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*Department of Otolaryngology, Head and Neck Surgery, Virginia Merrill Bloedel Hearing Research Center, University ofWashington; †Department of Speech and Hearing Sciences, University of Washington

Patrick Feeney, Ph.D., University of Washington, Otolaryngology, Head and Neck Surgery, V. M. Bloedel Hearing ResearchCenter, Box 357923, Seattle, WA 98195; Phone: 206-616-4692; Fax: 206-616-1828; E-mail: pfeeney@u.washington.edu

This study was supported by a grant from the Royalty Research Fund, University of Washington.

Detection of the Acoustic Stapedius Reflexin Infants Using Wideband EnergyReflectance and Admittance

M. Patrick Feeney*Chris A. Sanford†

Abstract

This study examined the measurement of the contralateral acoustic stapediusreflex in six-week-old infants and adults using wideband shifts in admittanceand energy reflectance (YR). The reflex activator was bandpass noise from2,500 to 11,000 Hz presented at a maximum spectrum level of 51 dB SPLmeasured in the ear canal. Reflexes were detected by calculating a cross-correlation between one-twelfth-octave measurements of YR for the highestactivator level and responses to lower levels. The reflex-induced shifts in YRfor the infant ears were similar in pattern to adult responses but were noisy atfrequencies below 1000 Hz. Infant reflexes were more successfully detectedwhen the cross-correlation was calculated from 1000 to 8000 Hz, whereas adultreflexes were more successfully detected for a cross-correlation from 250 to2000 Hz. This method may be useful in capturing the most robust frequencyregion for acoustic reflex detection across postnatal middle ear development.

Key Words: Acoustic reflex, acoustic impedance tests, infant

Abbreviations: ANSI = American National Standards Institute; DPOAE =distortion product otoacoustic emissions; NICU = neonatal intensive care unit;Peak Ytm = peak-compensated static acoustic admittance; YR = admittanceand energy reflectance

Sumario

Este estudio examinó las medidas del reflejo acústico estapedial contralateralen adultos e infantes de seis semanas de edad, utilizando cambios de bandaancha en la admitancia y en la reflectancia de la energía (YR). El activadordel reflejo fue un ruido de pasabanda desde 2.500 a 11.000 Hz, presentadoa un nivel espectral máximo de 51 dB SPL, medido en el canal auditivo. Losreflejos fueron detectados calculando una correlación cruzada entre unamedición de un duodécimo de octava del YR para la intensidad más alta delactivador y las respuestas para el nivel más bajo. Los cambios inducidos porel reflejo en el YR para los oídos de los infantes tuvieron un patrón similar alas respuestas de los adultos pero estuvieron interferidos por ruido a frecuenciaspor debajo de 1000 Hz. Los reflejos de los infantes fueron mejor detectadoscuando la correlación cruzada se calculó de 1.000 a 8.000 Hz, mientras quelos reflejos de los adultos de detectaron mejor con correlaciones cruzadas desde250 a 2.000 Hz. Este método puede ser útil para la identificación de la región

The acoustic stapedius reflex is a three-or four-neuron arc in the lowbrainstem, which is activated by

suprathreshold levels of sound andculminates in the bilateral contraction of thestapedius muscle (Borg, 1973). When thestapedius muscle contracts, the impedance ofthe normal middle ear changes, and thereforean indirect, noninvasive measurement of theacoustic reflex is the related change inacoustic impedance or admittance of themiddle ear. The presence of an acoustic reflexreveals information about the afferentauditory system, the function of the auditorybrainstem, the integrity of the VIIth (facial)nerve, which innervates the stapedius muscle,and the functional status of the middle ear.Therefore, the acoustic reflex is an importantphysiological test of auditory function and, assuch, is useful in the assessment of infants.For example, neonates with auditorydysynchrony may pass a screening of cochlearfunction using evoked otoacoustic emissionsbut may have poor functional hearing due tothis neural site of lesion. These infants canbe identified by the elevation or absence of theacoustic reflex (Hood, 1999).

The current American NationalStandards Institute (ANSI) standard forimmittance instruments (S3.39-1987) calls forthe use of a 226 Hz probe tone whenmeasuring admittance. This probe frequencyworks well for measuring acoustic reflexes inadults. However, studies that have used 220Hz or 226 Hz probe frequencies to measurethe acoustic reflex in neonates and younginfants have observed absent reflexes in 90%of infants (Keith, 1973; Allred, 1974; Jerger

et al, 1974; Bennett, 1975; Abahazi andGreenberg, 1977; Keith and Bench, 1978;Stream et al, 1978; Weatherby and Bennett,1980).

Higher probe frequencies (660 to 2000Hz) have been more successfully employed inobtaining acoustic reflex measurements withneonates and young infants (Weatherby andBennett, 1980; Bennett and Weatherby, 1982;Hirsch et al, 1992; Rhodes et al, 1999).Weatherby and Bennett (1980) reported that100% of 44 neonates had contralateralreflexes for a broadband noise activator whenprobe frequencies from 800 to 1800 Hz wereused. On the basis of those results, Bennettand Weatherby (1982) used a 1200 Hz probetone in measuring acoustic reflex thresholdsin 28 newborn infants. They were able tomeasure the acoustic reflex for a contralateralbroadband noise activator in 26 of 28 infants.Hirsch et al (1992) obtained ipsilateralacoustic reflex measurements with an 800 Hzprobe tone in infants who were about to bedischarged from a neonatal intensive careunit (NICU). The activators were a 2000 Hztone and a high-frequency noise band.Reflexes were absent in only 13 of 149 earstested (8.7%), and ten of these had abnormalauditory brainstem response (ABR) results.In another study examining hearingscreening in the NICU, Rhodes et al (1999)measured the ipsilateral acoustic reflex usinga 1000 Hz probe tone for 2000 Hz andbroadband-noise activators. Of the earstested, 82% had present acoustic reflexmeasurements for one or both of theactivators, while 83% passed an ABRscreening.

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frecuencial más robusta en la detección del reflejo acústico, durante eldesarrollo post-natal del oído medio.

Palabras Clave: Reflejo acústico, pruebas de impedancia acústica, infante

Abreviaturas: ANSI = Instituto Americano Nacional de Estándares; DPOAE= emisiones otoacústicas por productos de distorsión; NICU = unidad decuidado intensivo neonatal; Peak Ytm = pico de admitancia estático-acústicocompensado; YR = admitancia y reflectancia de la energía

These studies suggest the utility of usinga probe tone frequency higher than 226 Hzfor measuring the acoustic reflex in infants.However, there is currently no consensus onwhat probe frequency to use for young infants.Furthermore, it is not known if a singlefrequency would be ideal for all infants in theneonatal to six-month range, or if the idealprobe frequency would change with thedevelopment of the middle and outer ear overthe first six months of life (Keefe et al, 1993).

A method has recently been developedthat uses a wideband probe stimulus formeasuring the acoustic reflex by examiningchanges in admittance and energy reflectance(YR) of the middle ear (Feeney and Keefe,1999, 2001). This method has been shown toprovide a sensitive measurement of thecontralateral acoustic reflex threshold inadults, which is about 12 dB lower than thoseobtained using a 226 Hz probe tone (Feeney,Keefe, et al, 2003). As it is apparent that thefrequency region for the most sensitive reflexmeasurement changes with development(Weatherby and Bennett, 1980; Bennett andWeatherby, 1982), the use of a widebandprobe stimulus in infants should allow forreflex-induced shifts in middle ear functionto be monitored at the most robust frequency(or band of frequencies) as the reflex responsechanges with development. This should allowfor the most sensitive reflex measurement atany age, which would not be biased by theselection of a single probe frequency (e.g.,226 Hz). This should also facilitate theestablishment of normative reflex thresholddata across developmental ages, which wouldbe independent of the biasing effects of afixed probe frequency.

The purpose of the present study was toapply the wideband acoustic reflex method tothe measurement of contralateral acousticreflexes in young infants. Data from youngadults were obtained for comparison using thesame reflectance system and reflex activator.

MMEETTHHOODD

SSuubbjjeeccttss

Healthy infants having a normal birthhistory, born within two weeks of full-termdelivery, and with a negative family historyof hearing impairment were recruited for thestudy. Eight six-week-old infants, three boys

and five girls with an age range of 5.6 to 6.3weeks at test (mean = 5.9 weeks) wererecruited using the Infant Studies ParticipantPool at the University of Washington. Parentswho had agreed for their infant to participatein an infant hearing study were contacted byphone for participation in the study. Infantparticipants were also required to have (1) anormal otoscopic screening; (2) a normal 1000Hz admittance tympanogram (defined ashaving a peak within ±100 daPa of ambientpressure and with peak-compensated staticacoustic admittance [Peak Ytm] greater than0.6 mmhos at 1000 Hz using a +200 daPareference [Margolis et al, 2003]; and (3) anormal screening with distortion productotoacoustic emissions (DPOAE). The DPOAEscreening was conducted for f2 frequencies of2000, 3000, and 4000 Hz with a primaryfrequency ratio of 1.22. The levels of f1 andf2 were set at 65 dB SPL and 55 dB SPL,respectively. The pass criteria were definedas DPOAE levels ≥0 dB SPL and an emission-to-noise-floor ratio of ≥6 dB (Gorga et al,1999; Prieve, 2002).

Three adult subjects, one woman andtwo men ages 18.2 to 18.6 years, wererecruited from flyers posted on campus. Thesesubjects had a negative history of middle eardisorders and met the inclusion criteria of (1)a normal otoscopic screening, (2) a normal 226Hz admittance tympanogram defined ashaving a single peak within ±10 daPa ofambient pressure and with Peak Ytm between0.3 and 1.7 mmho (Margolis and Hunter,1999), and (3) pure-tone air-conductionthresholds ≤15 dB HL from 250 to 8000 Hzwith air-bone gaps ≤10 dB.

AAppppaarraattuuss aanndd SSttiimmuullii

All testing was completed in acommercial double-walled sound-treatedbooth. Otoacoustic emission screening wasconducted using a Mimosa Acoustics, modelDP-2000 DPOAE instrument with anEtymotic Research, model ER-10Cmicrophone system. Tympanometry wasconducted using a Grason-Stadler, Inc., model33 version II (GSI-33) immittance instrumentcalibrated to ANSI S3.39 (1987) standards.Pure-tone audiometry to meet the inclusioncriteria for adults was conducted using aMadsen, model 622 diagnostic audiometercalibrated for air- and bone-conduction testingto ANSI S3.6 (1996) standards.

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The wideband YR measurement systemconsisted of an Etymotic Research, modelER-10C microphone system; a CommunicationAutomation and Control, model 32C dataacquisition card with a 24 kHz samplingfrequency; and a personal computer. Theprobe signal consisted of 40 msec electricalchirps with a bandwidth from 200 to 10,000Hz that were generated by the dataacquisition card, routed to an attenuator(TDT-PA4), and then routed to a receiver inthe ER-10C probe, referred to as microphoneA. The overall level of the chirps was set at65 dB SPL for adult testing and 55 dB SPLfor infant testing as calibrated in a Zwislockioccluded ear simulator (Knowles Electronics,model DB-100). The microphone responsewas high-pass filtered at 225 Hz (64 dB/octave)to remove biologic and system noise. Themicrophone signal was then digitized at a 24kHz sampling rate using the data acquisitioncard and stored for data analysis.

The stimulus for reflex activation waspresented using a second ER-10C microphonesystem (microphone B), modified by themanufacturer to permit an additional 20 dBof receiver output, with a separate model32C data acquisition card and second personalcomputer. The stimulus was a digitallygenerated band of frozen noise with a lengthof 2048 samples at a 24 kHz samplingfrequency and 3 msec cosine-squared on-offramps. Its pass band was from 2.5 to 11 kHzwith a roll-off outside the pass band ofapproximately 64 dB per octave; similar to thenoise used in the infant-reflex study of Hirschet al (1992), but with a wider bandwidth.The stimulus was attenuated (TDT, PA-4)and then routed to a receiver in microphoneB. The microphone signal was then digitizedat a 24 kHz sampling rate using the dataacquisition card to record the activator levelin the subject’s ear canal implemented inMatlab. The maximum overall noise levelwas set at 90 dB SPL in the ear canal for adultand infant subjects. This level for infant earswas approximately equal to 68 dB SPLmeasured in 2 cm3 coupler. Given the 8500Hz bandwidth of the noise, this was at aspectrum level of 51 dB SPL in the ear. Pilottesting showed that the noise band at thislevel was high enough to elicit a contralateralacoustic reflex in about half of young adultswith normal hearing as measured with theGSI-33 immittance instrument using a 226Hz probe tone.

EEnneerrggyy RReefflleeccttaannccee CCaalliibbrraattiioonn

Calibration of the YR system wasconducted daily using the method describedby Keefe et al (1992). A set of six brass tubeswas used to calibrate for infant testing withinside diameters of 4.85 mm (approximatingthe diameter of the infant ear canal) andlengths ranging from 269 to 690 mm withrigid terminations. During calibration, anEtymotic ER10C-04 infant probe tip wasattached to the probe (microphone A) andthen inserted into each calibration tube toobtain a snug fit. A set of six brass tubes wasalso used to calibrate for adult testing withinside diameters of 8 mm (approximatingthe diameter of the adult ear canal) andlengths ranging from 486 to 918 mm withrigid terminations. During calibration, astandard ER-10C foam ear tip with adiameter of 14 mm and length of 14 mm(ER10C-14A) was attached to the probe andthen compressed and inserted into eachcalibration tube to a depth of approximately12 mm.

The calibration procedure calculated theThevenin source impedance and soundpressure of microphone A. This wasaccomplished by iterative comparison of themeasured sound pressure in each of the sixtubes to a tube model including viscothermallosses. Pressure spectrum measurements ina subject’s ear canal were combined with theThevenin parameters of the probe to obtainwideband impedance at the probe tip. Thecharacteristic impedance at the entrance ofthe ear canal was then estimated using anacoustic estimate of the ear canal area.Energy reflectance measurements wereobtained in one-twelfth-octave bands over afrequency range of 250 to 8000 Hz by acomparison of the impedance at the probe tipand the characteristic impedance of the earcanal using standard transformations (Keefeet al, 1992).

AAccoouussttiicc RReefflleexx DDaattaa CCoolllleeccttiioonn

One parent of an infant participant satin the sound-treated booth in a comfortablechair with the infant on his or her lap. Infantswere fed and changed as necessary andallowed to fall asleep in a supine position onthe parent’s lap prior to testing. It wasdesirable to make acoustic reflexmeasurements with the infant asleep because

the experimenter needed to hold both ER-10Cprobes still during testing; therefore, thismeasurement was obtained first after theinfant fell asleep. This was followed by 1000Hz tympanometry and DPOAE screening,which were both typically obtainable duringslight movement in the awake infant. Oncethe infant was asleep, an otoscopicexamination was conducted to ensure that theear canals were clear and to determine theappropriate size of the probe tip to seal thecanal. This was selected to be an ER10C-03,-04, or -05 infant probe tip with approximatediameters of 3, 4, and 5 mm respectively.

A probe tip was then attached tomicrophone B (the activator probe) andinserted into the infant’s right ear canal fora measurement of the activator-stimuluslevel. The probe was held in place by one ofthe researchers who held the probe cable.This technique was used to support the probe,which tended to be displaced due to its weightwith the infant in the supine position. Thegoal of this measurement was to calibrate thestimulus level in the subject’s ear canal fora maximum overall level of 90 dB SPL. Thenoise band was generated by the dataacquisition card using a custom signal-generation and measurement systemimplemented in Matlab. The stimulus waspresented to the subject’s ear canal forapproximately two seconds using one of thereceivers in microphone B at an attenuatedlevel, which was less than 90 dB SPL. Themicrophone response was low-pass filtered at225 Hz to reduce system and biologic noise,digitized using the data acquisition card andstored for analysis. The attenuator wassubsequently adjusted during data collectionto levels in the ear canal ranging from 90 to70 dB SPL by manual adjustment of theattenuator. Following the in situ calibrationof the activator stimulus, a probe tip wasattached to microphone A, and the probe wasinserted into the infant’s left ear canal forwideband YR measurements.

A similar procedure was used for the insitu calibration of the activator noise foradults. A standard adult tip (ER10C-14A)was attached to microphone B andcompressed to fit in the subject’s ear canalwith full insertion depth (14 mm), if possible,depending upon the ear canal shape and size.Measurement of the activator level proceededas for the infant subjects. Following theactivator calibration, a standard adult foam

probe tip was attached to microphone A andinserted into the subject’s left ear canal. Thecables for both microphones were attachedwith clips to the subject’s clothing, and eachadult subject was seated alone in the boothfor acoustic reflex testing. The probe tip wasallowed to expand for two minutes prior to YRmeasurement to seal the ear canal.

Several initial averaged baseline energyreflectance responses were evaluated todetermine if there was an age-appropriatereflectance response (Keefe et al, 1993) or aleaky probe fit resulting in lower or higherenergy reflectance than expected at thelowest one-third octave. A leaky probe fitwas suspected in adults if the energyreflectance was lower than 0.8 at 250 Hz. Ifsuch a measurement was obtained, the probetip was removed from the ear, reinserted, andallowed to expand for an additional twominutes prior to reflectance measurement.A leaky probe fit was suspected in an infantear if the energy reflectance was below 0.3or above 0.7 (Keefe et al, 2000) or if there wasan observable low-frequency shift in energyreflectance between consecutive averagedreflectance responses. If a leak was suspected,the next-larger probe tip was selected forinfant subjects, if appropriate, or the probewas repositioned and the reflectanceremeasured to achieve an acceptableresponse.

Testing commenced once theexperimenters determined that baselinereflectance responses were of the appropriatemagnitude and stability. When theexperimenter holding the ER-10C cablessignaled that the infant was sleeping quietly,or once the adult subject was seated quietly,a series of chirps was presented to thesubject’s left ear by the second experimenter.The microphone response was monitored todetermine if the subject’s state wassufficiently quiet for data acquisition. Theexperimenter then started data collectionand an artifact-reject algorithm was used tosample the level of intermittent noise toassess the validity of subsequent responsesto chirps (Keefe and Ling, 1998). Thisalgorithm allows for the real-time adjustmentof the artifact-rejection threshold, which wasraised for infant subjects compared to adults,given the differences in biological noise levelsto allow timely data collection. Eight validresponses to chirps were averaged acrossfrequency to form an energy reflectance

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baseline response. Immediately afteracquiring the baseline response, anotherenergy reflectance measurement wasobtained during the presentation of theactivator to the opposite ear at apredetermined level. The chirp series, underexperimenter control, was presented shortlyafter the activator onset and terminatedautomatically after eight valid responseswere obtained. These were averaged to formthe middle ear response in the presence of theactivator stimulus. The activator durationwas typically 2 to 3 sec depending on thesubject noise associated with themeasurement. One energy reflectancemeasurement in quiet (baseline) and one inthe presence of the activator (noise)constituted a baseline-noise pair.

An experimental run consisted of thepresentation of five baseline-noise pairs byvarying the noise level from 90 to 70 dB SPLin 5 dB steps. Two such runs were completedfor adults and infants unless the subject’sstate precluded additional measurements.YR shifts were then derived from thesemeasurements. An admittance shift is definedas the admittance magnitude in the activatorcondition minus the admittance magnitudein the baseline condition. The reflectanceshift is defined as the energy reflectance inthe activator condition minus the energyreflectance in the baseline condition. For oneinfant subject who was lightly sleeping (I1),a maximum activator level of 85 dB SPL wasused so that the activator would be lessalerting.

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The data from each experimental runwere examined to determine the presence ofa reflex response using a correlation method(Feeney and Keefe, 2001). In this methodeach shift for a given activator level wascross-correlated with the response from thehighest activator level used for that subject.The correlation method tested a response bydetermining whether the cross-correlationbetween it and the response at the highestnoise-stimulus level was sufficiently highand positive to exceed a certain value ρo. Thecross-correlation, r, was transformed toapproximate a normal distribution using theFisher’s Z transformation (Kleinbaum et al,1988). The following test statistic was usedto test whether the correlation exceeded ρo.

The value N represents the number ofone-twelfth-octave points in the cross-correlation that varied with the analysisbandwidth. A criterion value for ρo = 0.548was adopted, since the linear regressionbetween two variables with this correlationaccounts for 30% of the overall variance. Aone-tailed test with an alpha level of 0.05was used, and thus the correlation r > ρo if Z> 1.645. Thus, a significant cross-correlationbetween the shift observed for the highestactivator and a shift observed at a loweractivator level defined the reflex as presentat the lower activator level. Correlationswere calculated across bandwidths of 250 to2000 Hz (N = 37), 250 to 4000 Hz (N = 49),and 250 to 8000 Hz (N = 61) as in Feeney andKeefe (2001). Correlations across bandwidthsof 500 to 8000 Hz (N = 49) and 1000 to 8000Hz (N = 37) were also calculated to examinethe effect of negating low-frequencyinformation, and thus biological noise,expected to be higher for infants than adults.The cross-correlation value was expected todecrease to zero as the activator level wasdecreased, with the limit that the shiftbetween the baseline and activator conditionswas due to baseline variability. If there wereno significant correlations between reflectanceor admittance shifts for an experimental run,an additional correlation was obtainedbetween shifts at the highest activator levelfor repeated runs to determine the presenceof a reflex response for that level. A cross-correlation across each of the five bandwidthswas also calculated between the shift for thehighest activator and the difference betweentwo subsequent baseline pairs to test if theseYR responses would falsely be labeled a reflex.

RREESSUULLTTSS

Wideband acoustic data for infants wereaccepted as satisfactory if the

equivalent volume measurement was positiveat frequencies below 1000 Hz. Data for threeof the eight infants revealed negativeequivalent volumes (less than -3.0 cm3), whichbecame more negative with decreasingfrequency below 1000 Hz. This findingsuggests a leaky probe fit resulting in invaliddata (Keefe et al, 2000). Therefore, only thedata for the five infant subjects for whom

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equivalent volume measures were positivewill be considered further. One-twelfth-octavemeasurements of the impedance levelmeasured at the probe tip (in dB re 1 cgs ohm)and energy reflectance were averaged forthree baseline conditions in a single run foreach subject. The top panel of Figure 1 showsthe mean impedance level at ambientpressure for the adult and infant groups.The impedance level for the adults decreasedat approximately 6 dB per octave up to 3500Hz, consistent with a stiffness-controlledsystem, and increased above the resonancefrequency, consistent with a mass-controlledsystem. The mean impedance level for the six-week-old infants was higher at all frequenciesthan that of the adults and started out in thelow frequencies with a negative slope ofapproximately 6 dB per octave. There was asmall local maximum at around 450 Hz, after

which the impedance level decreased at aslope of about 4 dB per octave to 1000 Hz. Theslope then returned to approximately -6 dBper octave above 1000 Hz, and there was alocal minimum at 2000 Hz and localmaximum at 4000 Hz similar to the data forone month olds reported by Keefe et al (1993).The energy reflectance data at ambientpressure for the adults in the bottom panelof Figure 1 was typical of adult data withenergy reflectance near 1.0 in the lowfrequencies, decreasing with frequency to aminimum near 4800 Hz and increasing athigher frequencies to around 0.7 at 8000 Hz(Keefe et al, 1993; Voss and Allen, 1994;Margolis et al, 1999; Feeney, Grant, et al,2003). The group mean energy reflectance forthe six week olds was lower at all frequenciesthan for adult values. Infant energyreflectance was around 0.5 at 250 Hz,decreased with frequency to a minimum of0.03 at 2000 Hz, and then increased at higherfrequencies rising to 0.45 at 8000 Hz. Thesedata for adults and infants are in agreementwith the impedance level and energyreflectance data of Keefe et al (1993, figures1, 4) for adults and one-month-old infantscollected using a similar system.

Two of the three adult subjects did nothave a measurable acoustic reflex, andtherefore reflex data will only be presentedfor one adult subject. This is consistent withthe pilot data for this study suggesting thatthe bandpass noise at this level was onlyeffective in activating a contralateral acousticreflex in about half of young adults tested.

Figure 2 shows the one-twelfth-octaveshifts in energy reflectance (∆R =Reflectanceactivator - Reflectancebaseline) forthe adult subject (A1) and the five infantsubjects (I1 through I5). The parameter ineach plot is the overall level of the activatormeasured in the ear canal of each subject. Theplots labeled “Baseline” represent thedifference in energy reflectance between twobaseline conditions for each run during whichthe contralateral activator was not presented.Depending on how long the infant remainedsleeping, data collection involved thepresentation of all five activator levels as inthe case of I3 or as few as two levels for I1 andI5. The shifts in normalized admittancemagnitude (∆Y = [|Y| activator - |Y|baseline] / |Y| baseline) for the adult and fiveinfant subjects is shown in Figure 3 for thesame conditions displayed in Figure 2. Figure

FFiigguurree 11.. The upper panel shows the mean one-twelfth-octave impedance level in dB for the threeadults (solid line) and five infants (dashed line) forwhom valid data were obtained. The lower panelshows the mean energy reflectance for the samesubjects. The error bars represent ±1 SE with everythird data point shown for clarity.

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FFiigguurree 22 .. The one-twelfth-octave shifts inenergy reflectance (∆R =Reflectanceactivator -Reflectancebaseline) for theadult subject (A1) and thefive infant subjects (I1through I5). Theparameter in each plot isthe level of thecontralateral bandpassnoise stimulus. The plotslabeled “Baseline”represent the difference inenergy reflectancebetween two baselineconditions for each runduring which thecontralateral activatorwas not presented.

FFiigguurree 33 .. The one-twelfth-octave shifts innormalized admittance(∆Y = [|Y| activator - |Y|baseline] / |Y| baseline)for the adult subject (A1)and the five infant subjects(I1 through I5). Theparameter in each plot isthe level of thecontralateral bandpassnoise stimulus. The plotslabeled “Baseline”represent the difference inenergy reflectancebetween two baselineconditions for each runduring which thecontralateral activator wasnot presented.

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4 shows the mean values of ∆R (upper panel)and ∆Y (lower panel) for the 85 dB activatorcondition for the five infants. The maximumshift in energy reflectance is around 5% at1000 Hz, and the maximum shift inadmittance is around 10% of the baselineadmittance at 1414 Hz.

The cross-correlation coefficients betweenthe energy reflectance and normalizedadmittance shifts for the highest activatorlevel and lower levels are shown in Table 1for the conditions judged to result in anacoustic reflex by the correlation method(Fisher’s Z > 1.645). The cross-correlationswere obtained across five bandwidths from250 to 2000 Hz, 250 to 4000 Hz, 250 to 8000Hz, 500 to 8000 Hz, and 1000 to 8000 Hz. Thecorrelation coefficients decreased withactivator level for the adult subject asexpected and tended to be the highest forthe 250 to 2000 Hz and 250 to 4000 Hzbandwidths. The highest and in some casesthe only cross-correlations that weresignificant for the infant subjects tended tobe for the 500 to 8000 Hz and 1000 to 8000Hz bandwidths, although I2 had high cross-correlations for normalized admittancebetween the 90 dB activator condition and the85 and 80 dB SPL activator conditions acrossall bandwidths. None of the cross-correlationsbetween baseline differences and the shift forthe highest activator level were judged to bereflexes by the correlation test for either

Table 1. Cross-Correlation Coefficients for Activator Levels at Which an Acoustic Reflex Was Judged to BePresent by the Correlation Method for Energy Reflectance and Admittance Measurements across Five

Bandwidths

EEnneerrggyy RReefflleeccttaannccee BBaannddwwiiddtthh ((HHzz)) AAddmmiittttaannccee BBaannddwwiiddtthh ((HHzz))

Subject Level (dB) 250–2000 250–4000 250–8000 500–8000 1000–8000 250–2000 250–4000 250–8000 500–8000 1000–8000

A 1 85 0.98 0.83 0.85 0.85 0.86 0.98 0.96 0.91 0.88 0.83

80 0.93 0.69 0.71 0.73 0.95 0.92 0.80 0.73

75

70 0.75

I 1 80 0.90 0.91 0.80 0.82 0.97 0.98

I 2 85 0.96 0.83 0.83 0.82 0.88 0.91

80 0.87 0.90 0.90 0.88 0.89 0.93

I 3 85 0.71 0.72 0.72 0.83 0.94 0.92 0.94

80 0.92 0.97 0.91 0.92

75 0.96 0.87 0.94

70 0.73

I 4 85 0.89 0.77

80

I 5 85 0.76

Note: The cross-correlations were calculated between the highest activator level used for each infant and the levels listed in the table. Blank cellsrepresent conditions for which a reflex was judged to be absent.

FFiigguurree 44.. The upper panel shows the mean shiftsin energy reflectance for the five infant subjects forthe 85 dB activator condition (∆R = Reflectanceactivator– Reflectancebaseline). The lower panel shows themean shifts in normalized admittance for the fiveinfants for the same activator condition (∆Y = [|Y|activator - |Y| baseline] / |Y| baseline).

energy reflectance or normalized admittance.

DDIISSCCUUSSSSIIOONN

Data for two adult participants failed toyield an acoustic reflex based on the

correlation test. This is consistent with ourpilot data suggesting that the maximum levelof the noise activator was not of sufficientintensity to result in an acoustic reflex inabout half of young adults with normalhearing. In a previous study using the samemethod, acoustic reflexes were present in all34 young-adult subjects tested using 1000and 2000 Hz activators with an upper limitof 100 dB HL (Feeney, Keefe, et al, 2003).Thus, the failure to observe reflexes in thesetwo adults is attributed to the restrictedmaximum activator level, with a spectrumlevel of 51 dB SPL. However, an acousticreflex was measured for all infant subjects forthe same maximum activator level asmeasured in the ear canal.

The wideband pattern of subject A1’scontralateral reflex shifts in YR were similarto those previously reported using the samemethod. However, the first zero crossing forthe reflectance pattern for A1 was 1156 Hzfor the 85 dB activator condition that wasslightly higher than the average zero crossingfor young adults of 1000 Hz reported byFeeney, Keefe, et al (2003). The zero-crossingfrequency for reflectance provides an estimateof the middle ear resonance frequency(Feeney and Keefe, 1999). The first zerocrossing of the normalized admittance patternfor A1 for the 85 dB activator condition wasat 891 Hz. This was also somewhat higherthan the average zero crossing of admittancefor young adults that was reported to bearound 600 Hz by Feeney, Keefe, et al (2003).The 891 Hz zero crossing for admittance forA1 was lower in frequency than the first zerocrossing of the reflectance pattern (1156 Hz),which shows the effect of the distancebetween the probe and the tympanicmembrane on the ear canal admittancemeasurement. In contrast, energy reflectanceis relatively independent of probe position inthe ear canal (Stinson et al, 1982).

With the exception of I4, it was possibleto discern the first zero crossing of thereflectance pattern for each infant subject(aside from zero crossings in the lowfrequencies associated with noisy recordings),and this averaged 1637 Hz for the 85 dB

activator condition for which data wereavailable for the other four subjects.Compared with the average adult zerocrossing for reflectance of 1000 Hz (Feeney,Keefe, et al, 2003), this suggests a highermiddle ear resonance frequency for infantsthan adults. The average for the first zerocrossing of the normalized admittance patternfor the same four infants was 1156 Hz, whichis higher than for the adult pattern (891 Hz)but also lower than the related reflectancemeasurement in these infants (1637 Hz),again suggesting the effect of the ear canalon the admittance pattern of the acousticreflex.

Allowing for the differences in zero-crossing frequency between infant and adultreflectance and admittance shifts, thepatterns of reflex shifts for adults and infantsare remarkably similar. Both adult and infantreflectance shifts show a major increase inlow-frequency reflectance with minimal shiftsabove 2000 Hz. The admittance patternsshow a low-frequency decrease in admittancefollowed by sharp increase in admittance atfrequencies above the zero crossing. Themean positive peak in admittance for adultsis around 1000 Hz (Feeney, Keefe, et al, 2003),somewhat lower than for A1 at 1122 Hz.However, the first positive admittance peakaveraged across all five infants was at 1414Hz. As with adults, the amplitude of thispositive shift in normalized admittance isgreater than the negative shift, which occursbelow the cross-over frequency. For the 85 dBactivator condition in the present study, themagnitude of the positive normalized-admittance shift was approximately 10%compared to the magnitude of the low-frequency shift that was approximately 5%(Figure 4). This is important for the purposesof reflex detection because this larger shiftoccurs at frequencies above 1000 Hz that areless contaminated by biological noise thanlower frequencies, making the region above1000 Hz a logical frequency region to searchfor an infant acoustic reflex.

This concept is supported by the resultsof the correlation analysis for reflectance andadmittance (Table 1). The frequencybandwidth that most often resulted in thedetection of a reflex response for the infantsubjects was the one extending from 1000 to8000 Hz. All nine activator levels producinga reflex by reflectance measurement had asignificant correlation for the 1000 to 8000 Hz

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bandwidth, as well as seven out of the nineactivator levels for normalized admittance.Yet for the 250 to 2000 Hz bandwidth, onlyone out of nine activator levels yielded asignificant reflex for the reflectance method,and two out of nine activators for thenormalized admittance method.

The low-frequency region (<1000 Hz) ofthe infant YR shifts was noisy. This waslikely due in part to the inherent noisinessof this population, even during sleep. It wasnecessary to raise the artifact rejectionthreshold to enable expedient data collectionfor all infants, thus increasing the inclusionof noise. Additionally, the method of holdingthe probe cables used in this study may haveincreased the noise levels due to smallmovements of the examiner during testing.A leaky probe seal was responsible for theexclusion of acoustic reflex data from threeof the eight infants evaluated (37.5%). Incontrast, Keefe et al (2000) reported a leakyprobe fit in 270 of 2081 infant ears (13%)when making YR measurements. Thedifference between the studies in the successrate of obtaining valid YR data may be relatedto sampling error since the present studyhad a small number of subjects. Alternatively,the methodology of the present study, whichrequired the experimenter to monitor theprobe fit in both of the infant’s ears over alonger time period, could have led to thedifferences in measurement success. Negativeequivalent volume was used in both studiesas the criterion for suspecting a leaky probefit, but the analyses were conducted off-linefollowing data collection. A real-timeinspection of the wideband equivalent volumedata would facilitate detection of a leakyprobe fit in infants and allow for probeadjustments to be made during testing. Datacollection would also be facilitated by usingan ipsilateral acoustic reflex method so thatthe sleeping infant could be placed on itsside with one ear directed up as wassuccessfully employed by Hirsch et al (1992)and Rhodes et al (1999) in testing acousticreflexes in high-risk infants. This paradigmis attractive for acoustic reflex testing ininfants since only one ear would need to bemonitored for an appropriate probe sealduring testing. A recent study has shown theefficacy of obtaining wideband YR data usingan ipsilateral reflex paradigm in adults(Feeney et al, 2004). Additional work isneeded to refine this approach and apply it

to the infant population. The inclusion of a middle ear

measurement in newborn hearing screeningprograms has been suggested to assist theinterpretation of tests for screening hearingloss (Keefe et al, 2000; Keefe, Gorga, et al,2003; Keefe, Zhao, et al, 2003). For example,in programs utilizing otoacoustic emissionsas the hearing screening tool, a referral mayoccur due to the presence of a transientmiddle ear disorder or vernix caseosa in theear canal, both of which could impede theforward transmission of the acoustic signaland the reverse transmission of theotoacoustic emission. A valid test of middleear function could alert the examiner to thepossibility of a screening failure due to amiddle ear disorder. Or conversely, if themiddle ear test were normal, the failurewould alert the examiner of the likelihood ofa true positive test for sensorineural hearingloss. Both outcomes could be useful to theaudiologist for counseling parents and inplanning follow-up evaluations. If the middleear test included the measurement of theacoustic reflex, it could also be used to reducefalse negative results in infants with auditorydysynchrony, for whom the diagnostic profileis normal otoacoustic emissions but absentacoustic reflexes and auditory brainstemresponses (Hood, 1999). Furthermore, theotoacoustic emission and middle ear functiontests can be performed with the same basicprobe assembly containing receivers and amicrophone. Thus, a screening for inner eardisorders (otoacoustic emissions), middle eardisorders (wideband reflectance, admittanceand acoustic reflex), and neural disorders(acoustic reflex) could be obtained using asingle probe placement, making this apotentially efficient multifaceted screeningtechnique.

CCOONNCCLLUUSSIIOONN

The contralateral acoustic reflex for a high-frequency bandpass noise activator was

detected in six-week-old infants usingwideband measurements of energyreflectance and admittance. The generalpatterns of the YR reflex shifts were similarto those obtained in an adult subject with thesame activator and were similar to dataobtained from a larger group of young adultsusing tonal activators. This method of reflexmeasurement ensures that the most robust

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frequency region for reflex shifts can be usedto detect the reflex, regardless of changes inthe infant middle ear response duringpostnatal development. Data are needed frominfants in the birth to six-month age rangeto examine the development of the acousticreflex and wideband reflex test-retestreliability. Data are also needed from infantswith hearing loss due to middle ear, inner ear,or neural dysfunction. An ipsilateral reflexparadigm would be ideal for these types ofwideband measurements in infants, whichcould be combined with an otoacousticemission test for a comprehensive acousticmethod of screening inner ear, middle ear, andneural auditory function using a single probe.

AAcckknnoowwlleeddggmmeenntt.. The authors thank Douglas Keefefor providing software for acquiring acoustic trans-fer functions.

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