Ann M. Brown

The behavior of the acoustic distortion product, 2f1-f2, from the human ear and its relation to auditory sensitivity

The acoustic intermodulation distortion product 2f1-f2 (ADP) was measured in human subjects to investigate (1) the dependence of ADP level on stimulus parameters and (2) the relationship between ADP level and auditory sensitivity. The frequency ratio (f2/f1), at which ADP level is maximal, varies only slightly across frequency and subjects. The average optimal ratio is 1.225. Beyond the maximum, the ADP level declines with increasing f2/f1 ratio, at rates of up to 250 dB/oct. As the level of one stimulus is increased relative to the other, the ADP grows, saturates, and in most cases shows a bendover. Maximum distortion is generated when L 1 exceeds L 2. Growth rate and saturation point are dependent on which stimulus is incremented and on the level of the stationary stimulus. With optimal stimulus parameters (levels below 60 dB SPL; L 1 greater than L 2 by 15 dB; f2/f1 = 1.225), ADP levels are commonly 30 dB below L 2. Patterns of ADP level across frequency vary between subjects, but are repeatable within each subject. As the frequency of one or both of the stimuli is varied, changes in ADP level exhibit a broadly featured pattern with a fine structure superimposed upon it. This fine structure was compared with the features in the stimulus frequency emission spectrum in one subject. With appropriate stimulus parameters, half of our subjects show a statistically significant correlation across frequency, between ADP level and auditory sensitivity at the corresponding f1 frequency. Our results suggest that, with low levels of stimulation, ADP measurements could form the basis of an objective measure of cochlear function in human subjects.

Suppressibility of the 2 f1- f2 stimulated acoustic emissions in gerbil and man

The suppression tuning properties of the oto-acoustic distortion product emission, 2f1-f2 have been measured in the ear canal of gerbil and man. The results show the acoustic response to be suppressible in a similar, frequency-dependent manner in both species. Frequencies near to those of the stimulating tones are most effective in suppressing the response. Derived iso-suppression tuning curves have Q10dB values of between 1 and 6. Suppressor tones having frequencies near to f2 (the higher frequency stimulus) make a contribution to the tuning curve which is largely independent of the stimulus intensity and the frequency ratio between the two primary tones. Suppressors having f1-associated frequencies produce a variable amount of suppression depending on the stimulus parameters chosen. No specific suppression feature could be associated with suppressors near to 2f1-f2. The frequency selectivity of the acoustic DP generation mechanism shown by this study indicates a close association with the transduction mechanism. The demonstration of comparable signals in gerbil and man facilitates the direct transfer of laboratory results to the study of human ears.

Acoustic distortion products can be used to monitor the effects of chronic gentamicin treatment

Acoustic distortion has been measured from the ear canals of guinea pigs treated with the aminoglycoside antibiotic gentamicin. The correlation between pre- and post-treatment levels of distortion can be related to the extent of outer hair cell (OHC) pathology induced by gentamicin. All animals that were treated and show functional changes also show changes in OHC morphology. Changes in distortion properties are evident before clear changes in surface hair cell morphology are seen. In the early stages of intoxication, thin sections of the organ of Corti are necessary to reveal the underlying structural changes accompanying functional impairment. Where OHCs have been affected by gentamicin treatment, the greatest change in 2f1-f2 level is for distortion generated by widely spaced stimuli. With closely spaced stimuli, 2f1-f2 may be unchanged in level, but higher order distortion components are depressed. Functional evidence of mild OHC disruption is only revealed by stimulus levels at or below 60 dB SPL. With higher sound levels, apparently normal levels of distortion can be generated by ears in which there is clear evidence of OHC pathology. In a single, untreated animal with an extensive basal turn lesion, some 2f1-f2 distortion was generated when the stimulus frequency region lacked OHCs.

Two sources of acoustic distortion products from the human cochlea

The amplitude and phase of both stimulus-frequency and distortion-product emissions have been measured in healthy human ears. In some ears it is possible to model acoustic distortion products (DPs) generated by low-to-moderate stimulus levels as the vector sum of two components originating from distinct regions of the cochlea, one from the f2 place and the other, with a longer delay, from the distortion product place. The phase lag of the more delayed DP component is slightly greater than that of stimulus frequency emissions (SFEs) measured in the same ears.

Measurement of acoustic distortion reveals underlying similarities between human and rodent mechanical responses

The level of 2f1-f2 acoustic distortion product (ADP) measured in the meatus during two-tone stimulation was compared with N 1 thresholds measured at the round window for the guinea pig. A significant inverse relation was found between distortion level and N 1 threshold. A similar relationship has also been reported for ADP level and subjective thresholds in half the human ears measured [S.A. Gaskill and A.M. Brown, J. Acoust. Soc. Am. 88, 821-839 (1990)]. Guinea pig and human ADP levels behave similarly in response to varying stimulus parameters. The ADP levels grow to a maximum and decline with increasing stimulus separation. The decline is steeper in the human ear. In both species, ADP growth as a function of stimulus level is approximately 1 with covaried stimuli; more gradual with the level of f2 (L 2) alone increasing and steeper when the level of f1 (L 1) alone is increased. The latter slopes are strongly influenced by the level of the stationary L 2 and are less steep in the human ear. A link has been proposed between differences in ADP behavior and differences in auditory filter bandwidth in the two species. Guinea pigs show little intersubject variability in ADP level. They do not show the fine structure in distortion level across frequency or the variation in growth rate seen in human responses. Differences in organ of Corti fine structure may underly these differences.

Ear canal acoustic and round window electrical correlates of 2f1- f2 distortion generated in the cochlea

Close parallels have been found between the behaviour of the distortion product 2f1-f2 measured acoustically in the meatus (ACDP) and the 2f1-f2 component (CMDP) measured in the gross cochlear electrical response from the round window during two-tone stimulation. The two response modes show comparable growth with increasing stimulus intensity. Their group latencies and their frequency-selective susceptibility to suppression by a third tone are very similar. The common origin of these two responses is discussed.

Continuous low level sound alters cochlear mechanics: An efferent effect?

A slow, but reversible, alteration is described in the level of acoustic distortion emitted during continuous two tone stimulation. The nature of this change and the factors which affect it indicate that it may be due to the influence of cochlear efferents on the mechanical response of the cochlea to sound.

Contralateral and ipsilateral suppression of the 2f1−f 2 distortion product in human subjects

The amplitude of the 2f1-f2 distortion product was measured under four conditions in human subjects: during simple two-tone stimulation, during stimulation with a third, pure tone in the ipsilateral ear, during contralateral stimulation with broadband noise, and with combined ipsilateral third tone and contralateral noise. The amount of ipsilateral suppression produced varied with suppressor frequency but was maximum close to the f2 primary frequency as previously demonstrated using similar stimulus parameters [A. M. Brown and D. T. Kemp, Hear. Res. 13, 29-37 (1984); F. P. Harris, R. Probst, and L. Xu, ibid. 64, 133-141 (1992)]. Contralateral stimulation also suppressed the level of distortion. During ipsilateral suppression combined with contralateral stimulation, the amount of ipsilateral suppression of the distortion product was reduced. This interaction may indicate a common mechanism for ipsilateral and contralateral suppression. The decrease in ipsilateral suppression under contralateral stimulation was most pronounced at the peak of the ipsilateral suppression tuning curves and this produced some broadening of the function. In the majority of ipsilateral suppression curves, the peak ipsilateral suppression frequency was observed to shift upwards during contralateral stimulation. These results suggest that contralateral stimulation alters the frequency-selective properties of the cochlear response.

Acoustic distortion products (ADP) from the ears of term infants and young adults using low stimulus levels

It has been shown previously that relatively low stimulus levels (L1 = 55, L2 = 40 dB SPL) elicit recordable levels of ADP from healthy adult ears (Gaskill and Brown, 1990). We have used the same stimulus levels in healthy term infants within 6 days of birth. The results from the 38 infants who provided data from both ears (from a total of 66 attempted) are reported here. The data were compared with those from 12 adults aged 20 to 30 years with normal audiometric thresholds. ADP was measured across frequency with f2 (the high frequency tone) between 1 and 8 kHz (infants) and 0.5 kHz and 8 kHz (adults) and f1 (the low frequency tone) determined by f2/1.225. ADP was also measured at a range of different stimulus frequency separations. The mean level of ADP was higher in infant than in adult ears but this difference was not significant. The f2/f1 ratio of approximately 1.2 which has been widely adopted for use in adults is also an appropriate frequency separation for term infants.

Acoustic distortion as a measure of frequency selectivity : relation to psychophysical equivalent rectangular bandwidth

The magnitude of cubic intermodulation distortion generated when two tones are progressively separated in frequency reaches a broad maximum when the distortion frequency falls just over half an octave below the high-frequency stimulus (f2), when this distortion is measured with a microphone in the ear canal. For the component 2f1-f2, this peak occurs at an f2/f1 ratio of approximately 1.2. The tuning, magnitude, and mean group delay of this distortion peak was measured for a fixed f2 of 4 kHz at 40 dB SPL and a varied f1 at 55 dB SPL in eight human subjects with normal hearing. The distortion peak measures were compared with the frequency selectivity at 4 kHz of the same eight subjects derived using a forward-masking notched-noise paradigm. In the six subjects from whom good, repeatable levels of distortion were measured, a significant negative correlation was found between the tuning of the distortion peak and the psychophysical bandwidth at f2. It is concluded that the tuning of the distortion peak may provide an objective measure of frequency selectivity in the human cochlea.