The infancy of infant speech perception: the first decade of research.

Brain Behav. Evol. 16: 351-373 (1979)

The Infancy of Infant Speech Perception: The First Decade of Research1 Philip A. Morse Department of Psychology, University of Wisconsin, Madison, Wise.

Key Words. Infant speech perception • Auditory discrimination • Categorical discrimination • Perceptual constancy Abstract. Studies of infant speech perception have focused primarily on the ability of young infants (a) to make simple auditory discriminations, (b) to categori cally discriminate speech sounds and (c) to exhibit perceptual constancy for phonet ic categories. The first decade of developmental literature for stop consonants, vow els, and other speech and nonspeech contrasts is examined from these three perspec tives. In addition, research into the origins (ontogenetic and phylogenetic) of these abilities is discussed, along with the studies that have probed the psychological and physiological processes that underlie the infant’s speech perception accomplish ments.

1 Preparation of this review was in part supported by NICHD grants HD08240 and HD-03352.

Downloaded by: Karolinska Institutet, University Library 130.237.122.245 - 1/17/2019 1:03:08 PM

As evidence began to surface over the course of the past 20 years suggesting that the adult’s perception of our species-specific sounds is based on a complex code, subject to a variety of fascinating acoustic, phy siological, and cognitive processing constraints [see Pisoni, this volume], many investigators were eager to uncover the developmental course of these abilities during early infancy. However, it has only been in the last decade with the development of clever infant discrimination paradigms that the speech perception knowledge of the human infant (Infantus terribilis) has begun to yield to our curiosity. The three procedures that have been the most responsible for the advances in our understanding of infant speech discrimination are the operant nonnutritive, high-amplitude suck ing paradigm (HAS), the heart-rate orienting response paradigm (HR), and the visually-reinforced, head-turning paradigm (HT). In each of these

352

procedures the infant is presented with a familiar speech sound for a peri od of time and then his/her reaction is measured to the presentation of a series of novel sounds at the appropriate time in the experiment. To be more specific, in the HAS paradigm a speech sound (usually a single syllable) is played through a loudspeaker to the infant contingent upon the infant’s rate of hard (high-amplitude) sucking. Thus, the more the infant sucks on a nonnutritive nipple, the more presentations of that syllable the infant receives. As the infant ‘discovers’ this contingency, the rate (per minute) of high-amplitude sucking increases (acquisition). Even tually, the infant begins to satiate or habituate to the sound and the rate of high-amplitude sucking decreases. If the rate decreases in accordance with a predetermined criterion, then the sound is changed and a subse quent increase in responding relative to a no-change control condition is interpreted as evidence of discrimination of the two different sounds. This particular procedure has been found to be especially effective with infants between birth and 4 months of age. In contrast, in the HR paradigm the presentation of sounds is entirely under the control of the experimenter. If a speech sound or string of speech sounds is played at a moderate intensity, infants will generally re spond to the sound onset with heart-rate deceleration (orienting). With re peated trials of these same sounds, this initial orienting response (OR) tends to habituate. After a fixed number of trials (to permit the habitua tion of the cardiac OR), a novel sound is presented for a series of trials and if recovery of the OR (cardiac deceleration) occurs to the new sounds, then discrimination of the stimulus contrast is inferred. Variations of this procedure have been successfully employed with human subjects ranging in age from birth to adulthood. Finally, in the HT procedure the infant is seated on his/her parent’s lap and entertained at the midline by an assistant playing with an interest ing toy. From a speaker located in front and to the side of the infant (about 45°) is presented a constant speech sound as a background stimu lus in the study. After listening to this background sound for awhile, the presentation of a few tokens of a novel stimulus periodically interrupts the background stimulation. If the infant turns his/her head toward the speaker when this change occurs, then a visual reinforcer (e.g., an animat ed monkey or dancing bear) is illuminated within a darkened box for the infant to view. If the infant turns his/her head more frequently on those trials on which a change in sounds occurs, compared to no-change control trials, then discrimination of the contrast is inferred. This procedure has


Morse

353

been found to be primarily applicable with infants 5 or 6 months of age and older [see Kuhl, this volume]. Armed with these new procedures for studying infant perception, in vestigators began a decade ago to extend to infants the adult speech dis crimination phenomena discussed in the paper by Pisoni [this volume]. In particular, they have tended to ask primarily three types of discrimination questions of the young infant: (a) simple auditory discrimination ques tions, (b) categorical discrimination questions, and (c) perceptual constan cy or generalization questions. In studies which merely ask if the infant can distinguish two different speech sounds (e.g., [ba] vs. [da]), only a simple auditory discrimination question has been posed. In contrast, a study of categorical discrimination might involve the examination of the infant’s discrimination of pairs of stimuli selected from a [ba/da] synthes ized continuum, such that all pairs differ in equal acoustic amounts. How ever, in such a study some pairs are selected from within the same adult category (e.g., [ba]) and other pairs contain a member from two different categories on the continuum (e.g., [ba] and [da]). If the infant discrimi nates the £>e/u>ee«-category contrast but not the w/i/im-category contrast, then categorical discrimination of [ba] and [da] has been demonstrated [see Pisoni, this volume, for a more detailed discussion of categorical per ception], Finally, studies of perceptual constancy or generalization are concerned with the infant’s ability to demonstrate that his or her speech sound categories will tolerate not only the mild variations in the cues for that sound along an acoustic continuum (as demanded in categorical dis crimination studies), but also radically different variations in the cues which may signal that category. To continue our example, adult listeners may assign a speech sound to the [b] vs. [d] categories on the basis of the second formant transition cue employed in studies of categorical percep tion, but the particular cue can differ considerably depending on the vow els that accompany it in the syllable, the position of the consonant in the syllable, the pitch and vocal tract of the speaker who utters the sound, etc. Thus, a study of perceptual constancy might seek to determine if the infant’s [b] category also includes all or many of these context-dependent variations by showing that radically different acoustic exemplars of the [b] category are still considered by the infant part of the same phonetic category [see Kuhl, this volume, for a more extensive discussion of per ceptual constancy]. The safari which has been the product of these paradigms and ques tions has in its first decade been a very exciting one, filled with an impres


The Infancy of Infant Speech Perception

Morse

354

sive inventory of phonetic knowledge in the infant within only the first few months after birth. In fact, it has been relatively difficult to demon strate the absence of many of the adult speech perceptual phenomena out lined by Pisoni in this volume. The next section provides us with a chrono logical account of this safari and its major findings. We shall consider separately the findings related to the discrimination of stop consonants (place of articulation and voicing contrasts), vowels, other phonetic con trasts, and nonspeech sounds.

Place of Articulation Contrasts The first study of infant speech perception to begin our safari oc curred 1U years ago with Moffilt’s [1968, 1971] demonstration of the simple auditory discrimination of the place of articulation contrasts, [ba] vs. [ga], in 4- to 5-month-old infants using a HR paradigm. McCaffrey [1971] also reported evidence of place discrimination using a variation of the HR procedure in 3- to 7-month-olds. However, several methodologi cal problems with this study make his results difficult to interpret [Morse, 1974], Morse [1972] subsequently demonstrated, using the HAS proce dure, that infants could discriminate the [ba-ga] place contrast as early as 6 weeks of age. He also showed that these same-aged infants responded differently to the critical cues for [ba] and [ga] when they were presented in isolation. However, Eimas [1974a] was the first to demonstrate in two separate studies that young infants (2 and 3 months of age) discriminated the place continuum for [bae-dae-gae] categorically. In addition, he also found that infants, like adults, did not evidence categorical discrimination for the critical cues for these place contrasts when presented in isolation as nonspeech sounds. At about this same time, Leavitt et al. [1976] set out to replicate Morse's [1972] demonstration of place discrimination in 6-week-olds, but using a HR paradigm. When they employed the multi-trial procedure used by Moffitt [1971], which involved six familiarization trials each sep arated by long intertrial intervals (about 30 sec), they failed to obtain evi dence of place discrimination. However, when they employed a variant of the HR paradigm in which the infant is presented with a string of one sound (e.g., [ba]) followed by a string of a novel stimulus (e.g., [ga]), dis crimination of the place contrast employed by Morse [1972] was obtained


Studies of Stop Consonant Perception

355

in 6-week-olds. A similar set of contrasting results was observed by Miller ei al. [1977] for the place contrast [bu] vs. [gu], which differed only in the initial 8-32 msec release burst cue (rather than in the second or third formant transition cues). They also found no evidence of discrimination of this contrast when a multi-trial HR paradigm (similar to Mofjitt’s) was employed with 3- to 4-month-olds. On the other hand, strong evidence of this burst cue discrimination was obtained in these same-aged infants when 20 tokens of one syllable were followed by 20 tokens of the novel stimulus (20/20 HR paradigm). Encouraged by these results, Miller and Morse [1976] replicated Eitnas’ [1974a] finding of categorical perception for the place contrast [dae] vs. [gae] using a within-subjects 20/20 HR paradigm in 3- to 4-month-olds. In a study conducted concurrently in a different laboratory with different synthetic stimuli, Till (1976) obtained the same results using the 20/20 HR procedure in 4-month-olds. More recent studies of place of articulation discrimination have fo cused on the infant’s perception of these contrasts in positions in a sylla ble or string of syllables other than the initial position. For example, Jusczyk [1977] has shown using the HAS procedure that 2-month-olds can discriminate the [d]-[g] contrast in final position in consonant-vow el-consonant and vowel-consonant syllables. Furthermore, this discrimina tion is as strong as that obtained for the same contrast in the initial posi tion of a consonant-vowel-consonant syllable. Jusczyk and Thompson [1978] have also observed in similar-aged infants with the HAS paradigm that place contrasts that occurred in the second of two-syllable stimuli (e.g., [bada] vs. [baga]) were as discriminable as the same contrast in the initial syllables (e.g., [daba] vs. [gaba]). In addition, when these contrasts occurred in unstressed syllables they were no more difficult than ones oc curring in stressed syllables. Although these findings generally suggest no differences in discrimination between stress vs. non-stress contrasts, this may in part be due to the limitations of the HAS procedure in detecting relative differences in discrimination. Finally, using the HAS paradigm Williams [1977] found that young infants discriminated place contrasts in medial position in a vowel-consonant-vowel sequence, but only if a silent closure period preceded the consonant. In the only attempt to date to explore the perceptual constancy or generalization of the infant’s place categories, Fodor ei al. [1975] used a type of conditioned HT procedure with infants 3.5-4.5 months of age and syllables such as [pi], [ku], [pa]. They observed that conditioning pro ceeded more quickly if syllables with the same initial consonant were



iiimiimatiauub'tiOuuuanuuBuuBikio«

Morse

356

Voicing Contrasts The first study to demonstrate the infant’s ability to discriminate voicing contrasts in stop consonants was conducted by Eimas et al. [1970, 1971]. It was not only the first speech perception study to employ the HAS paradigm, but it was an historic first in its demonstration of categor ical discrimination in young infants. Eimas et al. [1970, 1971] found that 1- and 4-month-old infants discriminated the voicing contrast [ba] vs. [pa] categorically. This categorical discrimination finding was later repli cated by Eimas [1974a] in 2- and 3-month-olds with the HAS procedure and the alveolar contrast [da] vs. [ta]. In addition, Miller [1974] inde pendently replicated the original Eimas et al. [1970, 1971] [ba]-[pa] re sult using a similar procedure with infants 6- to 8-weeks of age. Finally, Trehub and Rabinovitch [1972] also provided confirmation of the 4- to 17-week-old infant’s ability to discriminate auditorily synthetic as well as natural tokens of [ba] and [pa] using the Eimas et al. [1970, 1971] HAS procedure. Although Eimas et al. [1970, 1971] were the first to successfully demonstrate voicing contrasts in infants, Yonas [see Doty, 1974] was en couraged by Moffitt’s [1968, 1971] earlier success with place discrimina tion using the HR paradigm and thus she attempted at about this same time to use the HR paradigm to test infant voicing discrimination. How ever, using the HR paradigm she found no evidence of discrimination of any voicing contrasts in 2-month-olds, although much older infants (10month-olds) did appear to discriminate the [ba]/[pa] contrast. Interesting-


grouped together (e.g., [pi/pu] vs. [ku]) as opposed to combinations in which different initial consonants were both reinforced (e.g., [pi] and [ku] vs. [pa]). However, since natural speech tokens were employed in this study, it is possible that this ‘generalization’ was based on an invariant acoustic cue in the [p] category, rather than on generalization across dif ferent acoustic cues for a given place category. In summarizing the research on infant place discrimination, a num ber of studies have documented the infant’s ability within the first few months of age to discriminate auditorily a variety of the aspects of place of articulation contrasts perceived by adult listeners, as well as to discrim inate these speech categories categorically. However, with the possible ex ception of the study by Fodor et al. [1975], we do not yet have any strong evidence of perceptual constancy or generalization for place of articula tion categories [of the sort discussed by Kuhl, in this volume].

357

ly, recent attempts in our own laboratory to demonstrate the discrimina tion of the voicing contrast, [ba] vs. [pa], using the 20/20 version of the HR paradigm, have also been unsuccessful with young infants. Initially, Roth and Morse [1975] conducted a within-subject categorical study [similar to the place study by Miller and Morse, 1976] using the same [ba]-[pa] contrasts employed by Eimas et al. [1970, 1971]. However, the 3- to 4-month-olds failed to yield any strong evidence of discrimination in the between-category condition. In an effort to investigate parametrically some of the components of the 20/20 HR paradigm that might have been responsible for this unsuccessful attempt, Miller and Hankes-Ruziclca [1978] carried out a series of HR experiments with 3- to 4-month-olds using synthetic and natural speech versions of the [ba] vs. [pa] contrast. However, they too were unsuccessful in demonstrating reliable group dis crimination for these voicing contrasts. Thus, in contrast to the strong ev idence of voicing discrimination in the first 4 months after birth obtained with the HAS paradigm, several studies using the HR paradigm(s) have consistently found no evidence of voicing discrimination in early infancy. Furthermore, this paradigm difference in the results for voicing contrasts has not been observed in the HR and HAS studies of infant place dis crimination. Some of the possible reasons for these patterns of results will be considered later when we probe the processes that may underlie the in fant’s discriminative performance in these paradigms. With the discovery that infants as young as 1 month of age discrimi nate voicing contrasts categorically [Eimas et al., 1971], researchers be gan to ask themselves whether this ability was inborn or depended upon the infant’s linguistic experiences. Although it is clear from studies of in fants’ early vocalizations that they are not yet producing in their speech these voicing contrasts [e.g., Kewley-Port and Preston, 1974], it is still possible that they develop these perceptual categories by listening to speakers around them. In English we employ the two voicing categories: voiced (e.g., [ba]) and voiceless (e.g., [pa]), but in some languages a third voicing category, prevoiced (e.g., a prevoiced [ba]) is employed. Thus, by studying the infant’s discrimination of nonnative language contrasts, we might be able to determine the role played by experience within a particu lar linguistic community. To this end, Streeter [1976] used the HAS pro cedure to test 2-month-old infants in Kenya whose parents spoke only Kikuyu, a Bantu language that employed the prevoiced/voiced distinction only. These infants nevertheless discriminated the English or foreign con trast (voiced/voicelcss), as well as the prevoiced/voiced contrast of Kiku-



Morse

uuiuuiufluumuuuuu

ibttitói

358

yu. However, unlike the American infants tested by Eimas et al. [1971], Streeter’s [1976] subjects failed to exhibit categorical discrimination for the foreign (voiceless) category. Eimas [1975b] tested American infants on an analogous task using the HAS procedure and the prevoiced/voiced alveolar ([d]) contrast which is not native to English. In a series of experi ments, he too observed reliable discrimination of this foreign contrast, but discrimination of this ‘novel’ category was also not categorical. Although the interpretation of this study is somewhat complicated by the large acoustic differences between the stimuli employed along the continuum and some uncertainty in the adult literature regarding the location of the prevoiced/voiced boundary, the pattern of results is strikingly similar to those observed by Streeter [1976]. Both investigators observed categori cal discrimination for the native category, but they found for the foreign category reliable between-category discrimination which failed to differ significantly from the within-category discrimination. In a study similar to Streeter’s [1976], Lasky et al. [1975] employed a multi-trial HR para digm in the speech discrimination of 4- to 6.5-month-old infants living in rural Guatemala, whose parents spoke only Spanish, a language with a prevoiced/voiced, but no voiced/voiceless contrast. As in Streeter’s [1976] study, their infants also discriminated the English (foreign) con trast. However, these same infants failed to discriminate their own native contrast and instead exhibited reliable discrimination within the prevoiced category. The cause of these anomalous findings is not completely clear, although methodological problems in the determination of the heart-rate results may have contributed to them [see Morse, 1978]. The results of a recent study reported by Eilers et al. [1979a] with infants of Cuban (Spanish) parents support the finding of Lasky et al. [1975] of discrimina tion across the foreign (English) boundary, but yielded the expected re sults for the Spanish (native) contrasts. Using the HT paradigm with 6-month-olds, they found comparable discrimination of the English (for eign) and Spanish (native) contrasts, with no discrimination within either the prevoiced or voiceless categories. In contrast, a group of Englishlearning 6- to 8-month-olds discriminated only the voiced/voiceless (English) stimulus pair and failed to discriminate between the prevoiced and voiced categories (foreign contrast). This pattern of discrimination for English-learning infants replicates that observed in an earlier HT study with 6-month-olds [Eilers et al., 1979b]. In this earlier study they tested American infants with eight different pairs of stimuli drawn from the voicing continuum and only observed discrimination at the voiced/


umumuui

359

voiceless (English) category boundary and not at the prevoiced/voiced boundary, even when the acoustic difference between stimuli [as in the Eimas, 1975b study] was very large. In summarizing these cross-language infant studies, the data suggest that if the foreign contrast is a voiced/voicelcss one, then infants will be able to discriminate it. However, if the foreign contrast is between the prevoiced and voiced categories, then infants (6-month-olds) do not dis criminate it. This finding is consistent with recent suggestions that the English (voiced/voiceless) boundary may be psychoacoustically more eas ily discriminable [Stevens and Klatt, 1974], The intriguing possibility re mains, however, that at 2- to 3-months of age [Eimas, 1975b] American infants are able to make this prevoiced/voiced discrimination, but perhaps with disuse or lack of appropriate linguistic experience this ability begins to disappear from the infant’s repertoire [Eimas, 1976; Trehub, 1976]. Clearly, additional probing is necessary to explore both these psycho acoustic and developmental hypotheses regarding the acquisition of voicing discrimination. In addition to the extensive studies of auditory and categorical dis crimination of voicing contrasts in syllable-initial position, some investi gators have explored the development of the voiced/voiceless distinction in multi-syllabic, non-initial position contexts in which the vowel duration cue may play an important role. For example, Trehub [1973] found using the HAS paradigm that 4- to 17-week-olds discriminated [aba] vs. [apa], but not [ataba] vs. [atapa]. More recently, Eilers [1977] and Eilers et al. [1977] employed three sets of contrasts: (1) a voice bar and vowel dura tion contrast ([at/a:d]), (2) a voice bar contrast only, with long vowel du rations ([a:t/a:d]), and (3) a vowel duration contrast only ([at/a:t]). The HAS procedure was used with infants 2- to 3-months of age, whereas the HT paradigm was employed with a group of 6- to 8-month-olds and a group of 12- to 14-month-olds. All these age groups were able to discrim inate the contrast that employed both voice bar cues and vowel duration cues. It is unlikely that this ability was due primarily to the vowel dura tion cue, since virtually all of the infants in the two younger groups and half of the infants in the oldest group failed to discriminate the [at/a:t] contrast. Whether infants discriminate the voicing contrast in final posi tion when the voice-bar cue is the primary feature available, is somewhat unclear; the youngest infants did, the middle age group did not, and half of the oldest group did. Eilers [1977] suggests that the youngest age group (HAS) results for this last contrast may be a false positive in light of the



Morse

360

HT findings for the two older groups. Although Eilers and her colleagues have made the first significant contributions to our understanding of the contextual constraints involved in the infant’s voicing discriminations, a great deal remains to be learned about the infant’s voicing categories, es pecially the perceptual constancy aspects of them, which as of yet remain unexplored.

Interest in studying the infant’s ability to categorize and discriminate vowels has been considerably less than that evidenced for stop consonant perception, perhaps because vowels are generally not perceived categori cally. In the first of the six studies conducted to date on infant vowel dis crimination, Treliub [1973] obtained evidence using the HAS paradigm that 4- to 17-week-old infants can discriminate auditorily the vowel con trasts [i] vs. [u], [i] vs. [a], [pa] vs. [pi], and [ta] vs. [ti]. Subsequently, Swoboda et al. [1976] demonstrated with the HAS procedure that 8-weekolds discriminate the continuum between [i] (as in beet) and [I] (as in bit) in a continuous or noncategorical manner. In other words, just as adult listeners do, infants discriminated both between and within catego ries of these vowels. Most recently, Swoboda et al. [1978] demonstrated with the same procedure and age range of infants, that if the durations of the [i] and [I] vowels are shortened, then infants (like adults) discriminate them more categorically. It has been suggested that in adults this increase in categorical responding reflects the role of short-term memory con straints in vowel perception. Thus, the results of the study of Swoboda et al. [1978] suggest that infants, too, appear to be subject to similar con straints in their categorization of vowels. Finally, in the most recent set of infant vowel studies, Kuhl [1976b] has shown that 1- to 4-month-old in fants in the HAS paradigm exhibit some perceptual constancy for the vowel categories [a] and [i], across different intonational patterns, i.e., they were able to discriminate the vowel change, despite variations in the pitch contours of both of the vowels. Kuhl has obtained similar and more impressive perceptual constancy results with the HT paradigm in 6-montholds using the vowels [a] vs. [i] [Kuhl, 1976c] and [a] vs. [3 ] [Kuhl, 1977] with variations in intonation and speaker [see Kuhl, this volume]. In sum, although the quantity of studies in infant vowel perception has been small, they have nevertheless revealed an impressive set of


Vowel Perception Studies


361

adult-like perceptual capabilities, which include evidence of simple audi tory, categorical, and perceptual constancy abilities in early infancy.

Research on speech sounds other than stop consonants and vowels has included studies primarily of fricatives, semi-vowels and liquids, and suprasegmental information such as intonation and stress. In the first study to demonstrate that the infant’s simple speech perceptual abilities might not be as advanced as the adult’s, Eilers and Minijie [1975], using the HAS paradigm, showed that infants between 4 and 17 weeks of age could discriminate auditorily the fricative contrasts [sa] vs. [va], [sa] vs. [5a] as in (‘she’), but not the voiced/voiceless fricative contrast [sa] vs. [za], Eilers [1977] replicated this discrimination failure in a second study using a more sensitive version of the HAS paradigm. She did find, howev er, that infants 9-19 weeks of age could discriminate this same contrast in final syllable position, provided that vowel duration served as an addi tional cue (i.e., [as/a:z] was discriminated, but [a:s/a:z] was not). Pursuing the development of these fricative discriminations, Eilers et al. [1977] tested 6- to 8-month-olds and 12- to 14-month-olds with the HT proce dure and found that in addition to discriminating the contrasts [sa/va], and [sa/Sa], and [as/a:z], both age groups were able to differentiate tire contrasts that 3-month-olds were unable to discriminate ([sa/za] and [a:s/ a:z]). However, when the fricative pairs [fi/0i] and [fa/0a] were present ed, the 6- to 8-month-olds failed to discriminate either contrast, but the 12- to 14-month-olds were able to distinguish [fi] from [0i], but still not [fa] from [0aj. Thus, taken together these studies by Eilers and her col leagues suggest that discrimination of some of the major speech contrasts employed by adults develop during early infancy. The difficulty with [f] and [0] has also been documented for 2-year-olds [Eilers and Oiler, 1976] and 4- to 5-year-olds [Abbs and Minijie, 1969], Finally, in a most recent study of fricative discrimination, Holmberg et al. [1977] observed with a modified HT procedure in which infants were trained across sever al days that 6-month-olds can discriminate not only [s] and [5], but also [f] and [0], and that furthermore they can generalize these discrimina tions across speakers and vowel environments. Although Holmberg et al. [1977] did find evidence of [f-0] discrimination in 6-month-olds with this modified HT technique, the [f-0] discrimination and generalization task


Studies of other Speech Contrasts

362

was by far the more difficult one for the 6-month-olds, thereby offering indirect support for the order of difficulty observed in the Eilers et al. [1977] less intensive assessment of fricative discrimination. Information about the infant’s perception of liquids and semi-vowels is very limited. Eimas [1975a] observed in 2- to 3-month-olds using the HAS procedure that [ra] vs. [la] was discriminated categorically. This is characteristic of English-speaking adults, but not of Japanese-speaking adults [Miyawaki et al., 1975]. In the only other related investigation, Trehub [1976] observed with the HAS procedure that 5- to 17-week-old English-leaming infants could discriminate the Czech [ra/za] contrast, which English-speaking adults found very difficult to distinguish. In the first study of the discrimination of the semi-vowel glides, [wa] and [ya], Jusczyk et al. [1977a] used the HAS procedure to demonstrate that 2-month-olds discriminated this contrast in two-syllable words, whether the contrast occurred in the initial or final syllable (e.g., [dawa] vs. [daya]) and whether it was the stressed or unstressed syllable (e.g., [dawa] vs. [daya] or [dawa] vs. [daya]). In addition, Hillenbrand and Minifie [1977] have found that 6- to 7-month-olds tested with the HT paradigm differentiated sounds that differed solely in the duration of the formant transitional cues and ranged from a stop consonant ([be]) to a semi-vowel ([we]) to a double vowel ([ue]). Finally, a few studies have ventured beyond segmental (consonant and vowel) discrimination questions to probe the infant’s perception of supra-segmental cues. In the first study of intonation perception, Kaplan [1969] assessed 4- and 8-month-old infants’ discrimination of rising vs. falling intonation contours on the sentence, ‘See the cat’. Measuring changes in heart rate variability, she found that both age groups changed their heart rate variability when presented with some of the changes in in tonation pattern, although she interpreted her data as suggesting that only the 8-month-olds discriminated the intonation contrast. Several years lat er, Morse [1972], testing 6-week-olds with the HAS procedure, found that they were able to discriminate a rising vs. falling intonation contour imposed on the syllable [ba]. More recently, Kuhl [1976b] has shown with the HAS procedure that 1- to 4-month-olds discriminate a monotone vs. a rise-fall pitch contour, but when a vowel variation ([a] and [i]) is in troduced into the two intonation categories, infants are unable to discrim inate the intonational difference, thereby failing to exhibit perceptual con stancy for these intonational contrasts. Another aspect of perceptual con stancy for intonation that no one has yet explored in infants is the extent


Morse

LiimuuumnnHin»M»t««niMi auauuEumü.M EfWfl»1.


¡umamrnn

363

to which their intonation categories are independent of the terminal pitch of the contour as they appear to be for adult listeners [Hadding-Koch and Studdert-Kennedy, 1964J. Finally, in the most recent study of supraseg mental perception in infants, Spring and Dale [1977] found that 1- to 4-month-olds discriminated in a modified HAS paradigm differences in the stress of two-syllable stimuli (e.g., [baba] vs. [baba]).

Studies of Nonspeech Contrasts

As investigators sought to determine if the infant’s perception of speech sounds differed from that for other sounds, they began by using nonspeech discriminations. For example, Morse [1972] found in an HAS study of 6-week-olds that they discriminated the second and third formant cues differently in nonspeech and speech [ba/ga] contexts. Eimas [1974a] then observed that infants 2 and 3 months of age discriminated the isolat ed second formant transition cues critical to the [bae/dae] contrast in a noncategorical manner, as adult listeners do. Finally, using a slightly dif ferent approach, Jusczyk et al. [1977b] found in an HAS study of 2-month-olds that the nonspeech sounds ‘pluck’ and ‘bow’ were discrimi nated categorically, as is also the case for adult listeners [Cutting and Rosner, 1974], When these nonspeech results are considered with the findings re ported over the last decade for infant speech sound discrimination, we find that not only can young infants auditorily discriminate almost all of the basic sound contrasts used by adult speakers (except some fricatives), but they also exhibit patterns of categorical and noncategorical discrimi nation for stop consonants, vowels, liquids, and nonspeech sounds that mirror those observed in more mature speaker-listeners. Finally, there is also preliminary evidence that infants may exhibit some degree of percep tual constancy for some of the phonetic categories [see Kuhl, this volume, for additional discussion of this point].

In retrospect, the first decade of research on infant speech perception has witnessed an impressive documentation of the infant’s ability to dis criminate the major speech sound contrasts characteristic of human adult


Origins and Underlying Processes

364

listeners. However, in addition to this primarily ‘adult-to-infant’ leg of the safari, some investigators have wandered off the beaten path and begun to ask what experiential, cognitive, and neurophysiological processes may underlie the development of these perceptual abilities. In other words, these researchers have probed beyond the question of whether infants perceive the same speech sound contrasts as adults do, and in so doing have extended our safari in infant speech perception into the domain of the peculiarities of the developing infant and the underlying processes governing his or her speech perception. As we have already seen in our discussion of voicing contrasts and in Trehub’s [1976] study of Czech contrasts, one direction that this inquiry has taken has been to use foreign speech sounds in determining the role of early linguistic experience. Although, at present, the role of psychoacous tic variables (e.g., the ‘special’ features of the English voiced/voiceless boundary) cannot be ruled out in these comparisons, many readers of these results reached the general conclusion that perhaps the development of these discriminative abilities did not depend very much upon the infant’s specific language environment. This prompted investigators to look for the origins of these abilities in two different directions. Some researchers began to look phylogenetically, testing human speech sound contrasts on monkeys [Morse and Snowdon, 1975; Sinnott et al., 1976; Waters and Wilson, 1976] and chinchillas [Kuhl and Miller, 1975; Kuhl, 1976a], These results are discussed in detail in the article by Kuhl in this volume. A second direction that was taken by Eimas and his colleagues [Eimas, 1975b] was to suggest, using the adaptation paradigm in adult listeners, that the neurophysiology of the auditory system possesses feature detec tors which are sensitive to the voicing, place, etc., features of human speech sounds and that these feature detectors may be there from birth [see Pisoni’s article in this volume for a brief discussion of this literature]. Ultimately, both lines of inquiry require a common understanding of how the neurophysiology of the auditory system actually does code the features of human speech sounds. This will necessitate more electrophysiological as well as behavioral research in nonhuman animals using com plex auditor}' signals in addition to the simple auditory signals that are traditionally employed in auditory neurophysiology. In addition, we need to learn a great deal more about the developing neurophysiology of the human infant’s auditory system. Hecox [1975] has recently reviewed what little is currently known about the normally developing auditory sys tem. Furthermore, a study by Graham et al. [1978] illustrates the import


Morse

365

ance of considering instances of aberrant development in learning about the normally developing nervous system. Graham et al. [1978] found, us ing a multi-trial HR paradigm, that an anencephalic (no cerebral cor tex) infant between 3 and 6 weeks of age attended to, remembered, and discriminated the [ba-ga] contrast employed by Morse [1972] and Leav itt et al. [1976] with 6-week-olds. This finding suggests that the neocortex is not essential either for this particular auditory discrimination, or for the psychophysiological-cognitive features of the multi-trial HR paradigm employed to test this infant. Finally, one very promising electrophysiological method that has re cently been found to tell us something of how the infant’s cortex differen tially codes various speech and nonspeech sounds has been the averaged evoked response (AER) based on the infant’s electroencephalographic (EEG) activity. By recording the electrical activity from the surface of the scalp over the auditory areas in the left and right hemispheres, Molfese and his coworkers have found that newborns, 5-month-old infants, 5-year-old children, and adults respond to speech sounds and words with a greater amplitude over their left hemispheres, and with a greater ampli tude over their right hemispheres to nonspeech sounds [Molfese, 1972; 1977; Molfese et al., 1975]. More recently, employing a factor-analytic method of analyzing AER data, Molfese has found that the right hemi sphere responds categorically to the voiced vs. voiceless, ([ba/pa]) voicing categories in adult [Molfese, 1978], preschool [Molfese and Hess, 1979], and 2- to 4-month-old infant [Molfese and Molfese, 1979] listeners, but not in newborns [Molfese and Molfese, 1979]. Specifically, surface mea surements taken over the auditory cortices of these listeners indicate that only the right hemisphere reliably exhibits different responses to betweencategory voicing contrasts, but similar responses to within-category conttrasts. In addition, another AER factor was found to reveal that both hemi spheres can differentiate all pairs of voicing contrasts, both within and between categories. It is interesting that although Molfese and his col leagues have found that these right hemisphere differences in categorical perception are highly replicable across a wide age range, they do not ap pear to occur in newborns for voicing cues. In contrast, similar AER studies of place of articulation contrasts ap pear to reveal differential discrimination of [bae] vs. [gae] over the left hemisphere, in adults and newborns [Molfese, personal commun.]. This latter finding is also consistent with a left hemisphere laterality observed by Entus [1977] with the HAS paradigm in infants 3- to 20-weeks of age



366

using the speech sound contrasts [ba], [da], and [ma], In contrast, Entus [1977] found that nonspeech (music) contrasts were discriminated better in the right hemisphere. Although a recent study by Khadem and Corballis [1977] has failed to replicate Entus’ [1977] laterality findings, Glanville et al. [1977] have observed a reliable left hemisphere advantage for the auditory discrimination of a place of articulation contrast in 3-month-olds using a HR paradigm. Finally, to further complicate matters, the right hemisphere effects for voicing categories may depend on the type of AER paradigm em ployed, since left hemisphere, categorical responding was obtained in adult listeners when an habituation/discrimination AER paradigm was employed [Molfese, 1977], Thus, the AER technique holds considerable promise, not only for tracing the development of cortical participation in different types of speech phenomena and contrasts, but perhaps even in different paradigms as well. In summary, these cross-cultural, animal, adaptation, neurophysiolo gical and electrophysiological studies of the origins of the infant’s audito ry and categorical abilities in speech perception have given us an exciting glimpse of how the developing nervous system codes the categories of hu man speech. In the same way that the early work of Moffitt [1968, 1971] and the demonstration of Eimas et al. [1971] of infant categorical dis crimination launched our safari in infant speech perception a decade ago, these studies of the origins of these abilities seem to have brought the re search to a new phase of inquiry which promises to be as exciting if not more so than the first one. A second and equally important domain of infant speech perception, that some researchers have ventured into, focuses on the processes which underlie the infant testing paradigms and therefore may contribute to positive or negative evidence of infant discriminative abilities. For exam ple, a number of investigators have been interested in what processes might be responsible for the infant’s pattern of sucking responses to the familiar and novel stimuli in the HAS paradigm. Trehub and Chang [1977] have demonstrated that the infant’s rate of sucking does not in crease systematically in an experiment as speech sounds are presented, unless the infant’s sucking is contingent upon the presentation of the sounds. Furthermore, the increase in sucking generally observed to a nov el stimulus also occurs only in the contingent arrangement. It is absent in a no-sound condition [Kuhl, 1976b] and also when the sounds are pre sented noncontingently, either periodically or randomly [Williams and


Morse

367

Golenski, 1978]. Thus, the HAS paradigm results cannot be attributed to cyclical changes in the infant’s sucking pattern, but truly reflect the ac quisition of control over the contingencies involved, coupled with perhaps an ‘interest’ in the novel speech sound. Interest in the processes underlying the infant HR paradigm was pre cipitated by the surprising observation that discrimination studies using the multi-trial HR paradigm (with approximately 30 sec of silence be tween trials) have generally succeeded in demonstrating discrimination with infants older than 4 months [e.g., Moffitt, 1971; Berg, 1974; Lasky et al., 1975], but generally failed with younger infants [e.g., Leavitt et al., 1976; Miller et al., 1977]. Suspecting that the long intertrial intervals may have imposed too much of a short-term memory burden on the younger infants, Leavitt et al. [1976] and Miller et al. [1977] employed a no-intertrial-interval paradigm in which a string of tokens of the familiar sound was followed immeditely by a string of the novel sound [e.g., the 20/20 paradigm of Miller and Morse, 1976], Using this no-intertrial-interval HR paradigm, 6-week-olds [Leavitt et al., 1976] and 3- to 4-month-olds [Miller et al., 1977] were able to discriminate the same place contrasts that infants of these ages fail to discriminate using the multitrial HR para digm. Thus, both of these studies suggest that underlying memory proc esses may be important in HR studies of speech perception with infants of different ages. A similar approach to the investigation of the memory constraints of the HAS paradigm was taken by Swoboda et al. [1976, 1978] in their studies of vowel perception. Since in the HAS procedure the timing be tween sound presentations is under the infant’s control, rather than the experimenter’s, Swoboda et al. [1976, 1978] examined the relationship between the infant’s discrimination performance and the duration of the silent interval that occurred for each infant between the last token of the familiar sound and the first opportunity to hear the novel sound. These authors found that over the range of 0-30 sec, the longer the silent inter val, the worse the infant’s ability to discriminate a sound change. Finally, using a somewhat different approach to the role of short term memory in the HAS paradigm, Spring and Dale [1977] in their study of stress discrimination developed a modification of the HAS procedure designed to minimize the memory constraints of this method. Instead of presenting the infant with repeated tokens of a novel stimulus following satiation on the initial stimulus (e.g., [ba, ba, . . . ba, ba/ba, ba, . . . ba, ba]) they played alternating tokens of the novel and initial stimuli to the



368

infant when the stimulus change occurred (e.g., [ba, ba, . . . ba, ba/ba, ba, . . . ba, ba]). Thus, both the HAS paradigm and the HR paradigm have major fea tures that involve the role of short-term memory processes, which may be studied in their own right as they affect discrimination. Specifically, not only can we begin to explore systematically developmental changes in these constraints, but we can evaluate them and use them in our assess ment of different types of stimulus contrasts, especially those stimuli which young infants appear unable to differentiate. Occasionally, such a manipulation may even have theoretical, as well as empirical implica tions, as in the case of the two studies by Swoboda et al. [1976, 1978]. In these two studies the authors examined the contributions of short-term memory constraints not only by looking at the silent interval/discrimination relationship, but by manipulating the duration of the vowel stimuli employed. It will be recalled that when long (300 msec) vowels were used [Swoboda et al., 1976], 8-week-old infants discriminated the vowels [i] and [I] continuously (both between- and within-category dis crimination). However, when shorter versions of these same vowels were employed (60 msec), thereby reducing the auditory short-term memory available to the listener [see Pisoni, this volume], these 8-week-olds dem onstrated more categorical-like discrimination [Swoboda et al., 1978]. Finally, one other aspect of our safari which has ventured into ques tions about the processes underlying the infant paradigms (and thus by extension, underlying the infant’s discrimination) has focused on the rela tionship between studies of adult adaptation to speech sounds [e.g., Eimas and Corbit, 1973] and the satiation or habituation phase of the HR and HAS procedures. It has been argued that the results of the speech sound adaptation studies suggest that humans may possess feature detectors in their auditory systems (similar to those observed in the sensory systems of other animals) which are tuned to the phonetic features of speech. Fur thermore, the presence of such feature detectors at birth would be suffi cient to explain the ability of very young infants to discriminate categori cally phonetic contrasts [Eimas, 1974b, 1975b], However, Eimas [1975b] has also speculated that perhaps the infant HAS paradigm, by virtue of its repeated presentation format, might involve the adaptation of these fea ture detectors in the same way that the adult adaptation paradigms do. If this is indeed the case, then this could have some interesting implications for the relative ease or difficulty of the infant’s discrimination of various speech contrasts along a feature continuum, such as voicing or place.


Morse

369

However, most adult adaptation studies typically employ several thou sand adapting stimuli, far more than the number of tokens of the familiar stimulus presented in most infant studies. Thus, in order to explore more directly the relevance of adaptation processes to the infant studies, Miller and Morse [1979] decided to assess the adaptation effects for adult listen ers using the number of stimuli and format of the HR (20/20) and HAS paradigms employed in infant categorical discrimination studies. They found that the HAS paradigm yielded reliable adaptation effects for place of articulation (along a [dae]/[gae] continuum), whereas the HR paradigm did not produce significant adaptation effects. These findings suggest that indeed, as Eimas [1975b] has speculated, adaptation may be an addition al process that underlies and perhaps facilitates the infant’s performance in the HAS speech discrimination paradigm. However, it does not appear to be necessary for the infant discrimination findings with the HR para digm. We are currently extending this work in our laboratory to deter mine if similar adaptation effects occur within these paradigms for voicing contrasts and if some of the relative difficulties in obtaining evidence of infant voicing discrimination with the HR paradigm, as opposed to the HAS procedure [Doty, 1974; Rotli and Morse, 1975; Miller and HankesRuzicka, 1978] may be related to these differences in adaptation effects for the HAS and HR paradigms. In conclusion, what began as a safari to explore the ability of the hu man infant to discriminate the basic contrasts of human speech that re search with adults has established, has revealed that infants can indeed detect most of the major speech contrasts at an early age and in most of the relevant cases categorically, and that investigators are now beginning to push the infant to answer these questions in terms of the perceptual constancy of these categorizations [see Kuhl, this volume]. In addition, a second phase of the infant speech perception safari, which was only hinted at a few years ago [Morse, 1974], has begun to emerge. In this phase we are beginning to wrestle with the multitude of exciting questions and problems that bear on the origins of the infant’s abilities observed over the course of the past decade and the processes (e.g., learning, memory, adaptation) which underlie the infant’s successful or unsuccessful demonstrations of discrimination of various contrasts at specific ages. To answer these questions will indeed require a multi-disci plinary effort, involving researchers in animal behavior, psychology, speech science, neurophysiology, and a plethora of developmental discip lines.



Morse

370

Abbs, J. and Minifie, F.: The effects of acoustic cues in fricatives on perceptual confusions in preschool children. J. acoust. Soc. Am. 45: 1535-1542 (1969). Berg, W.: Cardiac orienting responses of 6- and 16-week-old infants. J. exp. Child Psychol. 17: 303-312 (1974). Cutting, J. E. and Rosner, B. S.: Categories and boundaries in speech and music. Perception Psychophysics 16: 564-570 (1974). Doty, D.: Infant speech perception: report of a conference held at the University of Minnesota, June 20-22, 1972. Hum. Dev. 17: 74-SO (1974). Eilers, R.: Context-sensitive perception of naturally produced stop and fricative consonants by infants. J. acoust. Soc. Am. 61: 1321-1336 (1977). Eilers, R.; Gavin, W., and Wilson, W.: Linguistic experience and phonemic percep tion in infancy: a cross-linguistic study. Child Dev. (in press, 1979a). Eilers, R. and Minifie, F.: Fricative discrimination in early infancy. J. Speech Hear. Res. IS: 158-167 (1975). Eilers, R. and Oiler, D.: The role of speech discrimination in developmental sound substitutions. J. Child Lang. 3: 319-329 (1976). Eilers, R.; Wilson, W., and Moore, J.: Developmental changes in speech discrimina tion in infants. J. Speech Hear. Res. 20: 766-780 (1977). Eilers, R.; Wilson, W., and Moore, J.: Speech discrimination in the language-inno cent and the language-wise: a study in the perception of voice onset time. J. Child Lang, (in press, 1979b). Eimas, P.: Auditory and linguistic processing of cues for place of articulation by in fants. Perception Psychophysics 16: 513-521 (1974a). Eimas, P.: Linguistic processing of speech by young infants; in Schiefelbusch and Lloyd, Language perspectives - acquisition, retardation, and intervention, pp. 55-73 (University Park Press, Baltimore 1974b). Eimas, P.: Auditory and phonetic coding of the cues for speech: discrimination of the [r—1] distinction by young infants. Perception Psychophysics 18: 341-347 (1975a). Eimas, P.: Speech perception in early infancy; in Cohen and Salapatek, Infant per ception: from sensation to cognition, vol. II, pp. 193-231 (Academic Press, New York 1975b). Eimas, P.: Developmental aspects of speech perception; in Held, Leibowitz and Teuber, Handbook of sensory physiology: perception (Springer, New York 1976). Eimas, P. and Corbit, J.: Selective adaptation of linguistic feature detectors. Cogni tive Psychol. 4: 99-109 (1973). Eimas, P.; Siqueland, E.; Jusczyk, P., and Vigorito, J.: Speech perception in early infancy. Paper presented Meet. Eastern Psychol. Ass., April, 1970. Eimas, P.; Siqueland, E.; Jusczyk, P., and Vigorito, J.: Speech perception in infants. Science 171: 303-306 (1971). Entus, A.: Hemispheric asymmetry in processing of dichotically presented speech and nonspeech stimuli by infants; in Segalowitz and Gruber, Language devel opment and neurological theory, pp. 63-73 (Academic Press, New York 1977).


References

371

Fodor, J.; Garrett, M., and Brill, S.: Pi ka pu: the perception of speech sounds by prelinguistic infants. Perception Psychophysics 18: 74-7S (1975). Glanville, B.; Best, C., and Levenson, R.: A cardiac measure of cerebral asymme tries in infant auditory perception. Dev. Psychol. 13: 54-59 (1977). Graham, F.; Leavitt, L.; Strock, B., and Brown, J.: Precocious cardiac orienting in a human anencephalic infant. Science 199: 322-324 (1978). Hadding-Koch, K. and Studdert-Kenncdy, M.: An experimental study of some in tonation contours. Phonetics 11: 175-185 (1964). Hecox, K.: Electrophysiological correlates of human auditory development; in Co hen and Salapatek, Infant perception: from sensation to cognition, vol. II, pp. 151-191 (Academic Press, New York 1975). Hillenbrand, J. and Minifie, F.: Tempo frequency change as a cue in speech-sound discrimination by infants. Paper presented Biennial Meet. Soc. Res. in Child Development, New Orleans 1977. Holmberg, T.; Morgan, K., and Kuhl, P.: Speech perception in early infancy: dis crimination of fricative consonants. J. acoust. Soc. Am. 62: suppl. 1, p. S99(A) (1977) . Jusczyk, P.: Perception of syllable-final stop consonants by 2-month-old infants. Perception Psychophysics 21: 450-454 (1977). Jusczyk, P.; Copan, H., and Thompson, E.: Perception of glides in multisyllabic ut terances by infants. J. acoust. Soc. Am. 62: suppl. 1, p. S99(A) (1977a). Jusczyk, P.; Rosner, B.; Cutting, J.; Foard, C., and Smith, L.: Categorical perception of nonspeech sounds in 2-month-old infants. Perception Psychophysics 2: 50-54 (1977b). Jusczyk, P. and Thompson, E.: Perception of a phonetic contrast in multisyllabic utterances by two-month-old infants. Perception Psychophysics 23: 105-109 (1978) . Kaplan, E.: The role of intonation in the acquisition of language; Ph. D. Diss. Cornell University, Ithaca (1969, unpublished). Kewley-Port, D. and Preston, M.: Early apical stop production: a voice onset time analysis. J. Phonet. 2: 195-210 (1974). Khadem, F. and Corballis, M.: Cerebral asymmetry in infants. Paper presented Biennial Meet. Soc. Res. in Child Development, New Orleans (1977). Kuhl, P.: Speech perception by the chinchilla: categorical perception of synthetic al veolar plosive consonants. J. acoust. Soc. Am. 60: suppl. 1, p. S81(A) (1976a). Kuhl, P.: Speech perception in early infancy: the acquisition of speech-sound cate gories; in Hirsch, Eldredge, Hirsch and Silverman, Hearing and Davis: essays honoring Hallowell Davis, pp. 265-280 (Washington University Press, St. Louis 1976b). Kuhl, P.: Speech perception in early infancy: perceptual constancies for vowel cate gories. J. acoust. Soc. Am. 60: suppl. 1, p. S90(A) (1976c). Kuhl, P.: Speech perception in early infancy: perceptual constancy for the vowel categories/a/and/c/. J. acoust. Soc. Am. 61: suppl. 1, p. S39(A) (1977). Kuhl, P. and Miller, J.: Speech perception by the chinchilla: voiced-voiceless distinc tion in alveolar plosive consonants. Science 190: 69-72 (1975).



372

Lasky, R.; Syrdal-Lasky, A., and Klein, D.: VOT discrimination by four- to sixmonth-old infants from Spanish environments. J. exp. Child Psychol. 20: 215225 (1975). Leavitt, L.; Brown, J.; Morse, P., and Graham, F.: Cardiac orienting and auditory discrimination in 6-week infants. Devi. Psychol. 12: 514-523 (1976). McCaffrey, A.: Speech perception in infancy; Ph. D. Diss. Cornell University, Ithaca (1971, unpublished). Miller, C. and Hankes-Ruzicka, E.: A parametric investigation of the cardiac no-de lay discrimination paradigm and voice-onset-time discriminations in infants. Research Status Report II, 275-314, Infant Development Laboratory, University of Wisconsin (1978). Miller, C. and Morse, P.: The ‘heart’ of categorical speech discrimination in young infants. J. Speech Hear. Res. 19: 578-589 (1976). Miller, C. and Morse, P.: Selective adaptation effects in infant speech perception paradigms. J. acoust. Soc. Am. 65: 789-798 (1978). Miller, C.; Morse, P., and Dorman, M.: Cardiac indices of infant speech perception: orienting and burst discrimination Q. J. exp. Psychol. 29: 533-545 (1977). Miller, J.: Phonetic determination of infant speech perception; Ph. D. Diss. Uni versity of Minnesota, Minneapolis (1974, unpublished). Miyawaki, K.; Strange, W.; Verbrugge, R.; Liberman, A.; Jenkins, J., and Fijimura, O.: An effect of linguistic experience: the discrimination of /r/ and /!/ by na tive speakers of Japanese and English. Perception Psychophysics 18: 331-340 (1975). Moffitt, A.: Speech perception by infants; Ph. D. Diss. University of Minnesota, Minneapolis (1968, unpublished). Moffitt, A.: Consonant cue perception by twenty- to twenty-four-week old infants. Child Dev. 42: 717-731 (1971). Molfese, D.: Cerebral asymmetry in infants, children and adults: auditory evoked responses to speech and music stimuli; Ph. D. Diss. Pennsylvania State Uni versity, University Park (1972, unpublished). Molfese, D.: Infant cerebral asymmetry; in Segalowitz and Gruber, Language de velopment and neurological theory, pp. 21-35 (Academic Press, 1977). Molfese, D.: Electrophysiological correlates of categorical speech perception in adults. Brain Lang. 5: 25-35 (1978). Molfese, D.; Freeman, R., and Palermo, D.: The ontogeny of brain lateralization for speech and nonspeech stimuli. Brain Lang. 2: 356-368 (1975). Molfese, D. and Hess, T.: Hemispheric specialization for VOT perception in the preschool child. J. exp. Child Psychol. 26: 71-84 (1978). Molfese, D. and Molfese, V.: VOT distinctions in infants: learned or innate?; in Whitaker, Studies in neurolinguistics, vol. IV (Academic Press, New York, 1979). Morse, P.: The discrimination of speech and nonspeech stimuli in early infancy. J. exp. Child Psychol. 14: 477-492 (1972). Morse, P.: Infant speech perception: a preliminary model and review of the litera ture; in Schiefelbusch and Lloyd, Language perspectives - acquisition, retarda tion, and intervention, pp. 19-53 (University Park Press, Baltimore 1974).


Morse


373

Philip A. Morse, Department of Psychology, University of Wisconsin, Madison, WI 53706 (USA)


Morse, P.: Infant speech perception: origins, processes, and Alpha Centauri; in Minifie and Lloyd, Communicative and cognitive abilities - early behavioral as sessment (University Park Press, Baltimore 1978). Morse, P. and Snowdon, C.: An investigation of categorical speech discrimination by rhesus monkeys. Perception Psychophysics 17: 9-16 (1975). Roth, D. and Morse, P.: An investigation of infant VOT discrimination using the cardiac OR. Research Status Report I, Infant Development Laboratory, Uni versity of Wisconsin, pp. 207-218 (1975). Sinnott, J.; Beecher, M.; Moody, D., and Stebbins, W.: Speech sound discrimination by humans and monkeys. J. acoust. Soc. Am. 60: 687-695 (1976). Spring, D. and Dale, P.: The discrimination of linguistic stress in early infancy. J. Speech Hear. Res. 20: 224-231 (1977). Stevens, K. and Klatt, D.: Role of formant transitions in the voiced-voiceless dis tinction for stops. J. acoust. Soc. Am. 55: 653-659 (1974). Streeter, L.: Language perception of 2-month-old infants shows effects of both in nate mechanisms and experience. Nature, Lond. 259: 39-41 (1976). Swoboda, P.; Kass, J.; Morse, P., and Leavitt, L.: Memory factors in infant vowel discrimination of normal and at-risk infants. Child Dev. 49: 332-339 (1978). Swoboda, P.; Morse, P., and Leavitt, L.: Continuous vowel discrimination in normal and at risk infants. Child Dev. 47: 459-465 (1976). Till, J.: Infants’ discrimination of speech and nonspeech stimuli; Ph. D. Diss. Uni versity of Iowa, Iowa City (1976, unpublished). Trehub, S.: Infants’ sensitivity to vowel and tonal contrasts. Devi. Psychol. 9: 81-96 (1973). Trehub, S.: The discrimination of foreign speech contrasts by infants and adults. Child Dev. 47: 466-472 (1976). Trehub, S. and Chang, H.-W.: Speech as reinforcing stimulation for infants. Devi. Psychol. 13: 170-171 (1977). Trehub, S. and Rabinovitch, M.: Auditory-linguistic sensitivity in early infancy. Devi. Psychol. 6: 74-77 (1972). Waters, R. and Wilson, W.: Speech perception by rhesus monkeys: the voicing dis tinction in synthesized labial and velar stop consonants. Perception Psycho physics 19: 285-289 (1976). Williams, L.: The effects of phonetic environment and stress placement on infant discrimination of the place of stop consonant articulation. Paper presented 2nd Annu. Boston University Conf. on Language Development, Boston 1977. Williams, L. and Golenski, J.: Infant speech sound discrimination: the effects of contingent versus non-contingent stimulus presentation. Child Dev. 49: 213-217 (1978).

Sources of Confusion in Infant Audiovisual Speech Perception Research.

The first decade of organic spintronics research.

Organization in infant speech perception.

[Speech perception in the first two years].

The influence of infant-directed speech on 12-month-olds' intersensory perception of fluent speech.

The development of voicing categories: a quantitative review of over 40 years of infant speech perception research.

Refining Stimulus Parameters in Assessing Infant Speech Perception Using Visual Reinforcement Infant Speech Discrimination: Sensation Level.

Near-infrared spectroscopy: recent advances in infant speech perception and language acquisition research.

Health services and policy research in the first decade at the Canadian Institutes of Health Research.

Predicting individual variation in language from infant speech perception measures.

JBCB, the first decade.

Perception of the multisensory coherence of fluent audiovisual speech in infancy: its emergence and the role of experience.

A causal test of the motor theory of speech perception: a case of impaired speech production and spared speech perception.

Effects of recurrent otitis media in infancy on auditory perception and speech.

Reflections on the development of speech perception.

Editorial: The first decade of federal medicine.

Abortion and the NHS: the first decade.

The influence of dialect upon the perception of dysarthic speech.

Auditory-visual perception of speech.

Cranial chordoma in the first decade.

The journal and its first decade.

HIV infection and intervention: the first decade.

Lipids in psychological research: the last decade.

A decade of AIDS research.