JOURNAL OF EXPERIMENTAL CHILD PSYCHOLOGY 29, 282-293 (1980)

Developmental Changes in Infants’ Sensitivity to

Octave-Band Noises

SANDRA E. TREHUB, BRUCE A. SCHNEIDER, AND MAXINE ENDMAN

Centre for Research in Human Development, Erindale College, University of Toronto, Mississauga, Ontario, Canada LSL lC6

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 sensitiv- ity 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 sen- sitivity 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 by grants from the Medical Research Council of Canada and the Ontario Mental Health Foundation. Requests for reprints should be sent to Sandra E. Trehub, Centre for Research in Human Development, Erindale College, Univer- sity of Toronto, Mississauga, Ontario, Canada LSL lC6.

282 0022-0%5/80/020282-12$02.00/0 Copyright @ 1980 by Academic Press, Inc. All rights of reproduction m any form reserved.

INFANT AUDITORY SENSITIVITY 283

that the thresholds of 6- to 18-month-old infants to complex noise im- proved slightly with age but again, the frequency composition of their stimuli was unspecified. In contrast, Liden and Kankkunen (1%9) pre- sented 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 de- veloped 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 no- limit, forced-choice responding.

Subjects METHOD

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 frequen- cies 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 com- pleted 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 Hewlett- Packard 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.

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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 approx- imately 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 summa- tion 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 se- quentially. This randomization procedure was used to guarantee an ap- proximately 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 intro- duced 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 re- ceived 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

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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 frequen- cies. 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 complet- ing 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 octave- band 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 in- creases. 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 contrib- uted 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

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I ’ I I111111 I I I I I1111 200 500 1000 2000 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 affect- ing its asymptote), the estimates of stimulus detectability for the lower stimulus intensities will be less affected than the higher intensities.’ Con- sequently, 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 exceed- ing 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.

290 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 fre- quencies, 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 sensitiv- ity, 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 effec- tively 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 AUDITORY SENSITIVITY 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 (no- stimulus) 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 psy- chophysical functions in the sense that the percentage of correct head- turns 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 conten- tion 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 de- velopmental 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 fre- quencies 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 improve- ment 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. Further- more, minimal subject numbers render the threshold estimates in most of these studies questionable.

In a review of anatomical and physiological development of the audit- ory pathway, Hecox (1975) has attempted to relate structural changes to age-related improvement in sensitivity. He suggests that anatomical im- maturity 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 ac- knowledges 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 methodol- ogY*

REFERENCES Hecox, K. Electrophysiological correlates of human auditory development. In L. B. Cohen

& P. Salapatek (Eds.), Infant perception: From sensation to cognition. New York: Academic Press, 1975. Vol. 2.

Heinemann, E., & Chase, S. Stimulus generalization. In W. K. Estes (Ed.), Handbook of learning and cognitive processes. Vol. 2: Conditioning and behavior theory. Hillsdale, N.J.: Erlbaum, 1975.

Liden, G., & Kankkunen, A. Visual reinforcement audiometry. Acta Uto-Laryngofogica, 1969, 67, 281-292.

Moore, J. M., Thompson, G., &Thompson, M. Auditory localization of infants as a function of reinforcement conditions. Journal of Speech and Hearing Disorders, 197.5, 40, 29-34.

Moore, J. M., Wilson, W. R., &Thompson, G. Visual reinforcement of head-turn responses in infants under twelve months of age. Journal of Speech and Hearing Disorders, 1976, 41, 328-335.

Muir, D., & Field, J. Newborn infants orientate to sounds. Child Development, 1979, 50, 43 l-436.

Robinson, D. W., & Whittle, L. S. The loudness of octave-bands of noise. Acustica, 1964, 14, 24-35.

Spears, W. C., & Hohle, R. H. Sensory and perceptual processes in infants. In Y. Brackbill (Ed.), Infancy and early childhood. New York: Free Press, 1967.

INFANT AUDITORY SENSITIVITY 293

Suzuki, T., & Sato, I. Free field startle response audiometry, a quantitative method for determining hearing thresholds of infant children. Annals of Otology, Rhinology and Laryngology, 1%1, 70, 997-1007.

Taguchi, I., Picton, T. W., Orpin, J. A., & Goodman, W. S. Evoked response audiometry in newborn infants. Acta Oto-Lnryngologica Supplementurn, 1969, 252, 5-17.

Thompson, G., & Weber, B. A. Responses of infants and young children to behavior observation audiometry. Journal of Speech and Hearing Disorders, 1974,39, 140-147.

Wedenberg, E. Auditory tests on newborn infants. Acta Oto-Laryngologica, 1956, 46, 446-461.

REFERENCE NOTE

1. Wilson, W. R., Moore, J. M., &Thompson, G. Sound-$etd auditory thresholds of infants utilizing visual reinforcement audiometry. Paper presented at meetings of the American Speech and Hearing Association, Houston, 1976.

RECEIVED: December 19, 1978; REVISED: April 10, 1979.