cochlear dead regions in adults and children: diagnosis ... · the dead region. this is referred to...

Cochlear Dead Regions in Adults andChildren: Diagnosis and ClinicalImplications

Brian C. J. Moore, Ph.D.,1 and Alicja N. Malicka, Ph.D.2

ABSTRACT

The inner hair cells (IHCs) are the transducers of the cochlea;they convert mechanical vibrations to neural activity. When the IHCsand/or neurons are nonfunctioning over a certain region of the cochlea,this is referred to as a dead region. A dead region can be defined in termsof the characteristic frequencies of the IHCs and/or neurons immedi-ately adjacent to the dead region. Dead regions can be detected, andtheir limits can be determined, using the threshold equalizing noise(TEN) test or by measurement of psychophysical tuning curves (PTCs).Both PTCs and the TEN test can be used to assess children as young as7 years of age. The identification of dead regions can be helpful indetermining the appropriate form of amplification. For both adults andchildren with restricted dead regions (“holes”), benefit is obtained fromamplification of frequencies up to at least 4 kHz. For adults and childrenwith extensive continuous dead regions starting at a relatively lowfrequency (� 1.5 kHz) there may be little or no benefit fromamplification of high frequencies.

KEYWORDS: dead region, TEN test, psychophysical tuning curve,

amplification, hearing aid

Learning Outcomes: As a result of this activity, the participant will be able to (1) describe what a dead region

is and describe two methods for diagnosing a dead region for adults and children, and (2) list the implications

of dead regions for speech perception and the fitting of hearing aids.

1Department of Experimental Psychology, University ofCambridge, United Kingdom; 2School of Health andRehabilitation Sciences, The University of Queensland,Queensland, Australia.

Address for correspondence and reprint requests: BrianC. J. Moore, Ph.D., Department of Experimental Psychol-ogy, University of Cambridge, Downing Street, CambridgeCB2 3EB, United Kingdom (e-mail: [email protected]).

Proceedings of the Widex Audiology Congress; GuestEditor, Robert Sweetow, Ph.D.

Semin Hear 2013;34:37–50. Copyright # 2013 byThieme Medical Publishers, Inc., 333 Seventh Avenue,New York, NY 10001, USA. Tel: +1(212) 584-4662.DOI: http://dx.doi.org/10.1055/s-0032-1333150.ISSN 0734-0451.

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A dead region is a region in the cochleawhere the inner hair cells (IHCs) and/or theauditory neurons are functioning very poorly, ifat all.1–4 A sine wave that produces peak basilarmembrane vibration in a dead region may notevoke sufficient neural activity within the deadregion for that signal to be detected. However, ifthe signal is sufficiently intense, it may bedetected via IHCs and neurons adjacent to thedead region. In other words, the signal may bedetected via a place in the cochlea that is differentfrom the place where that frequency wouldnormally be detected. This is sometimes called“off-frequency listening” or “off-place listening.”Psychoacoustic tests fordiagnosingadead regionare basically tests for detecting when off-placelistening is occurring. Hence, a dead region canbedefinedoperationally as a region in the cochleawhere the IHCs and/or neurons are functioningso poorly that a tone producing peak vibration inthat region is detected by off-place listening.4

Using a frequency-to-place map,5–7 theboundary of a dead region can be defined interms of the characteristic frequency of theIHCs and/or neurons immediately adjacent tothe dead region. This is referred to as the edgefrequency (fe).1,3 We can distinguish severaltypes of dead regions:

1. A high-frequency (basal) dead region ex-tends upward from fe.2,8,9 When there is adead region for all frequencies above fe, thedead region is referred to as continuous.

2. A low-frequency (apical) dead region ex-tends downward from fe.10,11 When thereis a dead region for all frequencies below fe,the dead region is referred to as continuous.

3. A dead “hole” is a restricted dead region withlower and upper edge frequencies.12,13

4. There may be a continuous dead regionbelow a lower fe and above an upper fe,with a surviving “island” in between.14

5. Theremay be “patchy” dead regions (i.e., oneor more dead holes with surviving regions inbetween).

DIAGNOSIS OF DEAD REGIONS

Psychophysical Tuning Curves

The “gold standard” for detecting dead regionsis the psychophysical tuning curve (PTC). To

measure a PTC, the sinusoidal signal is fixed inlevel, usually at a low sensation level such as 10-dB sensation level. The masker is either asinusoid or a narrowband noise. The maskerlevel required just to mask the signal is mea-sured as a function of the masker center fre-quency. For normally hearing subjects, the tipof the PTC (the frequency at which the maskerlevel is lowest) usually lies close to the signalfrequency.15–19 Thus, the masker is most effec-tive when its center frequency is equal to thesignal frequency. However, when hearing-im-paired listeners are tested, PTCs have some-times been found whose tips are shifted wellaway from the signal frequency.2,12,20–27 Thishappens when the signal frequency falls in adead region. The masker is most effective whenits center frequency coincides with the charac-teristic frequency at which the signal is beingdetected (via off-place listening). Thus, it isassumed that the tip of the PTC falls at theboundary of the dead region (i.e., at fe).

There are several problems associated withthe measurement of PTCs. One is that theresults may be affected by the detection ofcombination tones produced by the interactionof the signal and masker.12,25 This can happenespecially for a person whose hearing at lowfrequencies is much better than at high fre-quencies. If the masker and the signal both fallin a region of hearing loss, they may interact toproduce a simple difference tone that falls in aregion of better hearing. This difference tonemay be heard even when the signal itself ismasked. To avoid this problem, Kluk andMoore25 recommended adding a fixed low-pass noise to the main narrowband masker.The noise level is chosen so that the low-passnoise does not mask the signal, but it does maskany combination tone produced by the interac-tion of the signal and the masker. A secondproblem with PTCs is that the results can beinfluenced by the detection of beats between thesignal and the masker, even when the masker isa narrowband noise.12,25 To avoid this problem,Kluk and Moore25 recommended the use of arelatively wide noise bandwidth, such as 300Hz. Such a noise has inherent rapid amplitudefluctuations that make it difficult to detect beatsbetween the signal and masker. However, theuse of such a wide bandwidth is problematic for

38 SEMINARS IN HEARING/VOLUME 34, NUMBER 1 2013

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low signal frequencies, where the normal audi-tory filters are relatively narrow.28 When thenoise bandwidth is larger than the auditory-filter bandwidth, this can result in a broadeningof the tip of the PTC, making it more difficultto determine the exact frequency at the tip. Forlow frequencies, a bandwidth of 160 Hzmay bea good compromise. If precautions are not takento prevent beats and combination tones frombeing used as a cue, PTCs with two or eventhree tips may occur, which makes the inter-pretation of the results very difficult.19,24,25,29

A practical problem for clinical applica-tions is that the measurement of PTCs can betime consuming. The traditional method ofobtaining PTCs is to measure the maskerlevel required for threshold for several maskerfrequencies. Depending on how many maskerfrequencies are used and on the method that isused to estimate the masked threshold, it cantake between 30 minutes and several hours tomeasure a PTC for a single signal frequency.A faster method is to use a masker that slowlysweeps in frequency, using a method resem-bling Bekesy audiometry, to “track” the mask-er level required for threshold.24,30–32 The fastPTC method described by Sek et al31,32

requires about 3 minutes to measure a PTC.However, subjects sometimes require a fewpractice runs before they give stable results.26

Software for measuring fast PTCs can bedownloaded from: http://hearing.psychol.cam.ac.uk.htm. The software runs on a PCwith a good quality sound card.

The Threshold Equalizing Noise Test

The threshold equalizing noise (TEN) test wasdeveloped as a simple and quick method fordiagnosing dead regions in the clinic.2,33 Thetest involves measuring the threshold for de-tecting a pure tone presented in a backgroundnoise called threshold equalizing noise. The sig-nal level is adjusted to determine the maskedthreshold, while the TEN level is fixed. Thenoise spectrum is shaped in such a way that thethreshold for detecting a pure tone in the noiseis approximately the same over a wide range oftone frequencies for people with normal hearingand for people with hearing loss but without anydead regions. The masked threshold is approx-

imately equal to the nominal level of the noise,specified as the level in a 132-Hz-wide bandcentered at 1000 Hz. The value of 132 Hzcorresponds to the equivalent rectangular band-width of the auditory filter, as determined usingyoung, normally hearing listeners, which is de-noted ERBN.

34 When the signal frequency fallsin a dead region, the signal will only be detectedwhen it produces sufficient basilar-membranevibration at a remote region in the cochlea wherethere are surviving IHCs and neurons. Becauseof the tuning of the basilar membrane, theamount of vibration at this remote region willbe less than in the dead region, and so the noisewill be very effective in masking it. Thus, thesignal threshold is expected to be markedlyhigher than normal. A masked threshold thatis 10 dB higher than normal is usually taken asindicating a dead region.2,33 For a positivediagnosis, the masked threshold of the signalin the TEN is also required to be 10 dB or moreabove the absolute threshold (i.e., theTENmustproduce at least 10 dB of masking). If the TENlevel/ERBN is set equal to or 10 dB above theabsolute threshold at the test frequency, thesecond criterion is automatically satisfied whenthe first criterion is satisfied.

In the first version of the TEN test,2 alllevels were calibrated in decibels sound pressurelevel (SPL) and the spectrum of the TEN wasbroad, so that the test could be used for a widerange of signal frequencies (0.125 kHz to 10kHz); this version is referred to as the TEN(SPL) test. In a later version of the test,33 alllevels were calibrated in dB hearing level (HL)and the spectrum was made narrower to reducethe loudness of the TEN; this version is referredto as theTEN(HL) test. TheTEN(HL) test canbe used for signal frequencies in the range 0.5 to4 kHz, and has to be used with specific head-phones, namely Telephonics TDH39, TDH49and TDH50 headphones (Telephonics; Farm-ingdale, New York). Recently, a version of theTEN test suitable for use with Etymotic Re-search ER-3A (Etymotic Research; Elk GroveVillage, Illinois) insert earphones has been de-veloped.35 This is called the TEN(ER3) test.

One drawback of the TEN test is that itdoes not give a precise estimate of the value offe. If the criteria for diagnosis of a dead regionare not met at a given test frequency but are met

DEAD REGIONS IN ADULTS AND CHILDREN/MOORE, MALICKA 39

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at the next higher test frequency, this suggeststhat the value of fe lies somewhere between thetwo test frequencies. Thus the precision inestimating the value of fe depends on thespacing of the test frequencies that are used.Also, the criteria for positive diagnosis of a deadregion may not be met when the test frequencyfalls only a little above fe. Thus, in general, it isdesirable to confirm the results of the TEN test,and to estimate the value of fe more precisely,using PTCs.

The Audiogram

To assess whether dead regions could be diag-nosed from the audiogram, Aazh and Moore36

and Vinay and Moore37 attempted to predictthe presence of dead regions based on theaudiogram. They compared the diagnosiswith that obtained using the TEN(HL)test.33 They showed that it was not possibleto achieve both high sensitivity and high speci-ficity when attempting to predict the presence/absence of a dead region from the audiogram.However, Vinay and Moore37 showed that, foreach test frequency from 0.5 to 4 kHz, 59% ormore of ears had a dead region when theabsolute threshold was above 70 dB HL.Thus, a hearing loss of 70 dB or more issuggestive of a dead region. However, a deadregion can be present at a given test frequencywhen the audiometric threshold at that fre-quency is 55 dB or less,2,37,38 whereas a deadregion may be absent when the hearing loss is85 dB or more.2,37 Thus, there is a large rangeof uncertainty. A very steep slope of the audio-gram, with thresholds worsening rapidly withincreasing frequency, is suggestive of a high-frequency dead region but does not provide areliable diagnostic method.37,39

IMPLICATIONS OF DEAD REGIONSFOR THE CHOICE OFAMPLIFICATIONCHARACTERISTICS FOR ADULTSThere have long been reports suggesting thatpeople with moderate to severe hearing loss athigh frequencies often do not benefit fromamplification of high frequencies, or even per-form more poorly when high frequencies are

amplified.40–45 It has been assumed in many ofthese studies that the subjects who did notbenefit from amplification of high frequencieshad reduced function or complete loss of func-tion of IHCs and/or neurons, but no direct testfor the presence of dead regions was conducted.

In two studies, adult subjects were testedfor the presence of dead regions using bothPTCs and the TEN test.8,9 All subjects hadhigh-frequency hearing loss, but some hadhigh-frequency dead regions and some didnot. The dead regions, when present, ap-peared to be continuous rather than patchy(i.e., they extended from fe upward, withoutany surviving “islands” of function). Also, thevalues of fe were rather low, so the deadregions were extensive. Generally, the subjectswith dead regions had more severe high-frequency hearing losses than those withoutdead regions. The speech stimuli were vowel-consonant-vowel nonsense syllables, usingone of three vowels (/i/, /a/, and /u/) and21 different consonants. The stimuli weresubjected to the frequency-gain characteristicprescribed by the “Cambridge” formula,46

which is roughly equivalent to a half-gainrule for midrange frequencies. The level ofthe input speech, before any amplification orfiltering was applied, was 65 dB SPL.

Subjects were tested using broadband stim-uli (upper frequency limit 7.5 kHz) and stimulithat were low-pass filtered with various cutofffrequencies. Changes in performance with cut-off frequency can be used to infer the extent towhich information was being extracted in dif-ferent frequency regions. In one experiment,8

the stimuli were presented in quiet. In anotherexperiment,9 the stimuli were presented insteady speech-shaped noise, using a speech-to-noise ratio chosen to give a moderate reduc-tion in performance relative to that measured inquiet. The general pattern of results was similarfor the two experiments, and we describe hereonly the results of the second experiment.

For subjects without dead regions, perfor-mance improved progressively with increasingcutoff frequency. This indicates that thesesubjects were able to make use of high-frequen-cy information. The mean score for the broad-band stimuli was 79%. For subjects with deadregions, scores tended to increase with


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increasing cutoff frequency up to 1.5fe to 2fe.For higher cutoff frequencies, scores tended toremain nearly constant or to decrease slightly.These results indicate that the subjects withdead regions extracted little or no informationfrom frequencies that were well inside the deadregions. Generally, the best scores for thesubjects with dead regions were well belowthose obtained by the subjects without deadregions when listening to broadband stimuli.

A study of Vestergaard47 provided databroadly consistent with the findings previouslysummarized. He tested experienced users ofhearing aids. He used the TEN(SPL) test todiagnose dead regions. He also measuredspeech intelligibility using low-pass filteredword lists, while subjects listened through theirown hearing aids, all of which provided sub-stantial amplification at frequencies where therewas hearing loss. To analyze his results, the earstested were divided into those showing high-frequency dead regions with fe in the range 0.75to 1.5 kHz, and ears showing either no deadregions or dead regions with fe at 3 kHz orabove. Ears showing evidence of dead “holes”were excluded from the analysis.

For the subjects with no dead regions ordead regions starting at 3 kHz or above,performance tended to improve progressivelywith increasing low-pass filter cutoff frequen-cy; the highest mean percent correct (86%)was achieved for the widest bandwidth. Forthe subjects with fe in the range 0.75 to1.5 kHz, the pattern was less consistent, butthe mean scores showed no clear trend withincreasing cutoff frequency above 1 kHz, andthe mean score for the widest bandwidth wasonly 58%.

Overall, these results support the idea thatsubjects with extensive continuous high-fre-quency dead regions do not make as effectiveuse of audible speech information at highfrequencies as subjects without such dead re-gions. The subjects tested by Vestergaard47 allhad experience using hearing aids that providedsubstantial high-frequency amplification. Thissuggests that the failure of the subjects withextensive high-frequency dead regions to bene-fit from the audibility of high frequencies wasnot due to lack of experience in hearing thosefrequencies.

Preminger et al39 investigated speech in-telligibility and subjective hearing aid benefitusing 49 experienced hearing aid users, withaudiometric thresholds poorer than 50 dB HLfor at least two frequencies and no audiometricthresholds poorer than 80dBHL.They used theTEN(SPL) test to diagnose dead regions, butusing a 15-dBmasked-threshold criterion ratherthan the 10-dB criterion recommended byMoore et al.2 Theymeasured speech intelligibil-ity using twoversions of theQuickSIN(signal innoise) test (Etymotic Research, Elk Grove Vil-lage, Illinois), one of which had increased gainabove1 kHz,with about 20dBand30dBofgainat 2 kHz and 3 kHz, respectively. Subjects withdead regions hadpoorer understandingof speechin noise than those without dead regions. How-ever, both groups showed a benefit from in-creased high-frequency gain. It should be notedthat the same frequency-gain characteristic wasused for all participants, unlike the studies ofVickers et al8 and Baer et al,9 where the frequen-cy-gain characteristic was tailored for each par-ticipant, based on their audiogram. The data ofPreminger et al are not inconsistentwith the ideathat, for participants with extensive continuousdead regions, there is benefit fromproviding gainfor frequencies up to about 1.7fe, but no benefitfrom providing gain at higher frequencies.

In some studies conducted more recently,dead regions were diagnosed using the TEN(SPL) test48 or a modified version of the TEN(HL) test.49,50 The subjects tested in thosestudies mostly had less severe hearing lossthan the subjects studied by Vickers et al,8

Baer et al,9 and Vestergaard.47 Also, the sub-jects had restricted dead regions (holes) ratherthan extensive continuous dead regions. Theresults generally showed that subjects both withand without dead regions benefited from theapplication of gain at high frequencies.

Overall, the results suggest that, for adultsubjects with continuous high-frequency deadregions with fe at 1.5 kHz or below, there isbenefit from amplification of frequencies up toabout 1.7fe, but no benefit of amplifying higherfrequencies. However, for subjects with deadholes, patchy high-frequency dead regions, orcontinuous dead regions with fe at 2 kHz orabove, amplification over the widest bandwidthpossible may be most beneficial.


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DIAGNOSIS OF DEAD REGIONS INCHILDRENMalicka et al51 assessed whether it is possible tomeasure PTCs reliably for school-age children.They used a fast method, similar but notidentical to the method of Sek and coworkers,as described earlier.31,32 Twelve normal-hear-ing children (7 to 10 years old) and five adultswere tested. Fast PTCs were measured for 1-and 4-kHz signals. The center frequency of thenarrowband noise masker either started belowthe signal frequency and was swept upward orstarted above the signal frequency and wasswept downward. Both upward and downwardsweeps were used for each subject and eachsignal frequency. The estimated tip frequencyof the PTC tends to be above the signalfrequency when the masker frequency sweepsupward and below the signal frequency when it

sweeps downward.31,51 Measurements were re-peated on a separate day to assess test-retestvariability.

All children were able to perform the task.Examples of some of the PTCs are shownin Fig. 1. Results are shown for upward fre-quency sweeps for signal frequencies of 1 kHz(top row) and 4 kHz (bottom row). The fastPTCs were smoothed by taking a two-pointmoving average of the masker levels at the turnpoints, and the smoothed PTCs were used toestimate the frequency at the tip. The frequencyat the tip was well defined for only 87% of thefast PTCs; for the other cases the PTCs had aflat region around the tip or had two or moretips. The variability in the estimated tip fre-quency was higher for the children than for theadults, but the difference was not statisticallysignificant. The absolute value of the difference

Figure 1 Examples of fast PTCs obtained from children with normal hearing. The age of the child is indicatedin each panel. The top and bottom rows show results for signal frequencies of 1 and 4 kHz, respectively. Thesignal frequency and level for each PTC are indicated by the position of the filled star. The thin lines are theraw fast PTCs, and the thicker lines are two-point moving averages. The tip frequencies estimated from themoving averages are shown in each panel. The data are from Malicka et al.51 Abbreviations: PTC,psychophysical tuning curve; SPL, sound pressure level.


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between the estimated tip frequencies for testand retest was about 14% at 1 kHz and 10% at4 kHz. When the masker frequency was sweptupward, the estimated tip frequency had a meanvalue that was shifted upward from the signalfrequency by about 11% at 1 kHz and 8% at4 kHz. When the masker frequency was sweptdownward, the estimated tip frequency had amean value that was shifted downward from thesignal frequency by about 8% at 1 kHz and 7%at 4 kHz.

There were only a few cases for childrenwhere the tip frequency was shifted down bymore than 20% or up bymore than 25%, relativeto the signal frequency. These values can there-fore be used as limits that can be expected formost children without dead regions. Shifts inthe tip frequency outside this range can be takenas indicating the presence of a dead region at thesignal frequency.

Malicka et al27 assessed the extent ofcorrespondence between results from theTEN(HL) test and fast PTCs. They alsoassessed whether the TEN(HL)-test criteriafor diagnosing a dead region should differ forchildren and adults. They tested eight normal-hearing children (16 ears) and 12 hearing-impaired children (21 ears), aged 7 to 13 years.For the normal-hearing children, maskedthresholds in the TEN(HL) were measuredfor five TEN(HL) levels (30, 40, 50, 60, and70 dB/ERBN). For the hearing-impaired chil-dren, the level of the TEN was selected sepa-

rately for each ear based on the highestacceptable level minus 5 dB.

The mean TEN(HL)-test results for thenormal-hearing children for TEN(HL) levelsof 40, 50, 60 and 70 dB/ERBN are shownin Fig. 2. The mean masked thresholds wereusually slightly below theTEN level/ERBN andwere never more than 5 dB above it. The resultsresemble those found for adults.2,33,35

All hearing-impaired children were able toperform the TEN test and fast PTCs. Examplesof PTCs for two hearing-impaired children are

Figure 2 Mean TEN(HL) test results for normal-hearing children for TEN(HL) levels of 40, 50, 60, and70 dB/ERBN. The data are from Malicka et al.27

Abbreviations: dB hearing level (HL); ERBN, equiva-lent rectangular bandwidth of the auditory filter, asdetermined using young, normally hearing listeners;TEN(HL), threshold equalizing noise with a spectrumchosen to produce equal masked thresholds in dBHL.

Figure 3 Examples of fast PTCs for two hearing-impaired children, one without a dead region at the signalfrequency (left) and one with a dead region at the signal frequency (right). The thin lines are the raw fastPTCs, and the thicker lines are two-point moving averages. Abbreviations: DR, dead region; PTC,psychophysical tuning curve; SPL, sound pressure level.


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shown in Fig. 3. For the case shown in the leftpanel, the tip frequency of the PTC (0.99 kHz)was close to the signal frequency (1 kHz)indicating that there was no dead region atthe signal frequency. For the case shown in theright panel, the tip frequency of the PTC (0.93kHz) was shifted well below the signal frequen-cy (1.5 kHz) indicating that there was a deadregion at the signal frequency.

Examples of the correspondence betweenthe PTCs and the TEN(HL)-test results areshown in Fig. 4. Eight ears showed no deadregion on the TEN(HL) test and no deadregion using PTCs. An example is shown inthe leftmost column of Fig. 4. The maskedthreshold in the TEN (open squares) wasalways less than 10 dB above the TEN level

of 60 dB/ERBN. The PTCs, determined usingsignal frequencies (filled symbols) of 1, 2, and4 kHz, all had tips (open symbols) close to thesignal frequency.

Nine ears showed a dead region on theTEN(HL) test that was confirmed using PTCs.An example is shown in the second columnof Fig. 4. The TEN-test criteria for a deadregion were met for signal frequencies of 0.5,0.75, and 1 kHz, as indicated by the area withdiagonal shading. For signal frequencies of1.5 kHz and above, the absolute thresholdwas too high for the TEN(HL) test to beperformed (as indicated by the cross-hatchedarea), but it seems very likely that a dead regionwas present. The PTC determined using asignal frequency of 0.25 kHz (middle panel)

Figure 4 Examples of results from hearing-impaired children who were tested using the TEN(HL) test andfast PTCs. In each column, the top panel shows results for the TEN(HL) test. Filled squares show absolutethresholds and open squares show masked thresholds in the TEN(HL). The TEN(HL) level is indicated in eachpanel. Diagonally shaded areas indicate frequency regions where the TEN(HL) test results suggested thepresence of a dead region. Cross-hatched areas indicate frequency regions where the absolute threshold wastoo high for the TEN(HL) test to be performed. The other panels show two-point moving averages of the fastPTCs. Solid symbols indicate the signal frequency, and open symbols indicate the estimated tip frequency ofthe PTC. The data are from Malicka et al.27 Abbreviations: ERBN, equivalent rectangular bandwidth of theauditory filter, as determined using young, normally hearing listeners; PTC, psychophysical tuning curves; SPL,sound pressure level; TEN(HL), threshold equalizing noise with a spectrum chosen to produce equal maskedthresholds in dB HL.


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did not show a clearly shifted tip, whereas thePTC determined using a signal frequency of0.5 kHz (bottom panel) showed a downward-shifted tip.

Three ears showed a dead region on theTEN(HL) test that was not confirmed usingPTCs; the PTCs did not show shifted tipswhen the signal frequency fell in a regionidentified as dead by the TEN(HL) test. Anexample is shown in the third column of Fig. 4.The findings for these three ears suggest thatthe TEN(HL) test can sometimes give false-positive results with children. This also some-times occurs with adults.2

For one ear, the TEN(HL)-test resultssuggested a restricted dead region (hole) around1 kHz, and probably a dead region above3 kHz. However, at 3 kHz the absolute thresh-old was so high that the TEN(HL) did not raisethe masked threshold above the absolutethreshold, so the result must be regarded asinconclusive. These results are shown in thefourth column of Fig. 4. For a PTC measuredwith a signal frequency of 1.5 kHz (bottompanel in the fourth column of Fig. 4), the PTCwas very “flat,” but the tip did appear to beshifted downward to about 0.7 kHz. This resultmight indicate that even the frequency regionbetween 1 and 3 kHz was dead. However,because the PTC is so flat, this result toomust be regarded as inconclusive.

Overall, Malicka et al27 concluded thatdead regions usually can be detected reliablyin children using the fast-PTC technique andthe TEN(HL) test. The most appropriatecriteria for diagnosing a dead region using theTEN(HL) test are those recommended foradults. For cases where the masked thresholdin the TEN(HL) is 10 to 15 dB above the TENlevel/ERBN, the diagnosis of a dead regionshould be confirmed via measurement of fastPTCs, because false positives can occur whenusing the TEN(HL) test alone.

BENEFIT OF AMPLIFICATION ATHIGH FREQUENCIES FORCHILDREN WITH DEAD REGIONSMalicka et al52 investigated the benefit of high-frequency amplification for children with vari-ous degrees of high-frequency hearing loss,

with and without dead regions. Dead regionswere diagnosed using both the TEN(HL) testand fast PTCs, as described by Malicka et al.27

The children, aged 8 to 13 years, were dividedinto two groups according to the severity oftheir hearing loss. Group MS had moderate tosevere loss (nine ears without dead regions andthree ears with dead regions). For this group,the dead regions, when present, were restrictedto the range 1.5 to 2 kHz. Group SP had severeto profound hearing loss (seven ears with high-frequency dead regions and one ear without adead region). For this group, the dead regions,when present, were extensive and continuous,with estimated values of fe between 0.75 and1.5 kHz. The stimuli were vowel-consonant-vowel nonsense syllables. They were subjectedto the frequency-gain characteristic prescribedby the Desired Sensation Level fitting meth-od53 for 65 dB SPL speech, and presented viaheadphones. The gains provided were similar tothose used in the children’s own hearing aids.However, the children were fitted with multi-channel compression hearing aids, whereas onlylinear amplification was used in the laboratorystudy.

The stimuli were either broadband or werelow-pass filtered with various cutoff frequen-cies. For ears with a dead region, the cutofffrequencies were selected to cover the rangefrom up to one octave below to well above theestimated value of fe. For ears without a deadregion, the cutoff frequencies were chosen to bein the range between 1 and 4 kHz, usually 1, 2,and 4 kHz and optionally 1.5 and 3 kHz. Thenumber of conditions varied between four andsix per ear.

The results for group MS are shownin Fig. 5. Consonant identification scores areplotted as a function of low-pass filter cutofffrequency. For all ears, the scores improved withincreasing cutoff frequency up to � 4 kHz andtended to flatten above that frequency. Therewas a significant difference between scores forcutoff frequencies of 1 and 2 kHz, and 2 and4 kHz, but there was no significant differencebetween scores for the cutoff frequency of4 kHz and the broadband condition. Thus,benefit was obtained for amplification of fre-quencies up to 4 kHz, but not for higherfrequencies.


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The results for group SP are shownin Fig. 6. For the one ear without any deadregion (7-L), performance was relatively poor,but it did improve with increasing cutoff fre-quency. Scores for the cutoff frequencies of 1, 2,and 4 kHz were 33, 39, and 42%, respectively.For the ears with dead regions, performancegenerally did not improve monotonically withincreasing cutoff frequency.Mean scores for theears with dead regions were 27, 31, and 30%, forthe cutoff frequencies of 1, 2, and 4 kHz,respectively. There was no significant effect ofcutoff frequency across these three values. Thusthe results provide no evidence for a benefit ofpresenting high-frequency speech components.For two ears (7-R and 11-L) performanceworsened somewhat when the cutoff frequencywas increased above 2 kHz.

To assess whether the changes in speechscore with cutoff frequency were related to the

values of fe, scores for speech low-pass filtered atfe were compared with scores for speech thatwas low-pass filtered at one octave and twooctaves above fe. The analysis was based on sixears (ear 8-L was excluded due to the largediscrepancy between the frequency at the tip ofthe PTC, and the value of fe estimated from theTEN(HL) test, leading to uncertainty as to thetrue fe). There was no significant effect of cutofffrequency. Thus, there was no benefit of ampli-fied frequency components above fe.

Summers54 has argued that the most ap-propriate amplification for subjects with a se-vere sloping hearing loss can be selected basedon the audiogram, and that a test for diagnosingdead regions is not required. He proposed,following common clinical practice, that am-plification should be provided only for frequen-cies where hearing thresholds were better than90 dB HL. However, this rule would not work

Figure 5 Consonant identification scores for individual subjects from group MS (group with moderate tosevere loss), plotted as a function of low-pass filter cutoff frequency. Error bars represent � one standarddeviation. The three upper rows show the results for the nine ears without dead regions, and the bottom rowshows results for the three ears with restricted dead regions. The edges of the dead regions are indicated bythe arrows close to the abscissa. The data are from Malicka et al.52 Abbreviation: DR, dead region.


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well for all of the children studied by Malickaet al.52 For example, for ear 7-R (Fig. 6),Summer’s rule would lead to no amplificationfor frequencies above 0.5 kHz, whereas 7-Ractually showed a benefit of amplifying fre-quencies up to 2 kHz.

The speech intelligibility index (SII)55 wasused to assess the effectiveness with which thechildren made use of audible speech compo-nents. The SII represents the proportion of thespeech spectrum that is audible for a givenspeech material (with a weighting functionrepresenting the relative importance of differ-ent frequencies). The SII is calculated from thespectrum of the speech signal and the subject’shearing thresholds. The SII has a value between

0 (none of the speech signal is audible) and 1(the speech signal is fully audible). The SIIincludes “corrections” to account for the self-masking of speech and distortions that canoccur at high listening levels. The SII valuescalculated by Malicka et al52 were based on theactual gains used for each ear.

To convert SII values to predicted speechscores, a transfer function relating scores to theSII needs to be estimated. The transfer functionwas derived by fitting a linear regression line tothe results for children with hearing im-pairment but without dead regions. This lineis shown in both panels of Fig. 7. The symbolsin Fig. 7 show the percentage of correct re-sponses for speech plotted against the

Figure 6 As Figure 4, but for group SP (with severe to profound loss). All ears except 7-L had extensivecontinuous dead regions with edge frequency at 1.5 kHz or below. The data are from Malicka et al.52

Abbreviation: DR, dead region.


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corresponding SII value. Each point representsone test condition and one ear. The left panelshows results for ears without dead regions(group MS—asterisks, group SP—open stars).The points are scattered roughly equally aboveand below the line, as expected. The scatterreflects individual differences in proficiency andrandom errors ofmeasurement. It is noteworthythat the deviations of obtained scores from theline are similar for group MS and for the singleparticipant in group SPwho did not have a deadregion.

The right panel of Fig. 7 shows the data forthe ears with dead regions. Most of the pointsfall below the line, indicating that obtainedspeech scores were lower than predicted fromthe SII. This was especially the case for groupSP, who had continuous rather than restricteddead regions. The deviations from the line aregreatest for higher values of the SII, whichcorrespond to cases where the cutoff frequencywas relatively high. The results indicate that thechildren with dead regions made less efficientuse of audible speech components than thechildren without dead regions.

CONCLUSIONSThe results obtained with children are broadlyconsistent with those obtained using adults.Children with moderate to severe hearingloss, without dead regions, or with restricteddead regions, showed a benefit from the provi-sion of amplification for frequencies up to4 kHz. However, there was no significantdifference in scores for the broadband conditionand the condition with low-pass filtering at4 kHz. For children with severe to profoundhearing loss, those with extensive continuousdead regions showed reduced benefit fromhigh-frequency amplification, and there waslittle benefit, or even a degradation, whenamplification was provided for frequenciesabove 2fe. The single participant in group SPwho did not have any dead region showed abenefit from providing amplification for fre-quencies up to 2 to 4 kHz.

The results may have important clinicalimplications for fitting hearing aids to childrenwith severe to profound hearing impairmentand extensive dead regions (fe � 1.5 kHz). Theresults suggest that it may be appropriate to

Figure 7 The symbols show percent correct scores for consonants plotted against the corresponding speechintelligibility index value. Each point represents one test condition and one ear. The left panel shows data forears without dead regions (group MS, with moderate to profound loss asterisks; group SP, with severe toprofound loss, open stars). The right panel shows data for ears with dead regions (group MS, open triangles;group SP, filled down-pointing triangles). The line in each panel is a linear regression line fitted to the data forall ears without dead regions. The data are from Malicka et al.52 Abbreviation: DR, dead region.


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provide amplification only for frequencies up toabout 2fe.

ACKNOWLEDGMENTS

The work of author B.M. was supported by theMedical Research Council (United Kingdom,grant G0701870).

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cochlear dead regions in adults and children: diagnosis ... · the dead region. this is referred to...

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