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Frequency and temporal analysis of contralateral acoustic stimulation on evoked otoacoustic emissions in humans
Hearing Research 145 (2000) 52^58 www.elsevier.com/locate/heares
Frequency and temporal analysis of contralateral acoustic stimulation on evoked otoacoustic emissions in humans N. Morand a
a;
*, S. Khalfa a , P. Ravazzani b , G. Tognola L. Collet a , E. Veuillet
b;c a
, F. Grandori b , J.D. Durrant
a;d
,
Laboratory `Neurosciences et Syste©mes Sensoriels', UMR CNRS 5020, Hoªpital Edouard Herriot and Universite¨ Claude Bernard (Lyon 1), Lyon, France b Biomedical Engineering Center CNR, Milan, Italy c Department of Biomedical Engineering, Polytechnic of Milan, Milan, Italy d Departments of Communication Science and Disorders and Otolaryngology, University of Pittsburgh, Pittsburgh, PA, USA Received 28 September 1999; accepted 20 March 2000
Abstract Previous studies have shown that the effect of contralateral acoustic stimulation (CAS) on ipsilateral evoked otoacoustic emissions (EOAE) depends somewhat upon the spectrum of the eliciting stimulus. The latency of the EOAE, however, is itself frequencydependent. Consequently, two general ways of analyzing the effects of CAS may be considered: by frequency band or by temporal segment. In this study, we analyzed the effects of CAS both ways in the same subjects, essentially simultaneously. The frequency analysis of the EOAE derived from the wavelet transform (WT). The WT is known to provide a robust approach to the analysis of non-stationary signals and was anticipated to avoid possible time^frequency confounds of the cochlear mechanical system. For comparison, a more basic analysis ^ using a temporal moving window ^ was employed. The results largely support earlier findings and confirm that in humans the greatest suppression of EOAEs by CAS is obtained for lower frequency and/or longer latency EOAE components. Despite expectations for the WT analysis, the more basic, temporal, analysis tended to yield the clearer results. ß 2000 Elsevier Science B.V. All rights reserved. Key words: Otoacoustic emission; Wavelet
1. Introduction The medial olivocochlear (MOC) system has been studied extensively in animals using direct electrical stimulation of the crossed olivocochlear bundle (COCB). Such stimulation induces changes in the cochlear response to acoustic stimulation, such as a decrease in the compound action potential (N1) (Fex, * Corresponding author. Laboratoire `Neurosciences et Syste©mes Sensoriels', 3 place d'Arsonval, Pavillon U, Hopital Edouard Herriot, 69003 Lyon, France; E-mail: [email protected] Abbreviations: CAS, contralateral acoustic stimulation; EOAE, evoked otoacoustic emission; MOC, medial olivocochlear; OAE, otoacoustic emission; OHC, outer hair cell; SL, sensation level; SPL, sound pressure level
1962). A reduction also has been seen in single-unit recordings from cochlear a¡erents in cats during contralateral acoustic stimulation (CAS) (Warren and Liberman, 1989a,b; Liberman, 1989). A similar observation was made using surface-recorded auditory brainstem responses in humans (Folsom and Owsley, 1985). Several studies, furthermore, have demonstrated that otoacoustic emissions (OAEs) (Kemp, 1979) are reduced by CAS (Collet et al., 1990; Veuillet et al., 1991). The fact that the OAEs radiate through the ossicular chain and tympanum (Kemp et al., 1986), nevertheless, raises the question of the middle ear's in£uence in this suppressive e¡ect, particularly the stapedial re£ex. There are now, however, several lines of evidence to show that the reduction of evoked OEAs (EOAEs) by CAS truly re£ects the functioning of the MOC system. In animals the e¡ect disappears after sectioning of
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the crossed olivocochlear bundle (Rajan, 1988) and is reduced in humans whose olivocochlear bundle presumably was cut after sectioning the vestibular nerve to treat severe vertigo in Me¨nie©re's disease (Giraud et al., 1995 ; Williams et al., 1993, 1994a,b). Even if the in£uence of the acoustic re£ex cannot be excluded completely, the CAS e¡ect observed in EOAEs cannot be attributed readily to middle ear muscle responses since the e¡ect has been shown to persist in subjects su¡ering Bell's unilateral loss of the stapedial re£ex (Veuillet et al., 1991). Finally, in guinea pigs, after section of the middle ear muscles themselves, the suppressive e¡ect on distortion product OAEs persists (Puel and Rebillard, 1990). Work to date in humans on the e¡ect of CAS has focused primarily on the overall amplitude of the EOAE (Collet et al., 1990, 1992). However, there is evidence that the suppression may have some degree of frequency speci¢city. Indeed, it has been shown that the greatest diminution of amplitude occurs in the same frequency range as the contralateral suppressor. Veuillet et al. (1991) have shown, using narrow band noise, that the suppression is greater in the 1000 Hz zone of the emission for contralateral stimuli of 1000 Hz, and in the 2000 Hz zone of the emission for contralateral stimuli of 2000 Hz. Similarly, the greatest decrease of EOAEs was observed when the frequency of a contralateral amplitude-modulated tone had a carrier frequency close to the frequency eliciting the EOAE (Maison et al., 1997). It also has been shown that the e¡ect of CAS, independent of the spectrum of the suppressor, is dependent upon the frequency of the EOAE probe stimulus. The latency of a given frequency of the EOAE, of course, is determined by the frequency^place transformation performed by the cochlea ^ the lower the frequency the longer the latency. The e¡ectiveness of CAS according to frequency component of EOAE was studied by Veuillet et al. (1992). They examined the e¡ect of CAS after conversion of the response into frequency bands of 200 Hz. Their results showed suppressive effects to be strongest for EOAE responses emitted around 0.7 and 3.1 kHz. They observed no e¡ect of CAS for EOAEs emitted at lower frequencies (Veuillet et al., 1992). These ¢ndings are in agreement with the results obtained using, in e¡ect, temporal ¢lters. Berlin et al. (1993), utilizing a moving 2 ms window, found that the contralateral suppression is more substantial than is evident from the overall RMS magnitude because the suppression is not equal at all latencies. The strongest e¡ects appear in later temporal zones, namely beyond 8-ms latency for click stimuli/elicitors. Furthermore, Ravazzani et al. (1997) observed recently that the e¡ect of CAS increases for EOAEs of longer latencies (i.e. longer post-stimulus time).
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However, even if the latency of a component of the EOAE is related to the frequency of the component itself, it is well known that emissions evoked by clicks show a `frequency dispersion' in time (Kemp, 1979; Wit and Ritsma, 1980 ; Wit et al., 1994). Therefore analysis of time^frequency properties of EOAEs is of particular interest. Recently, the wavelet transform (WT) (Mallat, 1989 ; Daubechies, 1990) has been found to provide a robust approach to time^frequency analysis, particularly that of non-stationary signals (e.g. bioelectric responses). The WT permits the analysis of the spectra of signals like otoacoustic emissions. Indeed, the use of this method for the description of time^frequency properties of EOAEs has been reported (Tognola et al., 1997, 1998) to yield a potentially more comprehensive analysis than the familiar Fourier transform. A WT-based spectral analysis is particularly attractive in a system like the cochlea with its traveling waves, wherein parts of the cochlear partition are moving at di¡erent times along the length of the basilar membrane. WT measures derive from a so-called time-scale view of the signal, namely from the decomposition of the signal into wavelets of all possible sizes. The WT, it thus can be reasoned, might be useful to avoid possible time^frequency confounds inherent to the pattern of vibration of the cochlear partition. Its performance in EOAE analysis, indeed, was found by Tognola et al. (1998) to be superior to that of the fast Fourier transform. The purpose of the present study, consequently, was to employ a method of analysis developed from wavelet theory to con¢rm ¢ndings of the previous studies and perhaps to determine more precisely the modi¢cation of EOAE frequency components by CAS. However, it seemed prudent to compare the results of the WT method with those obtained using a more basic method. For this purpose, we chose to use temporal analysis, in reference to the frequency-dependent latency characteristics of the EOAEs. We thus sought to compare, in e¡ect simultaneously, results of the two general modes of analysis: frequency, derived from the WT; temporal, derived from a moving window analysis. The speci¢c aim of this study, therefore, was to study the strength of the CAS e¡ect according to frequency band and/or temporal segment of the EOAE. 2. Materials and methods 2.1. Subjects This investigation was primarily a descriptive study and employed 59 subjects (29 males and 30 females, mean age = 24 years). All were subjects with negative otologic histories and with normal hearing sensitivity
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(i.e. hearing threshold levels 6 20 dB between 250 and 8000 Hz, tested at octave frequencies using conventional pure-tone audiometry). All subjects were tested in their right ear (RE). 2.2. Recording of EOAEs and exploration of MOC system The subjects were tested in a sound-treated room. The EOAEs were recorded and analyzed according to the method of Bray and Kemp (1987) using the Otodynamics ILO88 system. Stimuli were non-¢ltered clicks created by exciting the transducer with a direct-current pulse of 80 Ws duration at a rate of 50/s with 260 repetitions of four-click trains for two samples of each response. These samples were obtained essentially simultaneously, via multiplexing, but stored separately in bu¡ers `A' and `B'. The EOAEs were analyzed during the 20 ms following the onset of the click, but suppressing the ¢rst 3.2 ms to minimize stimulus artifact. The e¡erent re£ex was tested according to the protocol described by Collet et al. (1992): EOAEs were recorded for clicks1 presented at ¢ve sound pressure levels (peak equivalent) between 60 and 72 dB with and without CAS. The contralateral suppressor was a broadband noise (bandwith 500^8000 Hz) presented at 30 dB sensation level by a Madsen OB822 audiometer. The order of conditions was randomized. 2.3. Special analysis programs 2.3.1. Frequency^amplitude analysis Data were windowed 3.2^20.44 ms applying a cosine envelope, with a time resolution of 40 Ws. The response averages were treated with a second-order digital bandpass ¢lter set at 600^6000 Hz. The wavelet transform was used, again, in order to study the frequency content of the EOAEs in di¡erent frequency bands (Tognola et al., 1997; Mallat, 1989; Daubechies, 1990). This analysis utilizes the inverse WT to decompose the recorded emissions into elementary components (Tognola et al., 1997). More details can be found in papers from Wit et al. (1994); Tognola
1
The so-called 'non-linear` click mode of Bray and Kemp (Kemp et al., 1986; Bray and Kemp, 1987) was not employed in order to optimize sensitivity of recording of the EOAEs particularly at reduced SPLs. Grandori and Ravazzani (1993) have shown, indeed, that this 'non-linear` click paradigm, intended to minimize stimulus artifact and emphasize EOAEs resulting from non-linear growth with intensity, actually eliminates some valid response signal. At reduced stimulus levels, on the other hand, risk of contamination by the stimulus itself is greatly reduced. In any event, the primary observation here is that there were responses with and without CAS; stimulus artifact is not subject to in£uence by CAS.
et al. (1997, 1998). The frequency contents of the emissions were analyzed in 11 frequency bands (from 500 to 6000 Hz, step 500 Hz). At the end of the procedure, a bandpass ¢lter (¢nite inverse response of FIR ¢lter of order 100) was applied to each temporal component, with a bandwidth equal to the frequency range in which the components were wavelet antitransformed. This ¢nal ¢lter stage was added in order to reduce residual artifacts outside the studied bandwidth, intrinsic to the wavelet transform itself. In order to study the amplitude of each frequency component the root mean square value (RMS) of each frequency component was computed for all stimuli, for all intensities and for all subjects. For each frequency under analysis, the average of the components of the A and B bu¡ers was computed, and the RMS magnitudes of the components of the signal were estimated, yielding one RMS value per frequency. Also the RMS magnitude of the noise £oor was estimated, using nearly the same procedure. This estimate was obtained considering the di¡erence in the frequency components of the two trials, i.e. signal sampled and stored in bu¡ers A and B, divided by the square root of 2, in order to take into account both constructive and destructive e¡ects of the noise components on the recordings (Ravazzani et al., 1998). 2.3.2. Time^amplitude analysis The RMS of the average between A and B replicates was computed with a short time procedure similar to the method typical of fast Fourier analysis, similar to that used by Berlin et al. (1993). In this investigation the duration of, in e¡ect, each subwindow was set at 4 ms, with a temporal overlap of 3 ms. The ¢rst and the last windows started at 6 and 16 ms, respectively, for a total of 11 subwindows. Also in this case, the RMS amplitude of the noise was determined, essentially de¢ned and estimated as before. 2.4. Data analysis Before statistical analysis, data were sorted. We considered only RMS values of EOAEs exceeding the noise £oor by v5 dB. In the wavelet analysis, we included only frequency bands from 500 to 4000 Hz, as these were the least noisy bands. Furthermore, to compare frequency and temporal analysis, only results of subjects who had responses with both methods were considered. Di¡erences between EOAE magnitude with and without CAS were calculated. Further analyses were employed to determine slopes of the function of EOAE reduction over frequency or time, namely using lines computed by linear regression (as described below). Statistical testing was applied to determine whether the average of these slopes is signi¢cantly dif-
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ferent from zero. For this we employed the t-test with a Bonferroni correction in reference to the inherent repeated measures design, resulting from use of the two analyses (wavelet^frequency-based and temporal analyses) in the same subjects. 3. Results EOAE measurements were done successfully for each subject, but reproducible results were not obtained at all levels in all subjects. The results presented here were obtained from 18 subjects at a click SPL of 60 and 63 dB, 21 at 66 dB, 23 at 69 dB and 25 at 72 dB. 3.1. Frequency^amplitude analysis These ¢ndings, again, are results obtained using the WT. Fig. 1 shows the means and standard errors of RMS EOAE amplitudes after application of CAS (a negative value indicating a decrease of EOAE amplitude). The results reveal a decrease of EOAE with CAS, on average, for all frequencies bands and for all intensities. Furthermore we note a tendency for the greatest e¡ect of CAS to occur at low probe levels and at low frequencies. Consequently, the dB reduction in the EOAE appears to follow a slope function over frequency, namely sloping upward from low to high frequencies; this trend is most clearly evident for data
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obtained at the 63 dB SPL probe level. The slopes of these functions were evaluated subject by subject (mean = 0.327; S.E.M. = 0.297). The mean value was compared to zero which is the hypothetical value of the slope when the e¡ect of CAS remains the same for all frequency bands of the EOAE spectra. The mean slope, however, was not found to be statistically signi¢cant from zero (one-sample t-test, t = 1.051, df = 17, P = 0.285). 3.2. Temporal^amplitude analysis Results of the moving window, time-based analysis are shown in Fig. 2 and suggest also a dependence of the CAS e¡ect on probe intensity. Furthermore, the CAS e¡ect appears to be dependent on the time analysis window ^ the longer the latency, the greater the amount of suppression. Linear regression analysis of the mean functions suggested the steepest slope to occur at 66 dB SPL. We again evaluated slopes case by case to test the average slope (mean = 0.462; S.E.M. = 0.103) against zero and found a signi¢cant di¡erence (onesample t-test, t = 4.46, df = 20, P 6 0.001). 4. Discussion The results of this study indicate that e¡ect of contralateral suppression on EOAEs seems to be greater
Fig. 1. Relative EOAE amplitudes with CAS (i.e. EOAE amplitude with CAS^EOAE amplitude without CAS) for each frequency component between 500 and 4000 Hz (500 Hz steps) analyzed by the wavelet procedure. Left upper panel: 60 dB SPL ipsilateral click intensity (n = 18). Middle upper panel: 63 dB SPL ipsilateral click intensity (n = 18). Right upper panel: 66 dB SPL click intensity (n = 21). Left lower panel: 69 dB SPL click ipsilateral intensity (n = 23). Right lower panel: 72 dB SPL ipsilateral click intensity (n = 25).
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Fig. 2. Relative EOAE amplitudes with CAS (i.e. EOAE amplitude with CAS^EOAE amplitude without CAS) analyzed by a 4 ms moving window from 6 to 16 ms (thicks indicate the middle value of subwindows). Left upper panel: 60 dB SPL ipsilateral click intensity (n = 18). Middle upper panel: 63 dB SPL ipsilateral click intensity (n = 18). Right upper panel: 66 dB SPL click intensity (n = 21). Left lower panel: 69 dB SPL click ipsilateral intensity (n = 23). Right lower panel: 72 dB SPL ipsilateral click intensity (n = 25).
for EOAEs at low frequencies and/or of longer latencies. 4.1. Wavelet analysis Using the wavelet transform, we found, overall, the greatest decreases of EOAE amplitude for the lowest frequency components, as evidenced by the slope best¢t lines in Fig. 1. These results are descriptively consistent, in part, with preliminary results obtained by Veuillet et al. (1992), who found the greatest suppressive e¡ect around 0.7 kHz. Nevertheless they found also a signi¢cant e¡ect at 3.1 kHz. The main di¡erence between our study and that of Veuillet et al. is the analysis of EOAE frequency components. We used the WT which is known to give a better resolution than classic Fourier transform (Tognola et al., 1998). Nevertheless, we used an e¡ective bandwith of 500 Hz since reducing the resolution causes splattering in the frequency domain (i.e. due to increasing sidebands of the 'wavelet ¢lters`). Our method, consequently, was more accurate. Despite trends, however, the frequency-speci¢c e¡ects were not su¤ciently robust across subjects to produce signi¢cantly slope functions overall. 4.2. Temporal analysis The results of the temporal analysis appeared more orderly. These results essentially con¢rm those of Berlin
et al. (1993) and, more generally, are consistent with a systematic increase in EOAE suppression with decreasing frequency. This e¡ect was salient for all test intensities in the grouped data and was characterized by slopes of individual functions. Indeed, the slopes were found to be signi¢cantly di¡erent from zero overall. It is clear that the e¡ect of CAS is strongest for emissions of the longest latencies. 4.3. Comparison between frequency and temporal analysis of CAS e¡ect on EOAEs The results obtained with both methods thus showed a greater e¡ect of CAS on emissions of longer latencies or lower frequency components. It is worth considering possible bases for the di¡erences between the two methods and the apparently clearer trends indicated by the temporal analysis. The WT e¡ectively analyzes the involvement of each frequency component of EOAEs across time, giving a joint distribution of the energy of a signal in time and frequency. Consequently, the temporal evolution of elementary EOAE components can be described. Tognola et al. (1997) reported that EOAEs are characterized `by a mid/short duration with a progressive amplitude attenuation T with time'. Some components are characterized by a highly sustained activity particularly for the 1.0^1.5 kHz component. Also, even if a trend of greater latencies for lower frequencies is observed using wavelet analysis, it has
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been noted that there is a dispersion of frequency components across time, i.e. due to the basalward spread of excitation in the cochlea. Therefore, again, di¡erent places of the cochlea are active at the same frequency. From the presently reported results, it thus appears that WT analysis does not overcome this confounding e¡ect, as anticipated. In the ¢nal analysis, the ¢ndings of this study should not be interpreted as counter to the e¤cacy of WT analysis of EOAEs. The results simply failed to demonstrate new insights into the e¡ects of CAS on the EOAEs. Nevertheless, the study permitted con¢rmation of the latency-dependent e¡ects of CAS- and thus frequency-dependent e¡ects and suggest temporal analysis, akin to the approach of Berlin et al. (1993) as the method of choice in the study of CAS e¡ects. Lastly, although not critically assessed in this study, the e¡ects of CAS tended to be more robust at the lower probe levels, as expected from the literature, while the frequency/time-dependent e¡ects of CAS were qualitatively similar across probe level. Central tendencies of the results thus remained orderly across probe levels; this was true regardless of method of analysis. Acknowledgements The authors thank Simona Porati and Elena Vicario for their contribution in the data processing. We gratefully acknowledge the anonymous reviewers for helpful comments. This work was supported in part by a grant from the Ministry of Research and Technology to N.M. and was partially coordinated within the framework of the European concerted Action AHEAD (Biomedicine and Health Programme of the European Commission, Contract #PL951636). References Berlin, C.I., Hood, L.J., Szabo, P., Cecola, R.P., Rigby, P., Jackson, D.F., 1993. Contralateral suppression of non-linear click evoked otoacoustic emissions. Hear. Res. 71, 1^11. Bray, P., Kemp, D.T., 1987. An advanced cochlear echo suitable for infant screening. Br. J. Audiol. 21, 191^204. Collet, L., Kemp, D.T., Veuillet, E., Duclaux, R., Moulin, A., Morgon, A., 1990. E¡ect of a contralateral auditory stimuli on active micromechanical properties in human subjects. Hear. Res. 43, 251^262. Collet, L., Veuillet, E., Bene, J., Morgon, A., 1992. E¡ects of contralateral white noise on click-evoked emissions in normal and sensorineural ears: towards an exploration of the medial olivocochlear system. Audiology 31, 1^7. Daubechies, I., 1990. The wavelet transform, time-frequency localization and signal analysis. IEEE Trans. Inf. Theor. 36, 961^ 1005.
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Fex, J., 1962. Auditory activity in centrifugal and centripetak cochlear ¢bers in cat. Acta Physiol. Scand. 55, 2^68. Folsom, R.L., Owsley, R.M., 1985. N1 action potentials in humans: in£uence of simultaneous contralateral stimulation. Acta Otolaryngol. 103, 262^265. Giraud, A.L., Collet, L., Chery-Croze, S., Magnan, J., Chays, A., 1995. Evidence of a medial olivocochlear involvement in contralateral suppression of otoacoustic emissions in humans. Brain Res. 705, 15^23. Grandori, F., Ravazzani, P., 1993. Non-linearities of click-evoked otoacoustic emissions and the derived non-linear technique. Br. J. Audiol. 27, 97^102. Kemp, D.T., 1979. Evidence of mechanical non-linearity and frequency selective wave ampli¢cation in the cochlea. Arch. Otolaryngol. 224, 37^45. Kemp, D.T., Bray, P., Alexander, L., Brown, A.M., 1986. Acoustic emission cochleography practical aspects. In: Cianfrone, G., Grandori, F. (Eds.), Cochlear Mechanics and Otoacoustic Emissions. Scand. Audiol. (Suppl. 25), pp. 71^95. Liberman, M.C., 1989. Rapid assessment of sound evoked olivocochlear feedback: suppression of compound action potentials by contralateral sound. Hear. Res. 38, 47^56. Maison, S., Micheyl, C., Collet, L., 1997. Medial olivocochlear e¡erent system in humans studied with amplitude-modulated tones. J. Neurophysiol. 77, 1759^1768. Mallat, S.G., 1989. A theory for multiresolution signal decomposition: the wavelet representation. IEEE Pat. Anal. Mach. Intell. 11, 674^693. Puel, J.L., Rebillard, G., 1990. E¡ect of contralateral sound stimulation on the distortion product 2F1-F2: evidence that the medial e¡erent system is involved. J. Acoust. Soc. Am. 87, 1630^ 1635. Rajan, R., 1988. E¡ect of electrical stimulation of the crossed olivocochlear bundle on temporary threshold shifts in auditory sensitivity. I: Dependence on electrical stimulation parameters. J. Neurophysiol. 60, 549^568. Ravazzani, P., Locatelli, T., Cursi, M., Medaglini, S., Comi, G., Tognola, G., Grandori, F., 1997. Contralateral e¡ects of otoacoustic emissions as a noninvasive method to explore the human olivocochlear system. In: 14Th International Congress of EEG and Clinical Neurophysiology, Florence, August 24^30, p. 71 (abstract in Electroenceph. Clin. Neurophysiol. 103, 1997). Ravazzani, P., Tognola, G., Grandori, F., Ruohonen, J., 1998. Twodimensional ¢lter to facilitate detection of transient-evoked otoacoustic emissions. IEEE Trans. Biomed. Eng. 45, 1089^ 1096. Tognola, G., Grandori, F., Ravazzani, P., 1997. Time-frequency distribution of click-evoked otoacoustic emissions. Hear. Res. 106, 112^122. Tognola, G., Grandori, F., Ravazzani, P., 1998. Wavelet analysis of click evoked otoacoustic emissions. IEEE Trans. Biomed. Eng. 45, 686^697. Veuillet, E., Collet, L., Duclaux, R., 1991. E¡ect of contralateral acoustic stimulation on active cochlear micromechanical properties in human subjects: dependence on stimulus variables. J. Neurophysiol. 65, 724^735. Veuillet, E., Collet, L., Morgon, A., 1992. Di¡erential e¡ects of earcanal pressure and contralateral acoustic stimulation on evoked otoacoustic emissions in humans. Hear. Res. 61, 47^55. Warren, E.H., III, Liberman, M.C., 1989a. E¡ects of contralateral sound on auditory nerve response. I. Contributions of cochlear e¡erent. Hear. Res. 37, 105^122. Warren, E.H., III, Liberman, M.C., 1989b. E¡ects of contralateral sound on auditory nerve response. II. Dependence on stimulus variables. Hear. Res. 37, 105^122.
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Williams, E.A., Brookes, G.B., Prasher, D.K., 1993. E¡ects of contralateral acoustic stimulation on otoacoustic emissions following vestibular neurotomy. Scand. Audiol. 22, 197^203. Williams, E.A., Brookes, G.B., Prasher, D.K., 1994a. E¡ects of olivocochlear acoustic stimulation on otoacoustic emissions following vestibular neurectomy. Scand. Audiol. 22, 197^203. Williams, E.A., Brookes, G.B., Prasher, D.K., 1994b. E¡ects of olivocochlear bundle section on otoacoustic emissions in humans:
e¡erent e¡ects in comparison with control subjects. Acta Otolaryngol. (Stockh.) 114, 121^129. Wit, H.P., Ritsma, R.J., 1980. Evoked acoustical responses from the human ear: some experimental results. Hear. Res. 2, 253^261. Wit, H.P., van Dijk, P., Avan, P., 1994. Wavelet analysis of real and synthesised click-evoked otoacoustic emissions. Hear. Res. 73, 141^147.
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Frequency and temporal analysis of contralateral acoustic stimulation on evoked otoacoustic emissions in humans N. Morand a
a;
*, S. Khalfa a , P. Ravazzani b , G. Tognola L. Collet a , E. Veuillet
b;c a
, F. Grandori b , J.D. Durrant
a;d
,
Laboratory `Neurosciences et Syste©mes Sensoriels', UMR CNRS 5020, Hoªpital Edouard Herriot and Universite¨ Claude Bernard (Lyon 1), Lyon, France b Biomedical Engineering Center CNR, Milan, Italy c Department of Biomedical Engineering, Polytechnic of Milan, Milan, Italy d Departments of Communication Science and Disorders and Otolaryngology, University of Pittsburgh, Pittsburgh, PA, USA Received 28 September 1999; accepted 20 March 2000
Abstract Previous studies have shown that the effect of contralateral acoustic stimulation (CAS) on ipsilateral evoked otoacoustic emissions (EOAE) depends somewhat upon the spectrum of the eliciting stimulus. The latency of the EOAE, however, is itself frequencydependent. Consequently, two general ways of analyzing the effects of CAS may be considered: by frequency band or by temporal segment. In this study, we analyzed the effects of CAS both ways in the same subjects, essentially simultaneously. The frequency analysis of the EOAE derived from the wavelet transform (WT). The WT is known to provide a robust approach to the analysis of non-stationary signals and was anticipated to avoid possible time^frequency confounds of the cochlear mechanical system. For comparison, a more basic analysis ^ using a temporal moving window ^ was employed. The results largely support earlier findings and confirm that in humans the greatest suppression of EOAEs by CAS is obtained for lower frequency and/or longer latency EOAE components. Despite expectations for the WT analysis, the more basic, temporal, analysis tended to yield the clearer results. ß 2000 Elsevier Science B.V. All rights reserved. Key words: Otoacoustic emission; Wavelet
1. Introduction The medial olivocochlear (MOC) system has been studied extensively in animals using direct electrical stimulation of the crossed olivocochlear bundle (COCB). Such stimulation induces changes in the cochlear response to acoustic stimulation, such as a decrease in the compound action potential (N1) (Fex, * Corresponding author. Laboratoire `Neurosciences et Syste©mes Sensoriels', 3 place d'Arsonval, Pavillon U, Hopital Edouard Herriot, 69003 Lyon, France; E-mail: [email protected] Abbreviations: CAS, contralateral acoustic stimulation; EOAE, evoked otoacoustic emission; MOC, medial olivocochlear; OAE, otoacoustic emission; OHC, outer hair cell; SL, sensation level; SPL, sound pressure level
1962). A reduction also has been seen in single-unit recordings from cochlear a¡erents in cats during contralateral acoustic stimulation (CAS) (Warren and Liberman, 1989a,b; Liberman, 1989). A similar observation was made using surface-recorded auditory brainstem responses in humans (Folsom and Owsley, 1985). Several studies, furthermore, have demonstrated that otoacoustic emissions (OAEs) (Kemp, 1979) are reduced by CAS (Collet et al., 1990; Veuillet et al., 1991). The fact that the OAEs radiate through the ossicular chain and tympanum (Kemp et al., 1986), nevertheless, raises the question of the middle ear's in£uence in this suppressive e¡ect, particularly the stapedial re£ex. There are now, however, several lines of evidence to show that the reduction of evoked OEAs (EOAEs) by CAS truly re£ects the functioning of the MOC system. In animals the e¡ect disappears after sectioning of
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the crossed olivocochlear bundle (Rajan, 1988) and is reduced in humans whose olivocochlear bundle presumably was cut after sectioning the vestibular nerve to treat severe vertigo in Me¨nie©re's disease (Giraud et al., 1995 ; Williams et al., 1993, 1994a,b). Even if the in£uence of the acoustic re£ex cannot be excluded completely, the CAS e¡ect observed in EOAEs cannot be attributed readily to middle ear muscle responses since the e¡ect has been shown to persist in subjects su¡ering Bell's unilateral loss of the stapedial re£ex (Veuillet et al., 1991). Finally, in guinea pigs, after section of the middle ear muscles themselves, the suppressive e¡ect on distortion product OAEs persists (Puel and Rebillard, 1990). Work to date in humans on the e¡ect of CAS has focused primarily on the overall amplitude of the EOAE (Collet et al., 1990, 1992). However, there is evidence that the suppression may have some degree of frequency speci¢city. Indeed, it has been shown that the greatest diminution of amplitude occurs in the same frequency range as the contralateral suppressor. Veuillet et al. (1991) have shown, using narrow band noise, that the suppression is greater in the 1000 Hz zone of the emission for contralateral stimuli of 1000 Hz, and in the 2000 Hz zone of the emission for contralateral stimuli of 2000 Hz. Similarly, the greatest decrease of EOAEs was observed when the frequency of a contralateral amplitude-modulated tone had a carrier frequency close to the frequency eliciting the EOAE (Maison et al., 1997). It also has been shown that the e¡ect of CAS, independent of the spectrum of the suppressor, is dependent upon the frequency of the EOAE probe stimulus. The latency of a given frequency of the EOAE, of course, is determined by the frequency^place transformation performed by the cochlea ^ the lower the frequency the longer the latency. The e¡ectiveness of CAS according to frequency component of EOAE was studied by Veuillet et al. (1992). They examined the e¡ect of CAS after conversion of the response into frequency bands of 200 Hz. Their results showed suppressive effects to be strongest for EOAE responses emitted around 0.7 and 3.1 kHz. They observed no e¡ect of CAS for EOAEs emitted at lower frequencies (Veuillet et al., 1992). These ¢ndings are in agreement with the results obtained using, in e¡ect, temporal ¢lters. Berlin et al. (1993), utilizing a moving 2 ms window, found that the contralateral suppression is more substantial than is evident from the overall RMS magnitude because the suppression is not equal at all latencies. The strongest e¡ects appear in later temporal zones, namely beyond 8-ms latency for click stimuli/elicitors. Furthermore, Ravazzani et al. (1997) observed recently that the e¡ect of CAS increases for EOAEs of longer latencies (i.e. longer post-stimulus time).
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However, even if the latency of a component of the EOAE is related to the frequency of the component itself, it is well known that emissions evoked by clicks show a `frequency dispersion' in time (Kemp, 1979; Wit and Ritsma, 1980 ; Wit et al., 1994). Therefore analysis of time^frequency properties of EOAEs is of particular interest. Recently, the wavelet transform (WT) (Mallat, 1989 ; Daubechies, 1990) has been found to provide a robust approach to time^frequency analysis, particularly that of non-stationary signals (e.g. bioelectric responses). The WT permits the analysis of the spectra of signals like otoacoustic emissions. Indeed, the use of this method for the description of time^frequency properties of EOAEs has been reported (Tognola et al., 1997, 1998) to yield a potentially more comprehensive analysis than the familiar Fourier transform. A WT-based spectral analysis is particularly attractive in a system like the cochlea with its traveling waves, wherein parts of the cochlear partition are moving at di¡erent times along the length of the basilar membrane. WT measures derive from a so-called time-scale view of the signal, namely from the decomposition of the signal into wavelets of all possible sizes. The WT, it thus can be reasoned, might be useful to avoid possible time^frequency confounds inherent to the pattern of vibration of the cochlear partition. Its performance in EOAE analysis, indeed, was found by Tognola et al. (1998) to be superior to that of the fast Fourier transform. The purpose of the present study, consequently, was to employ a method of analysis developed from wavelet theory to con¢rm ¢ndings of the previous studies and perhaps to determine more precisely the modi¢cation of EOAE frequency components by CAS. However, it seemed prudent to compare the results of the WT method with those obtained using a more basic method. For this purpose, we chose to use temporal analysis, in reference to the frequency-dependent latency characteristics of the EOAEs. We thus sought to compare, in e¡ect simultaneously, results of the two general modes of analysis: frequency, derived from the WT; temporal, derived from a moving window analysis. The speci¢c aim of this study, therefore, was to study the strength of the CAS e¡ect according to frequency band and/or temporal segment of the EOAE. 2. Materials and methods 2.1. Subjects This investigation was primarily a descriptive study and employed 59 subjects (29 males and 30 females, mean age = 24 years). All were subjects with negative otologic histories and with normal hearing sensitivity
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(i.e. hearing threshold levels 6 20 dB between 250 and 8000 Hz, tested at octave frequencies using conventional pure-tone audiometry). All subjects were tested in their right ear (RE). 2.2. Recording of EOAEs and exploration of MOC system The subjects were tested in a sound-treated room. The EOAEs were recorded and analyzed according to the method of Bray and Kemp (1987) using the Otodynamics ILO88 system. Stimuli were non-¢ltered clicks created by exciting the transducer with a direct-current pulse of 80 Ws duration at a rate of 50/s with 260 repetitions of four-click trains for two samples of each response. These samples were obtained essentially simultaneously, via multiplexing, but stored separately in bu¡ers `A' and `B'. The EOAEs were analyzed during the 20 ms following the onset of the click, but suppressing the ¢rst 3.2 ms to minimize stimulus artifact. The e¡erent re£ex was tested according to the protocol described by Collet et al. (1992): EOAEs were recorded for clicks1 presented at ¢ve sound pressure levels (peak equivalent) between 60 and 72 dB with and without CAS. The contralateral suppressor was a broadband noise (bandwith 500^8000 Hz) presented at 30 dB sensation level by a Madsen OB822 audiometer. The order of conditions was randomized. 2.3. Special analysis programs 2.3.1. Frequency^amplitude analysis Data were windowed 3.2^20.44 ms applying a cosine envelope, with a time resolution of 40 Ws. The response averages were treated with a second-order digital bandpass ¢lter set at 600^6000 Hz. The wavelet transform was used, again, in order to study the frequency content of the EOAEs in di¡erent frequency bands (Tognola et al., 1997; Mallat, 1989; Daubechies, 1990). This analysis utilizes the inverse WT to decompose the recorded emissions into elementary components (Tognola et al., 1997). More details can be found in papers from Wit et al. (1994); Tognola
1
The so-called 'non-linear` click mode of Bray and Kemp (Kemp et al., 1986; Bray and Kemp, 1987) was not employed in order to optimize sensitivity of recording of the EOAEs particularly at reduced SPLs. Grandori and Ravazzani (1993) have shown, indeed, that this 'non-linear` click paradigm, intended to minimize stimulus artifact and emphasize EOAEs resulting from non-linear growth with intensity, actually eliminates some valid response signal. At reduced stimulus levels, on the other hand, risk of contamination by the stimulus itself is greatly reduced. In any event, the primary observation here is that there were responses with and without CAS; stimulus artifact is not subject to in£uence by CAS.
et al. (1997, 1998). The frequency contents of the emissions were analyzed in 11 frequency bands (from 500 to 6000 Hz, step 500 Hz). At the end of the procedure, a bandpass ¢lter (¢nite inverse response of FIR ¢lter of order 100) was applied to each temporal component, with a bandwidth equal to the frequency range in which the components were wavelet antitransformed. This ¢nal ¢lter stage was added in order to reduce residual artifacts outside the studied bandwidth, intrinsic to the wavelet transform itself. In order to study the amplitude of each frequency component the root mean square value (RMS) of each frequency component was computed for all stimuli, for all intensities and for all subjects. For each frequency under analysis, the average of the components of the A and B bu¡ers was computed, and the RMS magnitudes of the components of the signal were estimated, yielding one RMS value per frequency. Also the RMS magnitude of the noise £oor was estimated, using nearly the same procedure. This estimate was obtained considering the di¡erence in the frequency components of the two trials, i.e. signal sampled and stored in bu¡ers A and B, divided by the square root of 2, in order to take into account both constructive and destructive e¡ects of the noise components on the recordings (Ravazzani et al., 1998). 2.3.2. Time^amplitude analysis The RMS of the average between A and B replicates was computed with a short time procedure similar to the method typical of fast Fourier analysis, similar to that used by Berlin et al. (1993). In this investigation the duration of, in e¡ect, each subwindow was set at 4 ms, with a temporal overlap of 3 ms. The ¢rst and the last windows started at 6 and 16 ms, respectively, for a total of 11 subwindows. Also in this case, the RMS amplitude of the noise was determined, essentially de¢ned and estimated as before. 2.4. Data analysis Before statistical analysis, data were sorted. We considered only RMS values of EOAEs exceeding the noise £oor by v5 dB. In the wavelet analysis, we included only frequency bands from 500 to 4000 Hz, as these were the least noisy bands. Furthermore, to compare frequency and temporal analysis, only results of subjects who had responses with both methods were considered. Di¡erences between EOAE magnitude with and without CAS were calculated. Further analyses were employed to determine slopes of the function of EOAE reduction over frequency or time, namely using lines computed by linear regression (as described below). Statistical testing was applied to determine whether the average of these slopes is signi¢cantly dif-
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ferent from zero. For this we employed the t-test with a Bonferroni correction in reference to the inherent repeated measures design, resulting from use of the two analyses (wavelet^frequency-based and temporal analyses) in the same subjects. 3. Results EOAE measurements were done successfully for each subject, but reproducible results were not obtained at all levels in all subjects. The results presented here were obtained from 18 subjects at a click SPL of 60 and 63 dB, 21 at 66 dB, 23 at 69 dB and 25 at 72 dB. 3.1. Frequency^amplitude analysis These ¢ndings, again, are results obtained using the WT. Fig. 1 shows the means and standard errors of RMS EOAE amplitudes after application of CAS (a negative value indicating a decrease of EOAE amplitude). The results reveal a decrease of EOAE with CAS, on average, for all frequencies bands and for all intensities. Furthermore we note a tendency for the greatest e¡ect of CAS to occur at low probe levels and at low frequencies. Consequently, the dB reduction in the EOAE appears to follow a slope function over frequency, namely sloping upward from low to high frequencies; this trend is most clearly evident for data
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obtained at the 63 dB SPL probe level. The slopes of these functions were evaluated subject by subject (mean = 0.327; S.E.M. = 0.297). The mean value was compared to zero which is the hypothetical value of the slope when the e¡ect of CAS remains the same for all frequency bands of the EOAE spectra. The mean slope, however, was not found to be statistically signi¢cant from zero (one-sample t-test, t = 1.051, df = 17, P = 0.285). 3.2. Temporal^amplitude analysis Results of the moving window, time-based analysis are shown in Fig. 2 and suggest also a dependence of the CAS e¡ect on probe intensity. Furthermore, the CAS e¡ect appears to be dependent on the time analysis window ^ the longer the latency, the greater the amount of suppression. Linear regression analysis of the mean functions suggested the steepest slope to occur at 66 dB SPL. We again evaluated slopes case by case to test the average slope (mean = 0.462; S.E.M. = 0.103) against zero and found a signi¢cant di¡erence (onesample t-test, t = 4.46, df = 20, P 6 0.001). 4. Discussion The results of this study indicate that e¡ect of contralateral suppression on EOAEs seems to be greater
Fig. 1. Relative EOAE amplitudes with CAS (i.e. EOAE amplitude with CAS^EOAE amplitude without CAS) for each frequency component between 500 and 4000 Hz (500 Hz steps) analyzed by the wavelet procedure. Left upper panel: 60 dB SPL ipsilateral click intensity (n = 18). Middle upper panel: 63 dB SPL ipsilateral click intensity (n = 18). Right upper panel: 66 dB SPL click intensity (n = 21). Left lower panel: 69 dB SPL click ipsilateral intensity (n = 23). Right lower panel: 72 dB SPL ipsilateral click intensity (n = 25).
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Fig. 2. Relative EOAE amplitudes with CAS (i.e. EOAE amplitude with CAS^EOAE amplitude without CAS) analyzed by a 4 ms moving window from 6 to 16 ms (thicks indicate the middle value of subwindows). Left upper panel: 60 dB SPL ipsilateral click intensity (n = 18). Middle upper panel: 63 dB SPL ipsilateral click intensity (n = 18). Right upper panel: 66 dB SPL click intensity (n = 21). Left lower panel: 69 dB SPL click ipsilateral intensity (n = 23). Right lower panel: 72 dB SPL ipsilateral click intensity (n = 25).
for EOAEs at low frequencies and/or of longer latencies. 4.1. Wavelet analysis Using the wavelet transform, we found, overall, the greatest decreases of EOAE amplitude for the lowest frequency components, as evidenced by the slope best¢t lines in Fig. 1. These results are descriptively consistent, in part, with preliminary results obtained by Veuillet et al. (1992), who found the greatest suppressive e¡ect around 0.7 kHz. Nevertheless they found also a signi¢cant e¡ect at 3.1 kHz. The main di¡erence between our study and that of Veuillet et al. is the analysis of EOAE frequency components. We used the WT which is known to give a better resolution than classic Fourier transform (Tognola et al., 1998). Nevertheless, we used an e¡ective bandwith of 500 Hz since reducing the resolution causes splattering in the frequency domain (i.e. due to increasing sidebands of the 'wavelet ¢lters`). Our method, consequently, was more accurate. Despite trends, however, the frequency-speci¢c e¡ects were not su¤ciently robust across subjects to produce signi¢cantly slope functions overall. 4.2. Temporal analysis The results of the temporal analysis appeared more orderly. These results essentially con¢rm those of Berlin
et al. (1993) and, more generally, are consistent with a systematic increase in EOAE suppression with decreasing frequency. This e¡ect was salient for all test intensities in the grouped data and was characterized by slopes of individual functions. Indeed, the slopes were found to be signi¢cantly di¡erent from zero overall. It is clear that the e¡ect of CAS is strongest for emissions of the longest latencies. 4.3. Comparison between frequency and temporal analysis of CAS e¡ect on EOAEs The results obtained with both methods thus showed a greater e¡ect of CAS on emissions of longer latencies or lower frequency components. It is worth considering possible bases for the di¡erences between the two methods and the apparently clearer trends indicated by the temporal analysis. The WT e¡ectively analyzes the involvement of each frequency component of EOAEs across time, giving a joint distribution of the energy of a signal in time and frequency. Consequently, the temporal evolution of elementary EOAE components can be described. Tognola et al. (1997) reported that EOAEs are characterized `by a mid/short duration with a progressive amplitude attenuation T with time'. Some components are characterized by a highly sustained activity particularly for the 1.0^1.5 kHz component. Also, even if a trend of greater latencies for lower frequencies is observed using wavelet analysis, it has
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been noted that there is a dispersion of frequency components across time, i.e. due to the basalward spread of excitation in the cochlea. Therefore, again, di¡erent places of the cochlea are active at the same frequency. From the presently reported results, it thus appears that WT analysis does not overcome this confounding e¡ect, as anticipated. In the ¢nal analysis, the ¢ndings of this study should not be interpreted as counter to the e¤cacy of WT analysis of EOAEs. The results simply failed to demonstrate new insights into the e¡ects of CAS on the EOAEs. Nevertheless, the study permitted con¢rmation of the latency-dependent e¡ects of CAS- and thus frequency-dependent e¡ects and suggest temporal analysis, akin to the approach of Berlin et al. (1993) as the method of choice in the study of CAS e¡ects. Lastly, although not critically assessed in this study, the e¡ects of CAS tended to be more robust at the lower probe levels, as expected from the literature, while the frequency/time-dependent e¡ects of CAS were qualitatively similar across probe level. Central tendencies of the results thus remained orderly across probe levels; this was true regardless of method of analysis. Acknowledgements The authors thank Simona Porati and Elena Vicario for their contribution in the data processing. We gratefully acknowledge the anonymous reviewers for helpful comments. This work was supported in part by a grant from the Ministry of Research and Technology to N.M. and was partially coordinated within the framework of the European concerted Action AHEAD (Biomedicine and Health Programme of the European Commission, Contract #PL951636). References Berlin, C.I., Hood, L.J., Szabo, P., Cecola, R.P., Rigby, P., Jackson, D.F., 1993. Contralateral suppression of non-linear click evoked otoacoustic emissions. Hear. Res. 71, 1^11. Bray, P., Kemp, D.T., 1987. An advanced cochlear echo suitable for infant screening. Br. J. Audiol. 21, 191^204. Collet, L., Kemp, D.T., Veuillet, E., Duclaux, R., Moulin, A., Morgon, A., 1990. E¡ect of a contralateral auditory stimuli on active micromechanical properties in human subjects. Hear. Res. 43, 251^262. Collet, L., Veuillet, E., Bene, J., Morgon, A., 1992. E¡ects of contralateral white noise on click-evoked emissions in normal and sensorineural ears: towards an exploration of the medial olivocochlear system. Audiology 31, 1^7. Daubechies, I., 1990. The wavelet transform, time-frequency localization and signal analysis. IEEE Trans. Inf. Theor. 36, 961^ 1005.
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Fex, J., 1962. Auditory activity in centrifugal and centripetak cochlear ¢bers in cat. Acta Physiol. Scand. 55, 2^68. Folsom, R.L., Owsley, R.M., 1985. N1 action potentials in humans: in£uence of simultaneous contralateral stimulation. Acta Otolaryngol. 103, 262^265. Giraud, A.L., Collet, L., Chery-Croze, S., Magnan, J., Chays, A., 1995. Evidence of a medial olivocochlear involvement in contralateral suppression of otoacoustic emissions in humans. Brain Res. 705, 15^23. Grandori, F., Ravazzani, P., 1993. Non-linearities of click-evoked otoacoustic emissions and the derived non-linear technique. Br. J. Audiol. 27, 97^102. Kemp, D.T., 1979. Evidence of mechanical non-linearity and frequency selective wave ampli¢cation in the cochlea. Arch. Otolaryngol. 224, 37^45. Kemp, D.T., Bray, P., Alexander, L., Brown, A.M., 1986. Acoustic emission cochleography practical aspects. In: Cianfrone, G., Grandori, F. (Eds.), Cochlear Mechanics and Otoacoustic Emissions. Scand. Audiol. (Suppl. 25), pp. 71^95. Liberman, M.C., 1989. Rapid assessment of sound evoked olivocochlear feedback: suppression of compound action potentials by contralateral sound. Hear. Res. 38, 47^56. Maison, S., Micheyl, C., Collet, L., 1997. Medial olivocochlear e¡erent system in humans studied with amplitude-modulated tones. J. Neurophysiol. 77, 1759^1768. Mallat, S.G., 1989. A theory for multiresolution signal decomposition: the wavelet representation. IEEE Pat. Anal. Mach. Intell. 11, 674^693. Puel, J.L., Rebillard, G., 1990. E¡ect of contralateral sound stimulation on the distortion product 2F1-F2: evidence that the medial e¡erent system is involved. J. Acoust. Soc. Am. 87, 1630^ 1635. Rajan, R., 1988. E¡ect of electrical stimulation of the crossed olivocochlear bundle on temporary threshold shifts in auditory sensitivity. I: Dependence on electrical stimulation parameters. J. Neurophysiol. 60, 549^568. Ravazzani, P., Locatelli, T., Cursi, M., Medaglini, S., Comi, G., Tognola, G., Grandori, F., 1997. Contralateral e¡ects of otoacoustic emissions as a noninvasive method to explore the human olivocochlear system. In: 14Th International Congress of EEG and Clinical Neurophysiology, Florence, August 24^30, p. 71 (abstract in Electroenceph. Clin. Neurophysiol. 103, 1997). Ravazzani, P., Tognola, G., Grandori, F., Ruohonen, J., 1998. Twodimensional ¢lter to facilitate detection of transient-evoked otoacoustic emissions. IEEE Trans. Biomed. Eng. 45, 1089^ 1096. Tognola, G., Grandori, F., Ravazzani, P., 1997. Time-frequency distribution of click-evoked otoacoustic emissions. Hear. Res. 106, 112^122. Tognola, G., Grandori, F., Ravazzani, P., 1998. Wavelet analysis of click evoked otoacoustic emissions. IEEE Trans. Biomed. Eng. 45, 686^697. Veuillet, E., Collet, L., Duclaux, R., 1991. E¡ect of contralateral acoustic stimulation on active cochlear micromechanical properties in human subjects: dependence on stimulus variables. J. Neurophysiol. 65, 724^735. Veuillet, E., Collet, L., Morgon, A., 1992. Di¡erential e¡ects of earcanal pressure and contralateral acoustic stimulation on evoked otoacoustic emissions in humans. Hear. Res. 61, 47^55. Warren, E.H., III, Liberman, M.C., 1989a. E¡ects of contralateral sound on auditory nerve response. I. Contributions of cochlear e¡erent. Hear. Res. 37, 105^122. Warren, E.H., III, Liberman, M.C., 1989b. E¡ects of contralateral sound on auditory nerve response. II. Dependence on stimulus variables. Hear. Res. 37, 105^122.
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Williams, E.A., Brookes, G.B., Prasher, D.K., 1993. E¡ects of contralateral acoustic stimulation on otoacoustic emissions following vestibular neurotomy. Scand. Audiol. 22, 197^203. Williams, E.A., Brookes, G.B., Prasher, D.K., 1994a. E¡ects of olivocochlear acoustic stimulation on otoacoustic emissions following vestibular neurectomy. Scand. Audiol. 22, 197^203. Williams, E.A., Brookes, G.B., Prasher, D.K., 1994b. E¡ects of olivocochlear bundle section on otoacoustic emissions in humans:
e¡erent e¡ects in comparison with control subjects. Acta Otolaryngol. (Stockh.) 114, 121^129. Wit, H.P., Ritsma, R.J., 1980. Evoked acoustical responses from the human ear: some experimental results. Hear. Res. 2, 253^261. Wit, H.P., van Dijk, P., Avan, P., 1994. Wavelet analysis of real and synthesised click-evoked otoacoustic emissions. Hear. Res. 73, 141^147.
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