppression was reduced to a greater extent above the signal than below and where thresholds were elevated. Suppression for older subjects with normal hearing was poorer than would be predicted from their absolute thresholds, suggesting that age may have contributed to reduced suppression or that suppression was sensitive to changes in cochlear function that did not result in significant threshold elevation.

Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

A phenomenological model for the responses of auditory-nerve fibers: I. Nonlinear tuning with compression and suppression

A phenomenological model was developed to describe responses of high-spontaneous-rate auditory-nerve (AN) fibers, including several nonlinear response properties. Level-dependent gain (compression), bandwidth, and phase properties were implemented with a control path that varied the gain and bandwidth of tuning in the signal-path filter. By making the bandwidth of the control path broad with respect to the signal path, the wide frequency range of two-tone suppression was included. By making the control-path filter level dependent and tuned to a frequency slightly higher than the signal-path filter, other properties of two-tone suppression were also included. These properties included the asymmetrical growth of suppression above and below the characteristic frequency and the frequency offset of the suppression tuning curve with respect to the excitatory tuning curve. The implementation of this model represents a relatively simple phenomenological description of a single mechanism that underlies several important nonlinear response properties of AN fibers. The model provides a tool for studying the roles of these nonlinearities in the encoding of simple and complex sounds in the responses of populations of AN fibers.

A composite model of the auditory periphery for simulating responses to complex sounds

This paper presents a phenomenological model of the cochlea. It consists of a bank of nonlinear time-varying parallel filters and an active distributed feedback. Realistic filter shapes are obtained with the all-pole gamma-tone filter (APGF), which provides both a good approximation of the far more complex wave propagation or cochlear mechanics models and a very simple implementation. Special care has been taken in modeling nonlinear properties in order to mimic the responses of the cochlea to complex stimuli. As a result, the model reproduces several observed phenomena including compression, two-tone suppression, and suppression of tones by noise. The distributed feedback, based on physiological evidence from outer hair cell (OHC) functioning, controls the damping parameter of the APGF and provides good modeling of both low-side and high-side suppression. Responses to more complex stimuli as well as a study of the model's parameters are also presented. Areas of application of this type of model include understanding of signal coding in the cochlea and auditory nerve, development of hearing aids, speech analysis, as well as input to neural models of higher auditory centers.

The level dependence of response phase: observations from cochlear hair cells

Hair cell responses are recorded from third turn of the guinea pig cochlea in order to define the relationship between hair cell depolarization and position of the basilar membrane. Because the latter is determined locally, using the cochlear microphonic recorded in the organ of Corti (OC) fluid space, no corrections are required to compensate traveling wave and/or synaptic delays. At low levels, inner hair cells (IHC) depolarize near basilar membrane velocity to scala vestibuli reflecting the free standing nature of their stereocilia. At high levels, the time of depolarization changes rapidly from velocity to scala vestibuli to the scala tympani phase of the basilar membrane response. This change in response phase, recorded in the fundamental component of the IHC response, is associated with a decrease in response magnitude. The absence of this behavior in OC and outer hair cell responses implies that basilar membrane mechanics may not be responsible for these response patterns. Because these features are reminiscent of the magnitude notches and the large phase shifts observed in single unit responses at high stimulus levels, they provide the IHC correlates of these phenomena.

Basilar membrane motion in relation to two-tone suppression

It is proposed that two-tone suppression of rate responses in auditory-nerve fibres by a low-side suppressor cannot be explained in terms of basilar membrane motion. In a model, the amplitude of the mechanical response, either to the tone at characteristic frequency (CF), or to the CF tone combined with a second, lower frequency tone (a suppressor), is taken as the effective stimulus to inner hair cells (IHC), the voltage response of which is considered responsible for excitatory drive to auditory-nerve fibres. Many empirical mechanical and physiological effects are simulated accurately by the model, particularly phenomena observed in two-tone experiments using low-side suppressor tones, that authors have described as two-tone suppression. It is argued in this paper, however, that such phenomena strictly do not constitute suppression in the cochlear response and provide no explanation for rate suppression in nerve fibres. According to the model presented here and consistent with experimental data, suppression of the spike response to a CF tone in an auditory-nerve fibre by a low-side suppressor cannot be explained in terms of the mechanics of the BM. Conclusions by others that experiments support a mechanical explanation for low-side rate suppression are shown to be questionable. It is concluded that low-side suppression of neural responses is explicable only in terms of a non-mechanical factor derived from the response to the low frequency tone, that depresses responsiveness in fibres at the CF location. Adherence to the model of low-side neural rate suppression depending on reduced net mechanical response of the BM is contrary to experimental evidence; furthermore it overlooks a profound influence additional to synaptic drive, that is implied in the shaping of responses in auditory-nerve fibres.

Suppression in auditory-nerve fibers of cats using low-side suppressors. II. Effect of spontaneous rates

The responses of auditory nerve fibers with different spontaneous rates were studied in anesthetized cats, using harmonically related characteristic frequency (CF) tone and suppressor (SUP) tone (50-2000 Hz) as stimuli. The relative-response index, defined as the ratio of the maximum response level in the two-tone segment to the response level in the CF-alone segment, at or near the intensity of maximum suppression (i.e., where the two-tone rate was lowest), was dependent on fiber's spontaneous rate (SR). For all the SUP frequencies used, lower-SR fibers almost always showed values less than unity, while high-SR fibers almost always gave values near or greater than unity. The phase of maximum suppression was not dependent upon fiber SR. In one experiment, a pair of low- and high-SR fibers with the same CF (12 kHz) were recorded consecutively in the same electrode penetration, and were studied with the same stimulus parameters. Their temporal responses showed dramatic temporal resemblances, with very similar phases of suppression and response. But the relative-response indexes were different. The similarities in the lower- and high-SR fibers support the idea that the basic response and suppression patterns in all fibers are formed at or before the inner hair cell (IHC) stage, while the differences suggest that processes more central than the IHC receptor potential are important in determining the magnitudes of suppression, particularly in the lower-SR fibers.

Two-tone suppression in inner hair cell responses: correlates of rate suppression in the auditory nerve

Inner hair cell (IHC) recordings were made from second turn of the guinea pig cochlea where characteristic frequencies are approximately 4000 Hz. In order to compare IHC responses with rate suppression measured in the auditory nerve, suppressors were introduced that produced little or no response in the hair cell. The effects of a variable-frequency suppressor on a constant-frequency probe, placed near characteristic frequency, were also investigated since this paradigm is commonly used in single unit experiments. Resulting magnitude changes were measured in the fundamental component of the ac receptor potential and/or in the total dc produced in the region of temporal overlap between the two stimulus inputs. This latter component is especially important when considering how changes in IHC responses relate to decreases in discharge rate in single auditory nerve fibers. Since the ac receptor potential is filtered by the hair cell's basolateral membrane, the dc component probably controls transmitter release at the characteristic frequency of these second-turn IHCs. Based on results from these and previous experiments, a proposal is advanced to explain the evolution of two-tone suppression in the peripheral auditory system. The paper also discusses the use of excitatory versus non-excitatory suppressors and includes a description of two-tone suppression areas at the mechanical, IHC and single unit levels. The explanation of low-side suppression areas is of special interest since hitherto they have been difficult to model (Kim, 1985).

Physiological correlates of off-frequency listening

Recordings are made from inner hair cells (IHC) in the second turn of the guinea pig cochlea where characteristic frequencies (CF) are approximately 4000 Hz. Results from experiments using two stimulus inputs suggest that the characterization of two-tone suppression at this more-basal recording location is similar to the reported for IHCs in the third turn (Cheatham and Dallos, 1989, 1990a, 1990b). For example, introduction of a suppressor causes IHC frequency response functions to become narrower with the smallest magnitude reductions occurring between 1/2 to 1 octave below CF. In this frequency region, where suppression is minimal, it was also observed that suppressor magnitude was reduced by the probe. In other words, the mutual suppression of probe and suppressor may contribute to the sharpening of these functions. Since the peak of the frequency response function shifts to a lower frequency in the presence of the suppressor, these results may provide a physiological correlate of the psychophysical phenomenon known as 'off-frequency listening.'

Place-specific derived cochlear microphonics from human ears

The high-pass noise masking technique was used to obtain derived frequency-specific cochlear microphonics (CM) from subtracted waveforms to rarefaction and condensation stimuli recorded with a tympanic membrane electrode. Two characteristics suggest that the response is place-specific CM: the derived response retains the same frequency as the stimulating toneburst and the response follows the stimulus polarity. For click stimulation, derived neural responses make the place-specific CM difficult to observe except in the 2-1 kHz derived band. In contrast, place-specific CM evoked by 0.5 and 1 kHz tonebursts can usually be detected in at least three derived bands. The amplitude of the response is largest in the derived band with center-frequency (CF) just above that of the toneburst. This discovery of a place-specific CM offers the possibility of assessing (outer) hair cell function in the apical part of the human cochlea.