Pure-tonesensitivityof human infantsa)
Lynne Werner Olsho
Department
of Otolaryngology,
RL-30, Universityof
Washington,
Seattle,Washington
98195
Elizabeth G. Koch
UniversityofVirginia,Charlottesville,
Virginia22903
Elizabeth A. Carter
VirginiaCommonwealthUniversity,Richmond, Virginia23284
ChristopherF. Halpin and Nancy B. Spetner
University
of Virginia,Charlottesville,
Virginia22903
(Received7 October1987;acceptedfor publication29 June1988)
Pure-tonethresholdsat frequencies
rangingfrom 250 to 8000 Hz wereestimatedfor 3-, 6-, and
12-month-oldinfantsand for adults,usingthe Observer-based
Psycheacoustic
Procedure
(OPP). Soundswerepresented
monaurallyusingan earphone.Psychemetricfunctionsof
infantsweresimilarto thoseof adults,although3-month-oldshad shallowerfunctionsat
higherfrequencies.
The thresholdsof 6- and 12-month-oldinfantswere 10-15 dB higherthan
thoseof the adults,with the differencebeinggreaterat lower frequencies.This resultis in
generalagreementwith resultsfrom other laboratories.The thresholdsof 3-month-oldswere
15-30 dB higherthan thoseof adults.The greatestdifferencebetween3-month-oldsand adults
wasat 8000Hz. This thresholddifferenceis smallerthan that reportedin earlierbehavioral
studies;higherthresholdsat high frequencies
havebeenpreviouslyreportedfor newbornand
3-month-oldinfants.The relative contributionsof sensoryand nonsensoryvariablesto these
agedifferencesare discussed.
PACS numbers:43.66.Cb [NFV]
INTRODUCTION
One of the most fundamental
characterizations
of hear-
ing is the shapeof the audibilitycurve,the functionrelating
absolutesensitivityto acousticfrequency.It followsthat any
descriptionof auditorydevelopment
shouldaddressage-relatedchangesin the shapeof thiscurve.Severalstudieshave
Despitethe between-studyvariability in absolutevalue
of the threshold,severalof thesestudiessuggestthat, by 6
monthsof age, thresholdsare somewhatcloseto thoseof
adultsat higherfrequencies
(Trehub e! al., 1980;Schneider
et al., 1980;Sinnottet al., 1983;Nozza and Wilson, 1984).
Tomakethistrendmoreevident,
inFig.1,infantthresholds
described this curve for human infants older than 6 months
are plotted in decibelsre.'thresholdsof adults in the same
of age.The shapeof the audibility curve hasnot beenestablishedfor youngerinfants.
study. The results of Hoversten and Moncur (1969), the
For infants older than 6 months, the absolute value of
the reported infant thresholdvaries considerablyacross
studies.The averagethresholdsreportedare summarizedin
Table I. All but one of these studies (Hoversten and Moncur, 1969) usedsomevariation of Visual Reinforcement Au-
diometry (VRA) (Moore et al., 1975;Moore and Wilson,
1978), an operant discrimination paradigm, to obtain
thresholds. However, studies differ in the stimulus em-
ployed,the modeof stimuluspresentation(earphoneversus
loudspeaker), psychophysicalparadigm (two-alternative,
forced-choiceversusgo/no-go), and definition of threshold.
Some of these differences are also noted in Table I. While
thesefactorsundoubtedlycontributeto the variability between studies, there is no obvious relationship between
methodologicalvariablesand the thresholdsobtained.
A preliminary
reportof thesedatawasmadeat theFall 1986Meetingof
the AcousticalSocietyof Americain Anaheim,CA [J. Acoust.See.Am.
Suppl. I 80, S123 (1986) ].
1316
d. Acoust.Soc. Am. 84 (4), October 1988
only studynot usinga conditioningprocedure,clearlystand
out from the others.At the sametime, the rangeof thresholdsexpressedre: adultsis generallysmallerthan the range
of thresholdsexpressedin dB SPL. The agreementamong
someof the studiesin the shapeof the curvefor olderinfants
can also be seen.
There are reasonsto believethat the audibilitycurveof
youngerinfantswill differin shapefrom that of olderinfants
and adults.First, sincethe shapeof the audibilitycurvein
adultsis largely determinedby the characteristicsof the external and middle ears, and since at least some of these characteristieshave beenshown to changepostnatally in humans
(McLellan and Webb, 1957; Saunders et al., 1983), one
might expectto seepostnatalchangesin sensitivityon that
basis alone. Second, in other mammals, behavioral and
physiologicalthresholdstend to maturefirst at low to middle frequencies(reviewedby Rubel, 1978). By one estimate
(Javel et al., 1986), the auditory systemof a 3-month-old
human infant would be similar in maturity to the auditory
systemof a 6-week-oldcat. There are no behavioraldata for
catsbetween1 month of ageand adulthood,so it is difficult
to estimate when kitten thresholds
0001-4966/88/101316-09500.80
reach adult levels. How-
© 1988 AcousticalSocietyof America
1316
TABLEI. Summary
ofstudies
ofinfantabsolute
sensiti•
ity,6 to24months
postnatal
age.(VRA = visual
reinforcement
audiomerry,
infantreinforced
for
headturnby presentation
of mechanical
toy;2AFC = two-alternative,
forced-choice.
)
Mean threshold. dB S?L
re' adults) (,]andard dewarian)
Frequency(Ht)
Age
Study
Stimuli
Hoverslen and
30-s Iones.
Mencur119691
soundfield
3/
22
(roes.)
8
Procedure
250
500
1000
2000
4000
8000
10000 19000
Behavioral
observation
two o•ervers.
ascendingmethod
45
50
(33)
a
(58)
a
of hmlls
Trehub et al.
Continuou•
980)
89'
6
oclaveband
VRA
38'
33•
25
28
23
2AFC
b
24}
( 22)
b
( ]9}
a
a
a
a
a
noIs•. sound
18
a
field
74'
12
25'
I0
16
17
b
1161
1161
b
11tl
a
a
a
20
b
14
110)
19
a
a
a
a
76•
18
31d
b
a
Schneider eta/.
Conbnuoua
( 19801
octave-band
6
IO
27d
a
a
24'
1151
a
18
1151
a
1o)
VRA
24
41
22
38
2AFC
noise. sound
field
12.18
•41
a
24
Moore and
Wilson 119781
2•. 5%
warbled Iones
Earphone
Bergand
9
6-7
12-13
38
25
23
b
b
b
31
b
27
b
25
b
IR
18
16
9
6•7
b
b
b
I0
12-13
21
b
16
b
14
b
25CLms
Iones
23
VRA,
Srmlh 11983)
12
20
(11
(5.8)
go no-go
Earphone
6
6
I0
12
I0
(15)
(5.6)
20
12
19
14
7 lI
VRA,
go/no-go
Aslin (19831
O.5-•
tones.
round field
2-7/
frequency
38
(221
18.53}
30
1211
111.81
a
a
d. Acoust_Sec. Am,, Vol. 84. No. 4. October 1988
21
111)
{5.01
21
111)
18
21
112)
(7 I)
31
(23)
(10.21
36
(24}
19.01
(ll)
(4 5}
34
(27)
112.31
34
•5
37
31
25
123)
1261
(23)
123)
1181
14.08)
1317
(9)
(5.3)
20
161
(6.81
I 1-16/
frequency
23
1141
(5 61
19
6
17
(7)
(4.31
112}
(5.4)
(6)
(4 4)
I-s Iones,
sound field
b
115
14 I)
18
(7)
13.91
Sinnoll,
Pisoni.and
31
a
29
110
(4 8)
27
Sound field
19
VRA.
go/no-go
I0
Sound field
(5)
a
(7.5l)
(10.b}
(7.79)
10_971
Olshoet at: Pure-tonesensitivityof humaninfants
1317
TABLE I. (Continued.)
Mean threshold: dB SPL
(dB re: adults) (standard deviation)
Frequency(Hz)
Age
Study
Stimuli
Nozza and
Wilson
(1984)
N
5•-ms
tones,
earphone
(mos.)
screened
c
11, IK
12, 4K
6
Procedure
250
500
1000
2000
21
VRA,
go/no-go
17
12
23
20
(16)
(11)
18
19000
(10.00)
14
(11)
(3.62)
17 unscfeened
c
10000
(7)
(4.20)
(7.30)
12 screenedc
8000
16
(14)
(5.72)
unscreened
r
4000
22
(5)
(6.10)
15
(15)
(9.40)
(6)
(6.75)
No variabilityestimateavailable;thresholds
calculatedfrom grouppsychometric
functions.
No adults tested.
Eachsubjectcontributed(one) trial at eachof four levelsat eachof fivefrequencies.
Actualfrequency
200Hz.
Actual frequency400 Hz.
rTympanometric
screening,
criterionforinclusion
whenscreened
= pressure
peakgreaterthan -- 100mmH,O.
6 - 8 Month Old
ever,Ehret and Romand ( 1981) compareddetectionthreshS•-•ou
ndfield
•
Hoverslen & Mollcur, 1969
s
5O
Trehub el el., 1980
•
Schneider el eL, 1980
•
$innol! et el., 1983 (1-s)
ß
Sinnoirel el., 1983 (.5-s)
Earohone
•
• 2o
•.
1 ooo
Berg& Smith,1983
ß
Nozza & Wilson, 1984
•-
Currenl
10000
Frequency(Hz)
olds of 1-month-old
kittens to adult cats and found a differ-
encein sensitivityon theorderof 50 dB above10 000 Hz, but
only 40 dB at 1000Hz and 30 dB at 500 Hz. Thus, over the
agerangein cats(6-9 weeks)that supposedly
corresponds
to theperiodbetween3 and6 monthsin humans,changesin
high-frequencysensitivityare occurring.Third, the results
of the few studiesof younginfants'responsiveness
to sound
(Hutt et aL, 1968;Weir, 1976;Eiseleet al., 1975;Hoversten
and Moncur, 1969) suggest
that, prior to 3 months,infants
are relativelyinsensitiveto high-frequencysounds.
Only Hoversten and Moncur (1969) have reported
thresholds for 3-month-olds: 55 dB SPL at 500 Hz (43 dB re:
averagethresholdof youngadultstestedin thesamestudy)
10 - 12 Month Old
and 65 dB SPL at 4000 Hz (73 dB re: adult threshold).
60'
Sound
field
50-
•
Schrleider el el., 1980
Berg& Smilh,1983
Earohone
•
olds also increasedbetween 500 and 4000 Hz (see Table I
i1
Trehub
et
el.,
1980
ß
40-
However, for 8-month-oldstestedin the samestudy,thresh-
Berg& Smith,lg83
ß
Nozza & Wilson, 1984
andFig. 1). Because
thisistheopposite
ofwhatismostoften
reportedfor 6- to 18-month-olds,
Hoverstenand Moncur's
data do not providestrongevidencefor insensitivityto high
frequenciesat 3 months.Thus neithergeneralsensitivitynor
I ooo
1 oooo
Frequency (Hz)
FIG. 1.Averageinfant-adultthresholddifference
asa functionof frequency for severalstudiesandtwoinfantagegroups.Studiesareincludedif the
infantstestedwere older than 6 monthsof age,and if adult subjectswere
testedas part of the study.Within a study,onlyfrequencies
whereboth
infants and adults were tested are included.
1318
J. Acoust.Soc.Am.,Vol.84, No.4, October1988
the shapeof the audibilitycurve has beenestablishedfor
infantsyoungerthan 6 monthsof age.
The purposeof the currentarticleisto examinechanges
in theshapeof theaudibilitycurveof humaninfantsbetween
3 and 12 monthsof age.Thresholdswere estimatedover a
broader frequencyrange than in Hoversten and Moncur's
(1969) studyof 3-month-oldsand employeda conditioning
procedure.Unlike earlierstudies,the sameprocedure(Olsho et al., 1987b) was usedto obtain thresholdsfor infants at
all agestested.
Olshoet aL:Pure-tone
sensitivity
of humaninfants
1318
I. METHOD
nosedashavinghearingloss;(4) freeof colds;( 5) nooccur-
renceof middleearinfection
within3 weekspriorto testing,
andno morethantwo prior occurrences
of ear infections;
Thedatadescribed
werecollected
aspartof foursepaand (6) nofamily'history
of congenital
hearingloss.
rate studies:( 1) a preliminarystudyof.absolutethresholds
Sixadultswereincluded,aged18to 26. Eachadultlisandfrequency
discrimination
(Olsho,1984);(2) a longitutenedat each frequencyfrom 250 to 8000 Hz. None had
dinal study of frequencydiscrimination(Olsho et al.,
otherexperience
listeningin psycheacoustic
experiments.
1987b);(3) a cross-sectional
studyof frequency
discriminaNone reportedhearingproblems,and thresholdsweresimition (Olshoetal., 1987b);and (4) a cross-sectional
studyof
lar to thoseobtained
in ourlaboratory
in otherstudies
using
absolute
thresholds
at certainfrequencies
undertaken
forthe
a similarprocedurewith adults.
purposesof the presentarticle.The numbersof exclusions
fromeachofthefirstthreestudies
aregivenin therespective
articles.In the fourth group,five otherbabieswere tested
A. Subjects
who did not provide thresholds,either becausethe false
alarm rate wastoo high or becausethe psychemetricfunction was flat. Data from four other infants were lost due to
B. Stimuli and apparatus
The stimuliwerepuretonesgenerated
usinga Wavetek
(model171) oscillator.Toneburstsrangingin frequency
equipmentfailures.The total numberof infantsincluded
from eachstudyat eachageandfrequencyisshownin Table
from 250 to 8000 Hz, 500-ms in duration with 500 ms be-
II.
were 10 ms. Stimuli werepresentedin trainsof 10 bursts/
trial. The toneswere switchedby a Coulbourn(S84-04)
rise-fallgateand attenuatedby Coulbournprogrammable
Subjects
in thepreliminaryandfrequency
discriminationstudies
wereselected
andrecruitedasdescribed
by O1she (1984) and Olshoet al. (1987b), respectively.
In all
groups,eachinfantwastestedwithin2 weeksof therequired
age.In addition,all infantsubjects
metthefollowingcriteria
for inclusion,as reportedby their parents:(1) full-term
birth•with no complications
of deliveryor perinatalcourse;
(2) normalpostnataldevelopmental
course;(3) neverdiag-
TABLE II. Breakdown
of subjects
by age,frequency,
design,
andstudy.
Frequency
Design
Study
(Hz)
CS•
Lb
'84c
•87d
Currente
Total
250
500
1000
2000
4000
8000
10
5
17
17
5
10
0
10
11
0
10
0
6
0
6
0
5
0
0
15
22
0
10
0
4
0
0
17
0
10
10
15
28
17
15
10
3 mos.
tween bursts, were used. The rise and fall times of each burst
(S85-08) and manual (S85-02) attenuators.These devices
werecontrolledby an Apple II Plusmicrocomputer
using
locallydevelopedsoftware.The computeralsocontrolled
stimulusandtrial timingandrecordedobserver
judgments.
Two differenttypesof earphoneswere usedto deliver
soundsto the right ear. The subjectsin the preliminary
group listenedover TDH-49 headphonesin MX-41/AR
cushions,
whichwereheldin placebytwo elasticbandswith
velcro closures(after Moore and Wilson, 1978). The re-
mainingsubjects
(includingtheadults)weretestedusinga
ToshibaRM-3 or SonyMDR-E242 "WalkmanTM"-style
earphoneheld in the infant'sear with microporetape.The
response
characteristics
of theseearphones
aredescribed
by
Olshoetal. ( 1987a,b). Briefly,theresponse
isrelativelyflat
(within4 dB), from250to4000Hz, rollingoffbyabout8 dB
at 8000Hz. For all of thefrequencies
in thisstudy,theamplitude of the second harmonic was at least 45 dB below that of
thefundamental.
Bothearphones
werecalibratedusinga 6-
cm3coupler
(Bruel& Kjaer4152)witha 1-in.microphone
250
500
1000
2000
4000
8000
10
5
8
10
6
10
0
10
9
0
10
0
6
0
6
0
6
0
0
15
11
0
16
0
4
0
0
10
0
10
10
15
17
10
22
10
(type 4144) and a Bruel & Kjaer (2215) soundlevelmeter,
usingoctave-bandfilters,linear scale.The Walkman*Mstylephonesjust fit in the centeropeningof the MX-41/AR
cushion;the MX-41/AR
cushionwas usedto hold the small
earphonein placeduringcalibrations.No differences
in the
infantthresholds
obtained
withthesetwotypesofearphones
were noted. However, the number of infants ( 18 total) lis-
teningwith the TDH-49 earphonewas rather small. Am-
12 mos.
250
500
1000
2000
4000
8000
10•
3
3
11
4
11
0
7
7
0
7
0
6
0
0
0
10
10
4
0
0
10
10
10
0
0
0
0
11
0
11
0
11
11
ll
11
•CS = crosssectional.
Infant participated
at oneageonly.
bL = longitudinal.
Infantparticipated
at twoormoreages.
'84 = Olsho (1984). Infants were alsotestedin frequencydiscrimination.
'87= Olshoetal. (1987b).Infantswerealsotested
infrequency
discrimination.
"Current"refersto infantstestedat a singleagefor the purposes
of this
study.
1319
J. Acoust.
Sec.Am.,Vol.84, No.4, October1988
bient noiselevels (octave band) in the test room were mea-
suredusinga 6-cm3 coupler(Bruel& Kjaer4152) with a
1-in.microphone(type4144) and a Bruel& Kjaer (2215)
soundlevel meter, usingoctave-bandfilters, linear scale,
with the earphonein placeon the couplerat the baby'sapproximate location in the test booth. The levelswere 17 dB
SPL at 250 Hz, 11 dB SPL at 500 Hz, 10 dB SPL at 1000 Hz,
13.5 dB SPL at 2000 Hz, 15.5 dB SPL at 4000 Hz, and 15 dB
SPL at 8000 Hz.
Testingwasconductedinsidea single-walled,soundattenuatingroom(IAC). The infantsatona parent'slapin the
room, facinga window into the control booth and a video
Olshoeta/.: Pure-tone
sensitivity
of humaninfants
1319
camera.There wasa tablein front of the parentand infant.
trialscorrectandfourof thelastfiveno-signaltrialscorrect,
An assistant sat at the table to the infant's
the training phaseended.
During testing,the visualreinforcerwasactivatedonly
whenthe observerjudgedthat a soundhad beenpresented
whena signaltrial liad actuallyoccurred.Stimulusintensity
was variedduring testingto estimatethreshold,following
left. The "visual
reinforcer," a mechanicaltoy enclosedwith lights in a
smokedPlexiglasbox, waspositionedtb the infant'sright.
The mechanicaltoy couldnot be seenuntil the lightsinside
the box were turnedon undercomputercontrol.
PEST rules (Taylor and Creelman, 1967). The PEST rules
essentially
generatea binarysearch,attemptingon eachreC. Procedure
Infants were testedusingthe Observer-based
PsychoaeousticProcedure(OPP). Full detailsof the procedureare
describedby Olshoet aL ( 1987a);additionalinformationis
givenin Olshoet al. (1987b).
With the infant seatedon the parent'slap, the earphone
wasplacedon theinfant'srightear.The parentandanassistant whosatin the testroomworeheadsets
to preventthem
from hearingthe soundspresentedto the infant.The assistant manipulatedtoyson the tableto directthe infant'sgaze
at midline,but triednot to gettheinfantsoinvolvedwith the
toyson thetablethat theinfantignoredtheauditorystimuli.
Neither adult knewwhena trial wasin progress.
The observerwatchedthe infant through the window
and on the video monitor. When the infant was quiet and
attendingto the toyson the table,the observerbegana trial.
A flashingLED indicatedto the observerthat a trial wasin
progress.A train of toneburstswaspresentedto the infant
on a giventrial with a probabilityof 0.65. The observerdid
not knowwhethera stimuluswasbeingpresented
duringthe
trial. The observerwatchedthe infant during the trial and
made a judgment as to whether or not a soundhad been
presented.
The observerreceivedfeedbackat the conclusion
of each trial.
A testrun typicallylasted20 min.The session
consisted
of two phases,training and testing.During training, the
stimulusintensitywasbetween60 and 70 dB, dependingon
the frequency.If the observerjudged that a soundhad occurredon a signaltrial, thevisualreinforcerwasactivatedas
soonasthe observermadethejudgmentandcontinuedfor 4
s after the end of the stimuluspresentation.If the observer
misseda signaltrial, the reinforcerwas activatedfor 4 s at
the conclusionof the stimulustrain. If the observerjudged
that a soundhadoccurredona no-signaltrial, anerror (false
alarm) wasscored.If the observerjudgedthat no stimulus
hadoccurredona no-signaltrial, thiswasscoredasa correct
rejection,but in no casewasthe reinforceractivatedfollowing a no-signaltrial. Thus the observerwastrainedto usethe
infant's behaviorin anticipationof the onsetof the visual
reinforceras thebasisof thejudgment.The infant wasreinforced for respondingin sucha way that the observercould
makecorrectjudgments,sincethevisualreinforeerwasactivatedsoonerandfor a longerdurationwhenthe infantmade
an observable
anticipatoryresponse
on a signaltrial. Typical
behaviorsusedwereheadturns;changes
in facialexpression,
especiallyeyewidening;increasesor decreases
in activity;or
breaksin eyecontactwith the toysbeingmanipulatedon the
table. Head turns were more frequentlyobservedin 6- and
12-month-olds than in 3-month-olds, but other behaviors
were usedfor someinfantsin eachagegroup.Oncethe observerhad reacheda criterionof four of the last five signal
1320
J. Acoust.Soc. Am., Vol. 84, No. 4, October 1988
versal to halve the distance between the current
level and
levelof thelastreversal.The onlyexception
thatwe madeto
theseruleswasthat stimulusintensitystayedat a givenlevel
for at leastfour trialsbeforea decisionto changethe level
was considered.Thus, if the observerwas correct on three or
four trials at a givenlevel, the level went down on the next
trial by anamountspecified
by the PESTrules.If theobserv-
er wascorrectonfewerthanthreetrials,thelevelwentupon
the nexttrial. This aspectof the proceduremakesit more
conservativeand more resistantto brief lapsesof attention
on the part of the infant or the observer,sincea few missed
trials due to inattentiveness
would lead to one or two in-
creases in level rather than several increases. The observer
wasrequiredto maintaina falsealarmratebelow0.25during
testing.Testingwas continueduntil either 50 signaltrials
had beencompleted,or the observer'sfalsealarm rate exceeded0.25, or the infant'sstateprecludedfurther testing.
An exampleof an infant testprotocolis shownin Fig. 2.
Thresholdswereestimatedasthe70.7% "yes"pointon
thebest-fitting
psychometric
functionfor eachinfant,using
probit analysis(Finney, 1970) and maximumlikelihoodcriterion (Hall, 1968, 1981). The threshold from a run was
usedonly if ( 1) at least30 signaltrials had beenobtained;
(2) the falsealarm rate for the run was0.25 or below;and
(3) theslope
ofthe6est-fitting
psychometric
function
was
greaterthan zero. For about25% of the sessions,
thresholds
werenot obtained.Exceptfor the infantsnotedasexclusions
above,thresholds
weresubsequently
obtainedat a latervisit.
The entireprocedure(trainingandtesting)wasrepeatedon
all visits.Comparisonof thresholdsobtainedin first versus
subsequent
visitsrevealedno consistentdifferences.
Adult thresholdswereobtainedusingthe samegeneral
procedureexcept that the adults were instructedto raise
their handswhen they heard a train of tone bursts.An observerin the controlroomrecordeda "yes"response
when-
Observer Response
.9
40- :l]
•
60-
•
80
'Yes''No'
Tro'ning Signal Trial
Signal Tr'ol
No-Signal Tr'ol
III •ll
m
'o
1 oo
120
No s'gnol
<•03
O0
O0
0
03
01
01
0
ClO
ß
030
I0
(300
' 10I • •)10
' 30I ' 40[ I 50I ' 60I ' 70•
Trials
FIG. 2. Exampleof infantadaptivetrialIby-trialprotocol.The subjectwasa
&month-old testedat 500 Hz. Testingbeganon trial 18.
Olsho ot a/.: Pure-tonesensitivityof human infants
1320
ever the adult did so. The reinforcer was activated as feed-
back, with the same contingenciesas describedfor the
infants.Order of testingat the six frequencies
was randomized for eachsubject.
II. RESULTS
A. Psychemetric functions
We examinedage-groupaveragepsychometricfunctionsat eachfrequency(Fig. 3). The purposeof thisanalysis
wasto characterizeperformance
in thisprocedure;
notethat
thresholdswerecalculatedfrom individualbest-fittingfunctions,not from the age-groupaverages.The age-groupaver-
agefunctionswere obtainedby calculatingthe percentof
"yes"responses
on all signaltrialswithina 10-dBrangeof
thegivenstimulusintensityfor all subjects
at eachfrequency
and age.The functionsexhibitthree trends:
(1) Infants' functions,particularly thoseof 3-montholds,tend to be somewhatshallowerthan adults' functions.
Wherethereare differences
amonginfantagegroups,name-
ly, at higherfrequencies,
the 6- and 12-month-olds
tendto
havesimilarslopes,whichare steeperthan thoseof the 3month-olds.
(2) Infant functions often do not reach 100% "yes"
withinthe rangeof intensities
employed.In mostcases,the
slopeof theinfantage-average
psychometric
functionin the
regionof 50-60 dB isstill relativelysteep,implyingthat the
500
functionmightreach100% at still higherintensities.
However,it is possible
that the infantpsychometric
functions
actuallyasymptoteat a performancelevel lessthan 100%
"yes."
(3) In mostconditions,adult performanceisquitesimi-
lar to infantperformance
at low stimulusintensities.
As the
intensityof thestimulusisincreased,
however,adultperformance increasesat a faster rate than does infant performance.Ihe meaningof thisdifferenceis not clear.
We also examinedthe averageslopeof the individual
best-fittingpsychometric
functionsfor each age and frequencybearingin mind that the slopeestimateswerebased
on no more than 50 trials. Psychometricfunctionslopesincreasedprogressively
with age:The meanslopeandstandard
error for 3-month-oldswas 0.07 q- 0.14 z units/riB, for 6month-olds,0.13 +0.17 z units/dB, for 12-month-olds,
0.30 q- 0.02 z units/dB, and for adults0.38 q- 0.01 z units/'
dB. Them did not appearto be any consistentpatternof
changewith frequencyfor any of the agegroups.
We feelthatthepsychometric
functionslopes
of infants
are, in fact. somewhatshallowerthan thoseof older individ-
ualsbecausethis patternemergesin both the age-average
groupfunctions
andin the individualbest-fitting
functions.
Whetherthe agedifference
in slopeis frequency
specificis
not clearat this point. However,in general,the infant psychometricfunctionsare qualitativelysimilar to thoseof
adults.
1000
Hz
1.0
1.0
.g
.9
Hz
/
,.o.,_,..,,,.,
.3too
.8
.8
ß 12mo
.7
.7
ß Adult
.6
.6
.5
.5
.4
.4
.3
.3
.2
.1
!
.2
.2
.I
.1
0
0
ns 102030
2000
ns 1020
40 50 60
30 4050
4000
Hz
ns 10 20 30 40 50 60
60
8000
Hz
1.0
1.0
.9
.9
.8
.8
.8
.7
.7
.7
.6
.6
.6
.5
.5
.5
.4
.4
.4
.3
.3
.3
.2
.2
.2
.1
.1
.1
I
0
0
0
Hz
ns 10 20 30 40 50 60
ns 10 20 30 40 50 60
dB SPL
dB SPL
dB SPL
FIG.3.Average
psychometric
functions
for3-,6-,and12-month-old
infants
andforadults
atsixfrequencies.
Eachpointrepresented
combines
alltrials
withina 10-dBrangearoundthelevelindicated.
1321
J. Acoust.Soc Am., Vol. 84, No. 4, October 1988
elshe •t,tL: Pure-tonesons•bwtyof humaninfants
1321
B. Thresholds
The effectsof thisvariabilityare apparentwhenthe 12month-old versusadult comparisonsare considered.Not
only were the age IF(1,88) = 207.04,p <0.0001 ] and fre-
Averagethresholds,
estimatedfrom thebest-fittingpsychometricfunction for each subject,for each age and frequency [F(5,88) = 99.96, p<0.0001] effectssignificant,
quency,are shownin Fig. 4. For frequencies
above250 Hz,
but the interactionwassignificantaswell [F(5,88) = 2.65,
therewasa decrease
in averagethresholdsbetween3 and 6
p<0.05].
Given that the average thresholdshave not
months,whichwasmorepronounced
at higherfrequencies.
changed
between
6 and 12 months,this differencemust reBetween 12 months and adulthood, there was a further desult
from
the
decrease
in variability between 6 and 12
creasein threshold,but the changewasgreaterat lowerfrebetweenthe 12-month-olds
quencies.The averagethresholdsof 6- and 12-month-olds months.Pairwisecomparisons
and
adults
at
each
frequency
revealed
that all differences
were essentiallyidenticalat all frequencies.
were
significant
(p
<
0.05
in
all
cases).
However,
asthe sigComparisonof each age group to the adults revealed
nificantinteractionimplies,theagedifferencewassmallerat
that the difference between the 6-month-olds and the adults
wassmallerat higherfrequencies,
in agreementwith other
studiesof thisagegroup(Trehubet al., 1980;Sinnottet al.,
1983). However, this result did not hold for 3-month-olds:
The largestdifferencebetween3-month-oldsand adultsoccurred at 8000 Hz.
We testedthe significance
of thesetrendsusinganalysis
of variance,separatelycomparingeachgroupof infantsto
the adults.Becauseeachadult wastestedat all frequencies
while eachinfant wastestedat only onefrequency,the error
termsfor testingeffectsinvolvingfrequencywerecalculated
separatelyfor the two agegroupsand then pooled(Winer,
1971,pp. 371-378). For the 3-month-oldversusadult comparison,all the effectsin the ageX frequencyanalysiswere
significant:
age [F(1,119) = 459.03,p < 0.0001], frequency
[F(5,119) =48.73, p<0.0001], and ageX frequency
[F(5,119) = 2.8I,œ <:0.05]. Pairwisecomparisons
between
3-month-oldand adult meansindicatedsignificantdifferencesat eachfrequency,p < 0.01 for all comparisons.
However,thesignificantinteractionsupportsthe claimthat the 3month-olds'audibility curve doesnot parallel that of the
higherfrequencies.
To summarize,the averagethresholdsof 3-month-olds
arehigherthanthoseof adults,by 15-20dB between250and
4000 Hz, and by about 30 dB at 8000 Hz. By 6 months,
sensitivityat high frequencies
improves;at 250 Hz, thresholds are still elevatedby about 15 dB, but, at 8000 Hz, the
difference
between infants and adults is less than
10 dB.
Thusthe periodbetween3 and 6 monthsis markedby a 20dB improvementin sensitivityat 8000 Hz. Between3 and 6
months, the amount of thresholdimprovementincreases
regularlywith increasingfrequencyacrossthe frequency
range. Essentiallyno change occurs between3 and 12
monthsat 250 Hz. The only notablechangebetween6 and
12monthsisa decrease
in variability.The low-frequency
age
difference
is resolved between
12 months and adulthood.
Studiesof absolutethresholds
amongolderchildrensuggest
that low-frequencythresholdsmay not reach adult levels
until schoolageor later (e.g., Schneideret al., 1986;Elliott
and Katz, 1980).
adults.
For the 6-month-olds,the age [F(1,109)=52.62,
p • 0.0001] and frequency[ F( 5,i 09) = 29.36,p • 0.0001]
effectsweresignificant.However,the ageX frequencyinteraction was not significant[F(5,109) = 1.07,p > 0.25]. As
Fig. 4 shows,this failure to find a significantinteractionbetweenageand frequencyin this comparisonmay well have
beendue to the great variability betweeninfantsin some
conditions.In fact, pairwisecomparisons
did showthat the
6-month-olds differed from the adults at 250, 1000, and 2000
Hz (p <0.02 in eachcase),but not at 500, 4000, or 8000 Hz
(p > 0.2 in eachcase).
III. DISCUSSION
Two majorconclusions
canbedrawn from theseresults.
First, pure-tonethresholdsimprovebetween3 monthsand
adulthood.Second,the timing of this improvementis frequencydependent:Between3 and 6 months,the improvement is greaterat higher frequencies;improvementat low
frequencies
doesnot occuruntil sometime after 12months.
In general,the resultsfor 6- and 12-month-oldsare in
goodagreementwith thoseof previousstudies.This is evident in Fig. 1, wherethe resultsof this studyare plottedin
decibelsre: adult thresholds,along with the resultsof the
earlier studies. The infant-adult
•,
60
•
4o
A decreasein variability was also noted here between 6
=o 30
i-
differences observed here
are of the samemagnitudeas thosepreviouslyreportedin
conditioningstudiesusingearphones
andshowa similartendencyto decreaseat higherfrequencies.
and 12monthsof age,especiallyat 4000 and 8000 Hz. Nozza
and Wilson (1984) alsoreporteda decreasein variabilityat
4000 Hz between6 and 12 months,but only for infantswho
were not screenedby tympanometry on the test date (see
Table I). Thusat leastsomeof thevariabilityin performance
we see, particularly at high frequenciesat 6 months, may
I._•
3-month-olds
6-monlh-olds
12-menlh-olds•
Adults
lO
o
-10
•.•o
foo
tooo
:zooo
400o
•ooo
Frequency(Hz}
stem from variation in middle ear function.
FIG. 4. Meanthresholds
( _+1s.d.)asa functionoffrequency
for3-, 6-,and
This studyrepresents
the firstreportof absolutethresholds for 3-month-oldsover a broad frequencyrange. The
12-month-old
current results differ from those of Hoversten and Mortcur
322
infants and for adults.
J. Acoust. Soc. Am., Vol. 84. No. 4, October 1988
Olsho et al.: Pure-tone sensitivityof human infants
1322
(1969) in tworespects.
First,thethresholds
reportedhere
criminationof low frequencies,and that this effect could
are much lower. Three-month-olds achieved thresholds of
accountfor partof thedifference
betweeninfantsandadults
in frequencydiscrimination.
We would argueon severalgroundsthat thereis little
15-30 dB relativeto thoseof adultshere; Hoverstenand
Moncur (1969) reportinfant-adultdifferences
of 40-70 dB.
The useof visualreinforcement
in OPP undoubtedlyaccounts for some of this difference. Similar threshold im-
difference between 6-month-olds and adults in absolute sen-
nounced threshold elevation at 4000 Hz for 8-month-olds
sitivity.First, the sizeof the averagedifferencein thresholds
is not large,andsome6- and 12-month-olds
havethresholds
that fall within the adult rangeat eachfrequency.Second,
thepsychometric
functionsof infantsat theseagesarequite
similar to thoseof adults tested under similar conditions,
suggesting
that the functionreflectsa similarunderlying
processat eachage.Third, 6- to 12-monthoold
infantsdiffer
mostfrom adultsin the frequencyrangewherepracticeappearsto havethe greatesteffecton adult thresholds(Watson
et al., 1972;Zwislockiet al., 1958).Thusagedifferences
at
low frequencies
are likely to reflectpoorerinfant performanceunderapparentlymoredifficultlisteningconditions.
Sucharguments,however,do not accountfor the differ-
and that no otherlaboratoryhasreportedsucha resultfor
encesbetween 3-month-olds and older listeners.First, the
older infants.
agedifference
isconsiderably
largerin thiscase,andwerarely see3-month-olds
with thresholds
in theadultrange.Second, althoughindividualpsychometric
functionslopesare
quitevariable,theage-groupaveragepsychometric
function
of 3-month-oldsalsoappearsto growshallower,relativeto
olderlisteners,at higherfrequencies.
This suggests,
at least
to us,an insensitivesystem.While nonauditoryfactors,such
as lapsesof attention,mightalsoproducea flatterpsycho-
provementsare seenamong older infants when reinforcement proceduresare comparedto nonreinforcedbehavioral
observations(Moore et al., 1975). Second, the difference
between3-month-olds
and adultsis not particularlypronounced at 4000 Hz: Three-month-olds'
thresholds at 500
Hz averageabout23 dB higherthan thoseof adults,whileat
4000 Hz they averageabout 19 dB higher.Hoverstenand
Moncur (1969) report a 43-dB infant-adult differenceat
500 Hz and a 73-dB differenceat 4000 Hz. The reasonfor the
discrepancy
betweenthe studiesis not clear,but it shouldbe
notedthat Hoverstenand Moncuralsoreporteda pro-
The greatest difference between 3-month-olds and
adultsin the currentstudy (almost30 dB) occurredat 8000
Hz. It couldbe arguedon that basisthat humaninfantsare
similarto othermammalsin demonstrating
anearlyinsensitivity to high-frequency
sound.However,a simple"low frequencies
first" rule doesnot describethe patternreported
hereverywell.Despitetheearlyinsensitivity
at 8000Hz, by
6 monthsbehavioral
thresholds
at higherfrequencies
approachthoseof adults.Thusit is difficultto describeany
generaldevelopmental
sequence
for mammalianauditory
behavior.
Sucha description
awaitsa betterunderstanding
of the sources of infant-adult
differences in behavioral
metricfunction,
thesefactorswouldalsobeexpected
toproducea functionwith a lowei•ed
upperasymptote.
It isdifficult to draw any strong conclusionsabout an upper
asymptote
fromthesedata,sinceit isnotclearthatasymptotic performance
wasreachedat any frequencyexcept4000
thresholds,
aswellasa moredetailed
account
ofthedevelop- Hz. However, there is no evidence of a correlation between
ment of auditorybehaviorin other mammals.
To what extentcanthe agedifferences
describedbe at-
functionslopeand upperasymptoteamongthe 3-montholds,aswouldbepredictedif lapses
of attentionwererespontributedto sensory
immaturity?
Immaturityof theresponse siblefor the observeddifferences.Thus, althoughincomsystemand/or of the connections
betweenthe sensoryand
plete, the psychometricfunctiondata are consistentwith a
responsesystemsmay influencethe thresholdswe obtain
sensitivitydifferencebetween3-month-oldsandolderlisten(seediscussion
byOlsho,1986).Moreover,adultpure-tone ers.Third, evenif practiceor learningaccountfor the lowthresholds
areinfluenced
by learningandby othervariables frequency
thresholddifference,
suchfactorscannotexplain
notinherentin theauditorysystem.
Thesevariables
appear why the difference between 3-month-olds and adults is
to havea greatereffectat low frequencies
(Watsonet al.,
greatestat 8000 Hz.
If the differences between 3-month-olds and adults re1972;Zwislockietal., 1958).Onemightalsoarguethatthe
demands
of thedetection
taskin theOPP andVRA experi- flectimmaturityof theauditorysystemat 3 months,it isstill
mentsare actuallygreaterfor the infantsthan for the adults,
not clearwherethe immaturitylies.An obviouspotential
since the infant's attention is divided between the assistant's
contributor to the threshold differences between infants and
tabletop toys and the auditory stimulus. It is not clear
adultsis the resonance
of the externalear. Unfortunately,
whethertheeffectof dividedattentionwouldbefrequency the resultsof the only studyto examineinfantconcha-ear
dependent.
Thusthereare variablessuchas learningand canalresonance
directly (Kruger and Ruben, 1987) are at
attentionthat wouldbe expectedto affectadult perfor- variancewith predictionsbasedon ear canal lengthand
mance.In the caseof learning,at least,the effectmay be
frequencyspecific.
It would not be difficult to believe that these variables
wouldhavean exaggerated
effecton the performance
of an
individualwithlimitedprocessing
resources,
andcouldcontribute to the differences between infants and adults in detec-
tion thresholds.Along the samelines,Olshoet al. (1988)
foundthat adultsshowa greatereffectof practicein dis-
1323
J. Acoust.Soc.Am.,Vol.84, No.4, October1988
compliance(McLellan and Webb, 1957; Saunderset al.,
1983). At this point,then,it is difficultto assess
possible
external ear effects.In addition, thresholddifferencesat 250
or 500 Hz obviouslyrequireanotherexplanation.Middle ear
effects(Relkin, 1986), includingundetectedmiddleear effusions,must be consideredalong with immaturity of the
cochleaandauditorynervoussystem(Rubel etal., 1984), in
additionto the nonauditoryfactorsdiscussed
above.
Olshoota/: Pure-tonesensitivity
of humaninfants
1323
ACKNOWLEDGMENTS
This researchwassupportedby NIH grantNS24525 to
L. W. Olsho. The authors thank Trent Davis, Jay Gillen-
water, Pat Feeney,Cam Marean,andJoAnnChavira-Bash
for assistance
in data collection,graphics,and manuscript
preparation,and RichmondMemorialHospital,Martha
Jefferson
Hospital,and the Universityof VirginiaHospital
for helpwith subjectrecruitment.We alsothankEd Burns
for critical readingof the manuscript.
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