Contralateral suppression of DPOAE measured in real time

Contralateral suppression of DPOAE measured in real time

A.L. JAMES,� R.J. MOUNT� & R.V. HARRISON�y�Auditory Science Laboratory, Department of Otolaryngology and Brain and Behaviour Division, Hospital for Sick

Children and yDepartment of Physiology, University of Toronto, Toronto, Ontario, Canada

Accepted for publication 15 October 2001

JA M E S A.L . , M O U N T R.J . & H A R R I S O N R.V.

(2002) Clin. Otolaryngol. 27, 106–112

Contralateral suppression of DPOAE measured in real time

The aim of this study was to measure contralateral suppression of distortion product otoacoustic emissions (DPOAE)

in real time. A total of 10 human subjects were studied with a novel device to record DPOAE without signal

time averaging, using digital narrow band pass filtering. Real time DPOAE levels were recorded at 2f1-f2 using

primary tone settings of f2/f1¼ 1.22 and L1¼ 70 dB SPL, L2¼ 65 dB SPL, at five values of f2 between 2.2

and 7.7 kHz. An acoustic stimulus was applied intermittently to the contralateral ear to cause DPOAE suppression.

Characteristic features of contralateral suppression were identified and distinguished from small spontaneous

variations in the real time DPOAE signal. Magnitude of suppression increased with contralateral stimulus intensity.

Onset latency of suppression was around 43 ms (31–95 ms). Potential clinical applications are discussed in the

light of these findings, including a role in improving the specificity of neonatal hearing screening.

Keywords distortion product otoacoustic emission contralateral suppression latency

Distortion product otoacoustic emissions (DPOAEs) are

widely used in a variety of clinical settings, most commonly

in neonatal hearing testing.1 Their discovery was first reported

in 19792 and, since then, much work has been done on

defining optimum characteristics of the primary frequencies

that evoke these emissions, and on the interpretation of

emission data from normal and abnormal ears. Among their

many research applications, DPOAEs have been used to study

the function of the cochlear efferent neurones. These fibres

mediate suppression of otoacoustic emissions (OAEs) via the

medial olivocochlear pathway after acoustic stimulation of the

contralateral ear.3

To date, all commercially available devices that measure

DPOAEs use signal time-averaging to distinguish the small

DPOAE signal from physiological and other background

noise. We report a preliminary study of a novel DPOAE

device (Vivo 600DPR, Vivosonic, Toronto, Canada; http://

www.vivosonic.com), which discriminates DPOAE signals

using a digitally implemented narrow band pass filter,

thus allowing measurement of emissions in real time.4 In

addition to considerable research applications, we consider

that this may have significant advantages in certain clinical

settings.

Our previous studies with a prototype device have assessed

the accuracy and reliability of real time DPOAE measurement

in comparison with conventional signal averaging devices.5,6

We have used real time DPOAE measurement to assess the

latency and frequency specificity of contralateral suppres-

sion in an animal model.7,8 In this report, we demonstrate

the characteristics of real time contralateral DPOAE sup-

pression in human subjects and discuss potential clinical

applications.

Patients and methods

Real time DPOAEs were measured in 10 human subjects (ages

from 9 days to 60 years) using a Vivo 600DPR. Recordings

were made in the presence and absence of acoustic stimulation

of the contralateral ear. All measurements were made in a

sound-attenuating booth. Primary frequencies were set at f2/

f1¼ 1.22 for 5-values of f2 between 2.2 and 7.7 kHz, with

intensities of L1¼ 70 dB SPL and L2¼ 65 dB SPL. The dis-

tortion product emission at 2f1-f2 was recorded for periods

ranging from 15 s to 3 min.

Clin. Otolaryngol. 2002, 27, 106–112

106 # 2002 Blackwell Science Ltd

Correspondence: Professor R.V. Harrison, Auditory Science La-boratory, Department of Otolaryngology, Hospital for Sick Children,555 University Avenue, Toronto, Ontario M5G 1X8, Canada(e-mail: [email protected]).

pr i nc i ple s of dpoa e e st im at ion i n r eal t im e

The Vivo 600DPR uses a Kalman filtering paradigm to

estimate DPOAE signal levels. The process consists of three

steps: (i) modelling of the measured signal and background

noise; (ii) processing the measured signal using the properties

of the model to reduce the number of computational opera-

tions and processing time; and (iii) post processing that

includes signal presentation and recording.4

The Kalman filter represents the optimal solution to the

discrete-data linear filtering problem9,10 namely the instanta-

neous pressure state at the probe microphone due to the two

primary tones and the DPOAE. It recursively conditions the

current state estimate on all past measurements, and converges

very rapidly to the least square error. The optimal estimate is

equal to the best prediction of its value before the measure-

ment is taken, plus an optimally weighed correction factor

times the difference between the actual measurement and the

best prediction of its value before it is taken (called the

residual). If the a priori estimate error covariance is small,

the adaptive gain factor is small, and the gain weighs the

residual less heavily. Thus, any uncorrelated process or mea-

surement noise is filtered out. This process out-performs

traditional frequency and synchronous time averaging algo-

rithms in its capacity to track DPOAE level in real time

(Harrison et al., submitted). Empirical projections predict a

processing time for analogue-to-digital conversion and for

detection of DPOAE signal in noise in the order of 1 ms.

contr alat e r al dpoa e suppr e s s i on

To elicit contralateral suppression, real time DPOAE record-

ings were made with simultaneous presentation of a broad

band noise stimulus to the opposite ear. In three subjects,

narrow band noise signals (1/3 octave, centred at 1, 2, 3, 4 and

6 kHz) and pure tones (range 2–8 kHz) were also used as

contralateral stimuli. Stimuli were applied intermittently via

an Etymotic Research ER-2 transducer (Etymotic Research,

Elk Grove Village, IL). The signal duration was 300–1000 ms;

the ‘off’ period between stimuli was approximately 50%

greater than the stimulus duration. Rise/fall time was 0 for

noise signals and 4 ms for tones. Intensity of the stimulus was

set between 0 and 80 dB SPL. Stimulus onset was recorded in

synchrony with DPOAE recording.

cal i br at ion

Calibration of the DPOAE probe was performed in a 2-ml

coupler using a spectrum analyser (SR760 FFT, Stanford

Research Systems, Sunnyvale, CA). The contralateral broad

band noise stimulus was calibrated with a sound level meter.

Processing time of the Vivo 600DPR was calibrated using a

modulated acoustic signal in the 2 ml coupler, and found to be

in the order of 1–2 ms. This matched the empirical estimation

of the model as described above.

analy s i s o f r e sult s

DPOAEs recorded without contralateral stimulation were

plotted as DP-grams (i.e. DPOAE level versus f2 frequency).

Real time DPOAE recordings before and during contralateral

stimulation were analysed to reveal the magnitude of DPOAE

suppression at different intensities of contralateral stimulus.

The onset latency period between the start of contralateral

stimulation and the onset of DPOAE suppression was mea-

sured on averaged recordings. Tests for statistical significance

were made with the t-test using SIGMA STAT (SPSS,

Chicago, IL).

Results

dpoa e m ea su r em ent

Real time DPOAEs were recorded in all 20 ears. Examples of

DP-grams (i.e. DPOAE versus f2) from six ears in four

subjects are shown in Fig. 1. The emission level ranges from

0 to 27 dB SPL (solid lines) in these examples, which was a

characteristic finding. Typically OAE level was >10 dB above

the noise floor (dashed lines). Emissions were highest in the

youngest subject (age 9 days) and tended to be lowest in the

oldest subjects.

Figure 2 illustrates some typical forms of spontaneous

variation that occurred in OAE levels over different time-

courses. Amplitude changes of around 0.25 dB occurred every

1–2 ms (Fig. 2a) and those of around 1–2 dB occurred over

periods of 1–2 s (Fig. 2b). Regularly repeating larger spikes of

2–5 dB occasionally occurred at the same rate as the heart rate

Figure 1. DP-Grams from six ears in four subjects. DPOAE level(2f1-f2) (solid lines) and noise floor (dashed lines) are plotted againstf2 frequency.

Real time DPOAE suppression 107

# 2002 Blackwell Science Ltd, Clinical Otolaryngology, 27, 106–112

and are considered consistent with changes in OAE transmis-

sion from pulses of blood flow through the middle ear, altering

middle ear compliance (Fig. 2c). Occasionally, OAE levels

fluctuated in phase with respiratory rate. Although DPOAE

levels were typically stable during recordings (up to 3 min

duration), a downward drift of around 1–2 dB/h was usually

found over a whole recording session. This occurred inde-

pendently of contralateral stimulation.

det ect ion of contr alat e r al suppr e s s i on

The effects of intermittent contralateral stimulation on real

time DPOAE levels are shown in Fig. 3a. These recordings are

from three subjects at three different values of f2 (2.2, 4.4 and

6.6 kHz) with a 1-s duration contralateral broad band noise

signal of 50 dB SPL (indicated by the uppermost bar symbols).

In each case, a suppression of DPOAE can be seen to occur

following a short latency after onset of the contralateral

stimulus. At contralateral stimulus offset, DPOAE values

return to prestimulus levels with a similar latency. When

the magnitude of suppression was small compared with spon-

taneous variation in the DPOAE signal, averaging of multiple

responses from repeated stimulation was used. Figure 3b

shows examples of such averages corresponding to the adja-

cent real time traces. When no response was visible in an

averaged trace from a 3 min recording (up to 70 stimulations),

contralateral suppression was considered not to be detectable.

In the majority of cases, between one and 10 stimulations were

required to provide adequate data for quantification of the

suppression response. Using these criteria, suppression of real

time DPOAEs by contralateral broad band noise was found in

nine out of 10 subjects. In two subjects (ages 35 and 48 years),

suppression was only detected in one ear. Maximal suppres-

sion usually occurred when measuring the distortion product

at f2¼ 4.4 kHz. Suppression was also seen at f2¼ 2.2, 3.1, 5.4

and 6.6 kHz in some ears. Suppression was not detectable

when the DPOAE level was low, for example below 12 dB

SPL for f2¼ 4.4 kHz.

magn itude of contr alat e r al suppr e s s i on

The magnitude of suppression was found to vary between

subjects. For example, the averages in Fig. 3b show suppres-

sion of 4.0, 1.3 and 0.35 dB. However, there was little intra-

subject variation in suppression magnitude both between and

within recording sessions. This is shown clearly in the two

examples illustrated in Fig. 4, in which DPOAE magnitude

with and without contralateral stimulation is plotted in

successive recordings. In Fig. 4a, mean suppression¼1.15 dB� 0.13 SD in six successive 3-min recordings, and

in Fig. 4b, from the same subject 1 week later, mean suppres-

sion¼ 1.01 dB� 0.09 SD in eight successive recordings.

Although DPOAE level varies during each session (Fig. 5a,

mean¼ 18.7 dB SPL� 0.32 SD; Fig. 5b, 16.9 dB SPL�1.2 SD), the DPOAE level during contralateral stimulation

varies coincidentally, and there is no significant difference in

suppression magnitude between the two sessions (P> 0.05,

t-test).

Figure 5 shows the increase in magnitude of suppression

with increasing intensity of contralateral stimulus from one

recording session, a finding common to all subjects. In this

example, a small suppression response was seen with 10 dB

SPL stimulation, though more commonly �30dB SPL was

required to elicit suppression. There was no evidence of

significant acoustic cross-talk at intensities below 80 dB SPL.

To date we have not found a consistent suppression

response to contralateral pure tones in humans (though in a

parallel animal study, pure tone suppression has been found in

chinchilla7). However, contralateral stimulation with narrow

band noise at 50–60 dB SPL elicited DPOAE suppression in

the three subjects tested.

Despite the variation in magnitude of suppression between

subjects, the shape of the suppressed emission recording

remained consistent. As shown in Fig. 6, a single exponential

function fits both the suppression time-course and the recov-

ery phase.

Figure 2. Real time DPOAE traces at different time scales to showtypes of spontaneous variation in DPOAE signal levels (recordingsfrom one subject at f2¼ 4.4 kHz). Interval of 1.15 s between spikesin lowest trace (c) correlates with observed heart rate of 70 bpm.


108 A.L. James et al.

Figure 3. (a) Real time DPOAE recordings from three subjects during intermittent contralateral broad band noise (indicated by upperhorizontal bar). Recordings are at f2¼ (i) 6.6 kHz; (ii) 2.2 kHz; and (iii) 4.4 kHz. (b) Synchronized averages of 3-min recordings, whichincluded responses from up to 70 contralateral stimulus presentations. The averages are taken from the same recordings as the adjacent realtime examples.

Figure 4. Graphs from two separate recording sessions in onesubject (DM) showing stability of DPOAE level in successiverecordings (filled symbols). The suppressed DPOAE level duringcontralateral stimulation (open symbols) mirrors the unsuppressedlevel, showing stability of the magnitude of suppression. (Record-ings at f2¼ 4.4 kHz; longer periods between data-points were spentrecording DPOAEs at other frequencies).

Figure 5. Graph of magnitude of DPOAE suppression versusintensity of contralateral stimulus showing growth of suppressionwith larger stimuli.



lat ency of contr alat e r al suppr e s s i on

As was seen in Fig. 3, suppression of the DPOAE consistently

occurred after a short latency. To measure this latency, aver-

aged data from 3-min recordings were obtained to minimize

interference from spontaneous variation in the DPOAE signal.

An example is shown in Fig. 7 (upper), with the period of onset

of contralateral DPOAE suppression shown below on an

expanded timescale (synchronized average of 70 responses;

f2¼ 4.4 kHz; contralateral broad band noise 1 s duration at

50 dB SPL). Onset latencies were measured at the intercept

of linear fits, as illustrated (lower). In this example, onset

latency is 39 ms. The median onset latency from 20 three-

min recordings in four ears (f2¼ 4.4 kHz) was 43 ms (range

31–95 ms).

Discussion

r eal t im e dpoa e m ea su r em ent

It is possible to measure DPOAEs in real time, without the

time averaging that is the standard method of signal detection

in conventional DPOAE recording devices. Otoacoustic emis-

sions were detectable in all our subjects and, as with conven-

tional measurement, DPOAE levels tended to be higher in the

younger subjects.1 Emissions were highest in the youngest

subject; a neonate age 9 days. Previous studies have shown

real time DPOAE levels are similar to measurements made

with conventional devices and have similar intertest varia-

bility.6

DPOAE measurement was stable for the duration of

recording sessions (up to 45 min) but small fluctuations in

the real time signal were observed. There are obviously

multiple causes for such ‘spontaneous’ amplitude changes,

only some may relate directly to outer hair cell activity. Some

signal variability is attributable to cardiovascular and respira-

tory events. It is also likely that some variation can be

attributed to the limits of device accuracy in the presence

of fluctuating background and other physiological noise.

These variations are usually too small to prevent the detection

of DPOAE suppression in response to contralateral acoustic

stimulation.

contr alat e r al suppr e s s i on

Contralateral suppression of DPOAE was identified in all

subjects except one who had very low DPOAE levels. In

our neonatal subject, contralateral suppression was robustly

present and this finding is consistent with reports showing full

functional maturity of the olivocochlear contralateral reflex in

full-term neonates.11

We propose that the following features should be identified

to distinguish a contralateral suppression response from

Figure 6. Demonstration of exponential curve (continuous line)fitted to the falling and rising components of the DPOAEsuppression response. (Mean square error of fit<0.0001; MatlabSoftware).

Figure 7. Determination of onset latency for contralateral suppres-sion of DPOAE. Averaged suppression response from 70 contral-ateral stimulations (DPOAE at f2¼ 4.4 kHz; contralateral broadband noise stimulus at 50 dB SPL). Boxed section in upper panelshown with enlarged scale (lower). Linear regression used to fitstraight lines to baseline (first 30 ms after stimulus onset) and firstpart of suppression response (55–155 ms) (SIGMA PLOT Software).Onset latency (arrow) defined by intercept of straight lines¼ 39 ms.



normal baseline variation in real time DPOAE. First, suppres-

sion of DPOAE is synchronized with the contralateral stimu-

lus and occurs after a short latency following onset of the

contralateral stimulus. In humans, this latency is around

45 ms. Second, suppression appears to develop and decay

exponentially. Third, the suppression response is maintained

for at least the duration of the contralateral stimulus. To be

confirmed as a suppression response, maximum suppression

must be greater than spontaneous variations in DPOAE level.

Fourth, following contralateral stimulus offset, DPOAE

values return to prestimulus levels after a short delay.

Contralateral suppression of OAEs was first demonstrated

in humans in 198912 but, to date, assessment is not part of

mainstream clinical practice. Real time measurement may

allow assessment to be made more readily and should be

particularly feasible in younger subjects with larger emissions.

In many cases, suppression can be seen ‘live’ on the real

time display, and recording of three responses is often suffi-

cient to quantify magnitude and latency of suppression. Thus

contralateral suppression can be detected with only a few

seconds of recording. We consider this has potential as a

viable means of objective hearing assessment, based on the

concept that detection of contralateral suppression of

DPOAEs is indicative of intact brainstem connections and

normal function in the contralateral ear. We have shown that

the magnitude of contralateral suppression, measured with

this technique, increases with contralateral stimulus intensity,

and that suppression can be elicited with narrow band

noise. Thus frequency range-specific hearing is assessed in

the contralateral ear by this test. Limitations of the test are that

suppression is usually only detectable with a contralateral

stimulus �30 dB SPL and that OAEs are not reliably detect-

able with �40 dB HL hearing loss. However, these limitations

would not preclude its use in, for example, assessment of

those unable to co-operate with subjective tests (e.g. because

of behavioural abnormalities or functional hearing loss), or

indeed as an adjunct in neonatal hearing screening. As

presently used in neonatal hearing screening and diagnosis,

OAE measurements only indicate the functional status of

outer hair cells and middle ear.1 Thus they fail to detect

conditions affecting the inner hair cells and retrocochlear

causes of hearing loss. Assessment of contralateral suppres-

sion would improve specificity by detecting rare causes of

false negative results, such as auditory neuropathy (which

arises from inner hair cell/cochlear nerve disorders13) deaf-

ness from neonatal hyperbilirubinaemia (which probably

affects the cochlear nucleus14) and congenital auditory nerve

absence.

lat ency of contr alat e r al suppr e s s i on

Previous studies have measured the onset latency of the

olivocochlear reflex in humans with OAEs but accuracy has

been limited by the temporal resolution achievable with time

or spectral averaging. In a study by Moulin, using a conven-

tional DPOAE device, the suppression latency was found to be

<2.6 s, which was the temporal resolution of the technique.15

Using spontaneous OAEs, a value of 40–200 ms was obtain-

ed14 and with transient evoked OAEs, <40–140 ms.16 These

two studies had resolutions of 40 and 20 ms respectively. Our

measurement of mean onset latency of 45 ms is in line with

these findings, but should be more reliable given our temporal

resolution of 1–2 ms. Most of the suppression latency is

composed of the transmission time of the intercochlear neural

pathway from inner hair cells, via afferent cochlear neurones

and the cochlear nucleus, to the medial superior olive, olivo-

cochlear efferents and outer hair cells.3 Sound conduction

between the probes and both cochleas is unlikely to account

for >10 ms in total.17 As conduction along the afferent path-

way is fast (namely ABR latencies of wave II around 2 ms for

cochlea to the cochlear nucleus), and animal studies show

conduction along the myelinated efferent pathway is also

fast18, it is likely that much of the delay occurs in the

brainstem. Our findings in chinchilla suggest brainstem

transmission accounts for around 40% of the onset latency

(Harrison et al., submitted). It is likely that central synapses

are under the control of descending neurones19–21, thus levels

of attention22 might influence latency through neuromodula-

tion at the cochlear nucleus or medial superior olive.23 This

may account for the wide range of suppression latencies

observed (31–95 ms). Reduction of activity in descending

pathways is thought to occur during anaesthesia24 and anaes-

thesia may influence latency (unpublished observations on

chinchilla). We are presently assessing the utility of measuring

contralateral suppression for monitoring depth of anaesthesia

clinically.

Brainstem and cerebello-pontine angle disease affects the

magnitude of contralateral suppression25 and could be

expected to influence the suppression latency. Thus latency

measurement might have value in the monitoring of neuro-

otological conditions. The instantaneous monitoring of

cochlea function provided by real time DPOAE measurement

might assist hearing preservation during acoustic neuroma

surgery by giving the surgeon more rapid feedback of vascular

compromise to the cochlea (as has been suggested with

DPOAE phase changes26).

Conclusions

We have demonstrated the ability of a novel device to measure

DPOAE in real time, without time averaging. It provides an

instant record of changes in the emission, which has allowed

us to measure the onset and offset latencies of suppression of

DPOAEs using a contralateral acoustic stimulus. This tech-

nique is simple to perform and many potential clinical appli-

cations are envisaged.



Acknowledgements

This research was supported by MRC (Canada) and the

Masonic Foundation of Ontario.

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Contralateral suppression of DPOAE measured in real time