Correspondence between sharp tuning and two-tone inhibition in primary auditory neurones

Mathematical model of the auditory nerve response to stimulation by a micro-machined cochlea

We report a novel mathematical model of an artificial auditory system consisting of a micro-machined cochlea and the auditory nerve response it evokes. The modeled micro-machined cochlea is one previously realized experimentally by mimicking functions of the cochlea [Shintaku et al, Sens. Actuat. 158 (2010) 183-192; Inaoka et al, Proc. Natl. Acad. Sci. USA 108 (2011) 18390-18395]. First, from the viewpoint of mechanical engineering, the frequency characteristics of a model device were experimentally investigated to develop an artificial basilar membrane based on a spring-mass-damper system. In addition, a nonlinear feedback controller mimicking the function of the outer hair cells was incorporated in this experimental system. That is, the developed device reproduces the proportional relationship between the oscillation amplitude of the basilar membrane and the cube root of the sound pressure observed in the mammalian auditory system, which is what enables it to have a wide dynamic range, and the characteristics of the control performance were evaluated numerically and experimentally. Furthermore, the stimulation of the auditory nerve by the micro-machined cochlea was investigated using the present mathematical model, and the simulation results were compared with our previous experimental results from animal testing [Shintaku et al, J. Biomech. Sci. Eng. 8 (2013) 198-208]. The simulation results were found to be in reasonably good agreement with those from the previous animal test; namely, there exists a threshold at which the excitation of the nerve starts and a saturation value for the firing rate under a large input. The proposed numerical model was able to qualitatively reproduce the results of the animal test with the micro-machined cochlea and is thus expected to guide the evaluation of micro-machined cochleae for future animal experiments.

A 6 μW per channel analog biomimetic cochlear implant processor filterbank architecture with across channels AGC

A new analog cochlear implant processor filterbank architecture of increased biofidelity, enhanced across-channel contrast and very low power consumption has been designed and prototyped. Each channel implements a biomimetic, asymmetric bandpass-like One-Zero-Gammatone-Filter (OZGF) transfer function, using class-AB log-domain techniques. Each channel's quality factor and suppression are controlled by means of a new low power Automatic Gain Control (AGC) scheme which is coupled across the neighboring channels and emulates lateral inhibition (LI) phenomena in the auditory system. Detailed measurements from a five-channel silicon IC prototype fabricated in a 0.35 μm AMS technology confirm the operation of the coupled AGC scheme and its ability to enhance contrast among channel outputs. The prototype is characterized by an input dynamic range of 92 dB while consuming only 28 μW of power in total ( ∼ 6 μW per channel) under a 1.8 V power supply. The architecture is well-suited for fully-implantable cochlear implants.

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.

Development of single- and two-tone responses of anteroventral cochlear nucleus neurons in gerbil

Responses of anteroventral cochlear nucleus (AVCN) neurons in developing gerbils were obtained to single-tone stimuli, and two-tone stimuli elicited by best frequency probes presented over a range of intensities. Neurons displayed Type I, Type I/III, and Type III receptive field patterns. Best frequencies ranged from 1.5 to 10.0 kHz. Two-tone suppression (2TS) was first observed in 5 of 16 neurons examined at 14 dab. and in all neurons examined in gerbils aged 15 to 60 dab. Suppression areas grew larger, and discharge rate reductions became greater with age. Features of the two-tone responses that were highly correlated with single-tone responses across age groups include maximum rate reductions and suppression area thresholds. The intensity level of the CF probe-tone also influenced these features of 2TS. Maximum rate reductions to below spontaneous rate levels of activity were common across age groups. Results suggest that the cochlear amplifier is present and fundamentally adult-like by 15 dab for the regions of the cochlea coding the mid frequencies in gerbil. Over the subsequent week, contributions to the developing two-tone responses by the cochlear amplifier increase slightly. Two-tone responses are influenced by central inhibitory mechanisms as early as 14 dab.

Low-frequency suppression of auditory nerve responses to characteristic frequency tones

The effects of low-frequency (50, 100, 200 and 400 Hz) 'suppressor' tones on responses to moderate-level characteristic frequency (CF) tones were measured in chinchilla auditory nerve fibers. Two-tone interactions were evident at suppressor intensities of 70-100 dB SPL. In this range, the average response rate decreased as a function of increasing suppressor level and the instantaneous response rate was modulated periodically. At suppression threshold, the phase of suppression typically coincided with basilar membrane displacement toward scala tympani, regardless of CF. At higher suppressor levels, two suppression maxima coexisted, synchronous with peak basilar membrane displacement toward scala tympani and scala vestibuli. Modulation and rate-suppression thresholds did not vary as a function of spontaneous activity and were only minimally correlated with fiber sensitivity. Except for fibers with CF < 1 kHz, modulation and rate-suppression thresholds were lower than rate and phase-locking thresholds for the suppressor tones presented alone. In the case of high-CF fibers with low spontaneous activity, excitation thresholds could exceed suppression thresholds by more than 30 dB. The strength of modulation decreased systematically with increasing suppressor frequency. For a given suppressor frequency, modulation was strongest in high-CF fibers and weakest in low-CF fibers. The present findings strongly support the notion that low-frequency suppression in auditory nerve fibers largely reflects an underlying basilar membrane phenomenon closely related to compressive non-linearity.

Effects of acoustic trauma on the representation of the vowel "eh" in cat auditory nerve fibers

A population study of cat auditory-nerve fibers was used to characterize the permanent deficits induced by exposure to 110-115 dB SPL, narrow-band noise. Fibers in the region of acoustic trauma (roughly 1-6 kHz) showed a loss of sensitivity at best frequency (BF) of about 50-60 dB and an increased tuning bandwidth. A correlation between weakened two-tone suppression and loss of sensitivity was found for fibers with BFs above 1 kHz. Single-fiber responses to the vowel "eh" were recorded at intensities ranging from near threshold to a maximum of about 110 dB SPL. In normal cochleas, the temporal response patterns show a capture phenomenon, in which the first two formant frequencies dominate the responses at high sound levels among fibers with BFs near the formant frequencies. After acoustic trauma, fibers in the region of threshold shift synchronized to a broad range of the vowel's harmonics and thus did not show capture by the second formant at any sound level used. The broadband nature of this response is consistent with the broadened tuning observed in the damaged fibers, but may also reflect a weakening of compressive nonlinearities responsible for synchrony capture in the normal cochlea.

Auditory nerve of the normal and jaundiced rat. II. Frequency selectivity and two-tone rate suppression

This study is the continuation of the functional probing of the auditory periphery in the normal and jaundiced rat. Threshold tuning curves from normal rat auditory nerve fibers were comparable to those reported in other mammals. Life-long unconjugated hyperbilirubinemia does not appear to have a widespread, demonstrable effect on cochlear frequency selectivity and sensitivity as measured by the shapes of FTCs of single auditory nerve fibers. Most fibers from the jj Gunn rats had threshold tuning curves as sharp as those from control animals (Jj Gunn and Long-Evans). Any difference seems to lie in a greater threshold variability, particularly for the high-SR fibers, for the Gunn rat strain. Two-tone rate suppression, particularly above CF, was detected in most fibers from the three groups of rats. The optimal suppression frequency (SF) as a function of CF displayed the same progression. Suppression thresholds at any given CF were generally higher for high-SR fibers than for low-SR fibers for all three groups of animals.

Lower boundaries of two-tone suppression regions in the guinea pig

Lower boundaries of two-tone suppression regions were determined in single fibres of the guinea pig with a tracking algorithm as described by Schmiedt (1982). For a suppressee at CF having a level of 20 dB above the threshold of the tip, suppression at the high-frequency (hf) side of the FTC could almost always be found. With the method used, the percentage of fibres in which suppression could be found at the low-frequency (lf) side of the FTC decreased with decreasing CF. Moreover, the occurrence of lf-suppression decreased for lower suppressee levels for fibres with CF approximately 2-5 kHz. For each fibre the minimum level difference between lf-suppression boundary and tip threshold was larger than 20 dB, for the whole group of fibres the difference was 34 dB on average. The hf-suppression regions sometimes reached below the tip for fibres with CFs in the 4 kHz region. The frequency at the lowest level of the hf-suppression boundary, best suppression frequency or BSF, is related to the CF as: BSF = 0.55 + 1.13 CF. When the suppressee level increased, the lower boundary at the hf side shifted upwards with a rate greater than 1 dB/dB. On the whole the two-tone suppression data in the guinea pig agree with those found in other rodents.

Correlates of tone-on-tone masked thresholds in the chinchilla auditory nerve

In an attempt to determine neural correlates of tone-on-tone masking, discharge patterns of chinchilla auditory-nerve fibers were obtained in response to a set of two-tone stimuli for which behavioral masking had been previously measured (Long, G.L. and Miller, J.D. (1981): Hearing Res. 4, 279-285). The lowest masked thresholds in a sample of fibers were quantitatively similar to the chinchilla's behavioral masked thresholds. In addition, the neural data were in qualitative agreement with other previously-described characteristics of tone-on-tone masking, such as the contribution of cochlear distortion products and the upward spread of masking. It thus appears that the limitations imposed by peripheral frequency analysis determine the tone-on-tone masking pattern.

AP unmasking and AP tuning in guinea pig

Using a method introduced by Harris [6] AP unmasking was investigated in normal guinea pigs. AP unmasking existed in every guinea pig investigated and proved to be stable over measuring periods as long as 7 h. The average standard deviation of unmasking magnitude was 12%. AP unmasking strength was defined as the average unmasking magnitude across across suppressor levels from 0 to 100 dB SPL at constant masker and test-tone level and constant masker, test-tone and suppressor frequency. The relation between AP unmasking strength and AP thresholds was investigated. AP unmasking strength decreases with increasing AP threshold at the suppressor frequency. No relation with AP thresholds at other frequencies was found. AP unmasking areas were determined along with the corresponding AP tuning curves. High-frequency unmasking was found to be more prominent and more stable than low-frequency unmasking in guinea pig. From a comparison with other studies on AP unmasking and single fibre two-tone suppression it was concluded that a species difference exists with regard to the presence of low-frequency AP unmasking and low-frequency single fibre two-tone suppression.

Spontaneous and impulsively evoked otoacoustic emissions: indicators of cochlear pathology?

The first author's right ear produces a spontaneous otoacoustic emission (SOAE) at 7529 Hz and 16 dB SPL. An external continuous tone is able to suppress the SOAE. The 3 dB iso-suppression curve is broadly tuned and displaced, relative to the SOAE, toward higher frequencies. An audiogram notch exists at frequencies just below that of the SOAE. We explain the occurrence of both spontaneous and impulsively evoked OAEs in terms of disruption of active feedback mechanisms of the OHCs upon basilar membrane vibration. According to this hypothesis, each segment of the organ of Corti feeds back positively upon its segment of basilar membrane and negatively upon adjacent segments. If a patch of OHC loss exists, adjacent segments of the basilar membrane are released from the negative feedback and respond to an impulsive stimulus with exaggerated oscillations at their resonance frequencies, thus producing OAEs. At particularly sharp transitions between normal and abnormal regions of the organ of Corti SOAEs may be generated.

Lateral inhibition in the anteroventral cochlear nucleus of the cat: a microiontophoretic study

The spontaneous firing rates of non-prepotential (NPP) units of the anteroventral cochlear nucleus are quite low so it has not been possible to determine whether side band tones are inhibitory when presented alone. Microiontophoretically-applied excitatory amino acids can be used to excite non-spontaneous cells directly. Using this technique it can be shown that side band tone bursts 1/2 to 3/4 octave above the characteristic frequency (CF) of a NPP unit inhibit the amino acid-induced firing. Side band tones which inhibited the amino acid-induced firing were beyond the tuning curve. Side band tones within the tuning curve produced excitation. Both, however, usually reduced the activity evoked by a CF tone burst (i.e., two-tone interaction). The data suggests that lateral inhibition and two-tone interactions are separate phenomena in the auditory system and that lateral inhibition may play a critical role in determining the shape of the tuning curve of NPP units.

AP unmasking and AP tuning in normal and pathological human ears

In a group of seven normal and eight abnormal hearing subjects three-tone AP unmasking experiments and/or AP tuning experiments were performed during electrocochleography. In the unmasking experiments the Shannon forward masking paradigm was applied, i.e. two simultaneous tones (the masker and the suppressor) influence a test tone in a forward masking procedure. Frequency and intensity of the suppressor were varied. It appeared that unmasking effects are clearly present, i.e. the suppressor stimulus can reduce masking of the AP. This effect, however, is very variable, in normal ears as well as in pathological ears. AP tuning quality deteriorated with increasing hearing loss but no correlation was found between AP unmasking parameters and hearing loss. It seems that AP suppression areas shift less upward (or not at all) than the AP tuning curve does upon increasing the test tone level. These results raise some questions about the intercorrelation of the triad: hearing loss, quality of tuning and suppression effects, as single fibre experiments and psychophysical investigations suggest.

Efferent neural control of cochlear mechanics? Olivocochlear bundle stimulation affects cochlear biomechanical nonlinearity

We confirm the report of Mountain (Mountain, D.C. (1980): Science 210, 77-72) that stimulating the crossed olivocochlear bundle (COCB) can change the magnitude of the distortion product (f2-f1) in the ear-canal sound pressure. Our results are extended to include (2f1-f2) as well as (f2-f1) from anesthetized chinchillas with both middle-ear muscles sectioned. In contrast to Mountain's report, the polarity of the change can be either positive, negative or absent, depending on the choice of two-tone stimulus frequencies. The influence of two-tone stimulus level is also complex, but we have not yet seen the polarity of the COCB effect change with stimulus level. The magnitude and polarity of the change in (2f1-f2) are not simply related to those for (f2-f1). The effect of COCB stimulation is abolished when scala tympani is perfused with artificial perilymph containing 10(-5) M d-tubocurarine. These results demonstrate that the COCB effect is postsynaptic, probably mediated by outer hair cells. We suggest that the normal cochlea contains an active biomechanical mechanism which reduces the damping of the cochlear-partition motion and is modulated by activating the efferents. It is thus possible that the central nervous system may be able to control the dynamics of the motion of the cochlear partition.

Changes in endolymphatic potential and crossed olivocochlear bundle stimulation alter cochlear mechanics

Mechanical nonlinearity in the cochlea produces acuoustic distortion products that can be measured in the ear canal. These distortion products can be altered by changes in the endolymphatic potential as well as by stimulation of the crossed olivocochlear bundle, which provides efferent innervation to cochlear hair cells.

Relations between frequency selectivity and two-tone rate suppression in lizard cochlear-nerve fibers

Cochlear-nerve fibers innervating the apicial region of the alligator lizard basilar papilla show sharp frequency selectivity in response to single tones (measured with the frequency threshold contour, or FTC), and the phenomenon of two-tone rate suppression (TTRS) in response to two simultaneously presented tones (measured with the iso-TTRS contour, or ITC). The gross shapes of the FTCs, as characterized by the slopes of the sides and Q10dB, vary systematically with the fiber's characteristic frequency (CF). 'Fine-structural' features are also found: below CF, notches (frequency regions of relatively high threshold) occur in the FTC at frequencies related to CF. Above CF, a break frequency, which varies with CF, divides the FTC into segments of different slope. Features of the ITC also vary with CF. The detailed shapes of the FTCs and ITCs are related: lobes of the ITC interdigitate with notches in the FTC; the side of the FTC with steepest slope is closely associated with the side of the ITC with steepest slope. The close relation that is observed between sharp frequency selectivity and TTRS suggests that both phenomena arise from a common cochlear mechanism.