Download Developmental Changes in Infants` Sensitivity to Octave

JOURNAL
OF EXPERIMENTAL
CHILD
Developmental
SANDRA
Centre
for
PSYCHOLOGY
29, 282-293 (1980)
Changes in Infants’
Octave-Band
Noises
Sensitivity
to
E. TREHUB, BRUCE A. SCHNEIDER,
AND MAXINE
ENDMAN
Research
in Human
Development,
of Toronto,
Mississauga,
Ontario,
Erindale
Canada
College,
LSL lC6
University
Localization responses to octave-band noises with center frequencies at 200,
400, 1000, 2000, 4000, and 10,000 Hz were obtained from infants 6, 12, and 18
months of age. During an experimental trial, an octave-band noise was presented
on one of two speakers located 45” to each side of the infant. A head turn to the
noise (correct response) was rewarded by activating an animated toy on top of the
speaker. The intensity of the noise was varied over trials (method of constant
stimuli) to determine thresholds at each center frequency. Thresholds for the
lower frequencies were approximately 5-8 db higher in the 6-month-old infants
compared to the older infants. However, there were no consistent differences
among groups at the higher frequencies. Infant thresholds were found to be 20-30
db higher than adult thresholds at the lower frequencies. At the higher frequencies
thresholds for infants were approaching those of adults.
The present investigation was concerned with the auditory abilities of
infants, particularly the development of auditory sensitivity from 6 to 18
months of age. Although infants are thought to be less sensitive to sounds
compared to older children and adults (Spears & Hohle, 1%7), there is
little information concerning the nature and course of changes in sensitivity as a function of age. There is some preliminary
information about
sensitivity in the newborn period (Taguchi, Picton, Orpin, & Goodman,
1969; Wedenberg, 1956) but this does not shed light on the issue of
developmental
change.
A few studies have been specifically concerned with changes in sensitivity during infancy. Suzuki and Sato (1961) reported a decrease of
thresholds with age for three complex stimuli but their stimuli (cock
crowing, cow mooing, and cuckoo singing) were arbitrarily chosen and
inadequately specified. Wilson, Moore, and Thompson (Note 1) reported
The present research was supported
Canada and the Ontario Mental Health
Sandra E. Trehub, Centre for Research
sity of Toronto, Mississauga, Ontario,
0022-0%5/80/020282-12$02.00/0
Copyright
All rights
@ 1980 by Academic Press, Inc.
of reproduction
m any form reserved.
by grants from the Medical Research Council of
Foundation. Requests for reprints should be sent to
in Human Development, Erindale College, UniverCanada LSL lC6.
282
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AUDITORY
SENSITIVITY
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that the thresholds of 6- to 18-month-old infants to complex noise improved slightly with age but again, the frequency composition of their
stimuli was unspecified. In contrast, Liden and Kankkunen (1%9) presented pure tones of 250, 500, 1000, 2000, and 4000 Hz to children 3
months to 6 years of age and found age-related improvement
for all
frequencies studied. However, lack of uniformity
in test procedures,
small samples, and inadequate response specification limit the generality
of these findings.
The present study sought to ascertain changes in sensitivity between 6
and 18 months of age to octave-band noises with center frequencies at
200,400, 1000, 2000, 4000, and 10,000 Hz. This was accomplished with a
modification
of the visual reinforcement
audiometry
procedures developed by Moore, Thompson, and Thompson (1975). While Moore et al.
(1975) used a single loudspeaker and fixed response interval, the present
investigators explored the feasibility of using two loudspeakers and nolimit, forced-choice responding.
METHOD
Subjects
The subjects were 239 infants, 89 at 5.5-6.5 months of age (mean age =
6 months, 3 days), 74 at 11.5-12.5 months of age (mean age = 12 months,
5 days), and 76 at 17.5-18.5 months of age (mean age = 18 months, 5
days). All infants were born at term and resided in a suburban community
adjacent to Toronto. Infants were free of colds on the test day. Each
subject was tested individually
in two separate sessions at two frequencies of octave-band noise. Of the 89 infants at approximately 6 months of
age, 4 were excluded from the final sample for failure to reach a training
criterion (described under Procedure) and 20 were excluded for failing to
complete the first session because of fussing (N = 18), sleeping (N = l),
or illness (N = 1). Of the 65 remaining subjects, 14 completed both
sessions at 200 and 2000 Hz; 17 completed both sessions at 400 and 4000
Hz, and 18 completed both sessions at 1000 and 10,000 Hz. In addition,
several subjects completed only a single session: 3 at 200 Hz, 4 at 400 Hz,
1 at 1000 Hz, 4 at 2000 Hz, 1 at 4000 Hz, and 3 at 10,000 Hz. Of the 74
infants at 12 months of age, 2 failed to reach the training criterion, and 11
failed to complete the first session because of fussing. Of the remaining 61
subjects, 20 completed both sessions at 200 and 2000 Hz, 16 completed
both sessions at 400 and 4000 Hz, and 20 completed both sessions at 1000
and 10,000 Hz. There was 1 infant at 200 Hz, 1 at 400 Hz, and 3 at 4000 Hz
who completed only a single session. Of the 76 infants at 18 months of age,
2 failed to reach the training criterion, and 8 failed to complete the first
session because of fussing (N = 6), sleeping (N = l), or equipment failure
(N = 1). Of the remaining 66 infants, 16 completed 2 sessions at 200 and
2000 Hz, 19 completed both sessions at 400 and 4000 Hz, and 24 completed both sessions at 1000 and 10,000 Hz. There were 2 infants at 200
284
TREHUB,
SCHNEIDER,
AND
ENDMAN
Hz, 3 at 2000 Hz, and 2 at 4000 Hz who completed only the first session.
In addition, two adults, one male (25 years) and one female (23 years),
were tested in order to provide a rough adult comparison.
Apparatus
The output of a General Radio white noise generator (Model 1381) was
filtered by an Allison 2B bandpass filter. The filter was set for a l-octave
bandwidth around one of six frequencies (200, 400, 1000, 2000, 4000, or
10,000 Hz). The rate of falloff in energy on either side of the octave band
was approximately
30 db per octave. The output of the filter was routed to
be one of two electronic switches (Grason-Stadler
1287B) by means of
relay circuitry. Each switch had a rise-decay time equal to 25 msec to
eliminate clicks at onset and offset. The switching of the output from the
filter occurred only when the electronic switches were in the off mode.
The output of each electronic switch, after passing through a HewlettPackard 350D attenuator,
drove one channel of a stereo amplifier
(Marantz, Model 1060). The output of each channel, in turn, drove an
ESS-Heil
(Model AMTlAM)
speaker.
The speakers were placed in an Industrial Acoustics sound attenuating
chamber (single wall) 1.17 m away from the center of a chair which
occupied one corner of the room. Thus, one speaker was located 45” to
the right and another 45” to the left of the chair containing the infant and
mother. The calibration of sound-pressure levels was accomplished by the
placing of the microphone of a Bruel & Kjaer impulse sound-level meter
(Type 2204) at the approximate location occupied by the infant’s head.
This location was only approximate
since both the mother and baby
tended to shift their positions during an experimental session. Readings
were taken with a 0.5-in. microphone using weighting network C for the
200, 400, 1000 and 2000 Hz stimuli and the linear scale for the 4000 and
10,000 Hz stimuli. Movement of the microphone around the general area
occupied by the infant’s head produced readings within it 2 db of the
center reading. The ventilation system for the booth was turned off during
the 15-min sessions because of the high frequency hiss which it produced.
With the ventilation system off, the background noise level was about 28
db SPL (C scale).
Directly on top of each speaker at a height of about 1.02 m, was a
plywood and smoked glass enclosure (0.61 x 0.31 x 0.46 m) which
contained the toys that served as reinforcers. The glass side of this
enclosure (0.61 x 0.46 m) faced the infant. During reinforcement periods,
a light on the inside of the enclosure illuminated an animated toy (either a
Pluto dog or plush monkey) which was then activated for 4 sec. When
activated, the dog moved back and forth, bent down, and barked; the
monkey twirled around a horizontal bar. The dog was used above both
speakers during the first test session while the monkey was used at both
locations for the second.
INFANT
AUDITORY
SENSITIVITY
285
Procedure
At the beginning of a test session the mother was seated on the test
chair with the infant on her lap facing away from her. One experimenter
remained in the booth with the mother and infant and was seated approximately 1 m in front of them. A second experimenter remained outside the
booth to adjust the attenuation levels and record responses. During an
experimental session, both the mother and the experimenter in the booth
wore headphones over which a masking noise was presented to prevent
them from detecting which speaker was producing the test signal. A trial
was initiated only when the child exhibited midline orientation, that is,
only when he or she was looking at the experimenter in the booth. (Such
midline orientation was readily forthcoming; thus it was unnecessary for
the experimenter to engage in special activities to attract the infant’s
attention.) When midline orientation was obtained, the experimenter in
the booth pressed a button to initiate a trial. A sound was then presented
on one of the two speakers, and it remained on until the infant made a
head turn of 45” or more toward either side. This experimenter
then
pressed one of two buttons to indicate the direction of the head turn. If the
head turn was in the direction of the speaker producing the noise, the
noise was turned off and the toy above that speaker was illuminated and
activated for a period of 4 sec. If the head turn was in a direction away
from the speaker producing the noise, the noise was also turned off and an
intertrial interval of 4 set occurred. During this intertrial interval no signal
was presented even if the child returned to midline orientation.
Since the signal remained on until a response occurred, this procedure
becomes a two-alternative forced-choice signal detection task. (Since the
signal always remained on for over 1 set it is unlikely that energy summation affected detectability.)
To insure that all of the infants could perform
the task, a training criterion was employed with sound intensity well
above threshold. At the beginning of the first session, the sounds were
presented at an intensity of approximately 75 db (75,76,77,75,77,
and 80
db for octave-band noises with center frequencies of 200, 400, 1000,
2000, 4000, and 10,000 Hz, respectively). During the training period the
location of the sound was alternated between left and right speakers until
the child had made four successive correct responses. The intensity was
then reduced 10 db and the alternation continued until the infant again
made four successive correct responses. When this criterion was reached,
the actual test series began. A different frequency was employed in the
second session. The frequency pairs were 200-2000, 400-4000, and
lOOO-10,000 Hz. For any pair, the frequency to be used in the first session
was randomlv determined.
During the test session four different levels of the octave-band noise
were presented a total of 5 times each. The randomization of sound levels
286
TREHUB,
SCHNEIDER,
AND
ENDMAN
consisted of five random permutations of the four levels presented sequentially. This randomization
procedure was used to guarantee an approximately equal number of trials at each intensity level if the session
had to be terminated before completion of the 20 test trials. The speaker
on which the signal was to be presented was also randomized so that, on
any particular trial, the signal had a 0.5 chance of appearing at either
location. A sequence of 20 presentations was generated in this fashion and
then modified so that a sound did not appear on the same speaker more
than 3 times in a row and so that the sound appeared 10 times on the left
and 10 times on the right. These modifications of the random order were
introduced to minimize the occurrence of a response bias. The test levels
employed were 25,35,45, and 55 db, for 200 Hz: 11,21,31, and 41 db for
400 Hz; 12,22, 32, and 42 db for 1000 and 4000 Hz; 10,20, 30, and 40 db
for 2000 Hz; and 15, 25, 35, and 45 db for 10,000 Hz. These test levels
were selected partly on the basis of pilot testing and partly on the basis of
known adult thresholds.
At the conclusion of the first session there was a lo-min rest period
before the start of the second session during which parent and infant left
the booth and the monkey was substituted for the Pluto dog. At the
beginning of the second session the training criterion was again introduced with the exception that only two correct responses at each level
were required. The test levels were presented immediately
following
successful completion of this requirement.
In order to provide adult comparisons, two adult subjects were tested in
a similar experimental
situation at frequencies of 400, 1000, 4000, and
10,000 Hz. Adults were alone in the booth and used the push-button
apparatus (previously used by the experimenter in the booth) to initiate a
trial and to record the presumed location of the sound. The duration of the
reinforcer was reduced to 1 sec. Each adult received 40 trials at each of
nine intensity levels spaced 5 db apart for each of the four different
frequencies.
RESULTS
A check on systematic order effects was made for those subjects who
successfully completed both experimental
sessions. First, infants who
received the lower frequency first were separated from those who received the higher frequency first. For example, in the 1Zmonth group, 8
subjects heard the 200-Hz noise first and the 2000-Hz noise second, while
12 subjects heard the stimuli in the reverse order. Second, for all of the
subjects, the total number of correct responses for the lower frequency
was subtracted from the total number of correct responses for the higher
frequency. In terms of our example, the total number of correct responses
for the 200-Hz noise was subtracted from the total correct for the 2000-Hz
noise. If, for example, subjects tended to do better on the second session
INFANT
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287
than they did on the first, the difference scores for the subjects who
experienced the 200-Hz stimulus first should be higher than the difference
scores for the subjects who experienced the 2000-Hz noise first. The null
hypothesis, of course, is that the difference scores are the same for both
groups. A Mann-Whitney
U test (two-tailed) was used to test this
hypothesis for each of the age groups at each of the three paired frequencies. A two-tailed test was employed because it is plausible that infants
could improve over two sessions because of practice, or that they could
perform less well because of boredom or fatigue. Only the Mann-Whitney
test for the 1Zmonth group at lOOO-10,000 Hz was significant at the .05
level. For all other conditions, thep levels ranged from 0.16 to 0.98. Since
there were no demonstrable order effects except for a single group, results
were collapsed over order for the remaining analyses. Subjects completing only a single session were also included. Thus the number of infants at
each frequency and age group ranged from 17 to 24.
The upper panel of Fig. 1 shows the percentage of correct head turns
averaged across subjects as a function of the decibel level of the octaveband noises for the six different test frequencies. The parameter of the
psychometric function is the age of the test group. Each point in the upper
panel is based on a minimum of 85 trials. It can be seen that as the
intensity level increases the percentage of correct responses also increases. The percentage of correct responses never quite reaches 100%
even at the higher intensity levels. This is probably attributable
to
momentary lapses of attention to the task. Occasionally a child would
make a head turn which, in the experimenter’s
opinion, was directed at
the mother. Since it satisfied the response criterion, however, it contributed to the functions in Fig. 1.
For the frequencies between 200 and 4000 Hz, the psychometric
functions for the 6-month-old group are generally lower than those of
the 12- and 18-month-old groups. At 10,000 Hz, however, the 6- and
18-month-old groups are nearly equivalent, with the 12-month-old group
performing better than the other two.
The bottom panel of Fig. 1 indicates the psychometric functions for the
two adult subjects in this study. Unlike the children, the adults perform at
a 100% correct response level at the higher intensities. The increased
variability in these functions at the lower intensities as compared to the
infant functions (upper panel) can be attributed to the fact that each point
of adult data is based on only 40 trials while each point of infant data is
based on a minimum of 85 trials.
The psychometric functions can be used to determine threshold values
at each of the frequencies. Threshold for the infants was defined as that
intensity level at which the signal was detected 65% of the time. This
criterion was chosen for two reasons. First, since each point is based on a
minimum of 85 trials, the probability of a sound being correctly detected
I
I
I
RELATIVE
I
I
SOUND
I
1
PRESSURE
I
I
( SCALE
I
IN IOdB
I
I
I
STEPS)
M.B.
I
I
I
I
1
FIG. 1. Upper panel: Percentage of correct head turns as a function of decibel level of six test frequencies for infants 6. 12, and 18 months of age.
Lower panel: Percentage of correct responses as a function of decibel level of four test frequencies for two adults.
61
9fl
INFANT
I ’
200
AUDITORY
I I111111
500
289
SENSITIVITY
I
1000
2000
I
I
I I1111
5000
10.000
FREOUENCY
FIG. 2. Thresholds as a function of frequency for infants 6,12, and 18 months of age and
for two adults. Thresholds determined by Robinson and Whittle (1964) are also plotted.
65% of the time by chance alone is less than .002. (If the true probability is
0.5, the normal approximation
to the binomial distribution shows that the
probability of equalling or exceeding 65% correct is less than .002.) Such an
intensity level is clearly above threshold. Second, if inattentiveness is a
factor affecting the shape of the psychometric function (primarily affecting its asymptote), the estimates of stimulus detectability for the lower
stimulus intensities will be less affected than the higher intensities.’ Consequently, it is sensible to base the threshold estimates on the lowest
probability
of correct detection that can be regarded as significantly
different from chance responding. The 65% level cannot be used for the
adult data since, with only 40 trials, the probability of meeting or exceeding this value by chance alone is approximately
.03. Hence the more
conventional level of 75% was employed.
Figure 2 shows thresholds as a function of frequency for the three infant
groups. Since the percentage of correct responding for the 12-month-old
group at 10,000 Hz never dropped below 75%, it was not possible to
determine a threshold value using our criterion of 65%. Hence the value
shown in the graph is a linear extrapolation of the percentage of correct
* Consider an infant who is inattentive on 10% of the trials. A stimulus whose true
detection is 90 would have an estimated detection probability of .I x .5 + .9 x .9 = .86
since on 10% of the trials he would be performing at chance levels due to lack of attention
(.l x 3) while on 90% of the trials he would be 90% correct (.9 x .9). Hence our estimated
probability of detection would be lowered to 86%. However, for a stimulus whose true
detection probability was .70 the estimated detection probability would be .l x 3 + .9 x .7
= .68. Note that the accuracy with which we estimate the stimulus with the lower true
probability of detection is greater under this model, For a more complete discussion of
attention in signal-detection tasks see Heinemann and Chase, 1975.
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TREHUB,
SCHNEIDER,
AND
ENDMAN
responding from the two lowest intensities, and is connected to the other
threshold values for this age group by a broken line. Three features of this
graph should be noted. First, the threshold function for the 12- and
1%month-old groups appears to be fairly similar across the frequency
range explored in the present investigation.
Second, for the lower frequencies, the 6-month-old group appears to be approximately
5-8 db less
sensitive than the older groups. At the higher frequencies (4000 and 10,000
Hz), however, there does not appear to be any significant difference
between groups.
Figure 2 also indicates threshold values for the two adult subjects, as
well as thresholds determined by Robinson and Whittle (1964) using a
method of limits for octave-band noises presented from a single source
located in front of the subject. Similarities between our limited adult data
and data reported by Robinson and Whittle permit tentative comparisons
between infants and adults. It should be noted that the threshold functions
for the adult subjects are much flatter than for the infants. At the lower
frequencies, the differences between infant and adult thresholds are on
the order of 20-30 db. For the higher frequencies, however, thresholds for
the adults and infants are more nearly comparable.
DISCUSSION
In an attempt to specify the developmental course of auditory sensitivity, localization responses to octave-band noises with center frequencies
at 200,400, 1000,2000,4000, and 10,000 Hz were obtained from infants 6,
12, and 18 months of age.
Several methodological
conclusions emerge from this research. First,
the good psychophysical functions indicate that one can accurately and
reliably assess thresholds with the present technique for infants between 6
and 18 months of age. Moreover, the present thresholds are substantially
lower than those obtained with Behavior Observation
Audiometry
(Thompson & Weber, 1974) and are somewhat lower than those reported
by Wilson, Moore, and Thompson (Note 1) with Visual Reinforcement
Audiometry.
While the localization response may occur as early as the
neonatal period (Muir & Field, 1979), the response has not been effectively conditioned before 5 months of age (Moore, Wilson, & Thompson,
1977). Thus, 5-6 months of age may represent the lower age limit for this
technique. Second, reinforcement
of the localization response greatly
extends the number of localization responses which could otherwise be
obtained from infants in this age range (Moore, Wilson, & Thompson,
1976) so that it was possible in the present study to test most infants on as
many as 52 trials. Moore, Thompson, and Thompson (1975) have shown
that the specific nature of reinforcement is crucial, with animated toys
more effectively reinforcing localization responses than blinking lights or
social reinforcement.
In the present investigation frequent squeals of
delight lent further credence to the reinforcing efficacy of the animated
INFANT
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291
dog and monkey. Third, the present procedural modifications
of the
Moore et al. (1975) technique appear to be effective. The addition of a
second speaker and the elimination of a fixed response interval transform
the procedure into a two-alternative forced-choice signal detection task.
This has the triple advantage of eliminating the need for control (nostimulus) trials, minimizing concerns about response bias, and permitting
the inclusion of infants who are slow to respond.
As can be seen in Fig. 1 the present technique yields good psychophysical functions in the sense that the percentage of correct headturns increases with increases in stimulus intensity. Further support for
the validity of the technique derives from the similarity of threshold
functions between infants and adults at the higher frequencies (Fig. 2).
This comparability
of thresholds at 10,000 Hz adds weight to the contention that the greater differences obtained at the lower frequencies are not
merely attributable to the insensitivity of the technique.
A further advantage of the present technique is the relatively modest
attrition rate for infant subjects-27,
18, and 13% for infants 6, 12, and 18
months of age, respectively. Only 8 of the 239 infants tested did not meet
the training criterion and thus were untestable. This may reflect a developmental lag or impairment
in localization
ability, a gross hearing
deficit, or more simply, lack of interest in the stimuli or experimental task.
The remaining infants who did not complete a single session fussed or fell
asleep and were not retested. Thus there is no indication that these infants
were, in fact, untestable. The principal limitation of the technique is that
poor performance may be attributable either to decreased sensitivity or to
impaired localization ability.
So far as auditory sensitivity is concerned there appears to be an
orderly increase in infant sensitivity as the frequency of octave-band
stimulation increases such that thresholds at 200,400, 1000, and 2000 Hz
were substantially higher than those of adults, and thresholds at 10,000 Hz
were approaching those of adults. Comparisons of the three infant age
groups reveal that developmental
change is greatest for the lower frequencies where the 6-month-old infants performed substantially worse
than the 12- and l&month-old
infants. By contrast, the youngest and
oldest infants were nearly equivalent at 10,000 Hz. Thus developmental
changes in auditory sensitivity appear to be reflected largely in improvement at lower frequencies.
The pattern of increasing sensitivity with age corroborates the general
findings of previous research (Liden & Kankkunen,
1%9) but the
sensitivity-frequency
relation for infants stands in marked contrast to
results reported by other researchers. For example, Taguchi et al. (1%9)
found that neonatal thresholds for auditory-evoked
responses decreased
with decreasing frequency for stimuli of 500, 1000, and 2000 Hz. It should
be noted, however, that there have been no prior investigations of infant
292
TREHUB,
SCHNEIDER,
AND ENDMAN
sensitivity which included frequencies higher than 4000 Hz. In the few
studies with specified test frequencies, target responses (e.g., evoked
responses, auropalpebral reflexes, startles), stimulus parameters (pure
tones, band noises of varying duration) and age range of subjects have
differed considerably so that direct comparisons are impossible. Furthermore, minimal subject numbers render the threshold estimates in most of
these studies questionable.
In a review of anatomical and physiological development of the auditory pathway, Hecox (1975) has attempted to relate structural changes to
age-related improvement
in sensitivity. He suggests that anatomical immaturity in infancy might account for no more than a lo-db conductive
loss in sensitivity and raises the possibility of a greater sensorineural
recruiting type of loss (p. 158). Hecox speculates that changes in the
mechanical response characteristics of the basilar or tectorial membrane
may cause shifts in sensitivity or frequency-sensitivity
functions but acknowledges that the relevant research has simply not been done. In short,
there is no simple anatomical or physiological explanation for infants’
relative facility with high frequency sounds.
In conclusion, the present investigation confirms the fact that there are
important developmental
changes in auditory sensitivity. Moreover, the
region of maximal sensitivity in infancy, although undetermined as yet,
appears to be substantially different from that of adults. A significant task
of future research will be to specify the frequency limits of infants’
hearing, a task which appears to be feasible with the present methodologY*
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REFERENCE NOTE
1. Wilson, W. R., Moore, J. M., &Thompson,
G. Sound-$etd
auditory
thresholds
of infants
Paper presented at meetings of the American
Speech and Hearing Association, Houston, 1976.
utilizing
RECEIVED:
visual
reinforcement
December 19, 1978;
audiometry.
REVISED:
April 10, 1979.