Acquiring Phoneme Contrasts
4.3 Infants’ Perception of Laryngeal Contrasts
The contrasts under investigation in the studies cited above were mainly PoA contrasts. Apart from PoA contrasts, laryngeal contrasts have been of interest in the literature. Since ACT is one of the two cues focused on in the present work, the research conducted on infants’ perception of laryngeal contrasts will be given fairly broad space below (section 4.3) and also the few data published on length contrasts in first language acquisition will be looked at in more detail (section 4.4).
4.3 Infants’ Perception of Laryngeal Contrasts
Even though quite a lot of work has been done to unravel how infants per-ceive laryngeal properties, there is still no unequivocal explanation for this issue. Instead, there are many different studies providing different, some-times contradictory results. Drawing a coherent conclusion is aggravated by the fact that different test procedures are involved, age ranges of subjects vary and participants grow up in different linguistic environments. One of the few commonalities of these studies is that most of them use oral stops in syllable-initial position to test infants’ perception of laryngeal contrasts.
Nonetheless, there is a lot of variation among the stimuli used in the individ-ual studies. Stimuli differ with respect to their specific laryngeal properties (amount of voicing lead or lag), in PoA and in the context following the stop.
Furthermore, some researchers use naturally spoken stimuli while others use synthetic speech material. Despite these divergences, an attempt to clarify the picture will be presented in the following paragraphs.
Recall from the preceding chapters that vocal fold vibration may start at different intervals in different languages of the world and stops in two-way contrasting languages usually fall into two of three categories, namely prevoiced, voiceless unaspirated or voiceless aspirated. Consequently, re-search on infants’ perception of laryngeal contrasts is mostly devoted to find out whether and how young language learners discriminate and acquire (a) contrasts of the English and German type between voiceless unaspirated and voiceless aspirated stops (henceforth also referred to as ‘lag contrasts’) and (b) contrasts between voiced and voiceless unaspirated stops (‘lead con-trasts’ henceforth), as the tense/lax opposition is implemented, for example, in Dutch and French.
One of the earliest studies on the perception of laryngeal contrasts is the landmark paper by Eimas et al. (1971). Using the High Amplitude Suck-ing (HAS) method, they test whether 1- and 4-month-old American English infants are able to detect three laryngeal contrasts in stops. One pair of
syn-thetic stimuli crosses the adult English phoneme boundary (20 ms vs. 40 ms of ACT).14The other two stimulus pairs are within-category contrasts. One is a contrast within the lax category, opposing stops with a 20 ms-voicing lead to stops with neither voicing lead nor lag (ACT=0 ms). The other pair consists of two stops that both fall into the tense category, with 60 ms and 80 ms of ACT, respectively. The results reveal that both age groups are significantly better in discriminating the cross-category contrast than the within-category contrasts although the acoustic difference on a laryngeal continuum is the same in all three conditions (viz. 20 ms). Although it is assumed that neonates and very young infants are sensitive to a wider vari-ety of speech contrasts than adults, the findings of Eimas and his colleagues show that not all contrasts are perceived equally well by infants even in their very first months of life. Apparently, the short lag/long lag contrast is easier to discriminate than a contrast between stops with a voicing lead and one with a short voicing lag. Also, stops with different amounts of relatively long voicing lags are harder to distinguish than a short lag/long lag contrast.
The findings by Eilerset al.(1979a) support the assumption concerning lead contrasts. In a CHTP-experiment, 6- to 8-month-old English infants succeed to discriminate a short lag/long lag contrast (10 ms vs. 40 ms of ACT), but they fail to distinguish stops with a 20 ms-voicing lead from stops with a 10 ms-voicing lag. Interestingly, Spanish infants of the same age are able to distinguish both the lead and the lag contrast. Since Spanish is a so-called voice language, its stop system is marked by a lead contrast and lag contrasts are phonologically irrelevant. Accordingly, it seems that the exposure to the Spanish environment already had some influence on the Spanish infants’ perception (while the ability to discriminate the non-native lag contrast is also still existent).15 Support for the assumption that lead contrasts have to be learned with linguistic experience comes from Maye et al. (2002). English infants aged 6 and 8 months are familiarised with stops varying in their amount of prevoicing (between a voicing lead of 90 ms to 0 ms) either in a bimodal or in a unimodal distribution. The results reveal that only those infants who hear the stops bimodally distributed during familiarisation succeed in discriminating a [da]–[ta] contrast in the subsequent test phase.
So far it seems that the ability to discriminate a short lag/long lag con-trast is present from birth, but that lead concon-trasts are not discriminable by
14Eimaset al.(1971) assume that the category boundary that separates American En-glish lax and tense stops is located at an ACT value of approximately 25 ms.
15See Aslin & Pisoni (1980b) for a critique of that study, in which they mistrust the effects of early linguistic experience on infants’ discrimination capacities.
4.3 Infants’ Perception of Laryngeal Contrasts 83 very young infants. There are several studies providing counter evidence to this hypothesis. Aslin et al. (1981) test English 6- to 12-month-olds’
perception of stop contrasts along a laryngeal continuum, starting with a 70 ms-voicing lead and reaching up to a 70 ms-voicing lag. They discover that the infants are able to categorise contrasts that cross the English phoneme boundary as well as contrasts within one of the English categories. The re-sults imply that English infants are able to discriminate stops in the voicing lead region of the laryngeal continuum. Nonetheless, infants are better at discriminating cross-category contrasts.16
In a study by Lasky et al. (1975), Guatemalan Spanish infants at the age of 4 to 6.5 months fail to discriminate contrasts that cross the Spanish phoneme boundary (a 20 ms-voicing lead vs. a 20 ms-voicing lag) but they succeed in discriminating stops within the Spanish lead category (60 ms-vs. 20 ms-voicing lead). Subsequently, the authors infer that early percep-tion is not dependent on linguistic experience. Nonetheless, it is conceivable that the Guatemalan infants already developed some sensitivity to the pre-voicing cue, but that they have not yet learned the precise boundary location and that, instead, at that young age, categories are still rather broad, re-quiring further alignment. However, the data do not allow to verify this assumption since Lasky et al. (1975) did not test infants from other lan-guage backgrounds where the lead contrast does not play a role in phoneme categorisation.
The ability to discriminate a lead contrast is also attested for 2-month-old infants learning the Bantu language Kikuyu (Streeter 1976). Kikuyu distinguishes prevoiced and voiceless stops. In a HAS-experiment, the in-fants succeed in discriminating the contrast in labial PoA where Kikuyu does not have a laryngeal contrast. Besides, they also respond to a short lag/long lag contrast but not to a contrast within the long lag region of the laryngeal continuum.
Some more recent studies also provide evidence for infants’ sensitivity to both lead and lag contrasts – and, notably, not only in the first half year of life. A longitudinal study by Rivera-Gaxiola et al. (2005) tests infants’
neural responses to a lead and a lag contrast at 7 and at 11 months of age. In
16Although other studies (see below) also report stronger discrimination effects for lag than for lead contrasts, the weaker effect for the within-category contrasts (both tense and lax) in this particular study might also relate to the age of Aslinet al.’s (1981) subjects.
Infants from 6 to 12 months are tested without further subdivision into smaller age groups.
Thus, it might be that the younger ones are still able to discriminate within-category contrasts, while the older ones already lost this sensitivity, as the model of perceptual reorganisation would predict.
an electroencephalography (EEG) study measuring event-related potentials (ERPs) in a double oddball paradigm with the syllable [ta] (ACT=12 ms) as standard stimulus, they find that infants are sensitive to the deviant [da] (24 ms-voicing lead) as well as to the deviant [tha] (ACT=46 ms) both at the age of 7 and of 11 months. However, at 11 months, the infants show a stronger effect and a more mature response pattern to the native lag contrast than to the non-native lead contrast. Conboy et al. (2008) use the same contrasts in a CHTP experiment with 11-month-old English infants. Again, [ta] (ACT=12 ms) served as background stimulus and the subjects had to turn their heads whenever the signal changed either to [da]
(24 ms-lead) or to[tha](ACT=46 ms). Their behavioural results support the electro-physiological results by Rivera-Gaxiola et al.(2005). Conboy et al.
(2008) also report sensitivity to both contrasts with better performance for the native lag contrast than for the non-native lead contrast. Both studies provide further support for the assumption that a perceptual reorganisation in favour of the native phoneme contrasts takes place in the first year of life.
Besides, the findings show that the sensitivity to non-native contrasts is not completely lost, an insight that is in line with the fact that, with a sufficient amount of training, adults can re-learn to differentiate non-native contrasts (e.g., Werker & Logan 1985; Polka 1991; Andersonet al. 2003).
Zamuner (2006) provides data of Dutch infants’ perception of a lead contrast between alveolar stops. The lax stops are described as displaying a “significantly longer voicing in the consonant, and a significantly greater proportion of voicing during closure duration” (Zamuner 2006:87) and – in the case of stops in final position – as being preceded by longer vowels than the tense stops. The exact ACT and voicing lead values are not specified.
In several experiments conducted with the Switch Procedure, she shows that 10-month-olds succeed in discriminating the native laryngeal contrast in initial position. In final position, however, a position where Dutch phono-tactics require contrast neutralisation (devoicing), neither 10- nor 16-month-old Dutch infants succeed in discriminating the contrast. Zamuner’s (2006) findings show that, concerning the laryngeal contrast between prevoiced and voiceless stops, Dutch infants’ perception is in line with the native phoneme inventory towards the end of the first year of life. Yet, the study provides no insights concerning the developmental path towards this language-specific perception ability and, furthermore, it remains unclear whether the failure to distinguish the contrast in final position is due to the specific phonetic properties of the contrast, that is, low acoustic salience, or whether it can be attributed to the fact that Dutch has no laryngeal contrast in final position.
4.3 Infants’ Perception of Laryngeal Contrasts 85 Successful discrimination between vocoded versions of the naturally pro-duced VCV-sequences [aba] and [apa] by French 6-month-olds in a HTPP experiment is reported by Bertonciniet al. (2011). However, the study fo-cuses on cues available in degraded speech. The detailed information that is usually present in the spectrum and in the temporal structure of the sound signal is absent in Bertonciniet al.’s (2011) stimuli. No precise voicing lead and lag amounts are indicated. The finding suggests an early sensitivity to the native French lead contrast, but cannot answer the question whether the French participants learned to distinguish the native contrast on the basis of the linguistic input they were exposed to so far or whether the discrimination capacity was present already at birth.
Trehub & Rabinovitch (1972) are less interested in testing cross-category contrasts versus within-category contrasts, but their study provides evidence for infants’ extreme sensitivity to very small ACT contrasts. With the HAS procedure, they test 1- to 4-month-old infants from a Canadian English language background and find that the children are sensitive to three differ-ent ACT contrasts. Firstly, they discriminate a contrast between synthetic labial stops with ACT values of 20 ms and 80 ms, respectively. Secondly, they succeed in distinguishing a contrast between naturally spoken labial stops with ACT values of 15 ms and 45 ms, respectively. Finally, the infants are sensitive to a contrast between natural alveolar stops with ACT values of 30 ms and 40 ms, respectively. These results leave open whether perception is not categorical at all or whether the boundary is dependent on factors such as PoA and synthetic as opposed to natural speech.17 What is evident from these experiments is that infants are very good at detecting tiny ACT contrasts. They succeed in discriminating a contrast as small as 10 ms.
Evidence for infants’ extreme sensitivity to laryngeal properties also comes from Pegg & Werker (1997).18 They test Canadian English 6- to 8- and 10- to 12-month-old infants and Canadian English adults on a laryn-gealnon-phonemic contrast of English in syllable-initial position. A CHTP task is used to assess whether infants and adults are able to discriminate the syllables [t=a] and [d
˚a].19 [d
˚] is a realisation of underlying lax (‘voiced unaspirated’ in Pegg & Werker’s terms) /d/ which surfaces as voiceless[d
˚] in syllable-initial position. Its phonemic counterpart within the English
17Nittrouer (2001) claims that, in general, the nature of the stimuli does not have a significant influence on perception in this kind of experiments. At least in her data she observes no significant differences in speech perception when infants are presented with natural and synthetic stimuli, respectively.
18Note that in this study, the crucial cue is not ACT but the value of the second formant.
19The diacritics used by Pegg & Werker (1997) are adopted here.
two-way laryngeal contrast is the voiceless aspirated[th](e.g., ‘duck’ /d2k/
[d˚2k] vs. ‘tuck’ /t2k/ [th2k]). [t=] is a voiceless unaspirated variant of /t/
that only occurs after /s/, for example in a sequence like /sta/ (or in the word ‘stuck’ /st2k/ [st=2k]). Phonologically, [t=] and [th] are allophones of underlying /t/, while [d
˚] is a realisation of underlying /d/. Thus, the sounds [t=] and [d
˚] used in the tests by Pegg & Werker (1997) belong to two different categories. However, the contrast is not phonemic for there is no contrast between /t/ and /d/ following /s/, that is, there are no words starting with /sd/. Moreover, acoustically[d
˚]and [t=] are more similar than the two /t/-allophones (cf. table 4.1). The acoustic cue for the distinction of[d
Table 4.1: Mean ACT in ms and F2in Hz values used in the experiments by Pegg & Werker (1997).
Pegg & Werker’s (1997) study shows that English 6- to 8-month-olds are able to discriminate [t=a] from [d
˚a] whereas 10- to 12- month-olds do not respond to the contrast. The adult data are less clear: Although adults are able to discriminate the contrast in the CHTP, their performance is worse than adults’ typical performance in discrimination tasks with native phonemic contrasts. Additionally, preliminary tests suggest that adults’
discrimination performance is highly dependent on the method employed.
Pegg & Werker (1997) conclude from these findings that the perceptual reorganisation at the end of the first year of life depends on the phonological status of the contrast in question. They claim that only phonemic contrasts are maintained after the first 10 to 12 months (with adults being more flexible in their perceptual skills than infants). Regular exposure to non-phonemic native contrasts is considered insufficient for one-year-olds to keep the discrimination ability displayed earlier. It is obvious that this conclusion cannot be generalised. Rather, the specific acoustic properties of a contrast and not its phonological status seem to be decisive for discriminability.20 Yet, the findings by Pegg & Werker (1997) are a noteworthy demonstration
20Recall that the German allophones [ç]–[x] and [r]–[K] are easy to discriminate for German adults (and presumably also for infants) although the distinctions are not of phonemic importance (see also section 4.2.1).
4.3 Infants’ Perception of Laryngeal Contrasts 87 of the discrimination skills of 6- to 8-month-olds. Although the syllables tested are very similar and only differ significantly in the onset of the second formant, infants around a half-year of age apparently have no problems in detecting this minimal difference.
Yet another study explored infants’ perception of two laryngeal contrasts.
Burnset al.(2007; also cf. Burnset al.2003) use the visual fixation method to test English infants on their ability to discriminate a switch in the stim-ulus material from a labial stop with an ACT of 28 ms to a labial stop with an 8 ms-voicing lag and to a labial stop with a 48 ms-voicing lag. Although the authors claim that the 8 ms/28 ms-contrast represents a native French contrast, the study does not contribute to the issue of whether lead con-trasts are discriminable by very young infants.21 Instead, their data show that English 6- to 8-month-olds are able to discriminate a short lag/long lag contrast (8 ms vs. 28 ms) although it is assumedly not a native contrast.
Older English infants, aged 10 to 12 and 14 to 20 months, no longer distin-guish the contrast, but they are sensitive to the 28 ms/48 ms ACT contrast, which, according to the authors, straddles the Canadian English tense/lax phoneme boundary. Thus, the data by Burnset al.(2007) provide more sup-port for the assumption that initially infants are sensitive to short lag/long lag contrasts irrespective of the specific properties of the laryngeal system in their mother tongue. Furthermore, the findings show that the initial, apparently universal category boundary may be shifted in order to adapt to the native laryngeal categories. In this case, the boundary is not shifted leftward on the laryngeal continuum to replace the lag contrast by a lead contrast. Instead, it is slightly shifted to the right, maintaining a lag con-trast, but with increased ACT values.22 Thus, with Aslin & Pisoni (1980a) one could also speak of fine tuning or alignment of the laryngeal contrast.
A comparative overview over the diverse findings on infants’ perception of laryngeal contrasts is given in table 4.2.
21Burns et al. (2003) and Burns et al.(2007) refer to Canadian French. The French tense/lax distinction is usually assumed to contrast prevoiced to voiceless unaspirated stops (see Ladefoged 2006:148).
22Burns et al. (2007) compared the monolingual English infants to infants acquiring both French and English. The results suggest that the youngest bilingual infants behave like their monolingual peers: They succeed in distinguishing stops with an ACT of 8 ms and stops with an ACT of 28 ms but fail to discriminate stops with ACTs of 28 ms versus 48 ms. While older monolingual English children (10 to 12 and 14 to 21 months) seem to lose their sensitivity to the Canadian French contrast (8 ms/28 ms) and discriminate only the Canadian English contrast (28 ms/48 ms), bilingual toddlers of the same age ranges seem to be sensitive to both contrasts (albeit at a statistically only marginally significant level when it comes to the Canadian French contrast.)
Method Subjects i ii iii iv PoA
[1] HAS 1 & 4 -20/0 20/40 60/80 Lab
English × � ×
[2] HAS 1-4 20/80 Lab
English 15/45 Lab
30/40 Cor
�� �
[3] Heart 4-6.5 -60/-20 -20/20 20/60 Lab
Rate Spanish � × �
[4] HAS 2 -30/0 10/40 50/80 Lab
Kikuyu � � ×
[5] CHTP 6-8 -20/10 10/40 Lab
English × �
Spanish � �
[6] CHTP 6-12 -70–0 -50–20 0–70 Lab
English � � �
[7] ERP 7 & 11 -24/12 12/46 Cor
English � �
[8] Switch Dutch Cor
10 �
10 & 16 ×final
[9] VF English 8/28 28/48 Lab
6-8 � ×
10-12 × �
14-20 × �
[10] CHTP 11 -24/12 12/46 Cor
English (�) �
[11] HTPP 6, French � medial Lab
Table 4.2: Infants’ perception of laryngeal contrasts. Unless indicated oth-erwise, the data refer to stops in initial position. The third column provides infants’ age (months) and language background. Contrasts are assigned to four types: (i) contrast within the lead region of the laryngeal continuum; (ii) con-trast of prevoiced stops and stops with a short voicing lag; (iii) short lag/long
Table 4.2: Infants’ perception of laryngeal contrasts. Unless indicated oth-erwise, the data refer to stops in initial position. The third column provides infants’ age (months) and language background. Contrasts are assigned to four types: (i) contrast within the lead region of the laryngeal continuum; (ii) con-trast of prevoiced stops and stops with a short voicing lag; (iii) short lag/long