Two-tone Distortion Products Research Articles

Two-tone distortion products in hardware-efficient cochlea model based on asynchronous cellular automaton oscillator

Two-tone distortion products in hardware-efficient cochlea model based on asynchronous cellular automaton oscillator

Two-tone distortion at different longitudinal locations on the basilar membrane

When listening to two tones at frequency f 1 and f 2 ( f 2 > f 1), one can hear pitches not only at f 1 and f 2 but also at distortion frequencies f 2 − f 1, ( n + 1) f 1 − nf 2, and ( n + 1) f 2 − nf 1 ( n = 1, 2, 3 …). Such two-tone distortion products (DPs) also can be measured in the ear canal using a sensitive microphone. These ear-generated sounds are called otoacoustic emissions (OAEs). In spite of the common applications of OAEs, the mechanisms by which these emissions travel out of the cochlea remain unclear. In a recent study, the basilar membrane (BM) vibration at 2 f 1 − f 2 was measured as a function of the longitudinal location, using a scanning laser interferometer. The data indicated a forward traveling wave and no measurable backward wave. However, this study had a relatively high noise floor and high stimulus intensity. In the current study, the noise floor of the BM measurement was significantly decreased by using reflective beads on the BM, and the vibration was measured at relatively low intensities at more than one longitudinal location. The results show that the DP phase at a basal location leads the phase at an apical location. The data indicate that the emission travels along the BM from base to apex as a forward traveling wave, and no backward traveling wave was detected under the current experimental conditions.

Mechanical responses to two-tone distortion products in the apical and basal turns of the mammalian cochlea.

Mechanical responses to one- and two-tone acoustic stimuli were recorded from the cochlear partition in the apical turn of the chinchilla cochlea, the basal turn of the guinea pig cochlea, and the hook region of the guinea pig cochlea. The most sensitive or "best" frequencies (BFs) for the sites studied were approximately 500 Hz, 17 kHz, and 30 kHz, respectively. Responses to the cubic difference tone (CDT), 2F1 - F2 (where F1 and F2 are the frequencies of the primary stimuli), were characterized at each site. Responses to the quadratic difference tone (QDT), F2 - F1, were also characterized in the apical turn preparations (QDT responses were too small to measure in the basal cochlea). The observed responses to BF QDTs and CDTs and to BF CDTs at each site appeared similar in many ways; the relative magnitudes of the responses were highest at low-to-moderate sound pressure levels (SPLs), for example, and the absolute magnitudes grew nonmonotonically with increases in the level of either primary (L1 or L2) alone. The peak effective levels of the CDT and QDT responses were also similar, at around -20 dB re L1 and/or L2. In other respects, however, the responses to CDTs and QDTs and to BF CDTs at each site behaved quite differently. At low-to-moderate SPLs, for example, most CDT phase leads decreased with increases in either L1 or L2, whereas most QDT phase leads increased with increasing L1 and varied little with L2. Most CDT responses also varied monotonically with equal-level primaries (i.e., when L1 = L2), whereas most QDT responses varied nonmonotonically. Different responses also varied in different ways when F1 and F2 were varied. Apical turn QDT responses were observed over a very wide F1/F2 range (F1 = 1-12 kHz), but were usually largest for stimuli <2-4 kHz. Apical turn CDT levels decreased (at rates of approximately 40-80 dB/octave) only when the frequency ratio F2/F1 increased beyond approximately 1.4-1.5. In the basal turn and hook regions, the CDT levels depended nonmonotonically on F2/F1 with the eventual rates of decrease being approximately 200 dB/octave. Optimal frequency ratios for the CDT increased from (F2 < 1.1F1) to (F2 approximately 1.2F1) with increasing SPL in the basal turn, but were stable at around F2/F1 approximately 1.05 in the hook region. CDT phase leads tended to increase with increasing F2/F1 in all three regions of the cochlea, particularly at low-to-moderate SPLs. These findings are discussed in relation to previous studies of cochlear mechanics, physiology, and psychophysics.

Biophysics of the cochlea. II: Stationary nonlinear phenomenology.

Nonlinearities affecting cochlear mechanics produce appreciable compression in the basilar membrane (BM) input/output (I/O) functions at the characteristic frequency for sound-pressure levels (SPLs) as low as 20 dB (re: 20 microPa). This is thought to depend upon saturation of the outer hair cell (OHC) mechanoelectrical transducer (MET). This hypothesis was tested by solving a nonlinear integrodifferential equation that describes the BM vibration in an active cochlea. The equation extends a previously developed linear approach [Mammano and Nobili, J. Acoust. Soc. Am. 93, 3320-3332 (1993)], here modified to include saturating MET, with a few corrections mainly concerning tectorial membrane resonance and OHC coupling to the BM. Stationary solutions were computed by iteration in the frequency domain for a wide range of input SPLs, generating BM I/O functions, frequency response envelopes, and two-tone distortion products. Traveling-wave amplitude envelopes were also computed for a fixed suppressor and several suppressed tones in order to evidence the phenomenon of two-tone suppression (frequency masking) at the mechanical level. All results accord nicely with experimental data.

Two-tone suppression and distortion production on the basilar membrane in the hook region of cat and guinea pig cochleae

Two-tone suppression and two-tone distortion were investigated at the level of the basilar membrane in the hook region of cat and guinea pig cochleae using a displacement-sensitive laser interferometric measurement system. The system allowed measurements to be performed at physiological stimulus levels in the cochlear region tuned to 30–35 kHz in cat and 29 kHz in guinea pig. The amplitude of vibration of the basilar membrane due to a probe tone at the characteristic frequency (CF) was attenuated during the presentation of a simultaneous suppressor tone either above or below CF. The amount of suppression depended on the intensities of both probe and suppressor, and the relationship of the suppressor frequency to the CF. Suppressors at frequencies more than an octave below the CF attenuated the responses to the CF probe at a rate of up to 1 dB/dB, with little variation based on suppressor frequency. As the suppressor frequency was increased above CF the rate of suppression decreased rapidly. The lowest suppressor intensity at which attenuation of the probe response was observed did not vary in direct proportion to the probe intensity. This suppression threshold often varied only a few dB SPL when the probe was varied over a 20 dB SPL range. In a few instances the rate of attenuation was as much as a factor of two greater at the lowest probe intensities than at higher intensities. It is noteworthy that suppression was found when the frequency of the suppressor was either above or below CF in the same preparation. Low frequency suppressor tones suppress basilar membrane motion at the CF when the basilar membrane undergoes displacement toward either scala. The maximum suppression occurs around 100 /gms after the peak excursions caused by the low frequency biasing tone. Two-tone distortion products were often observed even at stimulus levels below those causing two-tone suppression at the site studied. The cubic difference tone (CDT) was the most prominent of the distortion products. The level of the CDT component varied nonmonotonically with the level of either of the primary tones. Responses at the difference frequency between the two primaries were usually below the noise floor of the recording system. The existence of both two-tone distortion and two-tone suppression was dependent on the presence of a cochlear nonlinearity.

Two-tone distortion in the basilar membrane of the cochlea

When humans listen to pairs of thnes they hear additional tones, or distortion products, that are not present in the stimulus. Two-tone distortion products are also known as combination tones, because their pitches match combinations of the primary frequencies (f1 and f2, f2 greater than f1), such as f2-f1, (n + 1)f1-nf2 and (n + 1)f2-nf1 (n = 1, 2, 3...). Physiological correlates of the perceived distortion products exist in responses of auditory-nerve fibres and inner hair cells and in otoacoustic emissions (sounds generated by the cochlea, recordable at the ear canal). Because the middle ear responds linearly to sound and neural responses to distortion products can be abolished by damage to hair cells at cochlear sites preferentially tuned to the frequencies of the primary tones, it was hypothesized that distortion products are generated at these sites and propagate mechanically along the basilar membrane to the location tuned to the distortion-product frequency. But until now, efforts to confirm this hypothesis have failed. Here we report the use of a new laser-velocimetry technique to demonstrate two-tone distortion in basilar-membrane motion at low and moderate stimulus intensities.

Mechanisms of salicylate ototoxicity

The ototoxic effects of salicylates, reversible hearing loss and tinnitus, are well documented. However, the pharmacological mechanisms underlying these changes in cochlear function are not well understood. The studies reported here were an investigation of the site and mechanism of salicylate ototoxicity through an examination of its effects on ionic, neural and mechanical aspects of cochlear transduction. Salicylate administration produced an intensity dependent reduction of the AP and SP, with the predominant effects occurring at low stimulus levels. In direct contrast, a significant increase was observed for corresponding CM responses, independent of stimulus intensity. Salicylates also reduced the magnitude of efferent induced shifts in the AP, CM and EP. Cochlear mechanics were altered as evidenced by the reduction in two-tone distortion products, electrically evoked emissions, and electro-phonic APs. These changes in cochlear function are attributed to a salicylate mediated increase in the membrane conductance of the outer hair cells. This change in membrane permeability interferes with the reverse transduction process, effectively reducing the gain of the cochlear amplifier. Results of single unit recordings suggest parallels between salicylate intoxication and noise trauma, which are discussed with regard to potential mechanisms of tinnitus generation.

Acoustic input-admittance of the alligator-lizard ear: nonlinear features.

The acoustic input-admittance at the alligator lizard's tympanic membrane varies with stimulus level; the magnitude of the variation can be as much as a factor of three. At 1.6 kHz, the frequency of maximum admittance magnitude, the admittance varies when the stimulus level exceeds 65 dB SPL. At frequencies above or below 1.6 kHz, larger SPLs are needed to produce admittance changes. With stimulus frequencies below 0.3 kHz or above 4.0 kHz the admittance is virtually constant for stimulus levels up to at least 100 dB SPL. The nonlinear behavior (a) is greatly reduced when the cochlear partition is destroyed, (b) does not return when the mechanical load of the partition is replaced, (c) is decreased by the introduction of proteolytic enzymes into the inner ear, and (d) is not affected by some manipulations that greatly reduce cochlear potentials. The results suggest that the mechanical properties of the cochlear partition are the source of the nonlinear admittance. Parallels between this phenomenon and two-tone distortion products in the ear canal (Rosowski et al. (1984): Hearing Res. 13, 141-158) suggest that the same nonlinear mechanical source that generates the level-dependent admittance also produces two-tone distortion products in the lizard ear canal. Published demonstrations of level-dependent admittance in mammalian ears, although rather different from these results, do not rule out the presence of a similar mechanism in the mammalian cochlea.

Cochlear nonlinearities inferred from two-tone distortion products in the ear canal of the alligator lizard.

Distortion products ( DPs ) evoked by two-tone stimuli at frequencies F1 and F2 were measured in the ear-canal sound pressure of the alligator lizard. The largest sound pressures measured, other than those at F1 and F2, where at the cubic difference frequencies 2F1-F2 and 2F2-F1. All cubic DPs were greatly reduced by destruction of the basilar membrane, which suggests that its nonlinear properties are the source of the DPs . Measurements following acoustic overstimulation show a complex relationship between the magnitude of DPs and cochlear state, as assessed by measurements of cochlear potential, and indicate the existence of multiple nonlinear sources within the inner ear. Relative magnitudes of the DPs and their dependence on stimulus level suggest that the inner-ear DP sources are cubic nonlinearities. The DPs are not highly sensitive to either average stimulus frequency or stimulus frequency separation, suggesting that the nonlinear processes are within the macromechanical processes of the inner ear. Contrary to some interpretations of ear-canal DP measurements in mammals, we conclude that DPs need not be associated with hair-cell processes and are not particularly useful indicators of cochlear health.

Mechanical and electrical mechanisms in the ear: Alligator lizard tales

The ear of the alligator lizard is similar in structure and function to mammalian ears, but somewhat simpler and more accessible to in vivo experimental studies. Our focus has been on signal transmission through the middle ear and the portion of the inner ear having hair cells with free-standing stereocilia. We have measured two-tone distortion products in the sound pressure and the input admittance at the tympanic membrane; velocity of the extrastapes, stapes, and basilar membrane; intracellular receptor potential of hair cells; and spike discharges of cochlear nerve fibers. Many features of these measurements have been represented by a model of the underlying mechanical and electrical mechanisms. Conclusions suggested by the experimental and theoretical work include: (1) Nonlinearities detected in the external and middle ear responses appear to be independent of mechano-electric transduction in the cochlea. (2) The nonlinearities observed in the receptor potential at low-stimulus levels are associated with hair-cell processes. (3) The frequency selectivity of the receptor potential is determined not only by the middle-ear—basilar-membrane mechanical system but also by stereociliary-tuft resonances whose frequencies depend on tuft morphology. [Supported by NIH and the Whitaker Health Sciences Fund.]

Cochlear models: Evidence in support of mechanical nonlinearity and a second filter (a review)

We have studied properties of a nonlinear one-dimensional model for motion of the basilar membrane. The model is able to reproduce a variety of physiological and psychophysical effects, and in some instances predictions of the model have been subsequently confirmed. We regard the agreement between model and nature as evidence that the major features of the model are qualitatively correct. Areas of agreement include generation and propagation of two-tone distortion products, two-tone suppression, and Zwicker's psychophysical masking period patterns. In order to reproduce all of the above effects, the model must contain the following major features: 1. (1) A mechanical nonlinearity. This nonlinearity is asymmetric, with membrane damping increasing during the same phase of membrane motion that elicits response activity in primary autidory fibers. 2. (2) An additional stage of sharpening, a ‘second filter’, between the physical variable controlling mechanical nonlinearity and the physical variable controlling neural excitation.

Two-tone distortion products in a nonlinear model of the basilar membrane.

Two-tone distortion products were studied in a computational model of the basilar membrane into which second- and third-order nonlinearities had been introduced. The displacement patterns produced by distortion products f2 − f1 and 2f1 − f2 resemble, in the neighborhood of the characteristic place for the frequency in question, displacement patterns produced by tones delivered at the stapes. This is not the case for distortion product 2f2 − f1. The dependence of both the amplitude and phase of distortion products f2 − f1 and 2f1 − f2 on frequency spacing and amplitude of the two primary tones has features in common with results from psychophysical experiments. The 2f1 − f2 distortion product shows a nonmonotonic behavior similar to that demonstrated in psychophysical experiments. The phase of the 2f1 − f2 distortion product changes with intensity in a manner similar to that observed psychophysically. In the model, a cancellation-tone method overestimates the amplitude of the 2f1 − f2 distortion product, again in agreement with results from psychophysical experiments. These areas of agreement between the model and psychophysical data support the notion that psychophysically observed distortion products 2f1 − f2 and (under some conditions) f2 − f1 are the result of nonlinear mechanical properties of the basilar membrane.