Basilar membrane correlates of the combination tone 2f 1 -f 2

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.

Mechanical responses of the mammalian cochlea

Recent findings in auditory research have significantly changed our views of the processes involved in hearing. Novel techniques and new approaches to investigate the mammalian cochlea have expanded our knowledge about the mechanical events occurring at physiologically relevant stimulus intensities. Experiments performed in the apical, low-frequency regions demonstrate that although there is a change in the mechanical responses along the cochlea, the fundamental characteristics are similar across the frequency range. The mechanical responses to sound stimulation exhibit tuning properties comparable to those measured intracellularly or from nerve fibres. Non-linearities in the mechanical responses have now clearly been observed at all cochlear locations. The mechanics of the cochlea are vulnerable, and dramatic changes are seen especially when the sensory hair cells are affected, for example, following acoustic overstimulation or exposure to ototoxic compounds such as furosemide. The results suggest that there is a sharply tuned and vulnerable response related to the hair cells, superimposed on a more robust, broadly tuned response. Studies of the micromechanical behaviour down to the cellular level have demonstrated significant differences radially across the hearing organ and have provided new information on the important mechanical interactions with the tectorial membrane. There is now ample evidence of reverse transduction in the auditory periphery, i.e. the cochlea does not only receive and detect mechanical stimuli but can itself produce mechanical motion. Hence, it has been shown that electrical stimulation elicits motion within the cochlea very similar to that evoked by sound. In addition, the presence of acoustically-evoked displacements of the hearing organ have now been demonstrated by several laboratories.

Two-tone distortion on the basilar membrane of the chinchilla cochlea

Basilar membrane responses to pairs of tones were measured, with the use of a laser velocimeter, in the basal turn of the cochlea in anesthetized chinchillas. Frequency spectra of basilar membrane responses to primary tones with frequencies (f1, f2) close to the characteristic frequency (CF) contain prominent odd-order two-tone distortion products (DPs) at frequencies both higher and lower than CF (such as 2f1-f2, 3f1-2f3, 2f2-f1 and 3f2-2f1). For equal-level primaries with frequencies such that 2f1-f2 equals CF, the magnitude of the 2f1-f2 DP grows with primary level at linear or faster rates at low stimulus levels, but it saturates or decreases slightly at higher levels. For a fixed level of one of the primary tones, the magnitude of the 2f1-f2 DP is a nonmonotonic function of the level of the other primary tone. For low intensities of the variable tone, the 2f1-f2 DP grows at a rate of approximately 2 dB/dB with f1 level and 1 dB/dB with f2 level. DP magnitudes decrease rapidly with increasing primary frequency ratio (f2/f1) at low stimulus levels. For more intense stimuli, DP magnitudes remain constant or decrease slowly over a wide range of frequency ratios until a critical value is reached, at which DP magnitudes fall with slopes as steep as -300 dB/octave. As stimulus level grows, DP phases increasingly lag for large f2/f1 ratios, but exhibit leads for small f2/f1 ratios. Cochlear exposure to an intense tone that produces large sensitivity losses for the primary frequencies (but only small losses for tones with frequency equal to 2f1-f2) causes a substantial decrease in magnitude of the 2f1-f2 DP. This result demonstrates that the 2f1-f2 DP originates at the basilar membrane region with CFs corresponding to the primary frequencies and propagates to the location with CF equal to the DP frequency. 2f1-f2 DPs on the basilar membrane resemble those measured in human psychophysics in most respects. However, the magnitude of basilar membrane DPs does not show the nonmonotonic dependence on f2/f1 ratio evident in DP otoacoustic emissions.

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 microseconds 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.

Basilar membrane responses to two-tone and broadband stimuli

The responses to sound of mammalian cochlear neurons exhibit many nonlinearities, some of which (such as two-tone rate suppression and intermodulation distortion) are highly frequency specific, being strongly tuned to the characteristic frequency (CF) of the neuron. With the goal of establishing the cochlear origin of these auditory-nerve nonlinearities, mechanical responses to clicks and to pairs of tones were studied in relatively healthy chinchilla cochleae at a basal site of the basilar membrane with CF of 8-10 kHz. Responses were also obtained in cochleae in which hair cell receptor potentials were reduced by systemic furosemide injection. Vibrations were recorded using either the Mössbauer technique or laser Doppler-shift velocimetry. Responses to tone pairs contained intermodulation distortion products whose magnitudes as a function of stimulus frequency and intensity were comparable to those of distortion products in cochlear afferent responses. Responses to CF tones could be selectively suppressed by tones with frequency either higher or lower than CF; in most respects, mechanical two-tone suppression resembled rate suppression in cochlear afferents. Responses to clicks displayed a CF-specific compressive nonlinearity, similar to that present in responses to single tones, which could be profoundly and selectively reduced by furosemide. The present findings firmly support the hypothesis that all CF-specific nonlinearities present in the auditory nerve originate in analogous phenomena of basilar membrane vibration. However, because of their lability, it is almost certain that the mechanical nonlinearities themselves originate in outer hair cells.

Responses to sound of the basilar membrane of the mammalian cochlea

Recent evidence shows that the frequency-specific non-linear properties of auditory nerve and inner hair cell responses to sound, including their sharp frequency tuning, are fully established in the vibration of the basilar membrane. In turn, the sensitivity, frequency selectivity and non-linear properties of basilar membrane responses probably result from an influence of the outer hair cells.

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.

The generation of combination tones

Combination tones can be explained in terms of linear travelling wave mechanics. The Pfeiffer model of the cochlea as two filters separated by a nonlinearity is assumed. The first filter is the travelling wave and the other two elements are located in the hair cell. This is sufficient to explain most of the neurophysiological and psychophysical properties of combination tones.

Nonlinear mechanical behaviour of the basilar membrane in the basal turn of the guinea pig cochlea

A capacitance probe is used to measure the vibrational displacement of the basilar membrane in the basal turn of the guinea pig. Nonlinear behaviour is exhibited in the region of the mechanical cut-off frequency similar to that previously described only for the squirrel monkey. The degree of nonlinearity appears to be directly correlated with the single unit threshold as reflected in the N1 action potential. Loss of neural sensitivity indicates loss of nonlinear behaviour in the mechanics which is undetectable shortly after death.

Combination tones and unmasking

Some important relationships between two auditory nonlinearities, unmasking and combination tone (CT) production, are described. An example of unmasking is provided by using a forward-masking paradigm where a baseline is first obtained by determining the threshold of a 2000 Hz signal preceded by a 2000 Hz masker. When a second tone (the suppressor) is then added concurrently with the masker, the signal can be easier to hear. At a suppressor level of 80 dB SPL, this unmasking occurs for suppressor frequencies around 2300 Hz. At a suppressor level of 55 dB SPL, the unmasking effect occurs at suppressor frequencies closer to the masker frequency. An example of CT production is provided by presenting two sinusoids (f1 and f2) as maskers in a forward-masking experiment. The threshold of the 2000 Hz signal is shown to decrease as f2/f1 is increased keeping 2f1 - f2 = 2000 Hz. This is consistent with the notion that the magnitude of the CR decreases as f2/f1 increases. We then provide evidence that CTs can produce ummasking effects similar to acoustic tones. Again a baseline was determined with a 2000 Hz signal amd masker (f1). Two higher-frequency sinusoids (f2 and f3) were added simultaneously to the masker, neither of which produced unmasking when presented individually. When 2f2 - f3 or f3 - f2 was approximately 2300 Hz, unmasking was observed. Next was explored 2f2 - f3 unmasking as a function of f3/f2, keeping 2f2 - f3 frequency fixed at 2300 Hz. As f3/f2 increases, the magnitude of the unmasking decreases. These CT unmasking effects suggest that the generation of CTs must be at the same site or peripheral to the site of suppression.

Cochlear tuning properties: concurrent basilar membrane and single nerve fiber measurements

Removal of perilymph from the cochlea has been reported to destroy the sharp tuning of cochlear neurons. That these changes are mechanical in origin is refuted by the concurrent recording of sharp neural tuning with broad basilar membrane responses from the same region of the partially drained cat cochlea. A second cochlear filter is therefore necessary.

The sharpening of cochlear frequency selectivity in the normal and abnormal cochlea

In the normal (anaesthetized) animal cochlea, the frequency threshold curves for single primary fibres are up to an order of magnitude sharper than the analogous function derived from various reported measurements of the basilar membrane amplitude of vibration. This enhanced neural frequency selectivity is found in the same species and under conditions similar to those in which the mechanical measurements are taken. The sharpening process (at least near threshold) appears to be linear and is not dependent upon lateral inhibitory mechanisms. The variability of the neural frequency selectivity and its vulnerability to metabolic, chemical and pathological influences suggests the hypothesis that the sharpening is due to some form of "second filter" subsequent to the relatively broadly tuned basilar membrane. All fibres recorded from in the cochlear nerve in the normal cochlea show this enhanced frequency selectivity; in contrast, in pathological cochleas, all fibres, or a substantial proportion, have high-threshold, broadly tuned characteristics, approximating to those of the basilar membrane. The frequency selectivity of normal cochlear fibres is adequate to account for the analogous psychophysical measures of hearing. It is proposed that loss of this normal frequency selectivity occurs in deafness of cochlear origin, accounting for widening of the critical band. A new hypothesis for recruitment is proposed on this basis.