FRICATIVE DISCRIMINATION IN EARLY INFANCY REBECCA E. EILERS and FRED D. MINIFIE
University of Washington, Seattle, Washington In three separate experiments using controlled natural stimuli and a high-amplitude sucking paradigm, infants' ability to detect differences b e t w e e n / s / a n d / v / , / s / a n d /f/, a n d / s / a n d / z / , respectively, was investigated. Evidence for discrimination was obtained for/s/versus/v/and/s/versus/~/but not for/s/ versus /z/. Implications for a theory of infant speech perception are discussed. Recent literature has revealed that infants below the age of four months have considerable ability to discriminate speech and speechlike stimuli. For example, it has been shown that infants can discriminate synthetic voiced and voiceless stops (Eimas et al., 1971), synthetic stops differing in place of articulation as cued by differences in second formant transition (Morse, 1972; and Moffitt, 1971), various naturally produced stops differing in place of articulation and voicing, and vowels differing in height and frontness (Trehub and Rabinovitch, 1972; Trehub, 1973). On the basis of these data, researchers have suggested that infants come "prewired" to perceive speech-sound contrasts, and some (Eimas et al., 1971) even suggest that infant perception is adultlike and linguistically relevant. To date, all the data dealing with infant perception of consonants are concerned with the perception of stops. Perception of other categories of phonetic elements (fricatives, affricates, liquids, nasals, and so on) may not follow the same patterns in infants. The present work was undertaken to describe infants' perception of fricatives using the high-amplitude sucking paradigm described by Trehub and Rabinovitch (1972). Fricatives were chosen as stimuli for the present study to extend our descriptive information of infants' speech perceptual skills and to provide data to compare with those already available for stops. Fricatives were chosen as the phonetic category of interest because available data suggest that infants may be more sensitive to features which cue discrimination of stops than to those requisite to fricative discrimination. For instance, a preference for stops is evidenced in infant production, with stop consonants being produced more frequently than fricatives in infant babbling and in the beginnings of meaningful speech (Oller et al., in press; Irwin, 1947). During the development of meaningful speech, children often substitute a stop for a fricative in CV syllables and delete the fricative member of a fricative-stop cluster in CCV 158
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syllables (Jakobson, 1968; Stampe, 1969). Furthermore, three- to five-year-old children show considerable evidence of perceptual confusion within the fricative category (Abbs and Minifie, 1969) indicating that the development of fricative perception may be incomplete even as late as five years of age. In addition, stop consonants are universal; they occur in all the languages of the world, whereas fricatives do not (Jakobson, 1968). All this evidence suggests that fricative perception is potentially different from the perception of stops. An investigation of fricative discrimination is particularly appealing due to the relative ease of varying a number of acoustic dimensions while holding others constant. Four fricative sounds were chosen for this experiment: /s/, /z/, /f/, and /v/. All were combined with the vowel /a/ to form CV test stimuli. METHOD
Procedural Rationale To determine if infants can discriminate between two auditory stimuli, procedures have been devised that make use of the infant's differential sucking response to novel stimuli. While an infant sucks on an artificial nonnutritive nipple, he is presented with an auditory stimulus contingent on his sucking rate. (In this experiment, infants are presented one syllable for every two sucks.) This syllable is the same for the entire first phase of the experiment. The basic assumption of the paradigm is that when yoked to the sucking response, the acoustic stimulus reinforces the sucking response. Thus, it is assumed that an infant will increase his sucking rate relative to a baseline sucking period (during which no stimulus is present) to increase the rate of the stimulus presentation. In these studies, infants typically go through a seven- to 12-minute acquisition phase in which sucking rate increases relative to baseline. Thereafter, infants begin to habituate to the first stimulus, that is, sucking rate drops off. After habituation, the stimulus is changed and if the infant can tell Stimulus 1 from Stimulus 2 and if the difference between Stimulus 1 and 2 is reinforcing, the infant will show an increased rate of sucking relative to the habituation period.
Apparatus The apparatus used in monitoring high-amplitude, nonnutritive sucking behavior consisted of a blind nipple attached to a pressure transducer. In our experiment, when an infant sucked on the nipple, a pressure differential was imposed across the pressure transducer, causing a change in voltage in the detection circuit of a programming unit. When the magnitude of the voltage change exceeded a preset level for a high-amplitude suck (specified for each infant after monitoring of one minute of nonreinforced sucking), the logic circuit of the programmer was activated. The logic circuit was set to require EILERS, ~IINIFIE" Fricative Discrimination 159
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two high-amplitude sucks by the infant prior to presenting one syllable of auditory reinforcement. When the infant sucked to criterion, the switching device in the logic circuit allowed one syllable from a continuously playing tape loop to be presented to the infant over a loudspeaker. Electronic counters recorded the number of high-amplitude sucks produced by the child in each time period (minute). Aside from the pressure transducer and the loudspeaker, all electronic equipment was housed in a control room adjoining the experimental room. A one-way mirror allowed the experimenter to observe the infant's behavior during the experimental session. Procedure The infant was placed in a reclining seat, loosely strapped and allowed to adjust to the experimental room for three to four minutes. An assistant then inserted one of three nonnutritive blind nipples (Evenflo, Nuk, or Binki pacifier) into the infant's mouth and held it there, gently replacing it immediately if the infant ejected it. The assistant was unaware of the infant's assignment to the experimental or control condition and could not hear the stimuli presented since music was played to the assistant via earphones for the entire experihaental session. In addition, the assistant did not initiate or maintain eye contact with the infant during the experimental session. These procedures reduced the likelihood that the assistant could communicate information to the infant through a non verbal mode that could affect the outcome of the experiment. As soon as sucking was initiated, the equipment in the control room was adjusted so that only the infant's high-amplitude sucks were counted as responses (approximately 80% of the infant's sucks). The infant's baseline sucking level was recorded during the next one-minute period. Sound stimuli were then presented at 75 dB SPL contingent on criterion sucking wherein an infant received one stimulus (one complete syllable) for every two high-amplitude sucks. The same syllable was repeated on criterion sucking throughout the acquisition and habituation periods. The first two minutes of contingent sucking were excluded from the determination of maximum prehabituation sucking rate so that the infant would have time to recognize the relationship between his sucking and the occurrence of the sound before meeting the habituation criterion. Stimulus presentation continued until the infant reached an habituation criterion, that is, two consecutive minutes during which the infant's criterion sucking rate fell below 80% of his maximum sucking rate. Following habituation, the sound stimulus was changed and the infant was permitted to suck for four additional minutes, after which the session was terminated. Evidence that an infant perceived the two stimuli as different was provided by an increase in posthabituation sucking rate as compared to a control group in which the stimulus was not changed after habituation criteria were met. Stimuli To obtain a variety of acoustic stimuli from which to select test items, we 160 1ournal o[ Speech and Hearing Research
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recorded utterances of an adult male speaker who produced eight tokens of each of the following syllables: /sa, za, ya, and va/. The entire corpus of syllables was subjected to computer analysis to extract and plot intensity contours, fundamental frequency contours, and syllable durations. The acoustic features were extracted via a Cepstrum analysis technique using the Sigma V computer in the Electrical Engineering-Computer Sciences Center at the University of Washington. Traces of computer printouts that were time locked with spectrographic displays from a Kay Electric sound spectrograph were used to visually match stimuli on noncritical dimensions. It was, therefore, possible to select syllables that differed mainly on only one dimension, or a select number of dimensions, thought to be important for adult perception of the fricatives being compared. Experiment I : / s a / v e r s u s / v a / . / v / w a s chosen for comparison w i t h / s / a s a test of two fricatives that were different in amplitude, spectrum, and voicing. The voiceless/s/has high-amplitude noise energy concentrated between about 4000 and 7000 Hz along the frequency spectrum. An antiresonance around 3500 Hz makes the spectrum drop rapidly to zero below 4000 Hz. By contrast, / v / i s characterized by a very low amplitude noise on which a harmonic series due to voicing is superimposed. Its spectral characteristics include a wide distribution of energy across the entire spectrum of speech frequencies with a series of very low amplitude resonances ( Strevens, 1960). Experiment II: /sa/ versus /fa/. The syllables /sa/ and /fa/ were highly similar in terms of intensity contours, fundamental frequency contours, and syllable duration. The elements /s/ and / f / are nearly identical in terms of amplitude, duration, and nature of the sound source ( both aperiodic), but differ primarily in terms of the spectral distribution of aperiodic energy at the mouth opening (Strevens, 1960; Heinz and Stevens, 1961). Whereas /s/ has the greatest concentration of noise energy above 4000 Hz, /f/ has much of its noise energy below 4000 Hz. Experiment III: /sa/ versus /za/. To obtain an acceptable match with the acoustic features o f / z a / , it was necessary to use a different/sa/ stimulus in Experiment III than was used in Experiments I and II. Although different, the /sa/ stimuli in Experiment III and Experiments I and II were judged acceptable /sa/ tokens by trained phoneticians. In fact, the judges perceived the tokens to be essentially identical. The syllable /za/ has a rising fundamental frequency contour at the phoneme boundary; /sa/ has a falling contour. This difference is due to the fact that during vocal tract occlusion for the production of the fricative /z/, the fundamental frequency of vibration is lower than when the vocal tract is open, as seen in the characteristic rise of the fundamental frequency immediately upon the cessation of the production of t h e / z / sound. / s / w a s contrasted w i t h / z / t o provide a pair discriminable primarily on the basis of the presence or absence of voicing. F o r / z / , voice onset occurs with the initiation of frication (VOT = 0), whereas for /s/ the voice onset lags behind the initiation of frication by 95 msec (VOT = +95). /s/ a n d / z / are EILEnS, MINIFIE: Fricative Discrimination 161
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nearly identical in terms of amplitude and duration (high amplitude, moderately long duration). Both fricatives have noise spectra concentrated between about 4000 and 7000 Hz. The primary difference between these two sounds is the presence or absence of voicing. The stimuli /s/, /z/, and /f/ as employed in our experiments all had sufficient energy to be heard easily. Typically, these sounds are differentiated by adult listeners on the basis of spectral characteristics and the presence or absence of voicing (Harris, 1958). Adults usually perceive the /v/ sound to differ f r o m / s / , / z / , a n d / f / i n several ways, but they primarily i d e n t i f y / v / o n the basis of the formant trajectories in the vocalic transitions.
Subjects The subjects were 84 infants between four and 17 weeks old identified by parental response to mail solicitation. Twenty-four infants completed Experiment I ( 16 in the experimental group where "order of stimulus presentation was counterbalanced and eight in the control group where no change in stimulus occurred following habituation) and 30 infants each completed Experiments II and III (20 in each experimental group and 10 in each control). Infants were excluded if they failed to suck (chewed on nipple), failed to suck following the second habituation minute, cried persistently, or if termination was requested by the parent. Approximately 60~ of the infant subjects completed the session in each of the three experiments. Infants were randomly assigned to either experimental or control groups. Approximately equal numbers of infants were excluded from the experimental and control groups in Experiments I, II, and III because they failed one or more of the inclusion criteria. The age range and median age, respectively of the subjects for each experiment was as follows: Experiment I, 4 to 15 weeks and 8.3 weeks; Experiment II, 4 to 17 weeks and 10 weeks; and Experiment III, 8 to 17 weeks and 12.4 weeks.
RESULTS The data for each infant consisted of criterion high-amplitude sucks per minute expressed as a percentage of the infant's maximum prehabituation sucking rate. Mean number of sucks for each of four consecutive minutes before habituation and for each of four consecutive minutes after the speech stimulus was changed are presented in Figures 1, 2, and 3. Notice that in Figures 1 and 2, the two experimental groups increase sucking rate relative to the control following the stimulus shift while in Figure 3 no difference is apparent between the experimental and control groups. Analysis of variance on the difference scores between the sum of the preshift and postshift data from Experiments I (comparing /sa/ and /va/) and II (comparing /sa/ and / f a / ) revealed significant posthabituation differences between the experimental groups (wherein the stimulus sound was changed after habituation) and control groups 162 Journal of Speech and Hearing Research
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Legend experimental e--4 control
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(wherein the stimulus sound remained the same after habituation). For Experiment I, where the stimulus presentation order w a s / s a / - / v a / a n d / v a / - / s a / v e r s u s / v a / - / v a / , F = 5.64, d[ = 2, 21; p < 0.01. A t test c o m p a r i n g / s a / - / v a / versus / v a / - / s a / was not significant (t = 0.70) while t tests c o m p a r i n g / v a / / s a / t o / v a / - / v a / a n d / s a / - / v a / t o / v a / - / v a / were significant (t = 3.16, p < 0.01 and t = 2.57, p < 0.05, respectively). For Experiment II, / s a / - / f a / and /fa/-/sa/ versus /fa/-/fa/, F = 4.79, df = 2, 27; p < 0.05. The t test for / f a / - / s a / versus /sa/-/fa/ was not significant (t = 0.48) while the ts f o r / f a / /sa/ versus /fa/-/fa/ a n d / s a / - / f a / v e r s u s / f a / - / f a / w e r e significant (t = 2.8, 0.05, t = 2.4, p < 0.05, respectively). For Experiment III (comparing /sa/ and / z a / ) , no significant posthabituation differences were found (F = 0.10, d / = 2,27). Further analyses failed to reveal any differences attributable to sex, age, prehabituation sucking rate, or the number of minutes required to reach habituation. EILERS, MINIFIE: Fricative Discrimination
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163
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Experiment II FZGtraE 2. Mean sucking rates expressed as a percentage of the maximum prehabituation sucking level for four minutes before and four minutes after the sound change. (In control groups sound was not changed. ) DISCUSSION Failure to obtain differences between the experimental and control groups in Experiment III must not be taken as concrete evidence of the infants' inability to d i s c r i m i n a t e / s / a n d / z / . As has been pointed out by Trehub (1973) and others, presently available infant discrimination paradigms contain an inherent limitation. They do not differentiate between cases where the infant can perceive stimuli as different but does not find that difference reinforcing and cases where the infant does not perceive the two stimuli as different. However, ff it is the case that infants can perceive the difference b e t w e e n / s / a n d / z / b u t do not find the difference reinforcing, this fact in itself would be extremely interesting in that it would suggest that the infant is much more than a passive recipient of speech information. It would suggest that the infant may actively choose which differences in the acoustic signal he will attend to. The child may narrow the range of interpretable sounds by ignoring certain speech contrasts. It may be the case, then, that the high-amplitude sucking technique demonstrates discriminations that are salient to the infant rather than discriminations that are possible for the infant. 164 ]ournal of Speech and Hearing Research
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100
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Experiment III F m ~ 3. Mean sucking rates expressed as a percentage of the maximum prehabituation sucking level for four minutes before and four minutes after the sound change. (In control groups sound was not changed. ) If, on the other hand, one assumes that the data obtained from the highamplitude sucking procedure accurately reflect the infants' ability to differentiate among acoustic stimuli, one can conclude that infants' discrimination of fricative sounds does not rigorously follow an additive feature model. Rather, the nature of the acoustic cue appears to be more important for discrimination than the absolute number of cues available. The syllable /va/ differs from / s a / i n voicing, distribution of spectral energy, and intensity./sa/differs from /Ia/ on only one of these dimensions (spectral distribution), and /sa/ differs f r o m / z a / mainly on the basis of another single dimension (voicing). Infants were readily able to discriminate between /va/ and /sa/ and between /sa/ /Sa/, but no evidence of discrimination could be found for /sa/ and /za/, a discrimination cued by presence or absence of voicing. If the number of cue differences were important, one would assume similar perceptual responses to /sa/-/Sa/ and /sa/-/za/, both pairs of which differed on only one crucial feature. The absence of evidence of discrimination for /sa/-/za/seems on the surface to be in direct contrast to the results of Eimas et al. (1971) who claimed that infants could differentiate b e t w e e n / b / a n d / p / (in CV syllables) on the basis of voice onset time. However, a closer examination of the Eimas et al. data EILERS, MINIFIE: Fricative Discrimination
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165
from synthetic stimuli suggests that o u r / s / - / z / c o n t r a s t is not truly comparable with their/p/-/b/contrast. Although the Eimas et al. study examined the difference between the English p h o n e m e s / p / a n d / b / , the actual phonetic content of their stimulus /p/ included a period of aperiodic energy following the plosive burst. This stimulus would be transcribed as [ph] in IPA symbology. The Eimas et al. /13/ did not, however, have true prevoicing and would be transcribed as [p] in IPA symbology. As Butterfield and Cairns (1974) and Stevens and Klatt (1974) have pointed out, voice onset time (VOT) is not the only cue for discrimination along the "continuum" between 0 VOT and +40 VOT. In [p], VOT +20, a first fonnant transition is present while in [phi, VOT +40, it is absent or markedly shortened. The possibility exists that the infants in the Eimas et al. (1971) study were discriminating the stimuli on the basis of presence or absence of the rapid spectral change associated with the vocalic transition and not on the basis of VOT per se. Further support for the hypothesis that infants may be discriminating on the basis of presence or absence of first formant transition and not on the basis of VOT can be found in Butterfield and Cairns' (1974) report of 10 comparisons chosen for study with infants (conducted by Eimas and Moffitt) along the VOT dimension. Of 10 comparisons, only two yielded significant evidence of discrimination by infants ( + 2 0 versus +40 and +10 versus +60). In both cases where discrimination was demonstrated, in one member of the pair a first formant transition was present and in the other the transition was either absent or greatly reduced. Consequently, it is impossible to tell whether the children heard the difference on the basis of VOT or on the basis of presence or absence of first formant transition. In fact, the results suggest that the presence or absence of the first formant is the crucial factor in this discrimination. Eimas et al. (1971) argued that evidence of discrimination was obtained bec a u s e / b / a n d / p / r e p r e s e n t e d different phoneme categories. None of the other comparisons (tested by Eimas and Moffltt in the study cited by Butterfield and Cairns) could be shown to be discriminable by infants even though the comparisons crossed other "phoneme" boundaries (at least for languages other than English). For instance, infants did not seem to discriminate between [b], VOT = --40, and [p], VOT 0. In this comparison, VOT is probably the only available cue for discrimination. Our /s/ and /z/ are comparable to phonetic [b] and [p], VOT = 0 and V O T - - - - 4 0 , respectively. No studies have demonstrated discrimination on either contrast to date. The data reported by Butterfield and Cairns (1974) indicate that infants reacted to stop consonant stimuli, differing only in VOT, in much the same way as the infants in the present study reacted during the fricative discrimination /sa/-/za/ wherein VOT is unconfounded. Our study thus offers further support for the hypothesis that infants are unable to discriminate on the basis of VOT alone. Even in our data, the possibility exists that the /va/-/sa/ and /sa/-/fa/ discriminations are cued in infants by differences in formant transitions even though formant transitions do not seem to provide the main cue for adult perception in the case of these fricative contrasts. 166 Journal o[ Speech and Hearing Research
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In conclusion then, a reexamination of all the d a ta from studies of infant discrimination of consonants can be explained as resulting from infants' sensitivity to acoustic properties of f o r m a n t transitions. Thus far, d a ta r e p o r te d for bot h stops and fricatives do not support the notion th a t infants can differentiate a m o n g phonetic units which differ only in VOT. Rather, differences in either presence or extent of the first f o r m a n t or direction of the second f o r m a n t as in the/ba/-/ga/ discrimination reported for infants b y Morse (1972) have b e e n p r e s e n t in every case w h e r e discrimination has been demonstrated. ACKNOWLEDGMENT This research was supported by Contract NIH-NICHD-NOL-HD-3-2793. The authors wish to express their appreciation to Leslie A. Wieman and John Folkins for their technical assistance. Requests for reprints may be directed to Rebecca E. Eilers, Senior Research Associate, Child Development and Mental Retardation Center, wJ-10, University of Washington, Seattle, Washington 98195. REFERENCES A~Bs, M., and MINIFm, F., The effects of acoustic cues in fricatives on perceptual confusions in preschool children. 1. acoust. Soc. Amer., 45, 1535-1542 ( 1969 ). BUTTEnFmLD, E., and CAmNS, G., Whether infants perceive linguistically is uncertain, and if they did, its practical importance would be equivocal. In R. Sehiefelbnseh and L. Lloyd (Eds.), Language Perspectives: Acquisition, Retardation and Intervention. Baltimore, Maryland: University Park Press (1974). ErMAS, P., SEQUELaND, E., jvsczY~, P., and VmORITO, J., Speech perception in infants. Science, 171, 303-306 ( 1971 ). HARRIS, K., Cues for the discrimination of American English fricatives in spoken syllables. Lang. Speech, 1, 1-7 (1958). HEINZ, J., and STEVENS, K., On the properties of voiceless fricative consonants. 1. acoust. Soc. Amer., 33, 589-596 ( 1961 ). IRWIN, O., Infant speech: Consonant sounds according to manner of articulation. 1. Speech Hearing Dis., 12, 402-404 (1947). JAKOBSON, R., Child Language, Aphasia and Phonological Universals. Mouton: Hague (1968). MOFFITT, A., Consonant cue perception by twenty- to twenty-four-week-old infants. Child Develpm., 42, 717-731 ( 1971 ). MORSE, P., The discrimination of speech and nonspeech stimuli in early infancy. ]. exp. Child Psychol., 14, 477-492 (1972). OLLER, D., WIEMAN, L., DOYLE,W. and Ross, C., Child speech, babbling, and phonological universals. Papers and Reports on Child Language Development, Stanford Univ. (in press ). STAMPE, D., The acquisition of phonetic representation. Papers from the Fifth Regional Meeting of the Chicago Linguistics Society, Department of Linguistics, Univ. of Chicago, 443-451 (1969). STEVENS, K., and KLATT, D., Role of formant transitions in the voiced-voiceless distinction for stops. ]. acoust. Soc. Amer., 55, 653-659 (1974). STm~VENS,P., Spectra of fricative noise in human speech. Lang. Speech, 3, 32-49 (1960). TP,E~UB, S., Infants' sensitivity to vowel and tonal contrasts. Develpm. Psychol., 9, 91-96 ( 1973 ). TnEHVB, S., and RABINOVITCH,M., Auditory-linguistic sensitivity in early infancy. Develm. Psychol., 6, 74-77 (1972). Received June 7, 1974. Accepted July 14, 1974.
EILERS, MINIFIE: Fricative Discrimination
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University of Washington, Seattle, Washington In three separate experiments using controlled natural stimuli and a high-amplitude sucking paradigm, infants' ability to detect differences b e t w e e n / s / a n d / v / , / s / a n d /f/, a n d / s / a n d / z / , respectively, was investigated. Evidence for discrimination was obtained for/s/versus/v/and/s/versus/~/but not for/s/ versus /z/. Implications for a theory of infant speech perception are discussed. Recent literature has revealed that infants below the age of four months have considerable ability to discriminate speech and speechlike stimuli. For example, it has been shown that infants can discriminate synthetic voiced and voiceless stops (Eimas et al., 1971), synthetic stops differing in place of articulation as cued by differences in second formant transition (Morse, 1972; and Moffitt, 1971), various naturally produced stops differing in place of articulation and voicing, and vowels differing in height and frontness (Trehub and Rabinovitch, 1972; Trehub, 1973). On the basis of these data, researchers have suggested that infants come "prewired" to perceive speech-sound contrasts, and some (Eimas et al., 1971) even suggest that infant perception is adultlike and linguistically relevant. To date, all the data dealing with infant perception of consonants are concerned with the perception of stops. Perception of other categories of phonetic elements (fricatives, affricates, liquids, nasals, and so on) may not follow the same patterns in infants. The present work was undertaken to describe infants' perception of fricatives using the high-amplitude sucking paradigm described by Trehub and Rabinovitch (1972). Fricatives were chosen as stimuli for the present study to extend our descriptive information of infants' speech perceptual skills and to provide data to compare with those already available for stops. Fricatives were chosen as the phonetic category of interest because available data suggest that infants may be more sensitive to features which cue discrimination of stops than to those requisite to fricative discrimination. For instance, a preference for stops is evidenced in infant production, with stop consonants being produced more frequently than fricatives in infant babbling and in the beginnings of meaningful speech (Oller et al., in press; Irwin, 1947). During the development of meaningful speech, children often substitute a stop for a fricative in CV syllables and delete the fricative member of a fricative-stop cluster in CCV 158
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syllables (Jakobson, 1968; Stampe, 1969). Furthermore, three- to five-year-old children show considerable evidence of perceptual confusion within the fricative category (Abbs and Minifie, 1969) indicating that the development of fricative perception may be incomplete even as late as five years of age. In addition, stop consonants are universal; they occur in all the languages of the world, whereas fricatives do not (Jakobson, 1968). All this evidence suggests that fricative perception is potentially different from the perception of stops. An investigation of fricative discrimination is particularly appealing due to the relative ease of varying a number of acoustic dimensions while holding others constant. Four fricative sounds were chosen for this experiment: /s/, /z/, /f/, and /v/. All were combined with the vowel /a/ to form CV test stimuli. METHOD
Procedural Rationale To determine if infants can discriminate between two auditory stimuli, procedures have been devised that make use of the infant's differential sucking response to novel stimuli. While an infant sucks on an artificial nonnutritive nipple, he is presented with an auditory stimulus contingent on his sucking rate. (In this experiment, infants are presented one syllable for every two sucks.) This syllable is the same for the entire first phase of the experiment. The basic assumption of the paradigm is that when yoked to the sucking response, the acoustic stimulus reinforces the sucking response. Thus, it is assumed that an infant will increase his sucking rate relative to a baseline sucking period (during which no stimulus is present) to increase the rate of the stimulus presentation. In these studies, infants typically go through a seven- to 12-minute acquisition phase in which sucking rate increases relative to baseline. Thereafter, infants begin to habituate to the first stimulus, that is, sucking rate drops off. After habituation, the stimulus is changed and if the infant can tell Stimulus 1 from Stimulus 2 and if the difference between Stimulus 1 and 2 is reinforcing, the infant will show an increased rate of sucking relative to the habituation period.
Apparatus The apparatus used in monitoring high-amplitude, nonnutritive sucking behavior consisted of a blind nipple attached to a pressure transducer. In our experiment, when an infant sucked on the nipple, a pressure differential was imposed across the pressure transducer, causing a change in voltage in the detection circuit of a programming unit. When the magnitude of the voltage change exceeded a preset level for a high-amplitude suck (specified for each infant after monitoring of one minute of nonreinforced sucking), the logic circuit of the programmer was activated. The logic circuit was set to require EILERS, ~IINIFIE" Fricative Discrimination 159
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two high-amplitude sucks by the infant prior to presenting one syllable of auditory reinforcement. When the infant sucked to criterion, the switching device in the logic circuit allowed one syllable from a continuously playing tape loop to be presented to the infant over a loudspeaker. Electronic counters recorded the number of high-amplitude sucks produced by the child in each time period (minute). Aside from the pressure transducer and the loudspeaker, all electronic equipment was housed in a control room adjoining the experimental room. A one-way mirror allowed the experimenter to observe the infant's behavior during the experimental session. Procedure The infant was placed in a reclining seat, loosely strapped and allowed to adjust to the experimental room for three to four minutes. An assistant then inserted one of three nonnutritive blind nipples (Evenflo, Nuk, or Binki pacifier) into the infant's mouth and held it there, gently replacing it immediately if the infant ejected it. The assistant was unaware of the infant's assignment to the experimental or control condition and could not hear the stimuli presented since music was played to the assistant via earphones for the entire experihaental session. In addition, the assistant did not initiate or maintain eye contact with the infant during the experimental session. These procedures reduced the likelihood that the assistant could communicate information to the infant through a non verbal mode that could affect the outcome of the experiment. As soon as sucking was initiated, the equipment in the control room was adjusted so that only the infant's high-amplitude sucks were counted as responses (approximately 80% of the infant's sucks). The infant's baseline sucking level was recorded during the next one-minute period. Sound stimuli were then presented at 75 dB SPL contingent on criterion sucking wherein an infant received one stimulus (one complete syllable) for every two high-amplitude sucks. The same syllable was repeated on criterion sucking throughout the acquisition and habituation periods. The first two minutes of contingent sucking were excluded from the determination of maximum prehabituation sucking rate so that the infant would have time to recognize the relationship between his sucking and the occurrence of the sound before meeting the habituation criterion. Stimulus presentation continued until the infant reached an habituation criterion, that is, two consecutive minutes during which the infant's criterion sucking rate fell below 80% of his maximum sucking rate. Following habituation, the sound stimulus was changed and the infant was permitted to suck for four additional minutes, after which the session was terminated. Evidence that an infant perceived the two stimuli as different was provided by an increase in posthabituation sucking rate as compared to a control group in which the stimulus was not changed after habituation criteria were met. Stimuli To obtain a variety of acoustic stimuli from which to select test items, we 160 1ournal o[ Speech and Hearing Research
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recorded utterances of an adult male speaker who produced eight tokens of each of the following syllables: /sa, za, ya, and va/. The entire corpus of syllables was subjected to computer analysis to extract and plot intensity contours, fundamental frequency contours, and syllable durations. The acoustic features were extracted via a Cepstrum analysis technique using the Sigma V computer in the Electrical Engineering-Computer Sciences Center at the University of Washington. Traces of computer printouts that were time locked with spectrographic displays from a Kay Electric sound spectrograph were used to visually match stimuli on noncritical dimensions. It was, therefore, possible to select syllables that differed mainly on only one dimension, or a select number of dimensions, thought to be important for adult perception of the fricatives being compared. Experiment I : / s a / v e r s u s / v a / . / v / w a s chosen for comparison w i t h / s / a s a test of two fricatives that were different in amplitude, spectrum, and voicing. The voiceless/s/has high-amplitude noise energy concentrated between about 4000 and 7000 Hz along the frequency spectrum. An antiresonance around 3500 Hz makes the spectrum drop rapidly to zero below 4000 Hz. By contrast, / v / i s characterized by a very low amplitude noise on which a harmonic series due to voicing is superimposed. Its spectral characteristics include a wide distribution of energy across the entire spectrum of speech frequencies with a series of very low amplitude resonances ( Strevens, 1960). Experiment II: /sa/ versus /fa/. The syllables /sa/ and /fa/ were highly similar in terms of intensity contours, fundamental frequency contours, and syllable duration. The elements /s/ and / f / are nearly identical in terms of amplitude, duration, and nature of the sound source ( both aperiodic), but differ primarily in terms of the spectral distribution of aperiodic energy at the mouth opening (Strevens, 1960; Heinz and Stevens, 1961). Whereas /s/ has the greatest concentration of noise energy above 4000 Hz, /f/ has much of its noise energy below 4000 Hz. Experiment III: /sa/ versus /za/. To obtain an acceptable match with the acoustic features o f / z a / , it was necessary to use a different/sa/ stimulus in Experiment III than was used in Experiments I and II. Although different, the /sa/ stimuli in Experiment III and Experiments I and II were judged acceptable /sa/ tokens by trained phoneticians. In fact, the judges perceived the tokens to be essentially identical. The syllable /za/ has a rising fundamental frequency contour at the phoneme boundary; /sa/ has a falling contour. This difference is due to the fact that during vocal tract occlusion for the production of the fricative /z/, the fundamental frequency of vibration is lower than when the vocal tract is open, as seen in the characteristic rise of the fundamental frequency immediately upon the cessation of the production of t h e / z / sound. / s / w a s contrasted w i t h / z / t o provide a pair discriminable primarily on the basis of the presence or absence of voicing. F o r / z / , voice onset occurs with the initiation of frication (VOT = 0), whereas for /s/ the voice onset lags behind the initiation of frication by 95 msec (VOT = +95). /s/ a n d / z / are EILEnS, MINIFIE: Fricative Discrimination 161
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nearly identical in terms of amplitude and duration (high amplitude, moderately long duration). Both fricatives have noise spectra concentrated between about 4000 and 7000 Hz. The primary difference between these two sounds is the presence or absence of voicing. The stimuli /s/, /z/, and /f/ as employed in our experiments all had sufficient energy to be heard easily. Typically, these sounds are differentiated by adult listeners on the basis of spectral characteristics and the presence or absence of voicing (Harris, 1958). Adults usually perceive the /v/ sound to differ f r o m / s / , / z / , a n d / f / i n several ways, but they primarily i d e n t i f y / v / o n the basis of the formant trajectories in the vocalic transitions.
Subjects The subjects were 84 infants between four and 17 weeks old identified by parental response to mail solicitation. Twenty-four infants completed Experiment I ( 16 in the experimental group where "order of stimulus presentation was counterbalanced and eight in the control group where no change in stimulus occurred following habituation) and 30 infants each completed Experiments II and III (20 in each experimental group and 10 in each control). Infants were excluded if they failed to suck (chewed on nipple), failed to suck following the second habituation minute, cried persistently, or if termination was requested by the parent. Approximately 60~ of the infant subjects completed the session in each of the three experiments. Infants were randomly assigned to either experimental or control groups. Approximately equal numbers of infants were excluded from the experimental and control groups in Experiments I, II, and III because they failed one or more of the inclusion criteria. The age range and median age, respectively of the subjects for each experiment was as follows: Experiment I, 4 to 15 weeks and 8.3 weeks; Experiment II, 4 to 17 weeks and 10 weeks; and Experiment III, 8 to 17 weeks and 12.4 weeks.
RESULTS The data for each infant consisted of criterion high-amplitude sucks per minute expressed as a percentage of the infant's maximum prehabituation sucking rate. Mean number of sucks for each of four consecutive minutes before habituation and for each of four consecutive minutes after the speech stimulus was changed are presented in Figures 1, 2, and 3. Notice that in Figures 1 and 2, the two experimental groups increase sucking rate relative to the control following the stimulus shift while in Figure 3 no difference is apparent between the experimental and control groups. Analysis of variance on the difference scores between the sum of the preshift and postshift data from Experiments I (comparing /sa/ and /va/) and II (comparing /sa/ and / f a / ) revealed significant posthabituation differences between the experimental groups (wherein the stimulus sound was changed after habituation) and control groups 162 Journal of Speech and Hearing Research
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Legend experimental e--4 control
100-. ~ . ~ 90
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Experiment I Fmtn~ 1. Mean sucking rates expressed as a percentage of the maxinmm prehabituation stacking level for four minutes before and four minutes after the sound change. (In control groups sound was not changed. )
(wherein the stimulus sound remained the same after habituation). For Experiment I, where the stimulus presentation order w a s / s a / - / v a / a n d / v a / - / s a / v e r s u s / v a / - / v a / , F = 5.64, d[ = 2, 21; p < 0.01. A t test c o m p a r i n g / s a / - / v a / versus / v a / - / s a / was not significant (t = 0.70) while t tests c o m p a r i n g / v a / / s a / t o / v a / - / v a / a n d / s a / - / v a / t o / v a / - / v a / were significant (t = 3.16, p < 0.01 and t = 2.57, p < 0.05, respectively). For Experiment II, / s a / - / f a / and /fa/-/sa/ versus /fa/-/fa/, F = 4.79, df = 2, 27; p < 0.05. The t test for / f a / - / s a / versus /sa/-/fa/ was not significant (t = 0.48) while the ts f o r / f a / /sa/ versus /fa/-/fa/ a n d / s a / - / f a / v e r s u s / f a / - / f a / w e r e significant (t = 2.8, 0.05, t = 2.4, p < 0.05, respectively). For Experiment III (comparing /sa/ and / z a / ) , no significant posthabituation differences were found (F = 0.10, d / = 2,27). Further analyses failed to reveal any differences attributable to sex, age, prehabituation sucking rate, or the number of minutes required to reach habituation. EILERS, MINIFIE: Fricative Discrimination
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163
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Experiment II FZGtraE 2. Mean sucking rates expressed as a percentage of the maximum prehabituation sucking level for four minutes before and four minutes after the sound change. (In control groups sound was not changed. ) DISCUSSION Failure to obtain differences between the experimental and control groups in Experiment III must not be taken as concrete evidence of the infants' inability to d i s c r i m i n a t e / s / a n d / z / . As has been pointed out by Trehub (1973) and others, presently available infant discrimination paradigms contain an inherent limitation. They do not differentiate between cases where the infant can perceive stimuli as different but does not find that difference reinforcing and cases where the infant does not perceive the two stimuli as different. However, ff it is the case that infants can perceive the difference b e t w e e n / s / a n d / z / b u t do not find the difference reinforcing, this fact in itself would be extremely interesting in that it would suggest that the infant is much more than a passive recipient of speech information. It would suggest that the infant may actively choose which differences in the acoustic signal he will attend to. The child may narrow the range of interpretable sounds by ignoring certain speech contrasts. It may be the case, then, that the high-amplitude sucking technique demonstrates discriminations that are salient to the infant rather than discriminations that are possible for the infant. 164 ]ournal of Speech and Hearing Research
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100
Legend
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90 ...';;~', 9
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Experiment III F m ~ 3. Mean sucking rates expressed as a percentage of the maximum prehabituation sucking level for four minutes before and four minutes after the sound change. (In control groups sound was not changed. ) If, on the other hand, one assumes that the data obtained from the highamplitude sucking procedure accurately reflect the infants' ability to differentiate among acoustic stimuli, one can conclude that infants' discrimination of fricative sounds does not rigorously follow an additive feature model. Rather, the nature of the acoustic cue appears to be more important for discrimination than the absolute number of cues available. The syllable /va/ differs from / s a / i n voicing, distribution of spectral energy, and intensity./sa/differs from /Ia/ on only one of these dimensions (spectral distribution), and /sa/ differs f r o m / z a / mainly on the basis of another single dimension (voicing). Infants were readily able to discriminate between /va/ and /sa/ and between /sa/ /Sa/, but no evidence of discrimination could be found for /sa/ and /za/, a discrimination cued by presence or absence of voicing. If the number of cue differences were important, one would assume similar perceptual responses to /sa/-/Sa/ and /sa/-/za/, both pairs of which differed on only one crucial feature. The absence of evidence of discrimination for /sa/-/za/seems on the surface to be in direct contrast to the results of Eimas et al. (1971) who claimed that infants could differentiate b e t w e e n / b / a n d / p / (in CV syllables) on the basis of voice onset time. However, a closer examination of the Eimas et al. data EILERS, MINIFIE: Fricative Discrimination
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165
from synthetic stimuli suggests that o u r / s / - / z / c o n t r a s t is not truly comparable with their/p/-/b/contrast. Although the Eimas et al. study examined the difference between the English p h o n e m e s / p / a n d / b / , the actual phonetic content of their stimulus /p/ included a period of aperiodic energy following the plosive burst. This stimulus would be transcribed as [ph] in IPA symbology. The Eimas et al. /13/ did not, however, have true prevoicing and would be transcribed as [p] in IPA symbology. As Butterfield and Cairns (1974) and Stevens and Klatt (1974) have pointed out, voice onset time (VOT) is not the only cue for discrimination along the "continuum" between 0 VOT and +40 VOT. In [p], VOT +20, a first fonnant transition is present while in [phi, VOT +40, it is absent or markedly shortened. The possibility exists that the infants in the Eimas et al. (1971) study were discriminating the stimuli on the basis of presence or absence of the rapid spectral change associated with the vocalic transition and not on the basis of VOT per se. Further support for the hypothesis that infants may be discriminating on the basis of presence or absence of first formant transition and not on the basis of VOT can be found in Butterfield and Cairns' (1974) report of 10 comparisons chosen for study with infants (conducted by Eimas and Moffitt) along the VOT dimension. Of 10 comparisons, only two yielded significant evidence of discrimination by infants ( + 2 0 versus +40 and +10 versus +60). In both cases where discrimination was demonstrated, in one member of the pair a first formant transition was present and in the other the transition was either absent or greatly reduced. Consequently, it is impossible to tell whether the children heard the difference on the basis of VOT or on the basis of presence or absence of first formant transition. In fact, the results suggest that the presence or absence of the first formant is the crucial factor in this discrimination. Eimas et al. (1971) argued that evidence of discrimination was obtained bec a u s e / b / a n d / p / r e p r e s e n t e d different phoneme categories. None of the other comparisons (tested by Eimas and Moffltt in the study cited by Butterfield and Cairns) could be shown to be discriminable by infants even though the comparisons crossed other "phoneme" boundaries (at least for languages other than English). For instance, infants did not seem to discriminate between [b], VOT = --40, and [p], VOT 0. In this comparison, VOT is probably the only available cue for discrimination. Our /s/ and /z/ are comparable to phonetic [b] and [p], VOT = 0 and V O T - - - - 4 0 , respectively. No studies have demonstrated discrimination on either contrast to date. The data reported by Butterfield and Cairns (1974) indicate that infants reacted to stop consonant stimuli, differing only in VOT, in much the same way as the infants in the present study reacted during the fricative discrimination /sa/-/za/ wherein VOT is unconfounded. Our study thus offers further support for the hypothesis that infants are unable to discriminate on the basis of VOT alone. Even in our data, the possibility exists that the /va/-/sa/ and /sa/-/fa/ discriminations are cued in infants by differences in formant transitions even though formant transitions do not seem to provide the main cue for adult perception in the case of these fricative contrasts. 166 Journal o[ Speech and Hearing Research
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In conclusion then, a reexamination of all the d a ta from studies of infant discrimination of consonants can be explained as resulting from infants' sensitivity to acoustic properties of f o r m a n t transitions. Thus far, d a ta r e p o r te d for bot h stops and fricatives do not support the notion th a t infants can differentiate a m o n g phonetic units which differ only in VOT. Rather, differences in either presence or extent of the first f o r m a n t or direction of the second f o r m a n t as in the/ba/-/ga/ discrimination reported for infants b y Morse (1972) have b e e n p r e s e n t in every case w h e r e discrimination has been demonstrated. ACKNOWLEDGMENT This research was supported by Contract NIH-NICHD-NOL-HD-3-2793. The authors wish to express their appreciation to Leslie A. Wieman and John Folkins for their technical assistance. Requests for reprints may be directed to Rebecca E. Eilers, Senior Research Associate, Child Development and Mental Retardation Center, wJ-10, University of Washington, Seattle, Washington 98195. REFERENCES A~Bs, M., and MINIFm, F., The effects of acoustic cues in fricatives on perceptual confusions in preschool children. 1. acoust. Soc. Amer., 45, 1535-1542 ( 1969 ). BUTTEnFmLD, E., and CAmNS, G., Whether infants perceive linguistically is uncertain, and if they did, its practical importance would be equivocal. In R. Sehiefelbnseh and L. Lloyd (Eds.), Language Perspectives: Acquisition, Retardation and Intervention. Baltimore, Maryland: University Park Press (1974). ErMAS, P., SEQUELaND, E., jvsczY~, P., and VmORITO, J., Speech perception in infants. Science, 171, 303-306 ( 1971 ). HARRIS, K., Cues for the discrimination of American English fricatives in spoken syllables. Lang. Speech, 1, 1-7 (1958). HEINZ, J., and STEVENS, K., On the properties of voiceless fricative consonants. 1. acoust. Soc. Amer., 33, 589-596 ( 1961 ). IRWIN, O., Infant speech: Consonant sounds according to manner of articulation. 1. Speech Hearing Dis., 12, 402-404 (1947). JAKOBSON, R., Child Language, Aphasia and Phonological Universals. Mouton: Hague (1968). MOFFITT, A., Consonant cue perception by twenty- to twenty-four-week-old infants. Child Develpm., 42, 717-731 ( 1971 ). MORSE, P., The discrimination of speech and nonspeech stimuli in early infancy. ]. exp. Child Psychol., 14, 477-492 (1972). OLLER, D., WIEMAN, L., DOYLE,W. and Ross, C., Child speech, babbling, and phonological universals. Papers and Reports on Child Language Development, Stanford Univ. (in press ). STAMPE, D., The acquisition of phonetic representation. Papers from the Fifth Regional Meeting of the Chicago Linguistics Society, Department of Linguistics, Univ. of Chicago, 443-451 (1969). STEVENS, K., and KLATT, D., Role of formant transitions in the voiced-voiceless distinction for stops. ]. acoust. Soc. Amer., 55, 653-659 (1974). STm~VENS,P., Spectra of fricative noise in human speech. Lang. Speech, 3, 32-49 (1960). TP,E~UB, S., Infants' sensitivity to vowel and tonal contrasts. Develpm. Psychol., 9, 91-96 ( 1973 ). TnEHVB, S., and RABINOVITCH,M., Auditory-linguistic sensitivity in early infancy. Develm. Psychol., 6, 74-77 (1972). Received June 7, 1974. Accepted July 14, 1974.
EILERS, MINIFIE: Fricative Discrimination
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