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VOT del Español to English: comparison of a language-disordered and normal child
Journal of Phonetics (1980) 8, 287-291
VOT del Espafiol to English: comparison of a language-disordered and normal child Z.S.Bond OhtJ University, Columbus, Ohio 43210, U.S.A.
Jean E. Eddey Apple Creek State Institute, U.S.A. and
Juan J. Bermejo Ohio University, Columbus, Ohio 43210, U.S.A. Received 30th October 1978
Abstract:
Voice onset time (VOT), the time interval by which the onset of phonation either precedes or follows the release of a stop occlusion, has been found to be a reliable parameter for differentiating the voicing contrast in numerous languages. The three relatively specific ranges of VOT production- prevoicing, short lag and long lagoccur when the phonemic boundaries for initial stop consonants in Spanish and English are compared. This study investigated the effects of second language learning on the voice onset time values employed for productions of word initial stop consonants. The productions of Spanish and English words by two native Spanish speaking siblings were compared . One child was delayed in the acquisition of Spanish, and both were learning English as a second language. VOT values were affected by second language learning, and delayed language development interfered with the acquisition of a new language specific phonemic contrast.
Introduction A claim commonly made in the investigation of the acquisition of a second language is that children acquire the phonetic and phonological structure of a new language with great facility. For example, Oyama (1976) found that immigrants who had arrived in the United States at an early age were perceived to have less accent than those who had learned English later in life. She felt this to be indicative of a "sensitive period for the acquisition of a nonnative phonological system" (p. 279) which Fatham (1975) stated might encompass the pre-teen years. Some research suggests that this "sensitive period" could begin quite early in life. Preston, Yeni-Komshian & Stark, (1967) reported that although at 12 months both Lebanese and American infants produced apical stop consonants in the short lag range, by age two these children began to exhibit values for voiced and voiceless stops which were phonologically appropriate to t}leir own specific languages. Studies by Siqueland,Jusczyk& Vigorito (1971), Usker & Abramson (1973), and Trehub & Rabinovitch (1972) suggest that by age four months, infants from English speaking environments had started to develop linguistic categories for bilabial stops that resembled adult English phonemic categories (Lasky, SyrdalLasky & Klein, 1975). Apparently language s~ecific phonological systems begin to develop at an early age. The purpose of this study was to investigate the effect of second language learning on the voice onset time values of word initial stop consonants in the speech of children. Voice onset time (VOT), the time interval by which the onset of vocal fold vibration either precedes .o r follows the release of the stop occlusion, has been found to be a reliable parameter for 0095-4470/80/030287+05 $)2.00/0
©1980 Academic Press Inc. (London) Ltd.
288
Z.S. Bond eta/.
differentiating the voicing contrast in numerous languages (Lisker & Abramson, 1964). There appear to be three relatively specific ranges of VOT for initial stop consonant production. In the voicing lead range, voicing precedes the release of the stop, and such stops have negative VOT values. Voicing begins 0 to +20 rns after the stop release in the short voicing lag range, and +60 ms after the release in the long voicing lag range (Kewly-Port & Preston, 1974). , All three of these VOT ranges occur when the phonemic boundaries for initial stop consonants in Spanish and English are compared. Spanish speaking adults typically produce voiced stops in the voicing lead ranges and voiceless stops in the short voicing lag range , whereas adult speakers of English produce the voiced stops in the short voicing lag range and the voiceless stops in the long voicing lag range (Lasky et al., 197 5). Perceptually, the Spanish phonemic boundaries also have lower VOT values for all places of articulation. Thus, Abramson & lisker (1973) concluded that phonemic categories are largely determined by experience . Apparently, bilingual experience readily affects the perception and production of stop consonants with respect to VOT. Streeter & Landauer (1976) found that Kikuyu-speaking school children learned to discriminate VOT values that were non-phonemic in Kikuyu following exposure to English, where these VOT values do differ phonemically. While linguistic experience can influence the ability to discriminate voicing categories, language switching appears to be easier for production than for perception. Caramazza eta/., (1973) used VOT as the linguistic parameter to examine the perception and production of six initial stop consonants by unilingual Canadian English, unilingual Canadian French and bilingual Canadian French-English adult speakers. Through spectrographic analysis , it was found that the bilingual speakers had learned to increase somewhat the amount of voicing lag required for the production of the English stop consonants. It was concluded that "the phonological processors the bilingual acquires for his second language are contaminated by properties accruing to his fust language" (p. 427). In the case of adult Spanish English bilinguals, this most commonly involves the use of the Spanish acoustic-phonetic feature of voicing lead in the production of English word-initial stops (Williams, 1977). In this study, we compared the VOT values of Spanish and English words produced by two Spanish speaking siblings. One child was delayed in the acquisition of Spanish, and both were learning English as a second language. Specifically , we posed two experimental questions: first , after six months' exposure to English would the subjects spontaneously begin to differentiate the Spanish and English VOT values; and second, would the productions of the language-disordered child differ from those of the normally developing child. Method
Subjects The subjects were two sisters, aged four and seven years . They were raised in a monolingual Spanish environment in South America until their family moved to the United States , approximately six months before they were involved in this study. After moving to the United States, Spanish continued to be spoken in the home , but both children were enrolled in schools where only English was spoken. The parents reported that the younger child had acquired Spanish normally and was beginning to speak English at school. They stated that the older child had spoken Spanish very little. She was enrolled in language therapy at the Ohio University Speech and Hearing Clinic where she was diagnosed as having a severe language disorder. At the time of this study, the older subject was beginning to produce twoword utterances in English.
289
VOT del Espanol to English
Stimuli and apparatus The 12 stimulus words used were three minimal pairs of English and Spanish words, with initial labial, dental, and velar stops. The vowels were matched as well as possible for the Spanish and English words containing a homorganic stop. The stimulus words were "paiio", "baiio", "Paul", "ball", "tia", "dia", "tear", "deer", "casa", "gasa", "coat", and "goat". Colored pictures of each stimulus word were used to elicit spontaneous productions of each token., The stimulus words were judged to be familiar to young children. A Sony tape recorder (TC 650) was used to record each subject's utterances. Wide-band spectrograms of each production were made on a Kay Sonagraph. Procedure The parents of the two subjects were interviewed concerning the subjects' past language development. The parents reported that the children would be more cooperative if they were tested together. The testing procedure was conducted in a sound treated therapy room at Ohio University Hearing and Speech Clinic. The two subjects, their mother, and two experimenters were present. Each subject was ftrst introduced to the Spanish and English words by native speakers of each language, and they then produced varying numbers of each stimulus item. Most utterances were produced spontaneously, although some words were produced following a prompting phrase such as "This is a ball, say " Spectrograms were made of both subjects' productions, and the duration of the voice onset time of the initial stop consonant in each stimulus word was measured. VOT was measured as the interval between the release of the stop, marked spectrographically by a "burst" of energy, and the onset of vocal fold vibration, marked spectrographically by regularly-spaced vertical striations. Results Because the number of productions of each token ranged from 0 - 6, it was difficult to make meaningful inter- and intra-subject comparisons. Therefore, the data were grouped according to the voiced/voicless category for each language and subject. Table 1 presents the number, means and standard deviations of the VOT values for the voiced and voiceless Spanish and English initial stops produced by S1 (C.A. 4 years) and S2 (C.A. 7 years): Ttests were performed to determine if the voiced/voiceless distinction maintained was significant within languages and between languages for each subject. Table I VOT distributions for [+voice 1 and [-voice 1 Spanish and English initial stops produced by S1 and S2 Spanish
English
+voice s1 N
II
X -0·68
s.d . 39-4 7
s:l
-voice sl
7
12
0
+24
27-98
14-79
+voice
s2
s1
5
5
+21-75
+ 15-75
+4·43
19-38
10-74
46-95
s2
11
-voice s1
11
s2 8
+37·16 +40-78 46
8-84
Both subjects appeared to maintain a significant voicing distinction for both Spanish and English respectively. When Spanish and English VOT values were compared for S1 , the VOT values for the voiced stop category were significantly different, (P
290
Z.S. Bond et aL
Conclusion The results indicate that both subjects maintained a significant voicing distinction for VOT in the short voicing lag range in Spanish and English. While both subjects showed a tendency to produce the appropriate longer voicing lag values for English, with the normal subject lengthening the VOT values for the voicless English stops, there does appear to be a difference between the VOT duration values produced by each subject. The range of VOT values exhibited by the normal subject extended into the long voicing lag range for English voiceless stops and into the pre-voicing range for voiced Spanish stops (see Fig. 1). The range of VOT values demonstrated by the language-disordered subject was basically confmed to the short voicing lag range for both Spanish and English (see Fig. 2). Therefore, although the means of VOT durations produced by each subject were similar, an examination of the range of productions indicates that the normally developing subject was beginning to approach the expected adult VOT values for each language.
;;; .D
E
z"
Voi ce-onset time
Figure 1
Histogram showing the distribution of VOT values for Spanish (dotted line) English (solid line) stops, as produced by a normally-developing child. The range of values extends from extensive pre-voicing to considerable delay in the onset of phonation .
., .D
E
z"
Voice -onset time
Figure 2
Histogram showing the distribution of VOT values, for both Spanish and English, as procued by the language-delayed child.
The mean VOT values for both subjects were concentrated in the short voicing lag range for both Spanish and English. These results appear to agree with reported data on the development of the voicing distinction in ·children. Zlatin & Koenigsknecht (1976) reported that developmental changes in the production of word-initial stops at all points of articulation occurred between two-year old children, six-year old children, and adults. They found that until the age of three, children's productions were unimodal in the short lag range of the VOT continum; after age three, bimodal VOT distributions began to emerge. Kewly- Port & Preston (1974) also found that children between the ages of two and 4 ~ produced both voiced and voiceless apical stops in the short voicing lag range. They suggested that the articulatory adjustments necessary to produce a stop in the short lag range were less complex than the articulatory control needed to produce pre-voiced stops, or stops with long voicing lags. Thus, the normal subject (C.A. 4 years) appears to be following the developmental
VOT del Espaiiol to English
291
pattern of extending the VOT durations from the short voicing lag range to the pre-voiced and long voicing lag ranges. However, the language delayed subject (C.A. 7 years) appears to be late in developing a bimodal VOT distribution, as the mean and range of her VOT productions were confmed to the short voicing lag range for both Spanish and English. Perhaps this delay in the development of appropriate adult values for either language is a function of her overall profile of delayed language development. Such a connection has been reported by Menyuk & Looney (1972), who found that children who had difficulty producing specific syntactic sentence structures also had difficulty producing phonological structures. Although only limited data have been reported, two conclusions are offered. It can be concluded fust, that VOT values are readily affected by second language learning, and second, that delayed language development also interferes with the acquisition of languagespecific acoustic phonetic characteristics of speech. A preliminary version of this paper was presented at the 1977 Convention of the American Speech and Hearing Association, Chicago, Illinois . We appreciate the assistance of the Speech and Hearing Clinic, Ohio University, Athens, and of the parents of the children we tested. References Abramson, ArthurS. & Lisker, L.(1973) . Voice-timing perception in Spanish word-initial stops. Journal of Phonetics 1, 1- 8. Caramazza, A., Yeni-Komshian, G., Zuriff, E. & Carbone, E, (1973). The acquisition of a new phonological contrast: The case of stop consonants in French-English bilinguals. Journal of the Acoustical Society of America 54, 421-428. Eimas, P. D., Siqueland, E. R., Jusczyk, P. & Vigorito, J., (1971). Speech Perception in Infants, Science 171, 303-306. Fatham, Ann, (1975). The relation between age and second language productive ability. Language Learning. 25, 245-255. Kewley-Port, Diane & Preston, Malcolm S., (1974). Early apical stop production : A voice onset time analysis. Journal of Phonetics 2: 195-210. Lasky , R. E., Syrdal-Lasky, A. & Klein, R. E. (197 5). VOT discrimination by 4 to 6 month old infants from Spanish environments. Journal of Experimental Child Psychology. 20. 215-225. Lisker, L. & Abramson, A.S., (1964). A cross-language study of voicing in initial stops: acoustical measurements. Word, 20, 384-422. Menyuk, P. & Looney, P., (1972). Relationships among components of the grammar in language disorders. Journal of Speech and Hearing Research 15, 395-406. Oyama, S. (1976). A sensitive period for the acquisition of a non-native phonological system. Journal of Psycholinguistic Research 5, 261-284. Preston, M. S., Yeni-Komshian, G. & Stark, R. E. (1976). Voicing in initial stop consonants produced by children in the prelinguistic period from different language communities. Johns Hopkins University School of Medicine, Annual Report of Neurocommunications Laboratory 2, 305-323. Streeter, L.A. & Landauer, T.K. (1976). Effects of learning English as a second language on the acquisition of a new phonemic contrast. Journal of the Acoustical Society of America 59, 448-451. Trehub, S.E. & Rabinovitch, M.S. (1972). Auditory-linguistic sensitivity in early infancy, Developmental Psychology 6, 74-77. Williams, L. (1977). The perception of stop consonant voicing by Spanish-English bilinguals. Perception and Psychophysics 21,289 - 297 . Zlatin , M. & Koenigsknecht, R. A. (1976) . Development of the voicing contrast. A comparison of voice onset time values in stop perception and production. Journal of Speech and Hearing Research 19, 93-111.
VOT del Espafiol to English: comparison of a language-disordered and normal child Z.S.Bond OhtJ University, Columbus, Ohio 43210, U.S.A.
Jean E. Eddey Apple Creek State Institute, U.S.A. and
Juan J. Bermejo Ohio University, Columbus, Ohio 43210, U.S.A. Received 30th October 1978
Abstract:
Voice onset time (VOT), the time interval by which the onset of phonation either precedes or follows the release of a stop occlusion, has been found to be a reliable parameter for differentiating the voicing contrast in numerous languages. The three relatively specific ranges of VOT production- prevoicing, short lag and long lagoccur when the phonemic boundaries for initial stop consonants in Spanish and English are compared. This study investigated the effects of second language learning on the voice onset time values employed for productions of word initial stop consonants. The productions of Spanish and English words by two native Spanish speaking siblings were compared . One child was delayed in the acquisition of Spanish, and both were learning English as a second language. VOT values were affected by second language learning, and delayed language development interfered with the acquisition of a new language specific phonemic contrast.
Introduction A claim commonly made in the investigation of the acquisition of a second language is that children acquire the phonetic and phonological structure of a new language with great facility. For example, Oyama (1976) found that immigrants who had arrived in the United States at an early age were perceived to have less accent than those who had learned English later in life. She felt this to be indicative of a "sensitive period for the acquisition of a nonnative phonological system" (p. 279) which Fatham (1975) stated might encompass the pre-teen years. Some research suggests that this "sensitive period" could begin quite early in life. Preston, Yeni-Komshian & Stark, (1967) reported that although at 12 months both Lebanese and American infants produced apical stop consonants in the short lag range, by age two these children began to exhibit values for voiced and voiceless stops which were phonologically appropriate to t}leir own specific languages. Studies by Siqueland,Jusczyk& Vigorito (1971), Usker & Abramson (1973), and Trehub & Rabinovitch (1972) suggest that by age four months, infants from English speaking environments had started to develop linguistic categories for bilabial stops that resembled adult English phonemic categories (Lasky, SyrdalLasky & Klein, 1975). Apparently language s~ecific phonological systems begin to develop at an early age. The purpose of this study was to investigate the effect of second language learning on the voice onset time values of word initial stop consonants in the speech of children. Voice onset time (VOT), the time interval by which the onset of vocal fold vibration either precedes .o r follows the release of the stop occlusion, has been found to be a reliable parameter for 0095-4470/80/030287+05 $)2.00/0
©1980 Academic Press Inc. (London) Ltd.
288
Z.S. Bond eta/.
differentiating the voicing contrast in numerous languages (Lisker & Abramson, 1964). There appear to be three relatively specific ranges of VOT for initial stop consonant production. In the voicing lead range, voicing precedes the release of the stop, and such stops have negative VOT values. Voicing begins 0 to +20 rns after the stop release in the short voicing lag range, and +60 ms after the release in the long voicing lag range (Kewly-Port & Preston, 1974). , All three of these VOT ranges occur when the phonemic boundaries for initial stop consonants in Spanish and English are compared. Spanish speaking adults typically produce voiced stops in the voicing lead ranges and voiceless stops in the short voicing lag range , whereas adult speakers of English produce the voiced stops in the short voicing lag range and the voiceless stops in the long voicing lag range (Lasky et al., 197 5). Perceptually, the Spanish phonemic boundaries also have lower VOT values for all places of articulation. Thus, Abramson & lisker (1973) concluded that phonemic categories are largely determined by experience . Apparently, bilingual experience readily affects the perception and production of stop consonants with respect to VOT. Streeter & Landauer (1976) found that Kikuyu-speaking school children learned to discriminate VOT values that were non-phonemic in Kikuyu following exposure to English, where these VOT values do differ phonemically. While linguistic experience can influence the ability to discriminate voicing categories, language switching appears to be easier for production than for perception. Caramazza eta/., (1973) used VOT as the linguistic parameter to examine the perception and production of six initial stop consonants by unilingual Canadian English, unilingual Canadian French and bilingual Canadian French-English adult speakers. Through spectrographic analysis , it was found that the bilingual speakers had learned to increase somewhat the amount of voicing lag required for the production of the English stop consonants. It was concluded that "the phonological processors the bilingual acquires for his second language are contaminated by properties accruing to his fust language" (p. 427). In the case of adult Spanish English bilinguals, this most commonly involves the use of the Spanish acoustic-phonetic feature of voicing lead in the production of English word-initial stops (Williams, 1977). In this study, we compared the VOT values of Spanish and English words produced by two Spanish speaking siblings. One child was delayed in the acquisition of Spanish, and both were learning English as a second language. Specifically , we posed two experimental questions: first , after six months' exposure to English would the subjects spontaneously begin to differentiate the Spanish and English VOT values; and second, would the productions of the language-disordered child differ from those of the normally developing child. Method
Subjects The subjects were two sisters, aged four and seven years . They were raised in a monolingual Spanish environment in South America until their family moved to the United States , approximately six months before they were involved in this study. After moving to the United States, Spanish continued to be spoken in the home , but both children were enrolled in schools where only English was spoken. The parents reported that the younger child had acquired Spanish normally and was beginning to speak English at school. They stated that the older child had spoken Spanish very little. She was enrolled in language therapy at the Ohio University Speech and Hearing Clinic where she was diagnosed as having a severe language disorder. At the time of this study, the older subject was beginning to produce twoword utterances in English.
289
VOT del Espanol to English
Stimuli and apparatus The 12 stimulus words used were three minimal pairs of English and Spanish words, with initial labial, dental, and velar stops. The vowels were matched as well as possible for the Spanish and English words containing a homorganic stop. The stimulus words were "paiio", "baiio", "Paul", "ball", "tia", "dia", "tear", "deer", "casa", "gasa", "coat", and "goat". Colored pictures of each stimulus word were used to elicit spontaneous productions of each token., The stimulus words were judged to be familiar to young children. A Sony tape recorder (TC 650) was used to record each subject's utterances. Wide-band spectrograms of each production were made on a Kay Sonagraph. Procedure The parents of the two subjects were interviewed concerning the subjects' past language development. The parents reported that the children would be more cooperative if they were tested together. The testing procedure was conducted in a sound treated therapy room at Ohio University Hearing and Speech Clinic. The two subjects, their mother, and two experimenters were present. Each subject was ftrst introduced to the Spanish and English words by native speakers of each language, and they then produced varying numbers of each stimulus item. Most utterances were produced spontaneously, although some words were produced following a prompting phrase such as "This is a ball, say " Spectrograms were made of both subjects' productions, and the duration of the voice onset time of the initial stop consonant in each stimulus word was measured. VOT was measured as the interval between the release of the stop, marked spectrographically by a "burst" of energy, and the onset of vocal fold vibration, marked spectrographically by regularly-spaced vertical striations. Results Because the number of productions of each token ranged from 0 - 6, it was difficult to make meaningful inter- and intra-subject comparisons. Therefore, the data were grouped according to the voiced/voicless category for each language and subject. Table 1 presents the number, means and standard deviations of the VOT values for the voiced and voiceless Spanish and English initial stops produced by S1 (C.A. 4 years) and S2 (C.A. 7 years): Ttests were performed to determine if the voiced/voiceless distinction maintained was significant within languages and between languages for each subject. Table I VOT distributions for [+voice 1 and [-voice 1 Spanish and English initial stops produced by S1 and S2 Spanish
English
+voice s1 N
II
X -0·68
s.d . 39-4 7
s:l
-voice sl
7
12
0
+24
27-98
14-79
+voice
s2
s1
5
5
+21-75
+ 15-75
+4·43
19-38
10-74
46-95
s2
11
-voice s1
11
s2 8
+37·16 +40-78 46
8-84
Both subjects appeared to maintain a significant voicing distinction for both Spanish and English respectively. When Spanish and English VOT values were compared for S1 , the VOT values for the voiced stop category were significantly different, (P
290
Z.S. Bond et aL
Conclusion The results indicate that both subjects maintained a significant voicing distinction for VOT in the short voicing lag range in Spanish and English. While both subjects showed a tendency to produce the appropriate longer voicing lag values for English, with the normal subject lengthening the VOT values for the voicless English stops, there does appear to be a difference between the VOT duration values produced by each subject. The range of VOT values exhibited by the normal subject extended into the long voicing lag range for English voiceless stops and into the pre-voicing range for voiced Spanish stops (see Fig. 1). The range of VOT values demonstrated by the language-disordered subject was basically confmed to the short voicing lag range for both Spanish and English (see Fig. 2). Therefore, although the means of VOT durations produced by each subject were similar, an examination of the range of productions indicates that the normally developing subject was beginning to approach the expected adult VOT values for each language.
;;; .D
E
z"
Voi ce-onset time
Figure 1
Histogram showing the distribution of VOT values for Spanish (dotted line) English (solid line) stops, as produced by a normally-developing child. The range of values extends from extensive pre-voicing to considerable delay in the onset of phonation .
., .D
E
z"
Voice -onset time
Figure 2
Histogram showing the distribution of VOT values, for both Spanish and English, as procued by the language-delayed child.
The mean VOT values for both subjects were concentrated in the short voicing lag range for both Spanish and English. These results appear to agree with reported data on the development of the voicing distinction in ·children. Zlatin & Koenigsknecht (1976) reported that developmental changes in the production of word-initial stops at all points of articulation occurred between two-year old children, six-year old children, and adults. They found that until the age of three, children's productions were unimodal in the short lag range of the VOT continum; after age three, bimodal VOT distributions began to emerge. Kewly- Port & Preston (1974) also found that children between the ages of two and 4 ~ produced both voiced and voiceless apical stops in the short voicing lag range. They suggested that the articulatory adjustments necessary to produce a stop in the short lag range were less complex than the articulatory control needed to produce pre-voiced stops, or stops with long voicing lags. Thus, the normal subject (C.A. 4 years) appears to be following the developmental
VOT del Espaiiol to English
291
pattern of extending the VOT durations from the short voicing lag range to the pre-voiced and long voicing lag ranges. However, the language delayed subject (C.A. 7 years) appears to be late in developing a bimodal VOT distribution, as the mean and range of her VOT productions were confmed to the short voicing lag range for both Spanish and English. Perhaps this delay in the development of appropriate adult values for either language is a function of her overall profile of delayed language development. Such a connection has been reported by Menyuk & Looney (1972), who found that children who had difficulty producing specific syntactic sentence structures also had difficulty producing phonological structures. Although only limited data have been reported, two conclusions are offered. It can be concluded fust, that VOT values are readily affected by second language learning, and second, that delayed language development also interferes with the acquisition of languagespecific acoustic phonetic characteristics of speech. A preliminary version of this paper was presented at the 1977 Convention of the American Speech and Hearing Association, Chicago, Illinois . We appreciate the assistance of the Speech and Hearing Clinic, Ohio University, Athens, and of the parents of the children we tested. References Abramson, ArthurS. & Lisker, L.(1973) . Voice-timing perception in Spanish word-initial stops. Journal of Phonetics 1, 1- 8. Caramazza, A., Yeni-Komshian, G., Zuriff, E. & Carbone, E, (1973). The acquisition of a new phonological contrast: The case of stop consonants in French-English bilinguals. Journal of the Acoustical Society of America 54, 421-428. Eimas, P. D., Siqueland, E. R., Jusczyk, P. & Vigorito, J., (1971). Speech Perception in Infants, Science 171, 303-306. Fatham, Ann, (1975). The relation between age and second language productive ability. Language Learning. 25, 245-255. Kewley-Port, Diane & Preston, Malcolm S., (1974). Early apical stop production : A voice onset time analysis. Journal of Phonetics 2: 195-210. Lasky , R. E., Syrdal-Lasky, A. & Klein, R. E. (197 5). VOT discrimination by 4 to 6 month old infants from Spanish environments. Journal of Experimental Child Psychology. 20. 215-225. Lisker, L. & Abramson, A.S., (1964). A cross-language study of voicing in initial stops: acoustical measurements. Word, 20, 384-422. Menyuk, P. & Looney, P., (1972). Relationships among components of the grammar in language disorders. Journal of Speech and Hearing Research 15, 395-406. Oyama, S. (1976). A sensitive period for the acquisition of a non-native phonological system. Journal of Psycholinguistic Research 5, 261-284. Preston, M. S., Yeni-Komshian, G. & Stark, R. E. (1976). Voicing in initial stop consonants produced by children in the prelinguistic period from different language communities. Johns Hopkins University School of Medicine, Annual Report of Neurocommunications Laboratory 2, 305-323. Streeter, L.A. & Landauer, T.K. (1976). Effects of learning English as a second language on the acquisition of a new phonemic contrast. Journal of the Acoustical Society of America 59, 448-451. Trehub, S.E. & Rabinovitch, M.S. (1972). Auditory-linguistic sensitivity in early infancy, Developmental Psychology 6, 74-77. Williams, L. (1977). The perception of stop consonant voicing by Spanish-English bilinguals. Perception and Psychophysics 21,289 - 297 . Zlatin , M. & Koenigsknecht, R. A. (1976) . Development of the voicing contrast. A comparison of voice onset time values in stop perception and production. Journal of Speech and Hearing Research 19, 93-111.