Investigating the role of articulatory organs and perceptual assimilation of native and non-native...

Investigating the Role ofArticulatory Organs andPerceptual Assimilation ofNative and Non-NativeFricative Place Contrasts

ABSTRACT: The perceptual assimilation model (PAM; Best, C. T. [1995]. Adirect realist view of cross-language speech perception. In W. Strange (Ed.),Speech perception and linguistic experience: Issues in cross-language research(pp. 171–204). Baltimore, MD: York Press.) accounts for developmental patternsof speech contrast discrimination by proposing that infants shift from untunedphonetic perception at 6 months to natively tuned perceptual assimilation at11–12 months, but the model does not predict initial discrimination differencesamong contrasts. To address that issue, we evaluated the ArticulatoryOrgan Hypothesis, which posits that consonants produced using differentarticulatory organs are initially easier to discriminate than those produced withthe same articulatory organ. We tested English-learning 6- and 11-month-olds’discrimination of voiceless fricative place contrasts from Nuu-Chah-Nulth (non-native) and English (native), with one within-organ and one between-organcontrast from each language. Both native and non-native contrasts werediscriminated across age, suggesting that articulatory-organ differences do notinfluence perception of speech contrasts by young infants. The results highlightthe fact that a decline in discrimination for non-native contrasts does not alwaysoccur over age. � 2014 Wiley Periodicals, Inc. Dev Psychobiol 56: 210–2272014.

Keywords: perceptual narrowing; infants; cross-language speech perception;fricatives; perceptual assimilation model; native-languageattunement; articulatory organs

INTRODUCTION

Classic cross-language speech perception findings with

adult listeners are fairly clear: they often have difficulty

categorizing and discriminating consonant contrasts not

found in their native language, though there are a range

of intriguing exceptions (e.g., Best, McRoberts, &

Goodell, 2001; Best, McRoberts, & Sithole, 1988;

Polka, 1991). Infants at 6–8 months of age, on the

other hand, successfully discriminate not only most

native contrasts but also many non-native ones, includ-

ing those that adults of their language environment

have difficulty discriminating (Aslin & Pisoni, 1980;

Eilers, Gavin, & Wilson, 1979; Trehub, 1976; Werker,

Gilbert, Humphrey, & Tees, 1981). Seemingly adult-

like attunement to native consonant contrasts begins to

Manuscript Received: 21 March 2013Manuscript Accepted: 7 December 2013Correspondence to: M. D. TylerContract grant sponsor: NIHContract grant number: DC00403Article first published online in Wiley Online Library

(wileyonlinelibrary.com): 6 January 2014DOI 10.1002/dev.21195 � � 2014 Wiley Periodicals, Inc.

Developmental Psychobiology

Michael D. Tyler1,2

Catherine T. Best1,3

Louis M. Goldstein3,4

Mark Antoniou1,5

1MARCS InstituteUniversity of Western Sydney

Penrith, New South Wales 2751, AustraliaE-mail: [email protected]

2School of Social Sciences and PsychologyUniversity of Western Sydney

Penrith, New South Wales, Australia

3Haskins LaboratoriesNew Haven, Connecticut

4Department of LinguisticsUniversity of Southern California

Los Angeles, California

5Department of Linguistics and ModernLanguages

Chinese University of Hong KongHong Kong

emerge by 10–12 months of age, when there is no

longer evidence for discrimination of many non-native

consonant distinctions (Werker & Lalonde, 1988;

Werker & Tees, 1984). For example, in a study that is

often cited in support of perceptual narrowing in early

development, there is a decline from 6, through 8, to 12

months of age, in the number of English-learning

infants who discriminated the Hindi unaspirated dental

versus retroflex stops / /-/ / and Nthlakampx velar

versus uvular ejectives /k0/-/q0/ (Werker & Tees, 1984).

A decline was also found for Zulu plosive versus

implosive bilabial stops /b/-/ / and voiced versus

voiceless lateral fricatives / /-/ / (Best & McRoberts,

2003). This pattern of decline is consistent with the

idea that there is perceptual narrowing in speech

perception over the first year, as a result of native

language experience.

Developmental decline in discrimination is not

observed for all non-native contrasts, however. Best,

McRoberts, LaFleur, and Silver-Isenstadt (1995) found

that English-learning infants at both 6–8 and 10–12

months of age successfully discriminated the non-native

Zulu dental versus lateral click contrast /|/-/||/, in

contrast to their replication of the earlier Werker and

Tees (1984) finding of developmental decline for

Nthlakampx velar versus uvular ejectives, which only

the 6-8-month-olds discriminated. Similarly, Best and

McRoberts (2003) found that both 6–8- and 10–12-

month-olds learning English discriminated a Tigrinya

bilabial versus alveolar ejective stop contrast /p0/-/t0/.A further complexity in the developmental story is

that some native contrasts appear to be initially difficult

to discriminate, with performance then improving over

time. For example, one study found that American-

English-learning infants’ discrimination of English /r/

and /l/ was mediocre at 6–8 months, equivalent to that

of Japanese-learning infants, but only American infants

showed improved discrimination at 10–12 months

(Kuhl et al., 2006). Another study found no evidence

for discrimination of the Tagalog (Filipino language)

alveolar-velar nasal stop contrast, /na/-/Ea/, by English-

learning infants aged 4–5, 6–8, or 10–12 months

(Narayan, Werker, & Beddor, 2010). On the other hand,

infants learning Filipino languages in which /na/-/Ea/ isa native contrast showed no evidence of discrimination

at 6–8 months, but succeeded at 10–12 months.

Additionally, the English alveolar stop versus interden-

tal fricative contrast /d/-/ð/ was discriminated equally

poorly by Canadian-English- and Canadian-French-

learning 6–8- and 10–12-month-olds (Polka, Colanto-

nio, & Sundara, 2001). Improved discrimination for the

English learners was achieved by 4 years of age

(Sundara, Polka, & Genesee, 2006), suggesting that

native-language attunement occurred between the ages

of 1 and 4 years. In contrast, the Canadian-French-

learning infants and children had poor discrimination of

the contrast at 6–8 months, 10–12 months, and 4 years,

and French-speaking adults were consistently less

accurate than the English-speaking adults. Moreover,

one study has even found a developmental decline for a

native contrast. Best and McRoberts (2003) found that

although both 6–8- and 10–12-month-olds were able to

discriminate English /s/-/z/, there was a significant

decline in the degree of discrimination at the older age.

On the surface, this latter finding seems to pose a

challenge for notions of perceptual narrowing.

Developmental theories of speech perception thus

face a difficult set of challenges. They must account

for why discrimination of both native and non-native

contrasts can initially be successful or unsuccessful,

may or may not improve or decline over time, and

why performance on native contrasts appears to follow

different developmental trajectories for different con-

trasts. But theoretical accounts can be difficult to

compare because they differ in focus, that is, one

account may focus on initial perceptual sensitivities

and another on developmental changes, and both

of these aspects might be investigated from the

perspective of native or non-native speech perception,

possibly using different procedures. We therefore

have concentrated on one model in the design of the

present study, the perceptual assimilation model

(PAM; Best, 1994a,b, 1995), but we will evaluate our

results in light of alternative models in the General

Discussion (the Native Language Magnet Theory-

Expanded, Kuhl et al., 2008; and Processing Rich

Information from Multi-dimensional Interactive Repre-

sentations, Curtin, Byers-Heinlein, & Werker, 2011;

Werker & Curtin, 2005). The present article has three

main goals: (1) To outline how PAM accounts for both

perceptual narrowing to native speech contrasts as well

as for variations in infants’ success at discriminating

different non-native contrasts at different ages. (2) To

test whether the articulatory organ hypothesis (AOH),

an expansion on PAM described in Best and McRo-

berts (2003), predicts infants’ initial discrimination of

native and non-native contrasts. (3) To provide much

needed data on infant discrimination of naturally

produced native and non-native fricative place of

articulation contrasts.

We follow Aslin and Pisoni (1980) in applying to

speech perception Gottlieb’s (1976) framework for

describing the possible trajectories that perceptual

abilities might display developmentally as a function of

language experience. At birth, discrimination of pho-

netic contrasts could range from perfect to poor. Those

discriminated perfectly at birth may undergo mainte-

nance with language experience or decline through lack

Developmental Psychobiology Infant Perception of Native and Non-Native Fricatives 211

of experience.1 Intermediate levels of initial discrimina-

tion can undergo facilitation (further improvement) or

maintenance with experience, or can decline through

lack of experience. Finally, poor discrimination will

remain unaffected in the absence of relevant experi-

ence, or will undergo induction (improvement) with

relevant experience [see Figure 5.4 in Aslin and Pisoni

(1980), for a depiction of this framework].

For PAM (Best, 1994a, b, 1995), the objects of

speech perception are articulatory gestures. In line with

Gibson’s (1979) ecological theory of perception, the

acoustic signal is thought to carry information about

those articulatory gestures, but information may also

come from other modalities such as vision or touch.

Although infants are exposed to low-frequency compo-

nents of maternal speech in the womb, which may result

in some prenatal attunement for prosodic properties

(e.g., intonation and rhythm) and possibly for some

vowels, infants’ initial consonant discrimination abilities

are generally unaffected by experience with their native

language environment. Successes or failures in phoneme

discrimination at a very early age therefore reflect

general pre-phonetic perceptual abilities. As they gain

experience with the ambient language environment,

infants begin to recognize that constellations of similar

articulatory gestures contrast with other constellations

and they begin to recognize the higher-order invariants

that characterize native phonetic categories.2 Later in

development (around 17–19 months), tuning of phonetic

categories is driven additionally by phonological catego-

ries that arise as a result of an expanding vocabulary

(see Best, Tyler, Gooding, Orlando, & Quann, 2009;

Mulak & Best, 2013; Studdert-Kennedy, 1991). Thus,

from a PAM perspective, discrimination of speech in

the first year of life is determined first by general pre-

phonetic perceptual abilities, and then additionally by

the recognition of native phonetic categories through

language-specific tuning, followed by emerging phono-

logical categories through lexical development. Impor-

tantly, it is not necessary for the infant to have had

experience with the specific phonetic characteristics of a

non-native phone for it to be perceived as an instance of

a native category. The non-native phone simply needs to

share some higher-order invariant information with the

native phonetic category.

As infant perception develops, it will be driven less

by pre-phonetic perceptual abilities and more by

phonetic (and phonological) categories. Perception via

those categories is highly efficient because they are

specially tuned to the properties of the ambient speech

environment. Phonetic categories are the result of

infants tuning in to those speech characteristics that are

necessary to distinguish them from other native speech

categories (i.e., higher-order invariants) and tuning out

within-category phonetic variability. Thus, while pho-

netic categories provide the infant with improved

speech perception efficiency, within-category differen-

ces become more difficult to detect. It is important to

note, though, that sensitivity to within-category differ-

ences is not lost altogether, according to PAM, and

they should remain detectable under the right condi-

tions.

Once an infant has developed phonetic categories, if

the phones of a contrast fall in regions of phonetic

space that have undergone language-specific tuning,

then the infant’s discrimination is likely to be affected,

either positively or negatively. If both phones in a

contrast (native or non-native) fall entirely within a

single developing native phonetic category then dis-

crimination will be poorer than if each phone falls

within a different native category or if one or the other

phone falls in a region of phonetic space that has not

(yet) undergone language-specific tuning.

In adult studies that have examined PAM, the

influence of attunement to the native language is

gauged by asking participants to categorize non-native

phones in terms of native speech categories and to rate

their goodness of fit. Patterns of assimilation of non-

native phones to native categories are then established

from those data and those assimilation patterns are

used to test PAM predictions for discrimination of

non-native contrasts (see, e.g., Best et al., 2001;

1Aslin and Pisoni (1980) used the term loss instead of

decline. We use the term decline because such contrasts may be

discriminable in adulthood under certain task conditions (for a

discussion, see Werker, 1995).2The term phonetic category is used widely in the literature,

but it is not clear whether all authors use the term in an

equivalent fashion, especially with respect to non-native speech

perception. In particular, some theorists appear to consider

phonetic categories to be innate and, therefore, that the infant’s

job is to discover which of those categories are relevant for the

language of their environment [for a lively debate on this issue,

see Nittrouer (2001), a comment by Aslin, Werker, and Morgan

(2002), and reply by Nittrouer (2002)]. In contrast, we conceive

of a universal phonetic space that is multidimensional and

encompasses all possible articulatory configurations of the

world’s languages (other theorists may prefer to consider acoustic

cues rather than articulatory configurations, but this does not alter

the general principle). Phonetic categories are not innate, but

rather emerge as the young infant carves out regions of phonetic

space that are utilized in the native-language. Initially those

native phonetic categories may be broad, such that there would

be little differentiation between good and poor exemplars, which

would then undergo additional fine-tuning throughout develop-

ment. Several adjacent points in the phonetic space may therefore

fall within a single phonetic category for an infant learning one

language, or at the intersection between two phonetic categories

for another language, or even in a region of phonetic space that

has not undergone language-specific tuning, for an infant learning

yet a third language.

212 Tyler et al. Developmental Psychobiology

Harnsberger, 2001; Strange et al., 1998; Tyler, Best,

Faber, & Levitt, in press). The challenge for predicting

infant discrimination from a PAM perspective is to

observe (and to predict) the point in development when

native-language phonetic tuning begins to exert an

influence on perception. This appears to occur, for most

contrasts, at some time between 6 and 12 months of

age. It is important to note that a shift to natively tuned

speech perception may occur without an observable

change in discrimination, or even with a change that

appears to be counterintuitive. For example, the

decline in discrimination for English /s/-/z/ (Best &

McRoberts, 2003) may reflect a shift from reliance on

pre-phonetic differences between stimuli at 6–8 months

to reliance on rudimentary natively tuned phonetic

categories at 10–12 months.

A shift to natively tuned perception would result in

a decline in discrimination for a pair of non-native

phones that are both assimilated to the same native

phonetic category. However, if they are assimilated to

two different native phonetic categories then discrimi-

nation could appear to be maintained at 12 months of

age, and might even improve between 6 and 12 months.

By this account, the shift to natively tuned perception

certainly does include many examples that are consis-

tent with the notion of perceptual narrowing, but it

might not always result in a decline in discrimination

for non-native phonemes.

Previous descriptions of PAM (e.g., Best, 1994a,

b, 1995) listed a range of ways that non-native phones

might be assimilated to the native phonological system

of adult perceivers (e.g., single category, category

goodness, two category, uncategorized–categorized, and

non-assimilable–non-assimilable). However, if the pho-

nological system does not develop until 17–19 months,

some of those assimilation types may not apply at

10–12 months.3 The question of interest in the present

study, for infants prior to the development of phono-

logical categories, is whether a non-native phone is

assimilated to a natively-tuned (i.e., a phonetic catego-

ry) or an untuned region of phonetic space. This results

in four possible non-native contrast assimilation types

for infants who have transitioned to natively tuned

perception: (1) Single-category assimilation occurs

when both phones in a non-native contrast are assimi-

lated to a single developing native phonetic category

and discrimination is predicted to be poor. (2) Two-

category assimilation occurs when each contrasting

non-native phone is assimilated to a different native

phonetic category and discrimination should be very

good. (3) Untuned–tuned assimilation occurs when one

non-native phone is assimilated to a developing native

phonetic category and the other falls in a region of

phonetic space that is not (yet) utilized for speech in

the native language, and discrimination should be very

good. (4) Untuned–untuned assimilation refers to the

case when both phones fall in an untuned region, and

in that case discrimination would depend on the infant’s

pre-phonetic perceptual abilities.

From this extension of PAM principles to infant

assimilation types, it is now possible to detail how

discrimination of non-native phones should develop as

a function of initial sensitivities and the contrast

assimilation type when infants transition to natively

tuned perception. The predictions are presented in

Table 1. A decline in discrimination over development

would be observed when a contrast that was initially

discriminated successfully becomes a single-category

assimilation after the development of phonetic catego-

ries. Contrasts that infants initially failed to discrimi-

nate would remain poor for single-category and

untuned–untuned assimilations, but would improve for

two-category and untuned–tuned assimilations. Initial

discrimination levels (from poor to good) would simply

be maintained for untuned–untuned contrasts, because

the non-native phones fall in a region of phonetic space

that has not undergone native-language tuning. Finally,

non-native contrasts that are initially discriminated well

will be maintained for two-category and untuned–tuned

assimilations, even though older infants may have

shifted to using higher-order information to achieve the

same performance.

PAM was devised to account for attunement to the

native language so it does not make clear predictions

about which contrasts will be easy or difficult to

discriminate at the earliest, that is, pre-phonetic period

of acquisition. Given the direct-realist foundations of

PAM, one possible source of predictions about this is

the AOH (Goldstein & Fowler, 2003; Studdert-Kennedy

& Goldstein, 2003), which was devised to explain

differences in contrast discrimination by young infants,

3For readers familiar with PAM, we have omitted the

category-goodness assimilation type because we have assumed

that phonetic categories are likely to be “broad and flat” at the

end of the first year of life, that is, to have little perceptual

differentiation within a category (for a discussion, see

Best, 1994a). Infants are unlikely to recognize differences in

category goodness until they have begun to develop phonological

categories between 15 and 19 months of age (see Best et al.,

2009). Furthermore, we propose that there is no difference

between uncategorized and non-assimilable phones for infants.

The difference between those two assimilation types is simply

whether they are heard as speech (i.e., fall inside the phonologi-

cal space) or as non-speech (see Best et al., 1988). We suggest

that the distinction between speech and non-speech might

develop, like category goodness, with the establishment of

phonological categories later in development. Alternatively, it

might appear a bit earlier, with the establishment of native

phonetic categories.


based on the principles of articulatory phonology

(Browman & Goldstein, 1990, 1992, 1995, 2000). The

articulatory organs are the lips, tongue tip, tongue

body, tongue root, velum, and larynx (specifically, the

glottis: the area circumscribed by the vocal cords).

These constricting organs are distinct and independent,

because the constrictions formed by these organs are

made with different parts of the vocal anatomy and a

constriction can be formed with one organ without

necessarily producing a constriction in another. In the

articulatory phonology view, the traditional phonetic

dimensions of place and manner of articulation corre-

spond, respectively, to the location and degree of a

constriction achieved by a specific articulatory organ.

Based on the infant lip and tongue motion imitation

studies of Meltzoff and Moore (1977), the AOH

proposes that infants can identify articulatory organs

from birth and, therefore, that contrasts involving

separate organs (between-organ contrasts; e.g., /b/ [lips]

vs. /d/ [tongue tip]) should be discriminated easily. In

contrast, those involving the same articulatory organ,

with differences in constriction degree (e.g., tongue tip

in /t/ vs. /s/), constriction location (e.g., /s/ vs. /u/), orphasing between organs (e.g., /t/ vs. /d/, phasing

between tongue tip and glottal gestures), require experi-

ence with language-specific distributions of those

parameters and should be much more difficult for

young infants to discriminate. Therefore, the AOH

would predict that initial discrimination of both native

and non-native contrasts is determined by whether the

contrast is between- or within-organ. One clear predic-

tion that can be made from applying AOH principles to

discrimination by young infants is that they should

never fail to discriminate a between-organ distinction,

even if it does not occur in the ambient language

environment. Furthermore, in general, between-organ

distinctions should be detected more easily than within-

organ distinctions prior to the influence of native-

language tuning.

Best and McRoberts (2003) tested whether discrimi-

nation of non-native within-organ contrasts would show

a developmental decline earlier and more dramatically

than non-native between-organ contrasts. They found a

decline in discrimination from 6–8 to 10–12 months on

discrimination of a series of within-organ glottal con-

trasts (plosive/implosive, plosive/ejective, and voiced/

voiceless) but no decline for a between-organ non-

native place of articulation contrast (lips vs. tongue

tip). Their finding of significantly poorer discrimination

at 10–12 than 6–8 months for native voiceless-voiced

/s/-/z/ was interpreted to suggest that organ effects may

not be restricted to non-native contrasts. Note, however,

that Kuhl et al. (2006) instead found an improvement

from 6–8 to 10–12 months in American-English-

learning infants’ discrimination of the primarily within-

organ /r/-/l/ contrast, which is not compatible with the

idea that discrimination of within-organ contrasts

declines even for native speech. In light of the Kuhl

et al. finding, we suggest here that the decline for

English /s/-/z/ observed by Best and McRoberts (2003)

may be due to a transition from untuned to natively

tuned perception. The focus of the present article is on

whether articulatory-organ-based differences can pre-

dict infants’ initial discrimination of non-native con-

trasts.

To summarize, PAM makes clear predictions about

how discrimination might change over development as

a result of native-language tuning. In principle, PAM

can account for the developmental trajectories observed

in the literature, but validation is difficult because it

requires information, such as categorization with good-

ness ratings, that is not accessible using current infant

research techniques. Firstly, it is necessary to know

whether the infant has transitioned to natively tuned

speech perception and, secondly, to know how the

contrast of interest has assimilated to the developing

native system. Tests of assimilation with adults can

provide some guidance, but there is no guarantee that

assimilation patterns at 12 months mirror those of the

mature adult perceiver. Indeed, by our reasoning they

would not be entirely parallel at those two ages because

the adult operates in a phonological domain that the

Table 1. PAM Predictions for Changes in Non-Native Contrast Discrimination Over Development as a Function of (i)

Discriminability Prior To Natively Tuned Speech Perception, and (ii) Assimilation to Developing Phonetic Categories After

Natively-Tuned Speech Perception (But Prior To the Development of a Phonological System)

After the Onset of Natively-Tuned Speech Perception

Single-Category Two-Category/Untuned–Tuned Untuned–Untuned

Pre-phonetic speech perception

Poor Maintenancea Induction Maintenance

Moderate Decline Facilitation Maintenance

Good Decline Maintenance Maintenance

aThat is, the contrast continues to be discriminated poorly.


12-month-old has not yet reached. In contrast, PAM

does not provide any concrete predictions about

whether particular contrasts might initially be perceived

well or poorly, but the AOH fills that gap, as it does

provide a clear prediction of moderate-to-good initial

discrimination of between-organ contrasts.

To test AOH predictions for initial discrimination,

we required a set of stimuli that would allow us to

compare discrimination of between- and within-organ

contrasts. Furthermore, to test PAM-based developmen-

tal predictions, it was necessary to investigate discrimi-

nation of both native and non-native contrasts. The

contrasts tested in Best and McRoberts (2003) involved

the same oral constriction but differed in a gesture of

the glottis (voiced versus voiceless lateral fricatives

/ /-/ /, voiceless aspirated versus ejective velar stops

/kha/-/k’a/, and unaspirated bilabial plosive versus

implosive stops /pu/-/ u/). Although they are certainly

within-organ contrasts, those particular contrasts do not

provide a simple test of organ-based predictions

because they involve coordination (phasing) of a

laryngeal gesture with a constriction made by a

different articulator (tongue tip, lips) at a different

location in the vocal tract. The English /r/-/l/ contrast,

tested by Kuhl et al. (2006), also involves phasing

between different organs. Although the constriction is

primarily within-organ, it also does not provide a

simple test because /r/ and /l/ are complex articulations

that involve secondary constrictions with other organs.

Specifically, English /r/ involves bunching of the tongue

body, lip rounding, and lip protrusion (Boyce & Espy-

Wilson, 1997; Guenther et al., 1999), whereas English

/l/ involves tongue tip contact (dental-alveolar), tongue

root retraction, and no lip rounding (Stone &

Lundberg, 1996). Finally, English /d/-/ð/ and /b/-/v/

(Polka et al., 2001) are within-organ and they are much

less complex than /r/ and /l/, but they do not provide a

minimal test of within-organ contrast perception be-

cause they each involve differences in both constriction

degree (stop/complete closure vs. fricative/critical clo-

sure) and location (alveolar ridge or lips vs. teeth).

For a basic test of organ-based predictions we

sought between-organ contrasts versus within-organ

contrasts that only involved a single active articulator,

that is, did not involve coordination between different

articulatory organs. We selected contrasts between

voiceless fricative pairs differing in constriction loca-

tion (within-organ) and/or articulatory organ (between-

organ). An additional advantage of selecting voiceless

fricative contrasts is that they appear to be more

difficult for infants to discriminate than other contrasts

such as stop-place distinctions (Eilers, Wilson, &

Moore, 1977; Holmberg et al., 1977 as cited in

Kuhl, 1980). Using such contrasts therefore provides a

situation in which young infants are most likely to have

difficulty discriminating both within- and between-

organ contrasts, in both the native and in non-native

languages. The choice of fricative contrasts for this

study will also provide much needed data on the

development of fricative place (constriction location)

perception in infants between 6 and 12 months of age.

The majority of studies on infant perception of non-

native consonant place contrasts have focused on stop

consonants and glides. Few studies have focused on

fricative place distinctions, not all of them assessing

developmental changes, and the findings have been

mixed regarding infants’ success on these contrasts

(e.g., Cristia, McGuire, Seidl, & Francis, 2011; Eilers

et al., 1977; Levitt, Jusczyk, Murray, & Carden, 1988;

Nittrouer, 2001), as we describe in the context of our

experimental stimuli later.

We tested discrimination of non-native fricative

place contrasts in Experiment 1 and of native fricative

place contrasts in Experiment 2. In each experiment

there were two groups of infants: (1) 6-month-olds,

who should not yet have undergone much, if any,

native-language tuning for consonant perception, and

(2) 11-month-olds, who should have begun to develop

native phonetic categories for consonants.

EXPERIMENT 1: DISCRIMINATION OF NON-NATIVE VOICELESS FRICATIVE CONTRASTS

The aim of our study was to test whether articulatory-

organ-based predictions can account for 6-month-olds’

discrimination and whether PAM can account for the

change in discrimination over development to

12 months of age. We began our investigation with

discrimination of non-native contrasts because they

allowed an exposure-free context for evaluating the

influence of articulatory organs, as the non-native

fricatives we selected for this experiment do not occur

in English even as allophones. Experiment 2 examines

organ effects from the complementary vantage point,

by using native fricatives to assess native-language

tuning due to exposure in the ambient input.

In Experiment 1, we minimized the influence of

native-language tuning by selecting between- and

within-organ non-native voiceless fricative place con-

trasts from an untuned region of phonetic space—

untuned–untuned assimilation types (see Tab. 1). There

are no fricative place contrasts in English that are

produced with the tongue body or the tongue body

versus tongue root. Thus, a set of fricatives produced

with those organs would be least likely to have been

influenced by native-language tuning, because they

would not overlap with phonetic distributions of any


native English fricative (or for that matter, stop or

approximant) place of articulation contrasts.

The target language we selected for this purpose was

Nuu-Chah-Nulth, which has a set of voiceless fricatives

that meet our criteria (Maddieson, 1984). Nuu-Chah-

Nulth is a Wakashan language spoken on Vancouver

Island, Canada, which has velar /x/ and uvular /x/fricatives, both produced with the tongue body but at

different constriction locations, and a pharyngeal /£/fricative, produced with the tongue root (Carlson,

Esling, & Fraser, 2001). We used the between-

organ /x/-/£/ and within-organ /x/-/x/ contrasts to test

6–8- and 10–12-month-old Australian-English-learning

infants’ discrimination. The inclusion of the uvular

fricative /x/ in both contrasts provides a crucial added

control—it is the base segment that serves as a

comparator for both within-organ and between-organ

change in a repeated measures design. To maximize the

opportunity for infants to detect differences between

the control and test items, we used a stimulus-alternat-

ing preference paradigm (Best & Jones, 1998; Houston,

Horn, Qi, Ting, & Gao, 2007; Mattock, Molnar, Polka,

& Burnham, 2008), in which the test trials consisted of

tokens of the target fricative alternating with tokens of

the habituation stimulus.

Method

Participants. Forty-eight English-learning infants from

Sydney, Australia, formed the sample for Experiment 1.

Half were aged 6 months, Mage¼ 28.8 weeks, range:

24.6–31.9, the other half 11 months, Mage¼ 48.6

weeks, range: 45.3–52.6. There were 13 females and 11

males in each age group. An additional 28 infants were

tested but their data were eliminated from the sample

due to equipment failure (n¼ 6), failure to habituate

(n¼ 1), exposure to a language other than English

(n¼ 1), having fixation times shorter than 1 s on any

individual trial (n¼ 6; Cristia et al., 2011), or spontane-

ous recovery to the control stimulus (fixation duration

on the second control trial that was more twice as long

as the first control trial [i.e., the last habituation trial];

n¼ 14), indicating they had not actually fully habituat-

ed to the control stimuli (Kitamura, Panneton, &

Best, 2013).

Stimuli and Apparatus. The stimuli consisted of four

tokens each of velar /x/, uvular /x/, and pharyngeal /£/fricatives from Nuu-Chah-Nulth, recorded and pre-

sented in a consonantþ /a/ syllable (e.g., /xa/). We

recorded multiple tokens of a female native Nuu-Chah-

Nulth speaker in her 60s producing multiple tokens of

the target syllables in an adult-directed speech register.

It should be noted that the /x/-/x/ distinction may be

disappearing from the speech of younger Nuu-Chah-

Nulth speakers, thus we selected our speaker from the

older generation that does not merge these two

consonants, and ensured that our speaker maintained

the contrast. Final tokens were selected from the

recordings, matched across the three consonant catego-

ries in duration, amplitude and pitch contour. One final

velar (/x/) token with a short vowel had to be elongated

by three pitch pulses. Relevant acoustic parameters of

the stimuli are shown in Table 2.

Stimulus presentation was controlled using a soft-

ware program developed at the MARCS Institute,

running on Windows XP. The stimuli were played to

the infants in the test room through Edirol MA-10A

speakers.

Procedure. Testing took place in a dimly lit sound-

attenuated room at the MARCS Baby Lab, University

of Western Sydney. Black curtains covered all four

walls of the room. The infant sat on the parent’s lap

facing a black 1900 computer monitor on a black table

that was draped with black material, under which the

speakers were also hidden. The parent holding the child

wore headphones and listened to music mixed with a

random list of the experimental stimuli. The experi-

menter and control computer were located in an

adjacent control room and the infant was observed via

a video camera positioned to right of the monitor,

aiming through a gap in the black draping. To set the

volume level for the stimuli in the testing room, the

habituation stimuli were concatenated and played in a

loop. The volume knob was adjusted until a sound

Table 2. Mean Acoustic Measurements for the Nuu-Chah-Nulth Syllables

Category

Fricative Vowela

Duration

(ms)

Duration Range

(ms)

Intensity

(dB)

Centroid

(Hz)

Duration

(ms)

Intensity

(dB)

F1

(Hz)

F2

(Hz)

Pharyngeal /£/ 214 179–257 61 2,491 532 80 810 1,392

Uvular /x/ 266 220–304 60 3,052 493 79 728 1,296

Velar /x/ 242 227–259 58 2,340 530 80 697 1,366

F1 and F2 refer to the first and second vowel formants, respectively.aVowel measurements are reported from the 50% point.


pressure level meter at the infants’ location registered

between 65 and 70 dB (A-weighting).

We used an infant-controlled visual fixation habitu-

ation procedure (see Best et al., 1988; Miller, 1983)

with stimulus-alternating test trials. A trial began when

the infant fixated on a flashing checkerboard pattern

(on the monitor facing the infant) for more than

400ms, at which point the checkerboard became

stationary and the auditory stimuli played. The experi-

menter pressed the space bar whenever the infant

fixated on the computer monitor, which was registered

by the experimental control program that also played

out the audio stimuli. If the infant looked away, the

stimuli stopped playing (after finishing any token that

had already begun playing) until the infant looked

back again. The trial ended when the infant looked

away for more than 2 s or if the total trial duration

reached 30 s.

Habituation trials consisted of all four tokens of the

uvular fricative /x/ presented in random order, with an

interstimulus interval of 1 s, and these trials were

repeated until the infant’s fixation time was lower than

a habituation criterion on two consecutive trials. The

habituation criterion was calculated as half the average

of the first two trials of the session. If the fixation time

was less than 1 s, the trial was repeated, and if the

habituation criterion was not reached by 20 trials, the

session was aborted. Test trials consisted of tokens of

the habituation stimuli alternating with tokens of the

test stimuli. These test trials were interleaved with

control trials, which were non-alternating, as they were

identical to the habituation trials. There were two

blocks of four test trials, with one between-organ and

one within-organ test trial and a control trial for each.

The last habituation trial was used as the control for

the first test trial. As there were two types of critical

test trial, between- versus within-organ, the presenta-

tion order was counterbalanced across infants; either

(1) control, between organ, control, within organ,

control, between organ, control, within organ, or (2)

control, within organ, control, between organ, control,

within organ, control, between organ. Mean fixation

times for each between- or within-organ test trial were

compared to the mean fixation time of the control trial

that immediately preceded it. This resulted in four

control–test pairs for each infant.

Results

To ascertain any effects of the counterbalancing factors

on the experimental variable, we conducted an initial

omnibus analysis of variance (ANOVA) on the data

using a 2� 2� (2)� (2)� (2) design with age (6- vs.

11-month-olds) and presentation order (counterbalanc-

ing factor: between organ followed by within organ vs.

within organ followed by between organ) as between-

subjects factors, and trial type (test vs. control), organ

(between vs. within), and block (first vs. second

presentation of a each control–test trial pair) as within-

subjects factors. This omnibus ANOVA found no

significant main effect or two-way interactions involv-

ing the presentation order counterbalancing factor;

however, one three-way interaction was significant,

indicating it mitigated the interactions involving two of

the experimental factors: Block�Presentation Order�Organ, F(1,44)¼ 6.21, p¼ .02, hp

2¼ .12. This interac-

tion indicates that, collapsed across age, combined

fixation times for test and control trials are generally

longer for the organ type that was presented first,

although that pattern reverses in the second block only

for the presentation order in which within organ is

followed by between organ. As this interaction does not

involve the trial type factor it is unlikely to have

affected the main results.

As for the experimental factors, there was not a

main effect of trial type but, importantly, there was a

significant interaction between block and trial type,

F(1,44)¼ 12.62, p¼ .001, hp2¼ .22. For the first block

the mean fixation times were longer for test than

control trials, Mtest¼ 7.49 s (SE¼ 0.69), Mcontrol¼ 5.03

s (SE¼ 0.41), but in the second block the pattern was

reversed, Mtest¼ 6.22 s (SE¼ 0.72), Mcontrol¼ 7.41 s

(SE¼ 0.96). This suggests that the infants either recov-

ered from habituation in the second block, or they

began to transition from a novelty to a familiarity

preference (see Cohen, 2004).

To investigate further whether infants’ fixation times

to control trials increased across the experiment, we

rearranged the data by trial order rather than factor.

Those data are presented in Figure 1. Note that the

columns representing test items are collapsed across

between- and within-organ trials, as they vary by

presentation order. The pattern of data in the figure

suggests that control-trial fixation times increase over

the course of the session, whereas test-trial fixation

times generally decrease. To test this, we subjected the

data to a 2� (4)� (2) ANOVA with age (6- vs. 11-

month-olds) as a between-subjects factor, and trial

position (1, 2, 3, or 4) and trial type (control vs. test)

as within-subjects factors. The pattern of increasing

fixation times for control trials and decreasing fixation

times for test trials was confirmed by a two-way

interaction between trial position and trial type,

F(3,138)¼ 5.82, p¼ .001, hp2¼ .11. This rearrange-

ment of the data also allows us to see whether we

introduced a confound by using the last habituation

trial as the first control trial in the first ANOVA. In a

recent review of habituation procedures, Oakes (2010)


pointed out that this practice may overestimate dishabi-

tuation to test trials because the last habituation trial is

low by design. From Figure 1, however, it can be seen

that although infants in our study show increased

fixation in the first test trial relative to the first control

trial (last habituation trial), in the second control trial

(following test trial 1) they return to the same level as

or lower than the first. It is only in the second block of

four trials that fixations to control trials increase and

those to test trials decrease, relative to baseline,

suggestive of a transition to a familiarity preference.

On the basis of this analysis, we chose to analyze the

data again, but using data from only the first block,

which should be the most sensitive to discrimination

abilities and less likely than the second block to exhibit

fatigue effects.

The results of that more focused 2� 2� (2)� (2)

ANOVA revealed a main effect of trial type,

F(1,44)¼ 18.01, p< .001, hp2¼ .29, such that infants

oriented longer overall to test trials than control trials.

Mean fixation times are presented in Figure 2. There

was also a significant Presentation Order�Organ

interaction, F(1,44)¼ 4.70, p¼ .036, hp2¼ .10, such

that mean fixation times were longer overall for test/

control trial pairs where the test trial involved the organ

difference that was presented first (e.g., for the within

organ! between organ presentation order, participants

oriented longer toward within-organ test/control trials

than to between-organ test/control trials). This effect

can be seen most clearly in Figure 1. For both 6- and

11-month-olds, the combined fixation times for test 1

and control 1 are longer than for test 2 and control 2.

Discussion

Our initial omnibus ANOVA, using two blocks of four

trials, showed a significant interaction between trial

type and block, such that infants apparently changed

their preference from looking longer to test than control

trials in Block 1, to looking longer at control than test

trials in Block 2. The reason for this shift is not clear,

but there are two reasonable possible explanations.

First, infants may have tired of the task during the

course of the test phase and preferred to listen to the

more familiar control trials that were the habituation

stimuli (Cohen, 2004), or they may have simply

recovered from habituation by block 2 due to the length

of the test phase. Whatever the source for the shift in

preference, the interaction showed that infant behavior

had changed by the second half of the test phase.

Therefore, we re-analyzed without the block factor,

using only the first and most informative block of the

test phase, for which infants showed the expected

preference for trials containing novel stimuli inter-

spersed with the habituation stimuli.

Results of that more focused ANOVA indicate that

infants looked longer to test trials than control trials in

the first exposure to the two novel test stimuli in the

first half of the test phase, suggesting that infants at

both 6 and 11 months can discriminate both the

between-organ /x/-/£/ and within-organ /x/-/x/ con-

trasts. Therefore, there was no evidence in this experi-

ment that 6-month-olds discriminated between-organ

contrasts more successfully than within-organ contrasts,

as had been predicted by the AOH.

0

1

2

3

4

5

6

7

8

9

10

11

12

6 month-olds 11 month-olds

Mea

n fix

atio

n tim

e (s

)

Control 1

Test 1

Control 2

Test 2

Control 3

Test 3

Control 4

Test 4

FIGURE 1 Mean fixation durations for 6- and 11-month-olds for each trial in the order that it

was presented in Experiment 1. Note that the test trials are averaged across the organ factor.

The control trials (black columns) show a generally increasing pattern of fixation durations and

the test trials (gray columns) a generally decreasing one. Error bars represent standard errors

of the mean.


Before explaining how PAM might account for

the observed developmental pattern between 6 and

11 months, it is useful to consider the results of an

adult perceptual assimilation study we conducted with

the Nuu-Chah-Nulth fricatives. In that study we found

that the voiceless pharyngeal fricative /£/ and the

voiceless uvular fricative /x/ were both assimilated to

English /h/ (Tyler & Best, 2010). There was a signifi-

cant difference in goodness-of-fit to /h/, which means

that the between-organ contrast was a category-good-

ness assimilation for the adults. The voiceless velar

fricative /x/ was uncategorized, with responses shared

primarily among /t /, /k/, and /h/, so the within-organ

contrast was an uncategorized–categorized assimilation.

According to PAM (Best, 1995), discrimination of

category-goodness assimilations should be moderate to

very good, while discrimination of uncategorized–

categorized assimilations should be very good.

Returning now to the current infant findings, we can

suggest possible assimilation types for the 11-month-

old infants, who should be transitioning to natively

tuned perception. For the within-organ contrast, /x/-/x/,the velar fricative /x/ would fall in an untuned region of

phonetic space and the uvular fricative /x/ would be

assimilated to the English [h] phonetic category—an

untuned–tuned contrast—leading to very good discrimi-

nation via natively tuned perception. In contrast, the

between-organ contrast, /x/-/£/, would have become a

single-category assimilation, because both phones

would have assimilated to English [h], and thus a

decline in discrimination would be predicted over

development. Although Figure 2 is suggestive of a

decline in discrimination for the /x/-/£/ contrast, therewas no significant interaction between age group,

organ, and trial type, so it must be concluded that

discrimination was maintained. Is it possible that

the transition to natively tuned perception occurs a little

later for fricatives than for other consonants, as

suggested by the lack of discrimination of the stop-

fricative contrast /d/-/ð/ by Canadian-English-learning

6–8- and 10–12-month-olds (Polka et al., 2001;

Sundara et al., 2006)? If that were the case, then we

would expect to see a significant decline in discrimina-

tion of this Nuu-Chah-Nulth contrast, relative to

6-month-olds, at a slightly older age such as 14 or

15 months or even 4 years (Sundara et al., 2006).

However, to fully clarify the effects of native language

experience on perception during the first year, specifi-

cally of voiceless fricative place distinctions, we need

also to examine English-learning 6- and 11-month-old

infants’ discrimination of comparable within- versus

between-organ English fricative contrasts, which was

the goal of Experiment 2.

EXPERIMENT 2: DISCRIMINATION OFNATIVE VOICELESS FRICATIVE CONTRASTS

In Experiment 1, we selected between- and within-

organ non-native contrasts from a region of an English-

learning infant’s phonetic space that is unlikely to have

undergone native-language tuning. Here, in Experiment

2, we investigated whether articulatory organ differ-

ences can explain 6-month-olds’ discrimination and

0

1

2

3

4

5

6

7

8

9

10

Between Organ / /-/ /

Within Organ / /-/x/

Between Organ / /-/ /

Within Organ / /-/x/


Mea

n fix

atio

n tim

e (s

)

Control

Test

FIGURE 2 Mean fixation durations for between- (/x/-/£/) and within-organ (/x/-/x/) Nuu-Chah-Nulth contrasts at 6 and 11 months of age. Error bars represent standard errors of the mean.


whether PAM can explain developmental changes in

infants’ perception of voiceless fricatives they have

experienced in ambient speech, by testing 6- and 11-

month-olds’ discrimination of between- versus within-

organ contrasts from their native language environment

of English.

The voiceless fricatives of Australian English (and

the primary articulatory organs) are /f/ (lips), /u/, /s/

(both tongue tip), / / (tongue tipþ tongue body), and

/h/ (glottis) (see Cox & Palethorpe, 2007, for a phonetic

description of Australian English). Clearly the within-

organ contrasts must involve the tongue tip and the

between-organ contrast must involve either the lips or

the glottis. Surprisingly, there is very little data on

infant perception of English fricative contrasts differing

in constriction location. In a widely cited study, using a

conditioned headturn procedure, Eilers et al. (1977)

found that while both 6-8- and 12-14-month-olds

discriminated /sa/-/ a/, neither discriminated /ua/-/fa/,and the only the older group discriminated /fi/-/ui/.

On the surface, the failure of young infants to

discriminate between-organ /f/-/u/ in either vowel

context is problematic for the articulatory organ-based

predictions. However, Holmberg et al. (1977, as cited

in Kuhl, 1980), also using a conditioned headturn

procedure, found that 6-month-olds discriminated

/ua/-/fa/ and /sa/-/ a/, and Levitt et al. (1988) reported

discrimination of /ua/-/fa/ and /ða/-/va/ by 2-month-

olds using a modified high-amplitude sucking para-

digm. As /f/ and /u/ naturally have less acoustic energy

than /s/ and / / (see, e.g., Jongman, Wayland, &

Wong, 2000), infants in the Eilers et al. study may have

been capable of detecting the spectral information that

signals the articulatory difference, but the intensity in

her stimuli may not have been sufficiently high to allow

robust discrimination (this has also been suggested by

Kuhl et al., 2008).

Here we tested 6- and 11-month-olds’ discrimination

of between-organ /ua/-/fa/ and within-organ /ua/-/sa/.To our knowledge, no study to date has tested infant

discrimination of /ua/-/sa/, perhaps because infants are

likely to respond to the large differences in acoustic

energy between those two fricatives. Such differences

in acoustic energy would be particularly striking in a

stimulus-alternating preference procedure, such as the

one used here, because tokens of low-energy /ua/ andhigh-energy /sa/ would alternate during the test phase.

If such a large difference in acoustic energy were

presented to infants they may show a preference for the

energy-varying test trials (e.g., alternating /ua/-/sa/)over the energy-stable control trials (e.g., repeated

tokens of /ua/), but it would not be possible to tease

apart whether discrimination was due simply to detec-

tion of acoustic energy differences or to other differ-

ences that are more specific to phonetic properties of

fricative place distinctions. To reduce the possibility

that infants would discriminate /ua/-/sa/ solely on

energy differences between the consonants, without

relying on the dynamic spectral information for place,

and also to minimize the possibility that they would

fail to discriminate /ua/-/fa/ simply due to their low

amplitudes, again without responding to the dynamic

spectral differences of interest, we amplified the frica-

tive portions of /f/ and /u/ and reduced that for /s/ (see

the method section for the stimulus measurements and

naturalness judgments). Paralleling our approach in

Experiment 1, we habituated English-learning infants

to /u/ and then tested their discrimination of it from

both the labiodental /f/ (between-organ contrast) and

alveolar /s/ (within-organ) fricatives using a stimulus-

alternating preference paradigm.

Method

Participants. The sample consisted of 48 English-

learning infants from Sydney, Australia. There were

two age groups, 6-month-olds, Mage¼ 28.6 weeks,

range: 25.4–32.1, and 11-month-olds, Mage¼ 48.5

weeks, range: 46.1–53.6. There were 13 females and 11

males in the 6-month-old group, and 10 females and 14

males in the 11-month-old group. An additional 33

infants were tested and their data were eliminated from

the sample due to failure to habituate (n¼ 4), equip-

ment failure (n¼ 8), experimenter error (n¼ 2), a

subsequent diagnosis of developmental delay (n¼ 1),

having fixation times shorter than 1 s on any individual

trial (n¼ 6), or recovery on the second control trial

(same criterion as Experiment 1), suggesting failure to

completely habituate (n¼ 12).

Stimuli and Apparatus. The English fricative stimuli

consisted of four tokens each of English /f/, /u/, and /s/,

recorded from an Australian female speaker in her late

50s, chosen to match the voice quality of the speaker

whose stimuli were used in Experiment 2. The

consonants were recorded and presented in a consonant

þ /a/ syllable (e.g., /fa/) that was produced in an adult-

directed speech register.

To ensure that any observed discrimination would be

due to acoustic characteristics of the fricatives, and not

to extraneous stimulus variables, we used Praat soft-

ware (Boersma & Weenink, 2009) to adjust the

durations of the consonants and vowels, using overlap-

add resynthesis, to ensure that they had a similar mean

and range as the stimuli used in Experiment 1. To

reduce the possibility that infants might discriminate

the /u/-/s/ contrast using intensity differences alone, /f/

and /u/ were amplified by 5 dB and /s/ was reduced by


6 dB (though this still left /s/ with somewhat higher

amplitude than the other two fricatives, to maintain

naturalness). Vowel intensities for all tokens were

adjusted to the same level of acoustic intensity. The

stimulus measurements based on the modified tokens

are presented in Table 3.

Procedure. The procedure was the same as in Experi-

ment 1.

Results

As in Experiment 1, we first analyzed the data using a

2� 2� (2)� (2)� (2) omnibus ANOVA with age

(6- vs. 11-month-olds) and presentation order (between

organ!within organ vs. within organ! between or-

gan) as between-subjects factors, and trial type (test vs.

control), organ (between vs. within), and block (first

vs. second presentation of test trials for each contrast)

as within-subjects factors. Again there was no signifi-

cant main effect of trial type or any other factors, but

there was a significant Block�Trial Type interaction,

F(1,44)¼ 13.94, p¼ .001, hp2¼ .24. For the first block

the mean fixation times were longer for test than

control trials, Mtest¼ 7.39 s (SE¼ 0.63), Mcontrol¼ 5.23 s

(SE¼ 0.31), but in the second block the pattern was

reversed, Mtest¼ 5.88 s (SE¼ 0.53), Mcontrol¼ 7.08 s

(SE¼ 0.62), as we had seen in Experiment 1. An

additional 2� (4)� (2) ANOVA with the data in trial

order showed the same Trial Position�Trial Type

interaction as in Experiment 1. As can be seen in

Figure 3, there is a generally increasing pattern of

fixation times for control trials and a generally decreas-

ing one for test trials. Furthermore, the fixations to the

second control trial appear to be similar to the first

control trial (i.e., the last habituation trial), indicating

that the infants were not simply showing regression to

the mean. Therefore, as in Experiment 1, we analyzed

results for the first block only.

A 2� 2� (2)� (2) ANOVA on the first block

only revealed a significant main effect of trial type,

Table 3. Mean Acoustic Measurements for the English Syllables

Category

Fricative Vowela

Duration

(ms)

Duration Range

(ms)

Amplitude

(dB)

Centroid

(Hz)

Duration

(ms)

Amplitude

(dB SPL)

F1

Hz)

F2

(Hz)

Labiodental /f/ 222 180–254 41 5,991 517 73 786 1,270

Dental /u/ 247 219–302 42 5,995 520 73 780 1,296

Alveolar /s/ 245 176–274 50 8,135 529 73 805 1,269

F1 and F2 refer to the first and second vowel formants, respectively.aVowel measurements are reported from the 50% point.

0

1

2

3

4

5

6

7

8

9

10

11


Mea

n fix

atio

n tim

e (s

)

Control 1

Test 1

Control 2

Test 2

Control 3

Test 3

Control 4

Test 4

FIGURE 3 Mean fixation durations for 6- and 11-month-olds for each trial in the order that it

was presented in Experiment 2. Note that the test trials are averaged across the organ factor. The

control trials (black columns) show a generally increasing pattern of fixation durations and the test

trials (gray columns) a generally decreasing one. Error bars represent standard errors of the mean.


F(1,44)¼ 11.00, p¼ .002, hp2¼ .20, with higher fixa-

tion for test trials than for control trials, across ages

and fricative contrasts. No other main effects or

interactions were significant and the mean fixation

times are presented in Figure 4.

Discussion

As in Experiment 1, infants in Experiment 2 looked

longer overall to test than control trials. Again, there

was no evidence for an influence of articulatory-organ-

based differences in 6-month-olds’ discrimination. Our

findings are inconsistent with Eilers et al. (1977), who

found that infants up to 12 months of age failed to

discriminate /u/-/f/, but they are consistent with Holm-

berg (1977, as cited in Kuhl, 1980) and Levitt et al.

(1988), who found that young infants discriminated

/u/-/f/.PAM would explain the developmental pattern in

one of two ways. Either, (1) the 11-month-olds

continue to discriminate the contrasts using the

same pre-phonetic abilities as the 6-month-olds, or;

(2) the 11-month-olds have transitioned to natively-

tuned perception and detect the differences using

their developing phonetic categories. There is some

suggestion in Figure 4 that the 11-month-olds dishabi-

tuated more strongly than the 6-month-olds, which

would lend support to the second possibility, but

there was no significant interaction between age

group and trial type so that possibility must remain

speculative.

GENERAL DISCUSSION

As we noted at the outset, the challenge for theoretical

approaches to experiential effects on infant speech

perception is reconciling seemingly divergent patterns

of discrimination and their development over time. In

this study, we aimed to test whether the AOH (Gold-

stein & Fowler, 2003; Studdert-Kennedy & Goldstein,

2003) might successfully predict 6-month-olds’ dis-

crimination of native and non-native speech contrasts,

to outline how the PAM (Best, 1994a,b, 1995; Best

et al., 1988) accounts for developmental changes in

discrimination, and to provide much needed new data

on infant discrimination of fricative place contrasts.

In our two experiments, we chose non-native

(Experiment 1) and native (Experiment 2) voiceless

fricative place contrasts that would provide a testing

ground for articulatory-organ-based discrimination pre-

dictions. We selected non-native contrasts that fell in a

region of the phonetic space that is unlikely to have

undergone native-language tuning for English-learning

infants, to narrow the possible influences of native-

language exposure on initial discrimination and to test

among the developmental mechanisms put forward in

the literature. The native contrasts, on the other hand,

are naturally constrained because they should both

eventually develop into two phonological categories,

even for within-organ contrasts.

Overall, English-learning 6- and 11-month-olds

looked significantly longer to test than control trials in

both Experiment 1 (Nuu-Chah-Nulth) and Experiment

0

1

2

3

4

5

6

7

8

9

10

Between Organ / /-/f/

Within Organ / /-/s/

Between Organ / /-/f/

Within Organ / /-/s/


Mea

n fix

atio

n tim

e (s

)

Control

Test

FIGURE 4 Mean fixation durations for between- (/u/-/f/) and within-organ (/u/-/s/) English

contrasts at 6 and 11 months of age. Error bars represent standard errors of the mean.


2 (English). On the basis of the AOH (Goldstein &

Fowler, 2003; Studdert-Kennedy & Goldstein, 2003),

we had predicted that between-organ contrasts

should always be discriminable and that discrimination

of within-organ contrasts should initially be poorer

than for between-organ contrasts, prior to native-

language attunement. That prediction was not upheld

in either experiment. Therefore, although organ-based

approaches provided a potentially useful way of

predicting which contrasts an infant might initially

discriminate well, we did not find support for this idea

in our experiments (see similar findings in Narayan

et al., 2010).

Turning to predictions for changes in discrimination

over time, the results showed maintenance of all

contrasts in both experiments from 6 to 11 months.

Here we will discuss how this result fits with PAM and

with two additional theories/frameworks of infant

speech perception that are most relevant to the issues at

hand: (1) the Native Language Magnet Theory-Expand-

ed (NLM-e; Kuhl et al., 2008); and (2) Processing Rich

Information from Multi-dimensional Interactive Repre-

sentations (PRIMIR; Curtin et al., 2011; Werker &

Curtin, 2005).

NLM-e (Kuhl et al., 2008) proposes that develop-

mental changes in speech perception result from infants

tracking the statistical regularities in the acoustics of

ambient speech (the native language environment).

Infants can initially discriminate “all phonetic units in

the world’s languages” (Kuhl et al., 2008, p. 998), but

there may be variation in the level of discrimination

achieved according to acoustic factors such as salience

(e.g., Burnham, 1986; see also Cristia et al., 2011), and

amplitude variation across different consonants. If

discrimination is not initially high for a given contrast,

it should improve over development if it is a native

contrast, but there should necessarily be a decline in

discrimination of all non-native contrasts, regardless of

how well they were discriminated initially. The results

for the native contrasts are consistent with NLM-e

because the initial high discrimination of the English

contrasts was maintained over development. However,

the results for Nuu-Chah-Nulth appear inconsistent

with NLM-e because we did not observe a decline over

development for either non-native contrast.

Like NLM-e, PRIMIR (Curtin et al., 2011; Werker

& Curtin, 2005) states that the learning mechanism

responsible for infant attunement to the language

environment is statistical learning, however, it takes

no stand regarding the type of information that is

“tracked” or perceived by the infant (e.g., acoustic or

articulatory). The framework is comprised of three

multidimensional interactive representational spaces.

Phonetic and non-phonetic (i.e., indexical) information

are stored in the General Perceptual Space, in which

clusters of phonetic and indexical categories form over

time through processes of statistical learning. The

Word Form Space stores exemplars of sound sequences

that become attached to meaning as the conceptual

system develops, and from that linkage the Phoneme

Space emerges as the children come to recognize

recurring segments across their developing lexicons. In

addition to these spaces, dynamic filters direct infants’

attention to particular subsets of the available informa-

tion. The filters are initial biases, task demands, and

developmental level. During the first year of life,

infants should demonstrate facilitation or maintenance

for native contrasts, similar to NLM-e, depending on

initial biases and task demands. A decline should be

observed for non-native contrasts, although PRIMIR

differs from NLM-e in positing that discrimination may

still be observed later in life if the demands of the task

are sufficiently low (e.g., Werker & Logan, 1985). A

decline in discrimination for only some native or non-

native contrasts but not others would be explained in

the PRIMIR framework via the intersection of the

formation of new representational spaces (Word Form

and Phoneme spaces) and the influence of the dynamic

filters, but to our knowledge the framework is not yet

sufficiently well specified to predict which particular

contrasts should show maintenance and which should

show decline.

According to PRIMIR, infants use the General

Perceptual Space and any phonetic categories devel-

oped during the first year of life for discrimination.

Globally, the predictions for PRIMIR would appear to

be similar to those of PAM. However, there is one key

difference between PRIMIR and PAM when consider-

ing discrimination of non-native contrasts. For PRI-

MIR, phonetic categories are established through

processes of statistical learning so non-native phones

could only be assimilated to a native category if they

fall within its statistical distribution of phonetic proper-

ties. PAM, on the other hand, proposes a Gibsonian

(Gibson & Gibson, 1955) process of perceptual learn-

ing. From this viewpoint, perceivers extract the infor-

mation relevant to their needs directly from the distal

objects and events in their environment. With complex

events, such as speech, they often must learn through

experience which dimensions and combinations of

information are crucial in their environment and thus

should be selectively attended to, versus which other

aspects of available information are irrelevant and need

not be attended to or extracted. By that mechanism

infants might assimilate a non-native phone to a native

category, even if it lies outside of any specific native

statistical distribution of phonetic properties, so long as

the infant recognized a higher-order invariant that was


consistent with a native category. The results for Nuu-

Chah-Nulth, therefore, could only be accounted for

within the PRIMIR framework via a combination of

initial biases and task demands—there could be no

influence of native-language attunement on discrimina-

tion of non-native contrasts.

For PAM, discrimination of both the Nuu-Chah-

Nulth and English contrasts at 6 months must be due to

initial biases. We argue that infants transition from

untuned to natively tuned perception between 6 and

12 months of age (or even later for certain consonant

contrasts; see Polka et al., 2001; Sundara et al., 2006).

Our data are not inconsistent with such a view, but we

did not observe a significant decline in discrimination

for the Nuu-Chah-Nulth uvular-pharyngeal contrast,

which would have been predicted for a single-category

contrast. This may be due to attunement to fricatives

occurring later in development (Polka et al., 2001;

Sundara et al., 2006), but it is also possible that the

11-month-olds showed a sensitivity to category good-

ness that was observed in adults by Tyler and Best

(2010). Such a finding would clearly falsify our

contention that native phonetic categories have little

within-category differentiation in infancy (see footnote

3). We acknowledge that more data are required to

evaluate our proposal.

In the context of perceptual narrowing, the critical

lesson is that attunement to the native language does

not involve a decline in discrimination for all non-

native phonetic contrasts. From the PAM perspective,

those heard as non-speech, that is, not assimilated as

speech elements (e.g., clicks for English-learning

infants; Best et al., 1988) will continue to be discrimi-

nated using domain-general perception, whereas those

heard as speech will be discriminated through the filter

of the native phonological system. If two non-native

phones are assimilated to two different native catego-

ries, then discrimination should remain excellent across

age (i.e., a PAM two-category assimilation; see Best,

1995), even if the non-native phones do not phonetical-

ly resemble any native phone (e.g., adult English-

speakers’ perception of Zulu voiced versus voiceless

lateral fricatives / /-/ /: Best et al., 2001). Note,

however, that the same contrast may be assimilated

differently by 11-month-olds than by adults, for exam-

ple as a single-category contrast or as an untuned–

untuned contrast, either of which could lead to a

temporary decline in discrimination (e.g., decline at

11 months in discrimination of the Zulu lateral fricative

voicing contrast / /-/ /: Best & McRoberts, 2003).

To continue toward an explanation of infant speech

discrimination abilities, we require data from cross-

language studies, such as those conducted by Polka

et al. (2001) and Kuhl et al. (2006), where infants from

different language-learning environments are presented

with the same stimuli. If infants are born with universal

perceptual biases, then young infants should show

similar discrimination patterns regardless of their

language environment. Effects of native-language tun-

ing are much easier to interpret when infants from

different language environments are presented with

exactly the same stimuli and divergent patterns of

discrimination are observed. For example, while we

have shown that English infants, whose language input

does not favor tuning to tongue-body and tongue-root

fricatives, discriminate the Nuu-Chah-Nulth stimuli, it

would be interesting to test Arabic-learning infants

because Arabic has a uvular and a pharyngeal fricative.

If the discrimination patterns we observed here are

universal at 6 months but language-specific at

11 months, then we should observe a developmental

decline for the velar-uvular (/x/-/x/) contrast by the

Arabic-learning infants. In contrast, the uvular-pharyn-

geal (/x/-/£/) contrast should be maintained for this

group as it would be developing into an Arabic two-

category distinction with native-language tuning. It

would be particularly interesting to extend these cross-

language comparisons to infants learning languages

with smaller consonant inventories, particularly inven-

tories with notable “gaps” in certain types of conso-

nants. In this regard, indigenous Australian languages

such as Arrente and Pitjantjatjara would be of particular

interest, as they lack fricative phonemes altogether, yet

on the other hand they tend to have multiple coronal

places for stops, nasals and approximants that are all

lacking in English. That is, Australian languages lack

an entire series of contrasts found in English (voiced

and voiceless fricatives), but conversely they have other

series of place contrasts lacking in most other lan-

guages including English (coronal stop place distinc-

tions).4

To conclude, articulatory-organ differences do not

appear to influence perception of speech contrasts by

young infants, and it is still not clear which factors

determine initial levels of discrimination, or whether

those levels are observed universally. Nevertheless, our

results provide another example of maintenance of

both non-native (Experiment 1) and native contrasts

(Experiment 2) across the second half of the first year

of life, for both within- and between-organ contrasts.

Moreover, our findings on 6- and 11-month-olds’

4Pragmatic issues, however, make this type of study very

unlikely. Many indigenous Australian languages are dying or

have died out already; the children are no longer acquiring them

naturally from birth. Even in languages that infants are still

learning, both practical and ethical concerns mean that conduct-

ing experiments would be highly problematic at best.


discrimination of non-native fricative contrasts from an

uncommitted region of phonetic space have shown

that perceptual attunement to the native language does

not necessarily entail perceptual narrowing, which

predicts a decline in discrimination of non-native

contrasts from a region of phonetic space that the

infants could not have encountered in their ambient

language environment.

NOTES

This research was supported by NIH grant DC00403 (PI: C.

Best). We thank Bryan Gick and Ian Wilson greatly for their

assistance in obtaining the Nuu-Chah-Nulth stimuli, Christian

Kroos for assistance with Praat stimulus adjustments for

Experiment 2, and all of the parents and babies who

participated in the study. We also grateful for the research

assistance of Suzana Bicanic, Lidija Krebs-Lazendic, Anna

Notley, Matthew Richardson, Dianne West, Susan Wijngaar-

den, and Bethany Wootton.

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