Attention Effects on Distortion- Product Otoacoustic Emissions with Contralateral Speech Stimuli

Transcription

1 J Am Acad Audiol 10 : (1999) Attention Effects on Distortion- Product Otoacoustic Emissions with Contralateral Speech Stimuli Genaya Kae Timpe-Syverson* T. Newell Decker, Abstract This study investigated the effect of selective attention on the distortion-product otoacoustic emission (DPOAE) level through the use of environmentally meaningful, contralateral auditory stimuli. Four different conditions were used for measurement : quiet, contralateral noise, contralateral speech (unattended), and contralateral speech (attended). A statistically significant suppression effect for both the noise and speech conditions was found. However, there was no support for an auditory selective attention effect on the distortion-product amplitude. Key Words : Attention, contralateral speech, contralateral suppression, distortion-product otoacoustic emissions Abbreviations : ABR = auditory brainstem response, ANOVA= analysis of variance, DPOAEs = distortion-product otoacoustic emissions, IHC = inner hair cell, MES = medial efferent system, OAEs = otoacoustic emissions, OCB = olivochlear bundle, OHC = outer hair cell, SOAEs = spontaneous otoacoustic emissions, TEOAEs = transient evoked otoacoustic emissions toacoustic emissions (OAEs) are believed to be a measure of outer hair 0 cell (OHC) motility, and when OHCs are undamaged, function is roughly quantifiable through the use of OAEs. The motility of the OHCs is accountable for the active micromechanical processes of the cochlea (Brownell et al, 1985). Much of the initial work on OHC electromotile responses was done on individual hair cells. Now, more is known about the OHCs acting as a group, including the fact that they are interconnected and can create a response that is the sum of multiple cells vibrating in synchrony (Ryan, 1997). This summation effect would therefore enhance the movement of the basilar membrane in a specific frequency range, and this is what is now referred to as the cochlear amplifier. The OHCs' amplification of basilar membrane movement presumably amplifies the signal to the inner hair cells (IHCs), thus creating a more sharply tuned response (Ryan, 1997). *Towson University Speech-Language-Hearing Clinic, Towson, Maryland ; tdepartment of Special Education and Communication Disorders, University of Nebraska-Lincoln, Lincoln, Nebraska Reprint requests : T. Newell Decker, University of Nebraska-Lincoln, Lincoln, NE The medial olivary complex otherwise known as the medial efferent system (MES) has been shown to have an inhibitory effect on the OHCs. Galambos (1956) first discovered a reduction in the compound action potential with electrical stimulation of the olivocochlear bundle (OCB) as it courses through the fourth ventricle. Mountain (1980) found a reduction in distortion-product otoacoustic emissions (DPOAEs) with electrical stimulation in that same anatomic region. Brown and Nuttall (1984) showed that electrical stimulation at the same level affected the IHC response in the guinea pig and could be linked to OHC inhibition. In fact, they proposed that contralateral stimulation "alters the membrane potential of the OHCs in the mammalian cochlea, producing a change in the mechanical properties of the stereocilia-tectorial membrane complex such that less mechanical stimulus is delivered to the IHC stereocilia" (p. 644). These studies have led to many others, which have further proven the role of the OHCs in cochlear micromechanics and ultimately the olivocochlear efferent system in inhibiting cochlear responses. Recently, contralateral suppression of OAE level has been studied quite extensively. Contralateral suppression occurs when an acoustic 371

2 Journal of the American Academy of Audiology/Volume 10, Number 7, July/August 1999 stimulus is presented to the contralateral ear and causes a decrease in amplitude of the emission measured in the ipsilateral ear. Rajan and Johnstone (1988) demonstrated that acoustic stimulation to the contralateral ear yields suppressed results similar to those found with electrical stimulation of the medial OCB. Veuillet et al (1991) found evidence that broad-band contralateral suppressors yielded larger amplitude reductions with increasing suppressor intensity in transient evoked otoacoustic emissions (TEOAEs). They found an average amplitude reduction of 3.77 db with a broadband contralateral stimulus. They reported that the "contralateral suppressive effect was strongest when the contralateral ear stimuli were narrow bands that were centered on the central TEOAE frequency" (p. 724). This effect was maximal with a center frequency of 2 khz. However, at higher levels of contralateral stimulation (above 60 db SPL in this case), this frequency specificity ceased. In contrast to a study conducted by Collet et al (1990), Veuillet et al (1991) did not find a suppressive effect until a narrowband contralateral stimulus reached 50 db SPL. Transient contralateral stimuli also proved to be effective suppressors at levels above 17.5 db SL, but varied by click rate. Hood et al (1996) also found an intensity effect on TEOAE suppression. They showed that as the level of contralateral stimulus intensity was increased, so did the amplitude of suppression ; however, they found that the level of ipsilateral stimulation also affected the amount of suppression. Their findings showed an increased amplitude reduction for lower-level ipsilateral clicks (50, 55, and 60 db peak SPL) at a 60 db SPL contralateral stimulus level than for higher-level clicks (65 and 70 db peak SPL). This finding was later confirmed by Veuillet et al (1996), who agreed that suppression occurs and that greater amplitude suppression occurs at low ipsilateral stimulus levels. Suppression studies have been quite successful in elaborating the role of the auditory efferent system. It is now a commonly held belief that the MES mediates the OHC activity, which, in turn, causes a suppression of OAEs with contralateral stimulation. Attention and the Efferent System "Attention involves the selective awareness of certain sensory messages with the simultaneous suppression of others" (Hernandez-Peon et al, 1956, p. 331). This process takes place within the nervous system, although it is not well understood. It has been suggested that there exists a peripheral "gating" mechanism for sensory input during selective attention (Hernandez-Peon et al, 1956). Hernandez-Peon et al (1956) demonstrated that with directed sensory attention, cochlear nucleus responses in cats were decreased in amplitude compared to those trials where no attention task was used. Both visual and olfactory stimuli elicited the same amplitude reduction effect in cats. They believed this to be an indication that sensory inputs not part of the attended stimulus are inhibited at the earliest levels of the sensory pathways, thus creating a reduction in the signal before ever reaching the central nervous system. A number of investigators have cited this effect as being due to the function of the OCB and efferent fibers (see, for example, Berlin et al, 1993). In 1980, Lukas conducted a study that also supported the peripheral gating mechanism theory. The subjects participated in two trials, one in which they were to listen to the tone pips while staring at a screen, the other in which they were to mentally count the visual stimuli presented. The second condition essentially caused selective attention to be focused away from the auditory stimuli. He found a decrease in amplitude and an increase in latency of wave V in the visually attended condition, which "suggests a functional role for the OCB in enhancing the signal to noise ratio by attenuating background, irrelevant acoustic stimuli" (Lukas, 1980, p. 449). Speech production has also been used as a selective attention device in attention studies. One such study by Papanicolaou et al (1986) showed a decrease in amplitude of wave V in the brainstem evoked potential. However, the study of attention effects in the peripheral auditory system using auditory brainstem response (ABR) is not the most appropriate method for a number of reasons. First, the use of higher intensity stimuli is not ideal when looking at attention via efferent fibers of the auditory system. Second, ABR is a measure of afferent activity, whereas attention effects are believed to be mediated by the efferent system. Last, ABRs require a significantly longer time to record than OAEs (Meric and Collet, 1994a). In contrast, OAEs are an efficient, noninvasive means of recording activity mediated by the OCB of the brain stem, which is the theorized source of the peripheral attention effect on cochlear responses. This attention effect is ulti- 372

3 Attention Effects on DPOAEsMmpe-Syverson and Decker mately controlled by the central nervous system but is mediated through the MES. The MES originates in the OCB of the superior olivary complex. It is here that the peripheral filtering mechanism is presumed to reside. The OHCs have been proven to be influenced by the efferent nerve fibers originating in the OCB and are, presumably, the source of OAEs. Therefore, stimulation of the OCB could reasonably cause a variation in emissions. Puel et al (1988) were first to observe a decrease in TEOAE amplitude in response to a visual attention task. This decrease, however, was only found at the highest frequency peak of the emission. Avan and Bonfils (1992) found a difference in amplitude of around 1.5 db for visual attention tasks in DPOAEs, but noted that it could be attributed to a number of factors, including variabilities between subjects, attention, and test frequency. Meric and Collet (1994b) compared the presence of spontaneous emissions (SOAEs) when recording TEOAEs under visual attention. They showed a decrease in TEOAE amplitude under visual attention in subjects without SOAEs but not in subjects with SOAEs. No attention effect was found on SOAEs alone. They concluded that the results supported a peripheral attention effect. Ferber-Viart et al (1995) found a consistent decrease in TEOAE amplitude in the presence of a visual attention task. They believe this confirms the "role of attention to enable recognition of significant stimuli against background noise" (p. 1078). Giard et al (1994) also conducted a study using TEOAEs but explored an auditory attention effect. They found a frequency-specific amplitude reduction effect. The emission was reduced when the TEOAE eliciting stimulus was ignored and the contralateral tone pips were attended. The effect was greatest when the auditory stimulus requiring attention had a frequency composition similar to the stimulus frequency. However, Michie et al (1996) found contradicting evidence for auditory attention. They did not find emission amplitude reduction when attending to contralateral auditory stimuli, but rather they found an increase in emission amplitude. Meric and Collet (1992) found a slight decrease in TEOAE amplitude when subjects were attending to a visual and contralateral auditory stimulus. Froehlich et al (1993) conducted a study using TEOAEs and alternating visual and auditory attention stimuli conditions as well. They showed amplitude decreases for both the visual and auditory selective attention conditions in two frequency bands: 960 to 1920 Hz and 1920 to 2880 Hz. The use of transient, environmentally meaningful stimuli has not been undertaken in any published study thus far. Speech is a transient stimulus that possesses environmental meaning for humans. It is an alternating periodic and aperiodic band pass filtered signal that has a wide frequency composition and intensity range. Giraud et al (1997) suggest that "olivocochlear efferents play an antimasking role in speech perception in noisy environments." They cite this as a result of their finding an increased ability for normal listeners, meaning listeners with an intact efferent system, to perceive speech in noise in the presence of contralateral noise. Their hypothesis is suggestive of a link between increased speech perception in noise and the presence of a contralateral suppression effect. However, it is obvious that many questions still exist regarding the role of the MES in contralateral suppression and attention. One of the questions that needs to be answered is whether or not using a more meaningful transient stimulus as a contralateral stimulus, while drawing the listener's attention toward it, would create a suppression effect. This study was designed to compare DPOAEs with standard contralateral suppression techniques to DPOAEs with contralateral selective auditory attention added to the suppressor stimulus. Subjects METHOD Subjects were 16 individuals, ranging in age from 19 to 35 years. All were native speakers of English. Pure-tone thresholds were no greater than 15 db HL from 250 to 8000 Hz. Middle-ear function was normal. Stimulus Materials Air-conduction screening was conducted using a GSI 61 audiometer with ER-3A insert earphones (ANSI, 1996). Pure tones from 250 to 8000 Hz were presented at 15 db HL. Acoustic immittance measures, including tympanometry and acoustic stapedial reflex thresholds, were conducted using a GSI 38 immittance system. A 226-Hz probe tone was used to screen for normal middle ear function. Pure tones of 500, 1000, 2000, and 4000 Hz were used to elicit ipsilateral acoustic stapedial reflex thresholds. 373

4 Journal of the American Academy of Audiology/Volume 10, Number 7, July/August 1999 The OAE system employed was the Virtual Corporation model 330. The complete system consisted of the hardware and software that produced the stimuli and recorded the responses. The recording probe was calibrated via a 2-cm3 coupler connected to a 1/-inch microphone on a Quest sound level meter. The DPOAE was elicited with primary frequencies separated by a f2/fi ratio of 1.21(Gaskill and Brown, 1992). Ll was presented at 65 db SPL; L2 was presented at 55 db SPL. The primaries in this study were swept from 500 to 8000 Hz, with the intensity held constant. An interval of 1/6-octave was used to collect and plot the emission. The emissions were represented as a DP Gram. An emission was considered present if there was a separation of at least 3 db between the noise floor and the distortion product (Lonsbury-Martin et al, 1990). The contralateral noise used was a steadystate broadband signal presented at 60 db SPL rms (Williams and Brown, 1995). This sound pressure level is below both the points of crossover and acoustic stapedial reflex thresholds (Collet et al, 1990; Veuillet et al, 1991 ; Moulin et al, 1993 ; Hood et al, 1996). A speech sample was recorded and also used as a contralateral stimulus. It consisted of a 10- minute story, which contained periodic occurrences of the word "red" in grammatically appropriate places. This stimulus was presented at 60 db SPL, like the preceding broad-band, continuous stimulus. It should be noted that the speech intensity varied by up to 24 db, so the actual level of presentation was the RMS level. The RMS level was determined by measuring all of the components of the speech message, including voiced and voiceless portions, frication, inspiration, and pauses, after digitization and compression. Experimental Procedure Subjects were requested to participate in one experimental session that lasted approximately 1 hour. Otoscopic examination was conducted to verify clear external ear canals and intact tympanic membranes. Acoustic immittance testing was completed, including tympanometry and acoustic stapedial reflex thresholds. Subjects were then seated in a chair in a sound-treated room for the remainder of the experiment. Airconduction sensitivity was screened at 15 db HL for frequencies of 250 to 8000 Hz, bilaterally. Following verification of qualifying criteria, a probe containing the microphone and speakers of the DPOAE equipment was placed in the right ear of each subject. Initial probe placement was done carefully in order to ensure a proper fit and decrease the necessity to reposition the probe before the final DPOAE measurement. None of the subjects tested required repositioning of the probe. An insert earphone (ER-3A) was placed in the subject's left ear. Noise levels were kept to an absolute minimum in the sound-treated room and the surrounding areas to ensure minimal ambient noise. A reclining chair was used to provide the subject with neck and head support, as well as encourage relaxation. The first trial consisted of routine DPOAE measurement absent of any contralateral stimulus. No response was required of the subject. The following instructions were given to each subject for the first, second, and third trials : "You will be hearing a series of sounds in either your right ear, your left ear, or both ears. No response is required." The second trial consisted of DPOAE measurement in the presence of contralateral steady-state broad-band noise. The DPOAE was measured in the right ear during simultaneous presentation of the noise in the left ear. No response was required of the subject. The third trial consisted of DPOAE measurement in the presence of a contralateral speech stimulus. The speech was played for 30 seconds prior to the DPOAE measurement. The DPOAE was measured in the right ear with the speech stimulus present in the left ear. Attention was not diverted to the speech stimulus. No response was required of the subject. The fourth trial consisted of DPOAE measurement in the presence of a contralateral speech stimulus that was attended. The subject was required to count the number of occurrences of the word "red" that were heard in the speech message ; this ensured the subject's attention to the left ear's speech message. The DPOAE was measured in the right ear during simultaneous presentation of the speech signal in the left ear. The subject was supplied with a paper and pencil to track the number of occurrences of the word "red" in the stimulus presentation. In addition, three simple multiple-choice questions regarding the content of the story were given to each subject after the measurement. The following instructions were given to each subject : "You will be hearing a series of tones in your right ear and a story in your left ear. Listen carefully to the story and tally the number of times you hear the word red in the story. A paper and pencil have been supplied for you to make slashes as the word occurs." 374

5 Attention Effects on DPOAEs/Timpe-Syverson and Decker Distortion-Product Measurement and Representation The probe microphone collected the cochlear intermodulation distortions in the external ear canal. The information was then sent through the preamplifier and amplifier and then passed to the signal averager in the microprocessor. The frequency response of the signal was analyzed by Fourier analysis and the amplitude was determined via spectral averaging. Eight spectral averages were conducted and the noise tolerance was set to 10 db SPL. The mean noise floor was compared to the 2f,41 emission during the analysis process to verify the presence of a DPOAE for each individual. The final representation of the 2f1 f2 emission and the median noise floor was displayed on a DP Gram, with frequency on the horizontal axis and db SPL on the vertical axis. Data Analysis Amplitudes of each individual subject's distortion-product measurements for each condition were compared within three frequency regions ( Hz, Hz, and Hz). Each trial was compared to the initial distortionproduct measurement where no contralateral stimulus was present. Amplitudes were observed for frequencies of 1000 Hz and higher on the DP Gram ; the frequency region below 1000 Hz was eliminated due to the variability in the presence of DPOAEs and the questionable existence of DPOAEs in that frequency region in the literature (Probst et al, 1990). Data tables were available through software, which enabled simple comparison of amplitudes. Repeated measures analyses of variance (ANOVAs) with alpha set at.05 were conducted to compare the absolute amplitudes across the four trials and across the three frequency regions, followed by the necessary post hoc testing. RESULTS P Grams were recorded for the 16 subjects D at F2 frequencies from 0.55 khz to 8.81 khz. Absolute amplitudes of the DPOAEs at 1000 Hz and above were put in tabular format by frequency region for each subject. The averages of the amplitude values in each of the three regions were submitted to a two-way repeated measures ANOVA (conditions X frequency region). The results of the analysis revealed a statistical difference between the main effects of fre- quency region (F = 5.48, p <.01) and listening condition (F = 7.29, p <.001). The interaction between frequency region and listening condition was not significant. It is well known that differences exist in DPOAE amplitudes between various frequency regions, so the finding of significance for that effect was not considered remarkable. In the absence of any interaction between frequency and condition, the amplitudes were then averaged across the frequencies and a mean amplitude was calculated for each of the four conditions (quiet, noise, speech, and speech/attention). In Table 1, each column represents the average amplitude for an individual condition and each row represents an individual subject. In addition, the average and one standard deviation for each condition across subjects was calculated and is also represented. See Figure 1 for a graphical representation of the average for each condition, respectively. Aone-wayANOVAwith repeated measures was performed on the collapsed data in Table 1 to determine differences between the four conditions. The ANOVA indicated a statistical significance between the four test conditions (F = 8.02, p <.0002). Post hoc analysis using the Sheffe test revealed a statistical significance (p =.05) between the quiet and noise conditions and the quiet and speech conditions (see Figure 2 for mean differences in individual conditions). The quiet condition had a higher absolute amplitude than the noise or speech conditions ; the quiet condition was 0.87 db higher than the noise condition and 0.62 db higher than the speech condition. DISCUSSION contralateral suppression effect has been A documented in many studies. Most of these studies have been conducted using TEOAEs and contralateral stimuli such as steady-state broad-band noise. Studies such as the one conducted by Veuillet et al (1991) showed that a broad-band contralateral stimulus caused an average amplitude reduction of a TEOAE of 3.77 db. In the same study, they reported a contralateral suppression effect when using a transient stimulus ; this effect decreased with increasing interstimulus interval. Berlin et al (1993) also found that transient clicks were effective contralateral suppressors for TEOAEs. Studies involving the use of DPOAEs have shown both an increase (Nieschall et al, 1997) and 375

6 Journal of the American Academy of Audiology/Volume 10, Number 7, July/August 1999 Table 1 Average Amplitudes for Each Individual Subject in Each Condition Subject DP Amplitude DPn Amplitude DPs Amplitude DPs+a Amplitude Average variance DP Amplitude = DPOAE amplitude in quiet ; DPn DPOAE amplitude with contralateral noise ; DPs = DPOAE amplitude with contralateral speech ; DPs+a = DPOAE amplitude with contralateral speech and attention. decrease (Puel and Rebillard, 1990) in amplitude in the presence of contralateral stimulation. Moulin et al (1993) suggest that an increase in DPOAE amplitude in the presence of contralateral stimulation may be attributable to the passive properties mixing with the active properties of the cochlea during higher levels of stimulation. They found that a contralateral stimulus of 60 db SPL can cause a suppression of approximately 1.0 to 1.5 db across the frequencies at primary levels of 50 db SPL; this is good supporting evidence of the lower level of suppression for DPOAEs found in the present study. It is important to note that they used contralateral broad-band noise as a contralateral stimulus. The results of this study helped to confirm the existence of a contralateral suppression effect for DPOAEs. Both steady-state broadband noise and a transient type stimulus (speech) were used as contralateral stimuli. Both stimuli did cause a statistically significant, although small, suppression of the DPOAE amplitude; the broad-band noise caused an aver v a Figure 1 Average amplitudes in each condition. DP : mean of DP only ; DPn: mean DP plus contralateral noise ; DPs: mean DP plus contralateral speech ; DPs+a : mean DP plus contralateralspeech with attention. Figure 2 Amplitude differences among the various means. 376

7 Attention Effects on DPOAEs/'Iimpe-Syverson and Decker age decrease of 0.87 db in amplitude and speech caused an average decrease of 0.62 db. This further supports the evidence that contralateral auditory stimulation of the MES, which is known to mediate the activity of the OHCs, does indeed suppress OHC activity. Lack of previous documentation of transient contralateral stimulation causing a suppression effect on DPOAEs prohibits an in-depth discussion of our findings. However, because of the very small amplitude differences found in most suppression studies, our findings seem noteworthy. The transient stimuli that have been used in the past have consisted primarily of tone pips and clicks. These stimuli have consistent on and off time and intensity and therefore are of a slightly different nature than speech. Speech is a transient signal with wide frequency and intensity variations (as much as 24 db in this study). Therefore, it is not surprising that the suppression provided by the type of stimulus introduced via the contralateral ear would be different than that produced by a steady-state broad-band stimulus. In this study, the suppression produced by the steady-state noise produced a larger suppression than the speech stimulus, although the difference in amplitude was not statistically significant. That result may be simply attributed to the consistent nature of the broad-band noise. Auditory attention effects have not been established for DPOAEs. Giard et al (1994) did find an attention effect on TEOAEs using contralateral tone pips. It is interesting to note that the effect was greatest when the tone pips had a similar frequency composition as the TEOAE eliciting stimulus. However, a similar study found an amplitude increase, causing an attention effect to be difficult to define (Michie et al, 1996). The results regarding attention effects on DPOAEs are not as well defined as the contralateral stimulus caused suppression results in this study. Post hoc testing did not show a significance for any comparisons with the attention condition. Although the quiet condition amplitude was 0.56 db higher and the noise condition was 0.32 db lower than the attention condition, the difference was not statistically significant. Therefore, these changes do not indicate that the orienting of attention caused any effect. This is further indicated by the lack of significant difference between the speech and attention conditions. The same stimulus was used in both of these conditions, at the same intensity level. The difference existed in the tasks assigned to the subject during the presentation. In the speech condition, the subject was asked to sit back and relax. In the attention condition, the subject was specifically told to attend to the stimulus, listening for content and the word "red." The subject was required to listen closely enough to recognize the word "red" and be able to answer simple multiple-choice questions on the content of the story. This was designed to ensure a difference in attention between the two conditions. However, attention proved to be difficult to control in the subjects. One subject even inquired as to what the word "red" meant in the story after the speech (unattended) condition ; therefore, it is difficult to determine if there was a difference between the subject's state of attention for the two conditions. Nevertheless, even if the subject gave cursory attention to the speech stimulus during the unattended condition, the attention condition could still theoretically yield a different result. However, the data indicated no significance between the speech and attention conditions ; in fact, the attention condition amplitude was 0.08 db higher than that of the speech condition. Not only is this value small, but the reverse effect from what was expected. Therefore, it is likely that there was no attention effect. Future studies should include an environmentally meaningful stimulus to which subjects will not be prone to attend without direction to do so. In conclusion, evidence suggests a MES mediated contralateral auditory suppression effect for DPOAEs at moderate intensity levels. This suppression is produced by both steadystate and environmentally meaningful noise. Therefore, the premise of decreased OHC motility due to contralateral stimulation is supported for DPOAEs as well as TEOAEs. However, the evidence is not suggestive of an auditory attention effect for DPOAEs when using environmentally meaningful stimuli. We believe there exists a need for further investigation in this area to establish or disprove a link between cortically based attention and the function of the cochlear OHCs. REFERENCES American National Standards Institute. (1996). American National Standard Specifications foraudiometers. New York: Acoustical Society of America. Avan P, Bonfils P. (1992). Analysis of possible interactions of an attentional task with cochlear micromechanics. Hear Res 57 : Berlin Cl, Hood LJ, Wen H, Szabo P, Cecola RP, Rigby P, Jackson DF. (1993). Contralateral suppression of non- 377

8 Journal of the American Academy of Audiology/Volume 10, Number 7, July/August 1999 linear click-evoked otoacoustic emissions. 71 :1-11. Hear Res Meric C, Collet L. (1994a). Attention and otoacoustic emissions : a review. Neurosci Biobehav Rev 18 : Brown MC, Nuttall AL. (1984). Efferent control of cochlear inner hair cell responses in the guinea pig. J Physiol 354: Brownell WE, Bader CR, Bertrand D, Ribaupierre YD. (1985). Evoked mechanical responses of isolated cochlear outer hair cells. Science 227: Collet L, Kemp DT, Veuillet E, Duclaux R, Moulin A, Morgon A. (1990). Effect of contralateral auditory stimuli on active cochlear micro-mechanical properties in human subjects. Hear Res 43 : Ferber-Viart C, Duclaux R, Collet L, Guyonnard F. (1995). Influence of auditory stimulation and visual attention on otoacoustic emissions. Physiol Behav 57: Froehlich P, Collet L, Morgon A. (1993). Transiently evoked otoacoustic emission amplitudes change with changes of directed attention. Physiol Behav 53 : Galambos R. (1956). Suppression of auditory nerve activity by stimulation of efferent fibers to cochlea. J Neurophysiol 19 : Gaskill SA, Brown AM. (1992). The behavior of the acoustic distortion product, 2f1 f2, from the human ear and its relation to auditory sensitivity. J Acoust Soc Am 91: Giard MH, Collet L, Bouchet P, Pernier J. (1994). Auditory selective attention in the human cochlea. Brain Res 633: GiraudAL, Collet L, Chery-froze S. (1997). Suppression of otoacoustic emission is unchanged after several minutes of contralateral acoustic stimulation. Hear Res 109: Hernandez-Peon R, Scherrer H, Jouvet M. (1956). Modification of electric activity in cochlear nucleus during "attention" in unanesthetized cats. Science 123: Hood LJ, Berlin CI, HurleyA, Cecola RP, Bell B. (1996). Contralateral suppression of transient-evoked otoacoustic emissions in humans : intensity effects. Hear Res 101: Lonsbury-Martin BL, Harris FP, Hawkins MD, Stagner BB, Martin GK. (1990). Distortion product emissions in humans. I : basic properties in normally hearing subjects. Ann Otol Rhinol Laryngol 99:3-14. Lukas JH. (1980). Human auditory attention : the olivocochlear bundle may function as a peripheral filter. Psychophysiology 17 : Meric C, Collet L. (1992). Visual attention and evoked otoacoustic emissions: a slight but real effect. Int J Psychophysiol 12 : Meric C, Collet L. (1994b). Differential effects of visual attention on spontaneous and evoked otoacoustic emissions. Int J Psychophysiol 17 : Michie PT, LePage EL, Solowij N, Haller M, Terry L. (1996). Evoked otoacoustic emissions and auditory selective attention. Hear Res 98 : MoulinA, Collet L, Duclaux R. (1993). Contralateral auditory stimulation alters acoustic distortion products in humans. Hear Res 65 : Mountain DC. (1980). Changes in endolymphatic potential and crossed olivocochlear bundle stimulation alter cochlear mechanics. Science 210: Nieschall M, Beneking R, Stoll W (1997). Increased amplitude of distortion product emissions in the human caused by contralateral low intensity acoustic stimulation. HNO 45: Papanicolaou AC, Raz N, Loring DW Eisenberg HM. (1986). Brain stem evoked response suppression during speech production. Brain Lang 27 : Probst R, Antonelli C, Pieren C. (1990). Method and preliminary results of measurements of distortion product otoacoustic emissions in normal and pathological ears. Ad Audiol 7: Puel JL, Bonfils P, Pujol R. (1988). Selective attention modifies active micromechanical properties of the cochlea. Brain Res 447: Puel JL, Rebillard G. (1990). Effect of contralateral sound stimulation on the distortion product 2F1 F2 : evidence that the medial efferent system is involved. JAcoust Soc Am 87: Rajan R, Johnstone BM. (1988). Binaural acoustic stimulation exercises protective effects at the cochlea that mimic the effects of electrical stimulation of an auditory efferent pathway. Brain Res 459: Ryan AF. (1997). New views of cochlear function. In : Robinette MS, Glattke TJ, eds. Otoacoustic Emissions : Clinical Applications New York: Thieme, Veuillet E, Collet L, Duclaux R. (1991). Effect of contralateral acoustic stimulation on active cochlear micromechanical properties in human subjects : dependence on stimulus variables. JNeurophysiol 65 : Veuillet E, Duverdy-Bertholon F, Collet L. (1996). Effect of contralateral acoustic stimulation on the growth of click-evoked otoacoustic emissions in humans. Hear Res 93 : Williams DM, Brown AM. (1995). Contralateral and ipsilateral suppression of the 2f1 f2 distortion product in human subjects. JAcoust Soc Am 97: