Syllables as signals for 2-day-old infants

INFANT BEHAVIOR AND DEVELOPMENT 13, 377-390 I 19901

Syllables as Signals for 2-Day-Old Infants

CHRISTINE MOON AND WILLIAM P. FIFER Columbia University and New York State Psychiatric Institute

Neonotes demonstroted two-stimulus ouditory operont discrimination leorning by oltering nonnutritive sucking potterns during on I 8-min session. Discriminative stimuli were two isolated, repeated syllables, and reinforcers were a recording of mother’s adult-directed speech and quiet. During the final 6 min, 16 of 20 subtects (M age = 55 hours) initiated bursts of sucking relatively more frequently during the syllable which signalled the availability of the recording of mother’s voice versus quiet. In oddition to providing new evidence of newborn speech perception ond learning capacities, the results suggest a useful method for investigating neonatol auditory perception.

audition discrimination learning mother’s voice

newborn nonnutritive sucking syllable

Studying infants younger than 1 or 2 months old has presented a challenge to developmental psychologists seeking behavioral means of assessing early perception and learning. The biological requirement for newborns to dedicate resources to rapid growth (Rovee-Collier & Gekoski, 1979) and the radical transition to the postnatal environment impose severe con- straints upon the behavioral responses available for study. One research approach pursued with some success over the past 25 years has been the measurement of sucking, a robust newborn behavior which is responsive to events other than nutritional events. The primary focus of investigation has been the relationship between sucking and auditory stimulation. The early hope that sucking would prove to be a sensitive measure of auditory and general behavioral competence available for assessment of individual differences has not been realized, but nonnutritive sucking research has contributed much to the new and prevailing appreciation of the neonate’s responsiveness to the environment.

This research was supported by NICHD grants ROl HD22817 to the first author and ROl HD20102 to the second. Stimuli were prepared at Haskins Laboratory with support of NICHD contract NOI HD52910. We would like to thank the Departments of Neonatology and newborn nursery personnel at North Central Bronx Hospital and Babies Hospital, Columbia-Presbyterian Medical Center for their cooperation and support in carrying out the experiments. We would also like to thank Robin Cooper and Tony D&Jasper for helpful comments on earlier versions of the manuscript.

Correspondence and requests for reprints should be sent to Christine Moon, Department of Psychology, Pacific Lutheran University, Tacoma, Washington 98447.

377

378 MOON AND FIFER

Early investigations of sucking during auditory stimulation in the U.S. followed a report in 1958 by Soviet researchers demonstrating that cessation of young infants’ sucking could be habituated, or that in some cases sucking could be increased by auditory stimulation (Bronshtein, Antonova, Kamenetskaya, Luppova, & Sytova, 1958; reviewed in Brack- bill, 1962). Over the next decade, American psychologists explored various parameters of acoustic stimulation and neonatal nonnutritive sucking, with mixed experimental results.

Several aspects of newborn nonnutritive sucking patterns have been assayed during auditory stimulation. Infants typically suck in bursts of individual sucks at a rate of between one and two sucks per second. The bursts typically last several seconds with pauses of approximately the same duration between bursts. Some sucking parameters that have been shown to change in response to sound stimulation are: duration of individual sucks within a burst; duration of the burst as a whole; duration of the pause between bursts; and probability of occurrence of a burst or pause, or cessation of burst or pause, given an auditory event.

In the first published U.S. neonatal sound/sucking study, Kaye and Levin (1963) reported a failure to find changes in sucking rate to presentations of a “moderately loud tone.” Subsequently, Lipsitt and Kaye (1964) published a report of classical conditioning of sucking by pairing a 93-dB square-wave tone with the opportunity to suck on a nonnutritive nipple. Also using loud stimuli (9OdB pure tones) with neonates, Keen (1964) was able to replicate, with attenuated effects, the Soviet findings for habituation of cessation of sucking. Comparing pre-stimulation to stimulation rate of sucks per unit time, not cessation of sucking, as a dependent measure, Kaye (1966) found that there was an increase over time, but only with the loudest of three square-wave stimuli (82.5, 94.0, and 104.0 dB).

The first sucking results with an acoustic stimulus that was not loud (6O-dB, 500-Hz tone) were based upon separate analyses of activity during bursts and pauses (Sameroff, 1967), and the author recommended future investigation of pattern of bursts and pauses rather than rate. Semb and Lipsitt (1968) also analyzed burst/pause sucking patterns and found that with stimulation (91dB square-wave tone) delivered specifically during bursts or pauses, newborns were more likely to begin sucking if they were pausing, and they were more likely to begin a pause if sucking. Sameroff (1971) analyzed burst and pause duration as a result of stimulation with 65-dB and 75dB pure tones and found that the two sucking states (burst and pause) were affected differently and were affected by both the onset and offset of the tones. In 1982, Fischel examined stimulation during bursts separate from pauses. She suggested that pauses and bursts were different contexts for responding to sound.

SYLLABLE SIGNALS 379

Despite indications that some sucking parameters showed promise, the use of sucking as an instrument for studying newborn sound detection has been pursued neither for auditory psychophysical development nor for clinical purposes. Butterheld and Cairns (1974) had investigated newborn syllable discrimination using the high-amplitude habituation of sucking (HAS) procedure developed with older infants (Eimas, Siqueland, Jusczyk, & Vigorito, 197 1; Siqueland & DeLucia, 1969). But Cairns and Butterfield (1975) summarized obstacles of using nonnutritive sucking in assessment of newborn auditory responding by pointing to high attrition rates, the need for expensive equipment and repeated sessions, and problems of interpretation of failure to respond. They concluded that one possible avenue for behavioral testing of newborns may be operant discrimination learning with a potent reinforcer. In such a procedure, the stimuli to be detected or discriminated would be used as signals for the availability ‘of a reinforcer that is sufficiently motivating for newborn infants to remain in the experimental session and show differential responding than when simply presented with the stimuli alone as in the HAS procedure. Recently, researchers in France have used the HAS procedure for syllable discrimination with 4&y-olds, and it may become more widely used in auditory discrimination experiments with neonates (Bertoncini, Bijeljac-Babic, Blumstein, & Mehler, 1987; Bertoncini, Bijel- jac-Babic, Kennedy, Jusczyk, & Mehler, 1988). Cairns and Butterfield’s (1975) call for a two-stimulus operant discrimination learning procedure was echoed by Jusczyk (1985) in a review of HAS application in which Jusczyk proposed discrimination learning as a more sensitive measure for obtaining information about the speech categories of very young infants.

Experiments conducted and described by Butterfield and Cairns (1974) indicated that, indeed, neonatal sucking could be operantly conditioned using auditory reinforcers, and that some sounds are more effective than other sounds as reinforcers. They described results in which infants emitted longer individual sucks when vocal/instrumental music was presented during sucks, relative to presentation between sucks. In other words, infants prolonged sucks when hearing music, and they shortened sucks when music occurred during pauses between sucks. For white noise, on the other hand, suck durations were relatively longer in the ‘between” rather than in the “during” condition (Butterfield & Siperstein, 1972). Contingent music with voices either alone or with instrumental background was more effective than instruments alone in affecting suck duration (Cairns dc Butterfield, 1975). The investigators also reported that isolated, repeated speech syllables were insufficiently reinforcing to motivate neonates to increase sucking rate, although the infants could discriminate between two syllables (Butterfield & Cairns, 1974). They did not publish any reports, however, of attempts to develop an auditory discrimination learning procedure.

380 MOON AND FIFER

There have been several subsequent demonstrations of neonatal learning in sucking/sound contingency experiments by DeCasper and colleagues in investigations of the relative reinforcing strength of sounds of the newborn environment. The primary dependent variable was the duration of pauses between bursts, the interburst interval (IBI). Infants were re- warded for either shortening or lengthening IBIS relative to their own pre-stimulation baseline. For example, for half of the subjects of Exper- iment 1 in DeCasper and Fifer (1980), all IBIS longer than baseline were followed by presentations of mother’s voice during the succeeding burst, whereas shorter IBIS were followed by a recorded unfamiliar female voice. For the other half of the subjects, the contingencies were reversed. After an initial acquisition period, infants had appropriately changed IBI and activated mother’s voice more frequently than the stranger’s voice. In subsequent IBI experiments, it has been shown that newborns demonstrate: the reinforcing value of a recording of intrauterine heartbeat sounds (DeCasper & Sigafoos, 1983); the failure of father’s voice to effect a change comparable to that of mother versus stranger (DeCasper & Prescott, 1984); and the preference for a story recited by mother prenatally versus a novel story (DeCasper & Spence, 1986).

Pursuing a strategy similar to the discriminative stimulus strategy suggested by Cairns and ButterGeld (1975), Fifer (198 1, with a subset of these results described in DeCasper & Fifer, 1980) found that neonates learned to respond to the presence versus absence of a pure tone (400- Hz tone not louder than 70 dB) that signalled the availability of a mother’s or an unfamiliar female’s recorded voice. Thirteen of 16 subjects, during the final third of sessions which ranged from 12 to 20 mins, initiated sucking bursts relatively more frequently during the signal (tone or no tone, depending upon condition) for mother’s voice. These results, and a more recent replication also using tone/no tone but with two versions of mother’s voice as reinforcers (Spence & DeCasper, 1987), indicated that newborns younger than 4 days of age can detect the tone and respond to it as a discriminative stimulus for the availability of mother’s voice, which is a sufficiently potent reinforcer to effect a change in newborn sucking patterns.

The ability of newborns to respond to the presence or absence of a tone suggested that they are capable of two-stimulus operant discrimination learning, with a nonnutritive sucking response. Moreover, a previous indication of the capacity was reported by Siqueland and Lipsitt (1966). They found, in a hybrid experiment using elicited head-turns reinforced with sucrose water, that 2- to 5day-olds demonstrated differential relative frequencies of ipsilateral head-turning after cheek-stroking. The two auditory stimuli were loud (80 and 90 dB) square-wave tones of two different fun&mental frequencies (23 and 230 Hz) which predicted availability of sucrose water upon turning.


We now report two-stimulus discrimination learning by 2day-old infants using nonnutritive sucking. The discriminative stimuli were isolated speech syllables, and the reinforcers were a recording of mother’s voice and quiet.

METHOD

Subjects Twenty infants served as subjects, ranging in age from 33 to 77 hours (M = 55.2 hours). Ten subjects were female, and all but four subjects were bottlefed. The following criteria were used in subject selection: Apgar scores of at least 8 at 1 and 5 min; 2500 to 4000 gms birth weight; and no pre- or perinatal medical complications. Twelve newborns comprised the put/pst group. For this group, 53 infants were tested in order to obtain data from 12 infants. Data will not be reported for subjects excluded for the following reasons: fussiness (n = 13), drowsiness (n = 7), unmeasureable bursts (n = 12), equipment failure or experimenter error (n = 6), alert but did not suck (n = 3). Eight subjects comprised the u/i group for which 14 infants were tested. Excluded were the following: 3 infants for drowsiness; 2 infants alert but did not suck; and 1 infant for emitting sucking bursts averaging longer than 30 s.1 Exclusion criteria are detailed in the Procedure section.

Apparatus Auditory stimuli were presented through standard Telephonics TDH-39 headphones which could be disinfected prior to each session. Infants sucked on a hospital feeding nipple connected to a pressure transducer via surgical tubing. Negative pressure was transmitted from the nipple to a pressure monitor and subsequently via an analog-to-digital converter to a microcomputer that stored pressure values and controlled stimulus presentation. A total of four channels of two continuously running tape recorders presented auditory stimuli gated through computer-controlled relays. A paper chart recorder provided a hard copy of sucking pressure patterns and stimulus presentation for on-line visual inspection.

The attrition rate for the pat/pst group was 77%, and for the a/i group it was 43%. While this difference could be due to the difference in stimuli which define the experimental conditions, it is more likely due to the fact that the a/i group was added after completion of the pat/pst group. With experience, the experimenters have become increasingly adept at recognizing stable newborn sleep/wake states prior to starting a session, resulting in increasingly fewer infants excluded during the session for falling asleep or crying. In addition, a technical problem was solved which accounted for exclusion of 12 infants in the pat/pst condition. These infants appeared to produce copious amounts of saliva during nonnutritive sucking, which interfered with sucking pressure measurement. The problem was solved in subsequent experiments by using nipples with larger holes. Exclusion of 4 pat/pst infants’ data was due to incomplete sessions when equipment failed. This equipment problem was subsequently remedied.

382 MOON AND FIFER

StimuIl

Mother’s Voice. After informed consent was obtained, each mother’s voice was tape recorded during a brief interview in her hospital room. Conversation was elicited with questions about events leading up to the infant’s delivery, reaction to the infant’s sex, and reaction of family members. The tape recording (usually about 5 min long) was edited to omit extraneous sound and pauses longer than 1.5 s. The tape was made using a Teat X3MKII reel-to-reel tape recorder and was re-recorded on one track of a Fostex X-15 Multitracker cassette recorder to provide 25 min of voice. Voices were presented at conversational loudness, approximately 70 dB.

Signals. Speech syllables were chosen as signals because infants have been reported to be particularly responsive to acoustically complex sounds as compared to simple stimuli like pure tones (for a review, see Aslin, Pisoni, & Jusczyk, 1983). For 12 infants, the syllable signals were /pact/ (“pat”) versus /pst/; they were chosen for maximum discriminability and compatibility with another study by Moon, Bever, and Fifer. These syllables were originally natural tokens spoken in a monotone male voice in order to be distinctive from a mother’s voice. They were subsequently edited to be 350 ms in duration with an interstimulus interval of 450 ms. Each string of syllables was recorded on a separate track of a 30- min stereo reel-to-reel tape and was presented by a continuously playing Teat X3MKII recorder. The syllables were equated for loudness by four adult listeners and were presented at conversational levels of loudness. The syllable /pa%/ was presented at 71 dB SPL (General Radio Sound Level Meter, Type 1551, Scale A, fast), and /pst/ was 63 dB (SPL, A). For eight subjects, the signals were /a/ and /i/ (“ah” and “ee”). These vowel sounds were repeated by a male-synthesized voice and were 500 ms in duration separated by 500 ms of silence. They were equated for conversational loudness by three adults and were presented at levels of 64 dB (/a/) and 60 dB (/i/) (General Radio Sound Level Meter, Type 155 lC, Scale A, fast).

Prueedure In order to maximize the probability of obtaining an alert and calm infant, sessions were conducted about 1 hour prior to feeding. Newborns were moved in their own bassinets to a quiet, dimly lit room adjoining the nursery. The lights were kept low to maximize the infant’s eye-opening. Infants were brought to a quiet, alert state with open eyes and adequate muscle tone by talking to them, gently massaging their limbs, or holding them upright. If they were fussy, they were tightly swaddled. The head end of the bassinet was elevated by about 20” to help maintain alertness.


Infants who were judged not to be calm or alert were returned to the nursery for possible later testing. Infants who did not reach the point of stimulus presentation were not counted in calculation of the attrition rate. Infants began the session swaddled in their blankets unless they appeared likely to become drowsy, in which case their blankets were loosely wrapped around them. For the infants who began a session, the earphones were placed securely over their ears, and the nipple was placed in the infant’s mouth. A 2-min period of adjustment followed for the infant to begin sucking in discrete bursts. After observing that burst/pause sucking was occurring, stimulus presentation began. While one experimenter monitored the recording equipment, the other, blind to the experimental condition, monitored the infant and held the nipple in the infant’s mouth. In the first 12 min of the session, if the infant’s eyes closed and muscle tone became flaccid, the experimenter monitoring the infant loosened the blanket to try to restore alertness. If the infant began to become active and irritable, the experimenter attempted to calm the infant by restraining the hands. These maneuvers were not permitted during the final 6 min of the session. Sessions lasted 18 min unless terminated prematurely. Sessions were terminated prematurely if the infant failed to emit meas- urable sucks due to fussiness, drowsiness, a bowel movement, or failure of sucking to be recorded as discrete bursts. Session termination occurred the second time that a 45-s period of nonsucking or unmeasurable bursts elapsed.

The sounds were presented contingent upon sucking pressure. A sucking burst was defined as -20 mmHg for a minimum of 2 s. Syllable signals were presented only when the computer detected the absence of a sucking burst for at least 2 s. The syllables were in strings such that 4 s of one syllable alternated with 4 s of the other syllable. If no sucking bursts occurred, the two syllable strings continued alternating. If the infant initiated a sucking burst during either string, the syllables were terminated (unless the string had been activated for less than 1 s, in which case it ceased after 1 s), and either mother’s voice or silence was presented for as long as the sucking burst continued. For example, if /p&l was the signal for the mother, and the infant sucked while hearing the /p&/ string, that string ceased and the infant then heard the mother’s voice for the duration of the sucking burst (usually several seconds). The signal for quiet would be /pst/, and if the infant sucked during that signal string, silence would be presented for the duration of the sucking burst. At the end of a burst, the 4-s syllable strings resumed alternation. Assignment of signal/voice pairing within condition was counterbalanced.

The termination of each burst was followed by a syllable string sequence. Each of the two strings had a random probability of beginning the sequence, with the prohibition of more than three occurrences of the same sting following successive bursts.

384 MOON AND FIFER

RESULTS The measure of two-stimulus discrimination learning was the relative frequency with which infants responded to the syllable signalling availability of mother’s voice compared to responding to the syllable signalling quiet. Previous results (DeCasper & Fifer, 1980; Fifer, 1981) have shown that the final 6 min of the l&min session is an appropriate period to test for greater responding to mother’s voice versus quiet. As previously assumed, during the earlier minutes of the session, infants would not yet have learned the sucking/voice contingency and would not be as likely to show consistent differential responding to the signals as they would during the final third part of the session. Therefore, a larger proportion of responses to the signal for voice as compared to the signal for quiet was expected in the final 6 min than in the first 6 min.

An individual infant’s response to the syllable signals for voice or quiet was expressed by the number of sucking bursts emitted in a 6-min period divided by the total number of signals presented in that period. For example, an infant who initiated sucking bursts during 16 of 20 signals for voice and 7 of 21 signals for silence would have ratios of 80% and 33%, respectively. Note that individual subjects’ burst/signal ratios need not sum to 100% (see Table 1).

An analysis of variance was conducted in a mixed 2 (Condition: pat/ pst, a/i) by 2 (Signal: mother, quiet) by 2 (Period: first 6 min, final 6 min) design with repeated measures on the last two factors. There were no significant main effects, and interaction effects were insignificant with the exception of Signal by Period (see Table 2).

During the final 6 min of the experimental session, 16 of 20 infants (binomial test, p = .006) initiated sucking bursts relatively more frequently during the signal for mother’s voice as compared to the signal for quiet, t(19) = 2.43, p = .025, two-tailed test. During the first 6 min, there was no differential responding to the two signals, f( 19) = 0.32, p > .lO. More frequent responding to the signal for voice during the final period could result from a tendency to increase across the session in responses to the signal for voice, to decrease to the signal for quiet, or both. Visual inspection of a graph of mean percent responding to the two signals suggests that infants tended to suppress responding to the signal for quiet (see Figure 1). In fact, neither the increase to the signal for mother nor the decrease to quiet is significant, although the latter approaches signif- icance, t(19) = 1.71, p = .lO.

In addition to the explicit two-stimulus discrimination learning measure, there was a second, implicit, sucking/sound contingency. Infants received synchronous reinforcement with either mother’s voice or quiet during sucking bursts. Emitting longer bursts during mother’s voice, relative to quiet or changing from first to final 6 min in burst duration in favor of


TABLE I lndividuol Infants’ Percent Responses to Signofs

First 6 Min Final 6 Min

Age lHoursl Voice Signal Mother Quiet Mother Quiet

pot/pst GROUP In = 121

I. 64 Pzt 2. 51 Pzt 3. 65 Pmt 4. 66 P=t 5. 33 P=t 6. 61 P=t 7. 59 Pst 8. 77 Pst 9. 58 Pst

IO. 47 Pst Il. 76 Pst 12. 62 Pst

M 59.9 SD 12.1

o/i GROUP In = 8) 13. 44 0 14. 38 o 15. 53 0 16. 41 o 17. 50 i 18. 56 i 19. 53 i 20. 50 i

M 48. I SD 6.4

COMBINED GROUPS in = 201 M 55.2 SD II.6

.I9 .29 .45 .44

.46 .54 .52 .4l 55 .39 .46 .41 .56 .53 .89 .68 .65 .67 .67 .50 .50 .59 .52 .28 .62 .65 .57 .62 .61 .74 .31 .I4 .58 .52 .83 .58 .58 .74 .52 .47 .42 .3B .52 .79 .50 .6l .43 .55

.52

.I2

.56 .7l .28 .29

.77 .69 .60 .50

.74 .50 .50 .46

.43 .33 .32 .30

.33 .3l .52 .37

.68 .66 .73 .70

.59 .48 .70 .I5

.35 .50 .55 .46

.56 .52

.I7 .16

.53

.I4

.55

.I4

.54

.I4

.56 .I7

.53

.I6

54 .46 .I6 .17

.49

.I8

.40

.I7

TABLE 2 Summory of Anofysis of Variance Results for Signal Response Data

Effect SS d.f. F P

Condition 0.015 I.18 0.338 .57 Signal 0.037 1.18 1.074 .31 Period 0.042 I,18 3.824 .07 Condition X Signol 0.019 I.18 0.536 .47 Condition X Period 0.018 1,18 1.632 .22 Signal X Period 0.044 1.18 4.901 .04 Condition X Signal X Period 0.000 I,18 0.037 .85

386 MOON AND FIFER

0.651

I? g 0.60. 9 co !! 0.55. v) 3 2 0.50.

- Mother’s Voice ---__. __-_- Quiet

First Final

6-Minute Periods

Figure 1. Mean percent of sucking bursts initiated during signals for mother’s voice and quiet for the first and final 6-min periods of the l&min session. Percent is calculated by dividing the number of bursts by the number of times the signal was presented during 6 min.

mother’s voice relative to quiet, would be a second indicant of the reinforcing value of the voice. An ANOVA of the same design as above was conducted using infants’ average burst durations while hearing voice or quiet during the first and final 6-min periods (see Table 3).

The analysis revealed one significant effect, an interaction between Condition, Period, and Signal. During the final 6 min, the pat/pst group demonstrated significantly longer bursts during mother’s voice than during quiet, t( 18) = 3.23, p < .Ol, two-tailed test. There was no such difference for the a/i group, t( 18) = 0.37, p > .lO. Neither the pat/pst nor the a/i group showed differential burst durations in the first 6 min, t( 18) = 0.27 and 1.11, respectively. For the pat/pst group, comparisons were made to determine whether infants lengthened bursts during voice or shortened them during quiet across the session. The mean burst duration to voice did not change from first to final 6-min period, t(l1) = 0.20, whereas mean duration to quiet declined, t( 11) = 2.49, p = .03.

DISCUSSION

The results provide evidence that infants between 1 and 3 days after birth are capable of two-stimulus auditory discrimination learning with the sound of mother’s voice and quiet as reinforcers. Neonates in the current study learned over time to begin a burst of sucking relatively more frequently during the syllable that signalled mother’s voice compared to the signal for quiet. Only one previous experiment has been published (Siqueland dc Lipsitt, 1966) in which infants shortly after birth demon-


Voice Sign01

TABLE 3 Individual Infants’ Mean Burst Durations

First 6 Min

Mother Quiet

Final 6 Min

Mother Quiet

pot/pst GROUP In = 12)

1. Part 2. P=t

3. P=t 4. P=t 5. Pzt

6. P=t 7. Pst 8. Pst

9. Pst IO. Pst I I. Pst

12. Pst

9.0 8.0 5.9 4.7 8.2 9.4 8.0 8.5 6.6 6.3 10.4 9.3 6. I 4.6 6.5 5.8

13.4 10.8 9.8 8.5 5.7 6.5 6.6 5.1 5.7 6.0 6.6 5.8

12.3 11.7 12.2 7.2 7.3 6.5 7.4 5.3 5.6 8.1 5.1 4.6 4.8 6.5 4.3 4.4 6.5 a.2 7.1 5.1

M 7.6 7.7 7.5 6.2 SD 2.7 2.1 2.3 1.7

o/i GROUP In = 81 13. 0

14. 0 15. 0

16. 0

17. I 18. I 19. I 20. I

9.1 13.2 9.0 14.6 II.8 10.6 7.0 9.1

9.2 6.9 9.6 10.0 14.4 13.8 12.1 13.7

7. I 5.8 6.4 7.2 4.2 3.8 3.9 4.0 8.1 5.7 10.1 4.6

10.2 7.5 9.9 8.1

M 9.3 a.4 8.5 8.9 SD 3.1 3.7 2.6 3.8

COMBINED GROUPS in = 201 M 8.3 8.0 7.9 7.3 SD 2.9 2.8 2.4 3.0

strated differential rates of response to two discriminative stimuli. In that case the response was an elicited one, and the reinforcer was a different sensory modality (taste) for the discriminative stimuli (audition). The current results show that newborns are capable of using two different speech sounds as signals of the availability of other, presumably highly salient, speech sounds. Moreover, neonates can actively change sucking patterns to affect the frequency of occurrence of those sounds.

The effectiveness of mother’s voice as a reinforcer is not surprising, given previous experimental results of sucking contingency procedures showing newborn responsiveness to group singing (Butterfield & Siperstein, 1972), female singing (DeCasper & Carstens, 198 l), and mother’s voice in comparison with a stranger’s voice (DeCasper & Fifer, 1980; Fifer,

388 MOON AND FIFER

1981). The current experiment extends evidence of newborn response to mother’s adult-directed speech. In the previous mother’s voice preference experiments, recordings of mothers’ speech have been obtained while they were reading a children’s story. An analysis of adults reading transcripts of spontaneous speech has shown the reading samples to be less highly intonated than the spontaneous speech (Remez, Rubin, & Ball, 1985). Samples of reading from a child’s book may have been more highly intonated than spontaneous speech and may therefore be closer to infant- directed speech, which older infants prefer to adult-directed speech (Cooper & Aslin, in press; Femald, 1985; Werker & McLeod, 1989). Prior to the current experiment, it was not known whether adult-directed speech could act as a reinforcer for newborns. It must be pointed out, however, that the infant responded relatively more to the voice than to quiet, and it is possible that newborns are neutral about their mothers’ adult-directed speech and negative about quiet. If the increase in responding to the signal for voice was significant, then the alternative explanation (voice/ neutral, silence/negative) would be less plausible, but the near-significant decline in responding to the signal for quiet, combined with a similar and even stronger pattern in the pat/pst burst duration data, does not allow the voice/neutral, silence/negative hypothesis to be rejected, even though intuitively it does not seem correct.

The results of the secondary measure of infant response, burst duration in the synchronous reinforcement contingency, are less straightforward than those of the signal discrimination contingency. One of the two groups (pat/pst) responded more to voice than to quiet, but the other (a/i) did not. Results from other experiments may offer clues to an explanation for the difference in groups. Fifer (198 1) used the tone/no tone discrimination procedure and obtained differential responding on the burst duration measure in favor of a mother’s voice. The pat/pst results replicate Fifer’s finding. DeCasper (personal communication, February 16, 1989) reported that none of the studies in his laboratory using the IBI procedure has resulted in synchronous burst duration differences to various reinforcer pairs. It is possible that if an infant has learned to obtain a reinforcer often, then that individual does not proceed to learn to obtain it relatively longer than the alternative. Perhaps learning the IBI contingency takes precedence in that procedure, and the synchronous relationship of sound to burst duration is not learned. In recent experiments in our laboratory, of the two sucking/sound contingencies (signal discrimination and burst duration), burst duration differences have been less robust. The signal discrimination contingency has proven to be more dependable. With large numbers of infants from combined experiments, we are beginning to see a pattern that suggests that younger infants (24-48 hours) tend to show differential responding on the signal contingency, and older infants (48-72 hours) are more likely to show the burst duration difference. In the current


experiment, besides having different signal pairs, the a/i and pat/pst groups

also differed in age, with the latter being older. The response pattern is consistent with the age hypothesis. Future experiments, systematically varying age while holding stimuli constant across groups, will be necessary to test whether older and younger neonates differ in this two-contingency sucking procedure.

In conclusion, the results of the experiment reported here demonstrate for the first time that auditory two-stimulus operant discrimination leaming is a capacity present shortly after birth. In less than 20 min, 2day- old infants learned that syllables were signals for hearing their mothers’ voice or quiet. They also learned to change sucking patterns in order to activate their mothers’ voice relatively more frequently than quiet. In addition to providing data about neonatal speech-sound discrimination and learning, this experiment offers a response to the call for a new method of investigating neonatal auditory perception.

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5 January 1989; Revised El November 1989 n

Syllables as signals for 2-day-old infants

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