From Universal to Language-Specific Speech PerceptionPerception

Although it has been demonstrated that infants’ speech perception is in-fluenced by maternal speech already prenatally (e.g., DeCasper & Spence 1986; DeCasper et al. 1994), the mothers’ utterances audible in the womb do not seem to affect the perception of single speech sounds. Rather, they lead to the formation of first prosodic preferences. In particular, neonates show a preference for the intonational patterns of their mother tongue and are able to discriminate their mother tongue from other languages with dif-ferent intonation patterns (e.g., Mehleret al. 1988; Moonet al. 1993; Nazzi et al. 1998). Regarding infants’ perception of segmental speech contrasts, all infants seem to begin from largely the same position with an initial phase of universal speech perception. In their first months of life, they are able to discriminate phonetic contrasts of all languages. Gradually, the universal perception develops into a language-specific perception of speech contrasts.

4.2 From Universal to Language-Specific Speech Perception 67 A landmark study by Werker & Tees (1984), which has remained in-fluential to date, provides the basis for this commonly accepted assump-tion. In a CHTP task the authors demonstrate that Canadian English 6-to 8-month-old infants are able 6-to discriminate two PoA contrasts in s6-tops which are not distinctive in the English phoneme inventory (Hindi dental [t”a]vs. retroflex[úa]and Thompson velar and uvular ejectives [k’i] vs. [q’i]⁴).

Older Canadian English infants, aged 10 to 12 months and Canadian En-glish adults fail to discriminate the contrasts whereas Hindi and Thompson adults and 11- to 12-month-olds reliably discriminate the respective native contrast. A control condition shows that the older English infants have no difficulties in discriminating the native PoA contrast between [ba] and [da].

Similar results are obtained in two preceding studies (Werker et al. 1981;

Werker & Tees 1983) for the Hindi laryngeal contrast between voiceless as-pirated[t^ha]and voiced aspirated[d^ha]. In sum, the experiments by Werker

& Tees (1984) reveal that 6- to 8-month-old infants – in contrast to adults – are able to discriminate non-native speech contrasts and that this ability is no longer observable at the end of the first year of life. Their findings had far-reaching consequences for the field of infant speech perception research.

Studies of the 1970s and early 1980s had yielded sometimes contradictory results and were problematic to compare due to different test methods and varying age ranges of the children (see also section 4.3). The findings by Werker and her colleagues seemed to suggest a generalisable pattern for the acquisition of speech contrasts, in particular, they were seen as evidence that the decline in sensitivity to non-native consonant contrasts in laryngeal quality and PoA occurs at around 10 to 12 months of age. This developmen-tal pattern is often termed ‘perceptual reorganisation’ (see Werker & Tees 1984). Apparently, infants start life with universal perception skills which decrease between 6 and 12 months in favour of a reorganised perception that is specialised to the discrimination of only native speech contrasts.

A strict interpretation of this development is referred to as universal theory by Aslin & Pisoni (1980a). It presumes that all phonetic contrasts existent in natural languages are discriminable at birth. The maintenance of the discrimination ability for a given contrast depends on whether the contrast is present in infants’ early linguistic experience or not. If it is a phonological contrast in the child’s ambient language, the distinction will be maintained whereas the absence of the contrast leads to a loss of the discriminability of this particular contrast. There has also been some debate

4Thompson is an Interior Salish language, spoken in south central British Columbia.

In other publications, the language is referred to as Nthlakampx.

on the nature of the decrease in discriminability in terms of whether it is a complete, irreversible loss or just an attentional deficit. Since studies with adults and, most notably, studies in second language acquisition have shown that adults are able to (re-)learn to distinguish foreign phoneme contrasts perceptually, the hypothesis of an irrevocable loss of discriminability was abandoned (see Aslin & Pisoni 1980a; and, e.g., Bradlowet al. 1997).

The paper by Werker & Tees (1984) generated a number of studies which aimed to put the hypothesis of perceptual reorganisation during the first year of life to test. There are indeed numerous studies on various consonant contrasts which yield similar results. For example, Werker & Lalonde (1988) find a significant decline in contrast discrimination for Canadian English 11-to 13-month-olds as against 6- 11-to 8-month-olds when tested on the Hindi PoA contrast between dental[d”]and retroflex[ã]. Kuhlet al.(2001) report the same pattern for (a) American English infants tested on a Mandarin Chinese contrast (alveolo-palatal affricate [tC^h] vs. alveolo-palatal fricative [C]) and (b) Japanese infants tested on the English [r]–[l] contrast. A study by Best & McRoberts (2003) adds three isiZulu contrasts to further support the developmental pattern of perceptual reorganisation. In particular, they contrast (a) a labial voiced stop [b] to a labial implosive [á], (b) a velar voiceless aspirated stop[k^h]to a velar ejective [k’] and (c) a lateral voiceless fricative[ì]to a lateral voiced fricative[Ð]. For all three contrasts, American English 6- to 8-month-olds are better in discriminating the isiZulu phonemes than 10- to 12-month-olds from the same language background.

A similar pattern has been found for the acquisition of phonemic vowel contrasts. While some suggest a different developmental time course with specialisation on the phonemic contrasts of the mother tongue already at the age of 6 months (e.g., Kuhlet al. 1992; Polka & Werker 1994) others claim that a robust effect of vowel reorganisation occurs only at 10 to 12 months, that is, at the same time when consonants are perceived phonemically (e.g., Polka & Bohn 1996; Cheour et al. 1998). The diverging findings are at-tributed at least partially to the specific phonetic properties of the stimuli used in the respective studies (cf. Polka & Bohn 1996). Thus, although the timeline may differ for vowels and consonants, the literature provides am-ple evidence for a perceptual reorganisation during the first year of life. In spite of the fact that most of the aforementioned studies are conducted with infants from English-speaking environments, the findings seem to support the suppositions made byuniversal theory, namely that all contrasts are dis-criminable at birth and that a contrast’s (ir)relevance in the native phoneme inventory leads to either the maintenance or the loss of the discrimination ability. There are, however, other studies which show thatuniversal theory

4.2 From Universal to Language-Specific Speech Perception 69 makes too broad generalisations and that the issue of contrast discrimination in early speech perception requires a more differentiated approach.

That a general assumption pertaining to the perceptual development of all contrasts is too simple a solution became explicitly apparent in 1988, when Catherine Best and her colleagues published their findings of American English-learning infants’ discrimination of contrasts between isiZulu click consonants.⁵ First of all, Best et al. (1988) demonstrated that American English adults had no difficulties in discriminating several isiZulu clicks even if the sounds were modified in amplitude, eliminating the features supposed to make the contrast most robust. The authors then tested English infants’

ability to discriminate the PoA contrast between the voiceless unaspirated apicodental click as in the syllable [Øa] and the lateral alveolar click as in [Ûa] in a visual fixation (VF) task. All infants, distributed to the four age groups 6 to 8, 8 to 10, 10 to 12 and 12 to 14 months, showed adult-like reliable discrimination of the click consonant contrast. In line with the assumptions of universal theory, the positive results for the 6- to 8-month-olds suggest that the discriminability of the isiZulu clicks is present in the first months of life. Beyond that, the findings suggest that the click distinction is not subject to decline during the second half of the first year of life. Although the contrast occurs neither as a phonemic nor as an allophonic distinction in English, discriminability is maintained, thus contradicting the predictions made byuniversal theory.

Yet another pattern of perceptual development is attested. It has been observed that for some contrasts, perception starts with a rather poor dis-criminability and only when the infants grow older and have had some ex-perience with their mother tongue, they learn to listen to the cues that are relevant for these particular phonemes. This was suggested already in 1979 by Eilerset al.(1979a), who focus on infants’ perception of prevoiced stops (see section 4.3). More recently, a study by Polka et al. (2001) examines Canadian English and Canadian French adults’ and 6- to 8- and 10- to 12-month-old infants’ ability to discriminate the contrast between the alveolar lax stop [d] and the interdental voiced fricative [D], which is a phonemic contrast in English but not in French. As expected, in a CHTP experi-ment, the French adults perform consistently worse than the English adults;

but neither a language effect nor an age effect is observed for the infants.

French 6- to 8- and 10- to 12-month-olds are as good as their English peers

5In their 1988 paper, Best and her colleagues speak of ‘Zulu’ when referring to the language used in their tests. Later they correct themselves and use ‘isiZulu’ instead (Best

& McRoberts 2003).

in discriminating [d] and [D] and all infants reach approximately the same discrimination level. Overall, the infants perform significantly worse than the English adults. The French adults’ discrimination level, on the other hand, is comparable to that of the infants.

Similar findings are reported by Narayan et al. (2010), who examine infants’ perception of a nasal contrast[Na]versus [na]. Velar[N]and alveolar [n] are contrasted phonemically in initial position in Filipino but not in English. Participants are Canadian English infants aged 4 to 5, 6 to 8 and 10 to 12 months and Filipino infants aged 6 to 8 and 10 to 12 months.

English infants of all three age groups fail to discriminate the contrast – as do the Filipino 6- to 8-month-olds. Filipino 10- to 12-month-olds, however, are able to distinguish the two nasals.

Thus, in addition to contrasts universally discriminable at birth, there are apparently other speech sound distinctions for which discriminability is only weakly developed in the first months of life. For that kind of contrasts, discrimination performance matures. It improves with increasing linguistic experience, if the contrast is relevant for the native phoneme inventory.

Polka et al. (2001:2199) also speak of a contrast ‘boost’ to describe this perception pattern. If, on the other hand, an initially hard-to-discriminate contrast is not part of the native phoneme inventory, discriminability may remain on the initial perception level or deteriorate.

In some recent studies, age-related improvement in discriminating native phoneme contrasts is found also for some contrasts that were considered to follow the ‘classical’ development of perceptual reorganisation with high discriminability from the start. For instance, Tsaoet al. (2006) find better discrimination of the Mandarin Chinese affricate/fricative contrast[tC^h]–[C]

in Mandarin-learning infants aged 10 to 12 months than in 6- to 8-month-olds. Similarly, Kuhl et al. (2006) report facilitated discrimination of the English [r]–[l] contrast for American English 10- to 12-month-olds compared to 6- to 8-month-olds’ performance.

The findings summarised in the preceding paragraphs show that uni-versal theory is not appropriate to cover the diverse developmental pat-terns observed for infants’ discriminative capacities. Aslin & Pisoni (1980a) mention an alternative approach, termed attunement theory, that allows to account for a broader range of contrasts. The theory proceeds on the as-sumption that infants start life with only partially developed discrimination skills. At least some of the phonetic contrasts occurring in the languages of the world are discriminable at birth, but linguistic experience is required to fine-tune the discriminative capacities to the precise phoneme contrasts of the mother tongue. Other contrasts which are not present in the infant’s

4.2 From Universal to Language-Specific Speech Perception 71 ambient language are either maintained at the original, possibly rather un-refined discrimination level or they become attenuated (cf. Aslin & Pisoni 1980a). Thus,attunement theory seems to be a more appropriate approach to account for the multiple variants found in early speech perception devel-opment than the proposal made byuniversal theory. At birth, a given con-trast is discriminable to a certain extent. If the concon-trast is a native phoneme contrast, its discrimination is fostered by the regular exposure to it. This may result in the simple maintenance of the discriminability, as seen in the control condition presented to English-learning infants by Werker & Tees (1984). English 6- to 8- and 10- to 12-month-olds succeed in discriminating the English phoneme contrast [b]–[d].⁶ Alternatively, discriminability of the native contrast may improve with increasing linguistic experience. Such a facilitative or enhancing effect is reported, for example, by Polkaet al.(2001) for the English [d]–[D]contrast or by Kuhlet al.(2006) for the [r]–[l] distinc-tion in English. Similarly, we have seen that Narayanet al.(2010) find that Filipino infants learn to distinguish the initially non-discriminable Filipino phoneme contrast between [n] and [N]. An improvement of discriminative capacities might also be achieved by the alignment of a certain contrast, as for instance demonstrated by Burnset al. (2007). They show that English 10- to 12-month-olds, in contrast to 6- to 8-month-olds, readjusted the cate-gory boundary to the English tense/lax opposition in stops to better match the adult boundary (see section 4.3 for details).

If a contrast that is (at least partially) discriminable at birth is not present in the infant’s mother tongue, there are also several options how fur-ther development may proceed. Firstly, the discriminability might be main-tained, as found for English infants’ and adults’ perception of the isiZulu click contrasts. Secondly, discriminability may become attenuated. The de-gree of the decline may differ for different contrasts. Andersonet al.(2003) examine American English 6.5- and 8.5-month-olds’ perception of the same Hindi and Thompson stop contrasts used by Werker & Tees (1984) and find different degrees of decline for the two non-native contrasts. At 8.5 months, English infants are still better at discriminating the velar/uvular ejective contrast of Thompson than the Hindi dental/retroflex stop contrast.

As became obvious, there are multiple ways in which speech contrasts are perceived by young language learners. It was shown that the categorical assumptions made byuniversal theory fall short of capturing the complete

6Note that in many cases it is difficult to determine the initial or final levels of percep-tual development when no comparative data exist (e.g., adults’ discriminative capacity) or due to test methods not sufficiently sensitive to provide gradual measures.

palette of perception patterns observed so far and that attunement theory is more appropriate to describe infants’ perceptual development. In order to explain why some (non-native) contrasts are better discriminated than others, Best and her colleagues developed a model which will be introduced in the following section.

4.2.1 The Perceptual Assimilation Model

After having found that not all non-native contrasts become indistinguish-able after the first half year of life, Bestet al.(1988) do not entirely reject the idea of perceptual reorganisation, but they put it into a more global frame of early speech perception. They assume that, in a first stage of speech perception called ‘prephonemic listening’, speech sound categories as organ-ised in adult phonology are irrelevant to the infants. Up to 6 to 8 months of age, infants pay attention to general auditory/phonetic characteristics of the speech signal. Only in the second half of their first year of life, the listening mode changes from prephonemic to ‘phonemic listening’. Children at this stage of speech perception start to analyse the speech input accord-ing to the categories of their native phoneme inventories. This means that children now try to map all incoming speech to phonemic categories of their mother tongue. However, as the data by Bestet al. (1988) suggest, it can-not be phonological relevance or linguistic experience alone that determines whether the ability to distinguish phonetic contrasts is maintained.⁷

Instead, for the stage of phonemic listening, a more refined explanation pattern is required which allows for qualitative distinctions of the contrasts tested. In several subsequent studies Best and colleagues (e.g., Best et al.

1995; Best et al. 2001; Best & McRoberts 2003) developed the so-called perceptual assimilation model (PAM). This model takes the relation of the non-native sounds to the respective native phoneme inventory into account.

Basically, there are four types of relations to which non-native contrasts can be assigned. According to this classification, discriminability in the phone-mic listening stage can be predicted. In other words, whether a perceptual decline in discrimination of a non-native contrast takes place in infancy is determined by the type of assimilation to which the respective contrast be-longs. The different ways in which non-native contrasts might be perceived according to PAM are listed in (1) below (cf. Bestet al.1988:352; Bestet al.

2001:777; Best & McRoberts 2003:186).

7Recall that in Best et al.’s (1988) study, English 6- to 14-month-olds and English adults are able to discriminate a contrast that does not occur in the English language, neither phonemically nor allophonically.

4.2 From Universal to Language-Specific Speech Perception 73 (1) Assimilation types according to PAM.⁸

a. SC = Single-Category Assimilation

Both phones of a non-native contrast can be assimilated as equal-ly-good tokens of a single native phoneme category. This assimi-lation type is expected to be discriminated poorly by adults and children who already reached the phonemic listening stage.

b. TC = Two-Category Assimilation

The phones of a non-native contrast are assimilated to two dif-ferent native phoneme categories. Discrimination is expected to be very good for this type of contrast.

c. CG = Category-Goodness Difference in Assimilation Although both phones of the non-native contrast are assigned to a single native phoneme category, one of the phones represents a

‘better’ exemplar of that category; that is, the non-native sounds differ in the degree of their similarity to the native phoneme category. The model predicts that contrasts of this type are discriminated rather well, but not as good as TC-contrasts.

d. NA = Non-Assimilable Sounds

The sounds of the non-native contrast bear no detectable simi-larity to any native phonemes. Therefore, they are processed in a rather general perception manner, in which listeners rely on auditory, non-speech properties of the sounds. Discrimination of such non-assimilable sounds is predicted to be very good.⁹ It should be kept in mind that this typology of contrast assimilation is irrelevant to listeners younger than 10 to 12 months of age. Only when in-fants have started to process the speech signal in a phonemically structured way, a decline in discrimination performance is expected for contrasts of the SC-type and possibly also for contrasts of the CG-type. Note that the main-tenance or loss of the discrimination ability for non-native contrasts remains always relative in this account. Non-native contrasts cannot be described absolutely as being discriminable or not. Discrimination is always depen-dent on the relation of the particular non-native contrasts to the respective native phoneme inventory. Or, to put it differently, a specific non-native contrast might be easy to discriminate for listeners from language A but

8For a more fine-grained classification see Bestet al.(2001:777).

9One might question whether it is reasonable to generally assume good discrimination for this class of sounds. It might very well be the case that discrimination of non-speech sounds depends heavily on their acoustic salience or other non-linguistic aspects.

not discriminable for listeners from language B.¹⁰ Moreover, PAM predicts

not discriminable for listeners from language B.¹⁰ Moreover, PAM predicts