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Revisiting perceptual sensitivity to non-native speech in a diverse sample of bilinguals

Authors:
Infant Behavior and Development 76 (2024) 101959
Available online 22 May 2024
0163-6383/© 2024 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Revisiting perceptual sensitivity to non-native speech in a diverse
sample of bilinguals
Victoria L. Mousley
a
, Mair´
ead MacSweeney
b
,
c
,
*
, Evelyne Mercure
a
,
d
a
Centre for Brain and Cognitive Development, School of Psychological Sciences, Birkbeck, University of London, London WC1E 7HX, United
Kingdom
b
Deafness, Cognition and Language Research Centre, University College London, London WC1H 0PD, United Kingdom
c
Institute of Cognitive Neuroscience, University College London, London WC1N 3AZ, United Kingdom
d
Department of Psychology, Goldsmiths, University of London, London SE14 6NW, United Kingdom
ARTICLE INFO
Keywords:
Bilingualism
Speech perception
Language acquisition
Perceptual narrowing
Visual habituation
Eye-tracking
ABSTRACT
Werker and Tees (1984) prompted decades of research attempting to detail the paths infants take
towards specialisation for the sounds of their native language(s). Most of this research has
examined the trajectories of monolingual children. However, it has also been proposed that bi-
linguals, who are exposed to greater phonetic variability than monolinguals and must learn the
rules of two languages, may remain perceptually open to non-native language sounds later into
life than monolinguals. Using a visual habituation paradigm, the current study tests this question
by comparing 15- to 18-month-old monolingual and bilingual childrens developmental trajec-
tories for non-native phonetic consonant contrast discrimination. A novel approach to the inte-
gration of stimulus presentation software with eye-tracking software was validated for objective
measurement of infant looking time. The results did not support the hypothesis of a protracted
period of sensitivity to non-native phonetic contrasts in bilingual compared to monolingual in-
fants. Implications for diversication of perceptual narrowing research and implementation of
increasingly sensitive measures are discussed.
1. Introduction
Perceptual narrowing is thought to be a robust and reliable process of human language acquisition (Werker & Tees, 1984).
Newborn infants are proposed universal listeners, able to discriminate phonetic contrasts in any language (Best & McRoberts, 2003;
Eimas et al., 1971; Friendly et al., 2014; Kuhl et al., 2006; Narayan et al., 2010; Streeter, 1976; Trehub, 1976; Werker & Tees, 1984;
Werker, 2018). Throughout the rst year of life, repeated exposure to native language sounds is thought to drive experience-dependent
organisation of infantsperceptual abilities. More specically, infants develop and then maintain their sensitivity to native language
phonetic contrasts (Kuhl et al., 2006; Werker & Tees, 1984) whilst at the same time, their sensitivity to most (but not all, see Best &
McRoberts, 2003) non-native contrasts declines. This developmental evolution is informed by patterns and variability in an infants
early language environment (Best et al., 2016; Kuhl et al., 2006; Werker & Curtin, 2005). However, the eld has focused primarily on
trajectories of perceptual narrowing in monolingual children who are learning one language and thus one set of phonetic rules. The
intricacies of perceptual narrowing in children learning two languages from birth, who make up a signicant portion of the worlds
* Correspondence to: Alexandra House, UCL Institute of Cognitive Neuroscience, 17-19 Queen Square, London WC1N 3AZ, United Kingdom.
E-mail addresses: v.mousley@bbk.ac.uk (V.L. Mousley), m.macsweeney@ucl.ac.uk (M. MacSweeney), e.mercure@gold.ac.uk (E. Mercure).
Contents lists available at ScienceDirect
Infant Behavior and Development
journal homepage: www.elsevier.com/locate/inbede
https://doi.org/10.1016/j.infbeh.2024.101959
Received 30 January 2024; Received in revised form 18 April 2024; Accepted 12 May 2024
Infant Behavior and Development 76 (2024) 101959
2
population, has been paid much less attention by comparison (for review, see Byers-Heinlein & Fennell, 2014; Costa &
Sebasti´
an-Gall´
es, 2014; Singh et al., 2022; Werker & Byers-Heinlein, 2008).
Some have theorised that the demands of learning two languages may drive a perceptual wedgein young bilinguals, such that
they might show a protracted period of perceptual openness to non-native speech compared to monolinguals (Petitto et al., 2012).
However, clear behavioural evidence dening the nuances of this potential advantage its developmental timeline, context-specic
constraints, and/or the extent to which it may be sensitive to variability in input has not yet been developed. The present study
compares monolingual and bilingual 15 to 18 month olds and the trajectory of their (declining) ability to discriminate non-native
phonetic contrasts. Further, visual habituation paradigms used to test perceptual narrowing hypotheses typically rely on the exper-
imenter to make button-presses to represent the infants looking behaviour. This approach can be prone to error and/or bias. In the
current study, an eye-tracking method was developed to measure infant looking time objectively and precisely.
The early language environments of monolinguals and bilinguals differ in many important ways. Bilinguals face more phonetic
variability overall than monolinguals (Singh & Tan, 2021), and the speech input they receive from adults is more variable than that of
monolinguals. For example, bilingual childrens parents may or may not be bilingual themselves and may show a wide range of
possible language use patterns. When speaking to their child, bilingual parents might always use one language, whilst others may
alternate between their languages depending on the context (e.g., English when with English-speaking friends, French at home). Some
bilingual parents speak one of their languages with an accent that could differ phonetically, phonologically, and/or rhythmically from
native speakers of the same language. Parents may or may not borrow words across languages or mix their two languages within
sentences or sentence to sentence (Byers-Heinlein, 2013, 2015). Some bilingual infants have experiences with many different speakers
of each of their languages, whilst others only have one adult that uses one of their languages with them (Gollan et al., 2015). There are
many ways to be bilingual, and there is a high degree of variability within the experience of learning two spoken languages.
Regardless of how many languages they learn, all hearing children must develop skills in speech perception to detect the acoustic
properties of speech, build native phonological repertoires, differentiate linguistic rhythms, and parse speech streams to identify and
learn words. Bilinguals must do all of this in two languages, acquiring two phonological repertoires with two sets of phonetic categories
(Bosch & Sebasti´
an-Gall´
es, 1997), lexicons, and statistical regularities (Byers-Heinlein et al., 2010). Unlike monolinguals, bilinguals
must also differentiate between their two native languages to learn them as two distinct systems (Byers-Heinlein et al., 2010; Werker,
2012).
It is also true that a bilingual childs experience with speech is split to some degree between their two native languages. They have
less experience with each of their native languages than a monolingual does with their one. This means that, compared to mono-
linguals, bilinguals have more to learn, must do so in more variable environments, and have less opportunity to practice (Bye-
rs-Heinlein & Fennell, 2014; Hoff, 2013; Werker et al., 2012). Despite this, bilinguals and monolinguals appear to reach early language
milestones on similar timelines, particularly when measured as totalor conceptual linguistic knowledge accounting for both bi-
lingualsnative languages (vocabulary: Bosch & Ramon-Casas, 2014; native language word segmentation: Orena & Polka, 2019; pitch
and tone perception within bilinguals: Singh & Foong, 2012; for review, see H¨
ohle et al., 2020).
All infants must be able to perceive the sounds of their native language(s) that comprise its lexicon. An English-learning infant, for
example, must be able to perceive the difference between /r/and /l/to then use these sounds to segment the English words that
contain them. Sensitivity to native phonetic contrasts is one domain under the umbrella of perceptual narrowing (for review, see Flom,
2014; Maurer & Werker, 2013). According to perceptual narrowing, infantsdiscrimination abilities for native contrasts are present
from birth and are evident throughout the rst year of life (Kuhl et al., 1992; Werker & Tees, 1984). Strengths of effects likely vary,
particularly in relation to the nature of a bilingualstwo languages, phoneme pairs tested, and the sensitivity of paradigms used, but
this native language trajectory appears to describe both monolinguals and bilinguals. Two studies have found that monolingual and
bilingual four month olds showed evidence of discrimination of a native vowel contrast (Bosch & Sebasti´
an-Gall´
es, 2003;
Sebasti´
an-Gall´
es & Bosch, 2009). Interestingly, in both studies, eight-month-old monolinguals but not bilinguals showed sensitivity to
the native contrast (/e/” – “/
ε
/; Bosch & Sebasti´
an-Gall´
es, 2003; /o/” – “/u/and /e/” – “/u/; Sebasti´
an-Gall´
es & Bosch, 2009).
When tested on a more distant (/e/” – “/u/) native contrast than in the original experiments, bilingual eight month olds did show
evidence of discrimination (Sebasti´
an-Gall´
es & Bosch, 2009). Another study using an anticipatory saccade paradigm suggested that
eight-month-old bilinguals were sensitive to a native vowel contrast (Albareda-Castellot et al., 2011), and evidence from
French-English bilinguals has also reported native vowel contrast discrimination in bilingual eight month olds (Burns et al., 2007;
Sundara et al., 2008). Overall, monolinguals and bilinguals can perceive native phonetic contrasts over the rst year of life, at least
from the fourth month to the twelfth, though they may also show slight differences in sensitivity around eight months.
The other relevant trajectory of perceptual narrowing is that of infants declining perceptual sensitivity to non-native phonetic
contrasts, driven by increased native language specialisation over the rst year of life. Research with monolingual infants suggests a
decline in perceptual sensitivity to non-native vowel contrasts between six and eight months of age (Kuhl et al., 1992; Polka & Werker,
1994) and to non-native consonant sounds between eight and 12 months (Best, 1994; Werker & Tees, 1984). According to these
patterns, an infant younger than eight months who is learning only Japanese, with no exposure to English, would be able to distinguish
between /r/and /l/, which are phonetically contrastive in English but not in Japanese. However, by approximately eight months,
they would be unlikely to show evidence of sensitivity to the /r/” – “/l/contrast. Much less is known about whether bilinguals follow
a similar trajectory. This is the focus of the current study.
On the one hand, it could be the case that monolinguals and bilinguals develop along similar non-native trajectory to each other.
One theoretical framework for bilingualism posits that bilinguals might acquire two language input systems that they assimilate into
one cognitive model for language (for review, see De Houwer, 1995). If bilinguals construct one overarching language system in which
they store and retrieve information in either of their two native languages, they may behave similarly to monolinguals on certain tasks.
V.L. Mousley et al.
Infant Behavior and Development 76 (2024) 101959
3
In some behavioural domains, evidence suggests similarities in monolinguals and bilingualslexical development (Pearson et al., 1993,
1995), comprehension of speech (Shook & Marian, 2012), and overall achievement of language milestones (for review, see Bye-
rs-Heinlein & Lew-Williams, 2018). A non-mutually exclusive view is that, while monolinguals and bilinguals show similar success in
the end goals of language development, there may be subtle differences in the paths they take towards native language specialisation.
Research with tonal languages has generated interesting hypotheses about the different potential paths of monolinguals and bi-
linguals. Tonal contrasts are minimal pairs based on pitch contours that convey information about the different meanings of lexical
items within syllables of the same segmental (e.g., consonant and vowel) structure. Research into the perception of non-native tonal
contrasts in monolingual and bilingual infants has shown that both groups appear to show U-shaped trajectories but along different
developmental timelines. Young infants seem to show an initial sensitivity to non-native sounds that reduces towards the end of the
rst year (see also Best, 1994; Kuhl et al., 1992; Polka & Werker, 1994; Werker & Tees, 1984); however, sensitivity could later
re-emerge, tracing a U-shapedtrajectory. In a study with Dutch-learning monolinguals, Liu and Kager (2014) reported evidence of
non-native contrast discrimination at ve to six months and 17 to 18 months but not at nine months. In a study of bilinguals learning
Dutch and another non-tonal language, Liu and Kager (2016) again reported the re-emergence of the ability to discriminate non-native
contrastive sounds, but this was observed among bilinguals at 11 to 12 months of age rather than 17 to 18 months as in the mono-
linguals. In contrast, a similarly designed study with a diverse bilingual sample did not nd evidence of group differences between
monolingual and bilingual infantsnon-native tonal contrast perception across the rst two years of life (Kalashnikova et al., 2023). In
sum, there is mixed evidence suggesting that monolingual and bilingual infants differ in their perception of non-native tonal contrasts
over the rst two years of life. In research investigating consonants as units of speech sound contrasts, it remains unknown what
trajectories monolinguals and bilinguals take towards non-native discrimination.
In studies of infants learning non-tonal languages, neurophysiological studies of consonant discrimination have shown different
neural responses to non-native consonant contrasts in bilinguals than in monolinguals towards the end of the rst year of life (Fer-
jan-Ramirez et al., 2017). For example, Petitto et al. (2012) found that 10- to 12-month-old bilingual infants showed similar left
inferior frontal cortex activation to native and non-native consonant contrasts. On the other hand, monolingual (English) infants of the
same age showed left inferior frontal cortex activation in response to native but not non-native contrasts. This study has been inter-
preted as evidence for the claim that bilingual infants can discriminate non-native phonetic contrasts until a later age than their
monolingual counterparts (Berken et al., 2017; Birdsong, 2018; Burnham et al., 2017; Costa & Sebasti´
an-Gall´
es, 2014; Jasi´
nska &
Petitto, 2014; Kandhadai et al., 2014; Kovelman et al., 2015; Liu & Kager, 2018; Potter & Saffran, 2015; Zadina, 2015). However, as
Petitto et al. (2012) do not report a behavioural measure of perceptual discrimination, it remains unknown whether bilingual infants
show a behavioural difference in their perceptual responses to non-native phonetic contrasts until a later age than monolinguals.
Mixed behavioural evidence exists for an effect of bilingualism on non-native contrast perception. For example, Liu and Kager
(2015) tested sensitivity to a non-native aspiration consonant contrast (/p/ /p
h
/) in eight- to nine-, 11- to 12-, and 14- to
15-month-old infants either learning Dutch (monolingual) or Dutch and French or Spanish (bilinguals). The consonant contrast was
non-native for all monolingual and bilingual infants. The results showed that, at eight to nine months, bilinguals but not monolinguals
discriminated the non-native consonant contrast. There were no group differences in discrimination in 11- to 12- or 14- to 15-month--
old monolinguals and bilinguals. On the other hand, Singh et al. (2017) tested 10- to 11.5-month-old monolingual (English) and
bilingual (English and Mandarin) infantslooking behaviour to native and non-native voiceless Hindi dental-retroex contrasts ("/ta/"
"/Ta/"). Using a "switch" variant of a visual habituation paradigm (Narayan et al., 2010), Singh et al. (2017) habituated infants to a
language sound (in this case, "/ta/") before playing the same language sound ("same" condition) and a contrastive sound ("switch"
condition). This facilitates a comparison of monolingual and bilingual infantsnovelty responses to a non-native phonetic contrast
(Narayan et al., 2010). Within-group analysis of the bilinguals revealed a signicant increase in xation time to the novel, contrastive
non-native stimulus that was not present in the same analysis of the monolingual group. These results suggest that 10- to 11.5-month--
old bilingual, but not monolingual, infants may be able to perceive non-native phonetic contrasts. However, the groups were not
statistically compared, and thus group differences cannot be claimed from this study alone (Makin & Xivry, 2019).
Further evidence was presented in an unpublished thesis by Casaus (2015), who conducted a cross-sectional study of monolingual
(either Spanish or Catalan) and bilingual (both Spanish and Catalan) childrens responses to non-native (Hindi) phonetic contrasts at
seven-, 12-, 15-, and 18-month-old time points. Using the switchvariant of a visual habituation paradigm as per Narayan et al.
(2010) and Singh et al. (2017), the infants were habituated to a Hindi dental "/ta/" syllable whilst black-and-white checkerboards were
presented. Once an infant habituated, indicated by a 40 % decrement in looking time to screen, a series of either dental "/ta/" syllables
or retroex "/Ta/" syllables were presented (Narayan et al., 2010). At 12 months, there was a non-signicant trend (p =.055) of a
Group x Trial interaction that predicted the outcome of looking time. Post-hoc results revealed that monolinguals did not look longer to
the non-habituated (retroex /Ta/) than the habituated (rental /ta/) sound, whilst bilinguals did, indicating potential non-native
contrast discrimination in the bilingual but not monolingual group. This nding broadly aligns with previous research reporting
bilingual discrimination of a non-native contrast at 12 months (Singh et al., 2017). At 15 months, results revealed a statistically
signicant Group x Trial interaction (Casaus, 2015). Post-hoc analyses suggested that the interaction was driven by the bilinguals
looking longer at the non-habituated (retroex "/Ta/") compared to the habituated sound (dental "/ta/"), whilst no difference was
observed in monolinguals. This suggests that at 15 months, bilinguals can discriminate the /ta//Ta/contrast, whilst mono-
linguals may not. Importantly, at 18 months, there was no indication in a within-group analysis that bilingual infants could
discriminate the non-native contrast (Casaus, 2015). Overall, these ndings suggest there is no evidence of monolingualsdiscrimi-
nation of a non-native consonant contrast at 12, 15, or 18 months, whilst there is evidence of discrimination in bilinguals at 12 and 15
but not 18 months.
These unpublished ndings may indicate a behavioural effect of a prolonged period of sensitivity to a non-native retroex
V.L. Mousley et al.
Infant Behavior and Development 76 (2024) 101959
4
consonant contrast amongst bilingual infants, though several key questions remain. Some research reports bilingual effects on
perceptual sensitivity for non-native language sounds in the beginning of the second year of life (e.g., Singh et al., 2017), whilst others
do not (e.g., Kalashnikova et al., 2023; Liu & Kager, 2015). The specic developmental trajectory of infantspotential sensitivity to
non-native contrasts within the 15- to 18-month age window also remains unclear, given that previous research has primarily been
cross-sectional (Kraemer et al., 2000). It is still unknown whether a potential group difference is linked to a bilingual experience more
broadly, or if it is specic to bilinguals learning language pairs that are linguistically similar (such as Spanish-Catalan bilinguals). The
present study addresses these questions.
1.1. Hypotheses
It was predicted that simultaneous bilingual infants exposed from birth to English and another language without a retroex
consonant would retain perceptual sensitivity for non-native phonetic contrasts longer than would monolingual infants. The stimuli
consisted of a non-native consonant contrast (Hindi dental "/ta/" versus retroex "/Ta/") used in past perceptual narrowing research to
report group differences between bilinguals and monolinguals in the rst and second years of life (Casaus, 2015; Petitto et al., 2012;
Singh et al., 2017; Werker & Tees, 1984; Werker et al., 1981). Eye-tracking was used to measure looking time differences to habituated
(dental "/ta/") and novel (retroex "/Ta/") tokens.
It was further predicted that the Hindi contrast would be imperceptible to all English monolingual infants tested, regardless of age
(15 to 18 months) (Werker & Tees, 1984). These infants were expected to look at both trial types (habituated same versus novel switch)
of the Hindi contrast equally. In contrast, a prolonged window of sensitivity to non-native contrasts for the bilingual group was
anticipated, reected by different looking patterns to habituated same versus novel switch tokens. A gradual decrease in bilingual
infantssensitivity to non-native phonetic contrasts across the age range of 15 to 18 months was expected.
The study design, which allowed for age to be examined as a continuous variable, permitted a direct test of whether monolinguals
and bilinguals demonstrated a difference in slope between the 15- and 18-month-old time points. This approach builds on previous
research, most of which is cross-sectional, providing the opportunity to make direct inferences about developmental trajectories
(Kraemer et al., 2000). A linear mixed effects model was constructed to test whether the looking time to the test phase was predicted by
the interaction of Group x Trial x Age. A signicant interaction was hypothesised, driven by bilingual infantsprolonged sensitivity
(compared to the monolingual group) to non-native consonant contrasts that was expected to decline with age.
1.2. Exploratory hypotheses
As reviewed above, bilingualism is a widely varying experience. If it is indeed the case that experience with two native languages
drives a period of protracted sensitivity to non-native phonetic contrasts, it is logical to predict that infants who experience their two
languages more frequently (e.g., 50 % Spanish, 50 % English), would retain this sensitivity longer than infants whose exposure to two
languages is less balanced (e.g., 20 % Spanish, 80 % English). A degree of bilingualism metric, used in previous developmental
research (Incera & McLennan, 2018; Nguyen et al., 2023; Mousley et al., 2022), can be calculated from in-depth language background
interviews with infantsparents. The impact of degree of bilingualism on bilingualsdeclining perceptual sensitivity to non-native
contrasts remains an open question.
If their parents are bilingual, bilingual infants are also exposed to different habits in language mixing, a behaviour dened as
switching languages mid-sentence or borrowing words from one language when using the other (see for example Byers-Heinlein et al.,
2020; Tsui et al., 2020). It could be that bilingual children who are exposed to more language mixing at home, and therefore more
unpredictable or variable speech streams, remain open to non-native contrasts later into life than do monolinguals.
1.3. Methodological aims
Previous perceptual narrowing research has relied on online computer mouse or key presses by the experimenter to calculate
habituation and record infant looking times (for example, see Estes et al., 2007; Fennell & Waxman, 2010; Graf Estes & Bowen, 2013;
Oakes et al., 2019; Polka et al., 2014; Singh et al., 2017; Sundara et al., 2008; for review, see Oakes, 2010). There are inherent dis-
advantages of button-press measurements for infant looking time. Collecting data with infants often requires monitoring many things
at once, which can make it challenging for the experimenter to capture looking time measurement precisely, especially if they are
working alone. Speech perception task trials also tend to be short, which could make the impact of even small imprecisions in
button-press measurement proportionately more signicant than tasks with longer trials. It is often the case that experimenters cannot
be blinded to the conditions of perceptual narrowing paradigms, such as visual habituation paradigms, where the experimenter knows
the design of the task and, from meeting the family, if the child is monolingual or bilingual. To address these issues and improve overall
precision of looking time measurement, the habituation software PyHab (Kominsky, 2019) was modied to accept Tobii TX300
eye-tracking input to serve as the primary coder. Eye-tracking is temporally precise, capturing blinks and rapid saccades away from
and back to the screen that cannot be captured by experimenter button presses. Because this software integration was novel, ofine
video recordings were used to validate eye-tracking looking time measurements. Manual, frame-by-frame measurements are more
robust to large movements than eye-tracking, which can offer benet as it is common for children participating in eye-tracking studies
to lean forwards, backwards, or make other big movements during the task (Tomalski & Malinowska-Korczak, 2020). More detail
about this software integration and its validity is included below.
V.L. Mousley et al.
Infant Behavior and Development 76 (2024) 101959
5
2. Method
2.1. Power analysis
To our knowledge, no published analyses have yet reported the estimated effect size of the potential bilingual effect on non-native
consonant contrast sensitivity in the second year of life. The necessary sample size to address this question was calculated using
unpublished data from a cross-sectional study of non-native phonetic perception amongst simultaneous bilinguals at seven, 10, 15, and
18 months of age (Casaus, 2015) using a visual habituation paradigm.
At 15 months, Casaus (2015) reported a signicant interaction of Group (monolingual versus bilingual) x Trial (same versus switch)
(F(1, 42) =6.27, p =.02). At 18 months, a within-participants ANOVA (bilinguals only) showed a null effect of test trial type (same
versus switch) on looking time (F(1, 19) =0.36, p =.55). The proposed analyses were designed to address whether or not monolinguals
and bilinguals demonstrated a difference in slope between the 15- and 18-month-old time points, which were tested cross-sectionally
in Casaus (2015). Casaus (2015)s interaction statistic at 15 months was used to calculate the effect size at the point of maximal effect.
At 15 months, Casaus (2015) reported a signicant Group x Trial interaction (15 months, F(1, 42) =6.27, p =.02) driven by increased
sensitivity to the non-native contrast in the bilingual compared to the monolingual group. As recommended by Lakens (2013), the
partial eta-squared value was calculated using the following formula:
η
2
p
=Fdfeffect
(Fdfeffect )+df error (in this case:
η
2
p
=6.211
(6.271)+42 =0.13). Using
Cohens (2013) equation for effect size of f (f =
η
2
p
1
η
2
p), an effect size of f =0.39 was calculated. A simulation-based power calculation for
repeated measures, mixed effects ANOVA (within-between interaction) was conducted using the protocol laid out by Lakens and
Caldwell (2021). Conducting 1000 simulations using parameter estimations from Casaus (2015), the analysis revealed that a sample
size of n=37 was required to detect the effect with 91.20 % power. Considering approximately 25 % attrition for an infant
eye-tracking study (Althaus & Marechal, 2012; Mercure et al., 2018; Pons et al., 2015), it was expected that approximately n=48
infants per group (N=96) would need to be tested.
2.2. Participants
Data from a total of N=95, 15 to 18 month olds was collected at the Birkbeck Babylab in central London. For failure to complete
the task, n=13 infants were excluded (n=6 monolinguals and n=7 bilinguals). For failure to reach habituation criteria within the
window of between nine and 33 trials, n=9 infants were excluded (n=4 monolinguals, n=5 bilinguals). For failure to look to the
same or switch phases for at least one second each, a further 11 infants were excluded (n=5 monolinguals, n=6 bilinguals). This left
a total of a N=62 infants who contributed data to analysis. This total sample size fell short of the target by n=12 participants (n=5
bilingual) because of the COVID-19 pandemic that was ongoing when the funding supporting this project ended (further detail about
implications found in discussion Section 4.1).
Of participants included in the analyses, n=30 were monolinguals learning English only (n=17 female, M =516.83 days or 16.99
months, SD =32.77 days or 1.08 months, range =459-577 days or 15.09 to 18.97 months). The n=32 bilinguals were learning English
and another language that did not contain a retroex consonant (n=16 female, M =518.31 days or 17.04 months, SD =33.36 days or
1.10 months, range =460-573 days or 15.12 to 18.84 months). Bilinguals non-English languages were Cantonese (n=2), Czech
(n=1), Danish (n=2), Dutch (n=2), French (n=2), Greek (n=1), Hebrew (n=1), Hungarian (n=1), Indonesian (n=1), Italian
(n=3), Mandarin (n=2), Polish (n=2), Russian (n=2), Spanish (n=6), Swedish (n=1), Twi (n=1), Welsh (n=1), and Yoruba
(n=1). There were no differences in the groupsages (t(59.86) =-0.18, p=.861), and average household income was matched across
groups. Mothersand fathersmedian level of education was a degree/higher national diploma in both groups. Ethical approval was
granted both by University College London (4966/003) and Birkbeck, University of London (181960) ethics committees.
To ensure no bilinguals had experience with the non-native retroex contrast, Phoible, an online repository of cross-linguistic
phonological inventory data from 2186 distinct languages (Morgan & McCloy, 2019) was used. Ten retroex consonants that
contain more than 0 % "Representation" in the database were selected and a searchable table of 1767 exclusion languages was created.
Infants with experience of these languages were excluded in the recruitment phase. Infants with more than 5 % exposure to third/-
fourth languages were also excluded.
2.3. Bilingualsearly language experiences
2.3.1. Language exposure questionnaire
Percentage of language exposure was calculated using the English adaptation of the Language Exposure Questionnaire (LEQ)
designed by Bosch and Sebasti´
an-Gall´
es (1997) and often implemented in developmental research on bilingualism (Carbajal &
Peperkamp, 2020; DeAnda et al., 2016; Kalashnikova et al., 2020; Mousley et al., 2022; Potter et al., 2018; Ramon-Casas et al., 2009;
Singh & Tan, 2021). The interview took approximately 15 minutes, during which parents were asked about a typical day in the childs
life for each day of the week across different periods of time since birth.
The number of hours a child hears their native languages was calculated, a percentage score was produced, and the inclusion
criteria of a minimum of 20 % exposure to the minority language and 80 % maximum exposure to the majority language was checked.
A measure of degree of bilingualismwas calculated as the percentage of exposure to the less dominant language divided by per-
centage of exposure to the more dominant language. For example, a child with 30 % exposure to English and 70 % exposure to
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6
Spanish would have a degree of bilingualism of 0.43. A score nearer to 1.0 indicates a 50/50 split in language exposure, whilst a score
nearer to 0.25 indicates a 20/80 split between native languages. The bilingual sample, on average, reported a degree of bilingualism of
M =0.43 (SD =0.19, range =0.25-0.75). Bilingualsaverage percentage of exposure to English was M =61.47 % (SD =21.03 %,
range =20-80 %). Bilingualsaverage percentage of exposure to a non-English language was M =38.53 % (SD =21.03 %, range =
20-80 %). The distribution of bilingualsdegree of bilingualism was normal (W =0.97, p=.413) and evenly spread between the
criteria of 20 % minority language minimum and 80 % majority language maximum.
2.3.2. Language mixing scale
Each bilingual parent also completed a language mixing scale (Byers-Heinlein, 2013; Byers-Heinlein et al., 2020; Tsui et al., 2020)
that took approximately three to ve minutes. Parents were asked to rate on a scale of zero (Never) to six (Always) how often they
mixed their language use. The questions asked about behaviours like borrowing words from another language or code switching
between two languages in the same sentence. Bilingual parents reported, on average, a language mixing score of M =13.28 (SD =7.48,
range =0-30) out of a total possible 36 points on the scale. The distribution of bilingual infantsparentslanguage mixing scores was
normal (W =0.97, p=.420).
2.4. Eye-tracking procedure
The eye-tracking task was the rst of a large battery to test the effects of bilingualism on emergent communication patterns. The
total protocol took approximately 1.5 to 2.5 hours from start to nish. The protocol began with a 10- to-15-minute eye-tracking session
that began with the non-native speech perception task reported here. Children sat on their parents laps approximately 65 centimetres
away from a Tobii Pro TX300 eye-tracker in a dimly lit, featureless room. The eye-tracker was set to a sampling rate of 120 Hz
(measurement accuracy: 0.4; screen size: 58.42 centimetres; aspect ratio: 16:9; screen resolution: 1920 ×1080). The tracking
equipment and stimulus presentation were controlled on a Dell 2018 desktop computer running Windows 10 via PsychoPy3 (Peirce
et al., 2019) and PyHab, a programme designed specically for infant looking time measurements during habituation paradigms
(Kominsky, 2019). A camera mounted directly above the horizontal midpoint of the screen recorded the childs behaviour.
2.5. Experimental paradigm
A classic switch variant of an infant visual habituation paradigm was used to compare monolingual and bilingual infantsnovelty
responses to a non-native, Hindi dental-retroex /ta/” – /Ta/phonetic contrast (Narayan et al., 2010). This design has been
implemented widely in previous research, allowing for the comparison of monolingual and bilingual infantsperceptual sensitivities to
phonetic contrasts in a non-native language (for example, see Narayan et al., 2010; Singh et al., 2017).
Auditory stimuli were presented through loudspeakers located behind the computer screen. A static picture of a black and white
checkerboard was presented on a screen concurrently with auditory stimuli. These stimuli have been used previously with a variety of
different speakers and are described in detail elsewhere (e.g., duration, average pitch, pitch minimum and maximum, pitch range, etc.)
(Casaus, 2015; Petitto et al., 2012; Werker & Tees, 1984; Werker et al., 1981). The task began with a non-social attention-getter (i.e., a
whirling water wheel, see Fig. 1) at the centre of the screen accompanied by an auditory tone sine wave for a xed duration of
12 seconds (following Casaus, 2015). This allowed for a baseline measure of infantsattention to the sequence at the beginning and end
of the task. The habituation phase followed the rst attention-getter (see Fig. 1). During the habituation phase, the programme
calculated and stored the childs peak looking time, calculated as the longest amount of time the child looked towards three
consecutive tokens of the habituation phase (Oakes, 2010). After every set of three tokens, starting with the sixth token, the childs
Fig. 1. Depiction of experimental paradigm. Note. ISI =interstimulus interval. 1 s/token =one second per token shown in the habituation phase,
broken into three "windows" of three trials each. Number of tokens in the habituation phase were variable (between six and 34) and depended on
when the infant looked to the screen 40 % less than their longest looking time measurement to a previous window.
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mean looking time (over three tokens) was compared to the peak looking time. If the childs looking time to the most recent window of
three tokens was equal to or less than 60 % of the childs peak looking time (Narayan et al., 2010), the child was considered
habituated. At this point, the programme ended habituation and proceeded with the presentation of the inter-stimulus interval, which
was a two-second ashing yellow light (see Fig. 1).
Next was the test phase, which contained two conditions: habituated same (dental "/ta/") and novel switch (retroex "/Ta/"). In
these conditions, infants were presented with short audio clips repeatedly articulating the same dental "/ta/" in the same phase and the
retroex "/Ta/" in the switch phase (the same stimuli used by Casaus, 2015). The retroex switch sound is a phonemic contrast to the
habituated same sound, but non-native speakers of Hindi do not perceive the two tokens as different from each other (Werker & Tees,
1984; Werker et al., 1981). If infants were able to perceive the non-native contrast, it was hypothesised they would show a novelty
response to the switch phase indicated by longer looking times to the screen (Narayan et al., 2010). If they could not perceive the
contrast, they were expected not to show a novelty response and instead look for a short amount of time to the switch phase which, to
them, sounded perceptually identical to the habituation phase. Within both conditions of the test phase, eight, one-second natural
syllables were presented whilst a concurrent black-and-white checkerboard was shown on the screen (see Fig. 1). Each test phase (same
and switch) had eight variations with diverse pitch and a variety of child-directed intonations. These were presented in random order
(Oakes, 2010). The phases were separated by a ashing yellow inter-stimulus interval to regain infantsattention.
2.6. Looking time measurement
Infant looking time was measured in two ways. A Tobii a TX300 remote eye-tracker was programmed to serve as the primary coder
using Python 3.6.9 and the GitHub package called psychopy_tobii_infant (https://github.com/yh-luo/psychopy_tobii_infant). To check
reliability of this measurement, looking time was also measured from video recordings of the protocol via ofine, frame-by-frame
coding (one frame =40 ms, look/no-look).
2.7. Eye-tracking measurement reliability
Frame-by-frame video coding was conducted in ELAN (ELAN, 2019). Using the frame-by-frame function (frame =40 milliseconds),
each childs gaze was binary coded as LOOK or NO-LOOK to the screen. When the child moved temporarily out of the webcam view,
the gaze was coded as NO-LOOK to be conservative. Each childs total looking time to the screen, calculated as the sum number of
seconds of looking in the LOOK tier, was extracted. The result was two measures of looking time for each phase of the task, one
calculated online by the eye-tracker and one calculated ofine by frame-by-frame coding. There was a strong correlation between the
two looking time measures in the test phase, indicating that the measures were highly reliable (r=0.71, p<.001, Fig. 2).
Fig. 2. Reliability of looking time measures. Note. LT =Looking time. Shaded grey =standard error.
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3. Results
3.1. Main analysis
The main hypothesis was tested with a linear mixed effects model using lme4 (Bates et al., 2015) in R (R Core Team, 2021) and the
lmerTest package (Kuznetsova et al., 2017). Assumptions of linear mixed effects models were met. The predictors included in the linear
mixed effects model were Group (monolingual versus bilingual), Trial (same versus switch), and Age (continuous, days), and the
outcome was looking time in seconds (full model: Looking Time ~ Group x Trial x Age +(1|ID)). The hypothesised interaction of Group
x Trial x Age was not signicant (p=.737). There were no main effects of group (p=.673), trial (p=.814), or age (p=.396). Finally,
the step() function of lmerTest was used to conduct backwards elimination of xed effects terms in the linear mixed effects model. The
results revealed the best tting model included only the random effect of individual.
3.2. Exploratory analyses
3.2.1. Potential discriminators
In alignment with previous research, the present study was conducted under the expectation of a novelty preference for the switch
phase if infants discriminated the contrast. Under this premise, to measure whether any children were potentially discriminating the
contrast, a novelty preference scorewas calculated as LT SwitchLT Same
LT Switch+LT Same. This identied potential discriminators whilst standardising for
individual differences in overall looking time to the test phase (see Fig. 3). Children who looked longer to the switch phase than to the
same phase would have a novelty preference score of more than zero. Children who looked either to the phases equally or to the same
phase more than to the switch phase would have a novelty preference score of equal to or less than zero.
A total of 31 (out of 62) children had a novelty preference score of more than zero, suggesting they may have been discriminating
the non-native phonetic contrast. Of them, nine were 15 month olds (n=6 monolinguals, n=3 bilinguals), three were 16 month olds
(n=2 monolingual, n=1 bilingual), 11 were 17 month olds (n=6 monolinguals, n=5 bilinguals), and eight were 18 month olds
(n=4 monolingual, n=4 bilingual) (total monolingual: n=18, total bilingual: n=13) (see Fig. 4).
3.2.2. Group differences in habituation
There were no group differences in overall amount of looking to the habituation phase (t(55.80) =0.71, p=.483) and a Pearsons
chi-square revealed no differences in the groupsaverage last habituation trial (p=.450) (see Fig. 5).
3.2.3. Within-group variability
Within the bilingual group, there was no relationship between novelty preference score and degree of bilingualism (t(30) =0.10,
p=.328) nor between novelty preference score and amount of parental language mixing (t(30) =1.71, p=.100). Of the n=13
potential bilingual discriminators, degree of bilingualism ranged from 0.10 to 0.66 (M =0.45, SD =0.17), language mixing ranged
from 4.5 to 29 (M =14.69, SD =7.25), and non-English languages included Cantonese (n=2), Danish (n=1), French (n=1), Greek
(n=1), Hebrew (n=1), Hungarian (n=1), Italian (n=2), and Mandarin (n=2), and Spanish (n=2).
4. Discussion
On the basis of neuroimaging data, it has been proposed that bilingual infants remain sensitive to foreign speech sounds later into
life than do monolinguals (Petitto et al., 2012). The theory posits that the strong linguistic demands of spoken language bilingualism,
Fig. 3. Infantslooking time to test phase by age. Note. LTSame =same phase. LTSwitch =switch phase. Shaded area =standard error. Bilinguals
represented by the dashed line and monolinguals by the solid line.
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9
including high phonetic variability and the need to navigate between two spoken languages, may drive a perceptual wedgein bi-
lingualsdeclining sensitivity to non-native speech sounds (Petitto et al., 2012). While this notion is supported by some neurophys-
iological studies showing distinct neural responses in bilinguals towards the end of the rst year of life, direct behavioural tests of this
proposal are limited. The present study lled this gap by testing whether monolinguals and bilinguals differed in the trajectories of
their declining sensitivity to a non-native phonetic contrast between 15 to 18 months. No signicant group differences were found.
Fig. 4. Distributions of novelty preference scores by group. Note. Novelty preference score calculated as looking time to switch phase minus looking
time to same phase divided by sum of total looking to same plus switch. Distribution of bilinguals represented by solid line and monolinguals by
dashed line.
Fig. 5. Distributions of habituation thresholds by group. Note. Final habituation trial =the point at which infants reached the 40 % decrement in
looking time and task moved to the test phase. Bilinguals represented by solid line and monolinguals by dashed line.
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4.1. No evidence of group differences in non-native contrast sensitivity
The main hypothesis predicted that bilingual infants, who were exposed to English and a second language from birth, would retain
perceptual sensitivity to a non-native phonetic contrast later into life than would monolinguals (e.g., per the Perceptual Wedge
hypothesis, Petitto et al., 2012). Considering previous research that suggests monolinguals are no longer able to perceive foreign
language sounds after 12 months of age (Best, 1994; Kuhl et al., 1992; Polka & Werker, 1994; Werker & Tees, 1984), it was expected
that all English-learning monolinguals between 15 and 18 months would not be able to perceive the foreign contrast (Hindi retroex
/ta/and /Ta/, as per Werker & Tees, 1984). Bilingual infants, to whom the contrast was also foreign, were expected to show
sensitivity to the non-native contrast in the younger months of the age window (e.g., 15, 16 months). This was expected to decline
towards the end of the age window (i.e., 18 months).
The main analyses did not support this hypothesis. There were no indications that monolinguals or bilinguals reliably discriminated
the foreign contrast at any age studied. While no criteria exist against which it can be conclusively determined if a child is discrim-
inating a phonetic contrast, a predominant assumption made in developmental speech perception research is that children will look
longer to new sounds than familiar and/or habituated ones (e.g., Albareda-Castellot et al., 2011; Bosch & Sebasti´
an-Gall´
es, 2003; Kuhl
et al., 2006; Narayan et al., 2010; Sebasti´
an-Gall´
es & Bosch, 2009; Singh et al., 2017; Werker & Tees, 1984). In the present study, a
novelty preference score was calculated for each child to determine if they did indeed look longer to the novel, contrastive /Ta/
token after habituating to the dental /ta/. The novelty preference calculation did not reveal any group differences. Approximately
half of all infants were identied as potential discriminators, and they were spread evenly across groups and ages (see Fig. 4).
The lack of group differences between monolinguals and bilinguals do not support the Perceptual Wedge hypothesis (Petitto
et al., 2012). The ndings also do not align with the pattern reported by Casaus (2015), who found 15-month-old bilinguals, but not
monolinguals, discriminated a non-native consonant contrast. One possibility is that ndings diverge because of differences in
analysis. Casaus (2015) used a cross-sectional design which permitted n=44 (n=22 monolinguals) in the 15-month-old group. In
contrast, the trajectory approach adopted here meant there was a smaller sample size at each month. Only 13, 15 month olds were
included in the current study. If the perceptual narrowing effect is limited to the 15th month alone, it is likely that the present study did
not have enough power within that limited age group to detect it. The strength of the approach is that it could capture variability in the
developmental trajectories of children across the 15-to-18-month age window, but it is likely not sensitive enough to capture small
effects at the level of a month-by-month comparison. Future research may consider testing large groups of younger infants to
determine, with enough power to test group equivalence (Lakens et al., 2018), if group differences shown previously are driven
predominantly by younger ages than studied here.
Importantly, the sample collected here was n=12 children short of the target recruitment size due to ongoing COVID-19 lab
closures while the funding supporting this work ended. However, if it were simply the case that the present study was unpowered,
there would likely be a higher number of potential bilingual discriminators of the contrast in the 15th and 16th month, even if the
difference were non-signicant, which is not reported. If data from the last n=5 bilingual and n=7 monolingual children had been
collected, and if all children showed strong effects that aligned with the hypothesis such that all bilinguals showed a novelty preference
and all monolinguals did not, the nal result would be exact group equivalence (n=18 potential discriminators in each group). For
this reason, it is unlikely that the addition of these last few children would signicantly change this studys results or the interpretation
of them.
Although it is thought to be a universal phenomenon, some perceptual narrowing research has failed to show evidence of phonetic
contrast discrimination in infants that aligns with typically proposed perceptual narrowing trajectories (e.g., Mazuka et al., 2013; Tyler
et al., 2014; for review, see Singh et al., 2022). Some evidence suggests that bilinguals may construct a single language system in which
they store and access information in both of their native languages (for review, see De Houwer, 1995). The notion of one cognitive
model for language, even in the case of bilingualstwo acquired language systems, could explain why monolinguals and bilinguals
show similarities at certain ages and/or on certain tasks (for review, see Werker et al., 2009), such as in the domains of lexical
development (Pearson et al., 1993, 1995) and speech comprehension (Shook & Marian, 2012). It could be that an alignment of ca-
nonical perceptual narrowing trajectories in monolinguals and bilinguals at certain points in development can also be explained by the
theory of one cognitive system that accommodates two language systems. Future work is required to provide direct evidence, as the
null results presented here cannot be interpreted as support of the null hypothesis (i.e., group equivalence). Future research may test
whether similarities exist between monolingual and bilingual infantsperception of non-native consonant contrasts by establishing
statistical group equivalence (Lakens et al., 2018).
We could also speculate that potential bilingual effects on non-native speech perception follow a U-shaped trajectory that cannot be
captured in the limited 15- to 18-month age window studied here. A non-linear trajectory in non-native contrast sensitivity would be
characterised by initial discrimination from birth to approximately seven months of age, followed by a decrease in sensitivity between
eight to nine months thought to accompany native language specialisation, and then later re-emergence of discrimination abilities. As
described in the introduction, such U-shaped patterns have been shown in both monolingual and bilingual infantssensitivity to native
and non-native tonal contrasts over the rst two years of life (Liu & Kager, 2014, 2016). For example, Liu and Kager (2014) found that
Dutch-learning monolinguals showed sensitivity to a non-native tonal contrast at ve to six months of age and at 17 to 18 months, but
not at nine months. Liu and Kager (2016) report a similar U-shaped trajectory in bilinguals learning Dutch and another non-tonal
language, such that they showed evidence of non-native tonal contrast discrimination at ve to six months and 11 to 12 months,
but not at eight to nine months. In this context, experience-related effects in infantsnon-native speech sound discrimination may be
understood as differences in the timing or nature of discriminatory re-emergence in bilinguals compared to monolinguals. The bi-
linguals in Liu and Kager (2016) showed evidence of sensitivity re-emergence about six months earlier (11 to 12 months of age) than
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11
the monolinguals in Liu and Kager (2014) (17 to 18 months). The approach of the present study to investigate differences between
15- to 18-month-old monolingual and bilingual infants is not well-suited to capture such broad, U-shaped effects that occur over the
rst years of life. Future large-scale, collaborative approaches (e.g., the ManyBabies project; Baumgartner et al., 2023; Frank et al.,
2017) may be most effective in investigating infantsdeclining and possibly re-emerging sensitivities to different types of native and
non-native language sounds on a larger developmental scale (e.g., Kalashnikova et al., 2023).
It is important to note that the U-shaped bilingual effects reported in previous perceptual narrowing literature may be sensitive to
both the sound inventories of bilingualstwo languages (e.g., tonal versus non-tonal) and to the types of stimuli used. Kalashnikova
et al. (2023) found no group differences in sensitivity to non-native Cantonese tonal contrasts between ve-, 10-, and 17-month-old
monolinguals (learning non-tone or pitch-accent languages) and bilinguals learning either two non-tone languages (Basque and
Spanish) or one tonal and one non-tonal language (Mandarin and English). While the lack of robust differences between 17-month-old
monolingual and bilingual infants in Kalashnikova et al. (2023) roughly aligns with the lack of group differences reported here, direct
comparisons are difcult to draw. Kalashnikova et al. (2023) tested sensitivity to non-native tonal contrasts in bilinguals learning one
tonal and one non-tonal language, whereas the current study tested non-native retroex consonant contrast discrimination in bi-
linguals learning two languages without a retroex consonant. Kalashnikova et al. (2023) also found that all monolingual and bilingual
infants across the three time points discriminated the tonal contrast, whereas the results of this study did not suggest all infants
discriminated the non-native retroex consonant contrast.
Perhaps more conceptually similar to this study are the stimuli used by Liu and Kager (2015), who tested sensitivity to a non-native
aspiration consonant contrast (/p/— “/p
h
/) in eight- to nine-, 11- to 12-, and 14- to 15-month-old infants either learning Dutch
(monolingual) or Dutch and French or Spanish (bilinguals). Liu and Kager (2015)s results showed that, at eight- to nine-months,
bilinguals but not monolinguals discriminated the non-native consonant contrast. There was no evidence of discrimination in
either group at 11- to 12-month and 14- to 15-month timepoints (Liu & Kager, 2015). Lack of non-native contrast discrimination in the
second year of life, in both monolingual and bilingual infants (Liu & Kager, 2015), broadly aligns with the lack of discrimination
amongst 15- to 18-month-old monolinguals and bilinguals reported here. The current study found no evidence of reliable differences
between monolingual and bilingual 15 to 18 month oldslooking behaviours that would indicate presence of the predicted group
differences on non-native phonetic contrast discrimination. Overall, future research is required to extricate fully the experience-related
effects that diverse groups of infants show in their trajectories of sensitivity to different types (e.g., tonal and non-tonal) of non-native
language sounds.
Another possible reason for the lack of coherence between the present study and previous studies in this area is that any difference
in non-native contrast discrimination between monolinguals and bilinguals may be present at a younger developmental age than that
studied here. The 15- to 18-month age window investigated was older than previous samples in which neurophysiological group
differences between monolinguals and bilinguals have been suggested (Petitto et al., 2012; Singh et al., 2017). One study reported that
10- to 11.5-month-old bilinguals looked longer to the switch than same phase of a non-native phonetic contrast (Singh et al., 2017).
This effect was not present in monolinguals (Singh et al., 2017). Further, Casaus (2015) reported a trend of a Group x Trial interaction
effect, driven by a signicant within-group bilingual discrimination effect at 12 months and a statistically signicant Group x Trial
interaction at 15 months. The 15- to 18-month-old age range was selected in the current study to test whether behavioural differences
persisted past the point of neural differences reported by the four- to 12-month-old literature (Petitto et al., 2012; Singh et al., 2017)
and to replicate the behavioural differences reported in unpublished research at 15 to 18 months (Casaus, 2015). However, it may be
that behavioural group differences in monolingual and bilingual childrens sensitivity to non-native phonetic contrasts are stronger at
the end of the rst year and beginning of the second year (between 12 and 15 months) than at later time points. Future work should
employ a developmental trajectory approach with this age range to determine if group differences between 12- to 15-month-old
monolingual and bilingual infantslooking to non-native contrasts is replicable, and if so, what characterises the groups different
trajectories.
4.2. No evidence of within-group bilingual variability effects
Within-group, differences in bilingualsearly language experiences were not related to their sensitivity to foreign language sounds.
Degree of bilingualism, calculated as percentage of exposure to minority divided by majority language, and exposure to parents
language mixing behaviours were not related to bilingualscontrast discrimination. When novelty preference was calculated for in-
dividual children, approximately half of all participants across the bilingual (and monolingual) groups showed a novelty preference
that may have been linked to contrast discrimination. However, amongst potential bilingual discriminators, there were no clear
patterns related to age, types of non-English languages (i.e., close versus distant language pairs), childrens degree of bilingualism, nor
parentslanguage mixing behaviours. The lack of relationships between bilingual experiences and novelty preference is not partic-
ularly surprising given that most of the bilinguals (59.37 %) did not show evidence of discriminating the non-native contrast. It is
important to test whether elements of variability in bilingual experiences relate to processes of interest when this variability is the
theoretical motivation for predicting bilingual differences from monolinguals. Measuring types of within-group variability can help
clarify the environmental conditions that may drive group differences. The shift away from treating bilingualism as a categorical
variable (for example, see Byers-Heinlein, 2015; Kremin & Byers-Heinlein, 2021) is another important consideration in future
perceptual narrowing literature.
Another difference between the present study and past work is the diverse composition of the bilingual group. To understand if
being raised bilingual has robust, cascading effects on the development of speech perception, it is necessary to test effects across
diverse groups of bilinguals learning a variety of languages. The experience of learning two languages that are in the same language
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Infant Behavior and Development 76 (2024) 101959
12
family and are therefore quite similar to each other, such as Spanish and Catalan, is likely to differ from learning two languages of
different families, such as Chinese and Arabic (Floccia et al., 2018). The two languages a bilingual infant hears can also be variable in
domains such as speech sound inventories, transitional probabilities, use of lexical stress, rhythm, and word order. Most previous
research on perceptual narrowing in infancy constrains the language pairings of bilingual groups. The results that do exist about the
effects of bilingualism on perceptual narrowing are derived primarily from bilinguals learning phonetically similar languages that are
predominantly romance and/or Germanic languages (for review, see Singh et al., 2022; for experimental examples, see Albar-
eda-Castellot et al., 2011; Bosch & Sebasti´
an-Gall´
es, 2003, 2005; Burns et al., 2007; Sebasti´
an-Gall´
es & Bosch, 2009; Sundara et al.,
2008).
One study reporting a bilingual effect on perceptual narrowing in 12- to 15-month-old children tested a sample of bilinguals
learning Spanish and Catalan, two languages that are phonologically close to each other and contain similar translational probabilities,
rhyme, and word order (Bosch & Sebasti´
an-Gall´
es, 1997; Casaus, 2015). It may be that those learning two phonologically close
languages lose the ability to perceive non-native contrasts later in life than those learning distant languages. In a study of bilinguals
learning two phonologically distant languages (Mandarin and English), Singh et al. (2017) found a bilingualism effect in 10- to
11.5-month-old participants. It could be that a bilingual effect on perceptual narrowing is related to the distance between the two
languages a bilingual is learning. Bilinguals learning two close languages that have a high degree of overlap with each other, such as
Spanish and Catalan, may remain more sensitive to subtle differences in speech sounds than those whose native languages are distant
and therefore rapidly identiable based on large phonological differences, like Mandarin and English. Group differences between
monolinguals and distant-language bilinguals may be best captured towards the end of the rst year whilst close-language bilinguals
may still differ from monolinguals towards the middle of the second year. This is an open question for future research.
The current study cannot empirically test whether language distance relates to trajectories of perceptual narrowing; however, it is
relevant that the n=13 potential bilingual discriminators were learning two languages with differing distances from each other. Some
were learning English and non-English language pairs that were distant, such as Cantonese, Greek, Hebrew, Hungarian, and Mandarin,
whilst others were learning languages more closely related to English such as Danish, French, Italian, and Spanish. It could be that the
heterogeneity of this studys bilingual sample masked differences that are language-pair specic. Future work should determine the
time at which bilinguals learning two phonologically distant language pairs are no longer able to discriminate non-native consonant
contrasts, as well as determine more specically the time at which learners of two similar languages lose this perceptual skill. In a
recent meta-analysis, Singh et al. (2022) surveyed 99 perceptual narrowing studies and reported that, of the 19 % that examined both
monolingual and bilingual infants, half sampled learners of English, Catalan, and Spanish. The present study adds to the existing
literature by investigating perceptual narrowing in a group of heterogenous bilinguals, but future research is clearly required to fully
understand the developmental effects of bilingualism on foreign speech sound perception.
4.3. Methodological considerations
The eye-tracking integration approach developed for this study may be of interest to future researchers using a habituation pro-
tocol. Visual habituation paradigms often rely on experimenter button-press measurements that are likely subject to error and
experimenter bias. The current study used an open source looking time and stimulus presentation system, PyHab (Kominsky, 2019), a
PsychoPy (Peirce et al., 2019) add-on. Instead of relying on button-presses, the software was amended to accept input from a Tobii
TX300 eye-tracker to serve as the primary gaze coder. Manual, frame-by-frame looking time measures were established to allow
reliability checks of the eye-tracking approach. These analyses clearly demonstrated that the eye-tracking method reliably measured
infantslooking time during the habituation paradigm. Agreement between methodstest phase measurements was high (r=0.71).
Unsurprisingly, disagreement between methods was generally higher in cases where children moved signicantly during the session.
There were four cases where measurement difference was greater than two standard deviations above the mean. In all of them, the
child moved signicantly down or back, and frame-by-frame analysis captured valid looking time measurement that eye-tracking did
not. Given that this occurred in 4 % of the sample tested, there does not appear to be a high degree of risk for unnecessary data loss,
though future research with this method may want to predict attrition accordingly. Instances where frame-by-frame coding can
recover lost eye-tracking data can also be addressed post-hoc; however, data replacement is complicated by the fact that visual
habituation is determined categorically and irreversibly by the online eye-tracker measurement.
Reliability of habituation decisions between measures were also examined. The habituation calculation was determined online by
eye-tracking; if a childs eye gaze was lost before or during habituation, the software would have advanced into the test phase
incorrectly, and this decision cannot be reversed post-hoc. Habituation windows were located in the experimental time course by
referencing habituation token length against the experiment start time. Frame-by-frame looking times were extracted from timestamps
in ELAN output les and infantstotal looking to each habituation window was summed. The trial at which infants reached habitu-
ation, according to frame-by-frame measurement, was calculated as it was online by the eye-tracker (i.e., as the point at which infants
overall looking to a habituation window fell to 60 % of the looking time of the longest three-trial window). In all but the four cases of
measurement disagreement referenced above, the frame-by-frame habituation trial matched the eye-tracker habituation trial, sug-
gesting high agreement between the measuresthreshold decisions. These four cases were excluded from nal analysis for failure to
look for at least one second to the same and switch phases, and thus mismeasurement in habituation did not confound the results.
Overall, the eye-tracking integration used in this study to measure infant looking behaviour provided valid measurement of infants
looking time during visual habituation paradigms. The method was generally robust, capturing infantseyes on the screen, even during
a habituation paradigm where infants became increasingly less interested in the stimuli. It also seems promising for research with
younger, less mobile infants, as valid data was collected in the present sample of highly mobile children who were old enough to crawl
V.L. Mousley et al.
Infant Behavior and Development 76 (2024) 101959
13
and walk. Future research should employ sensitive techniques, such as the eye-tracking integration used here, to re-examine the
proposed trajectories of monolingualsnarrowing to foreign language sounds. Precision of looking time measurement has previously
been shown to affect the detection of potential monolingual and bilingual group differences in discrimination to native language
sounds (see Albareda-Castellot et al., 2011). It is possible that more sensitive approaches than previously used will reveal greater
variability in infants perceptual narrowing trajectories than is currently thought. The stimulus presentation software is freely
available (https://github.com/jfkominsky/PyHab; Kominsky, 2019) and now contains a feature that allows the experimenter to select
eye-tracking.
5. Conclusion
This study addresses recent calls for diversication of perceptual narrowing research to include new methods and diverse samples,
such as bilinguals with a wide variety of language pairs (Singh et al., 2022). The results do not support the widely popular claim that
heterogenous bilinguals retain perceptual sensitivity to non-native contrasts for longer than monolinguals (Berken et al., 2017;
Birdsong, 2018; Costa & Sebasti´
an-Gall´
es, 2014; Jasi´
nska & Petitto, 2014; Kovelman et al., 2015; Potter & Saffran, 2015; Singh et al.,
2017; Singh & Tan, 2021; Zadina, 2015). Variability within bilingualsexperiences also did not show signicant relationships to
bilingualsdiscrimination of foreign language sounds. Lack of obvious bilingual effects could be related to the age range in which it was
measured, the wide variability in bilingualsnon-English languages, or to the more sensitive eye-tracking measurement than typically
employed by perceptual narrowing research. While there were no obvious group differences, approximately half of all 15 to 18 month
olds showed more looking time to the novel stimulus than the habituated stimulus, which may indicate discrimination of the foreign
language sound in both monolinguals and bilinguals. Finally, an adaptation of a free stimulus presentation software, PyHab, was
developed and validated for precise and objective measurement of infant looking time during a visual habituation paradigm. The
adaptation has been integrated with the software and is publicly available for use.
Funding
This work was supported in part by the the Wellcome Trust Senior Research Fellowship awarded to M.M. [100229/Z/12/Z]. V.L.M
was supported by a Research Excellence Scholarship (previously a Graduate Research Scholarship and an Overseas Research Schol-
arship) from University College London.
CRediT authorship contribution statement
Victoria L Mousley: Writing review & editing, Writing original draft, Visualization, Project administration, Methodology,
Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Mair´
ead MacSweeney: Writing review &
editing, Supervision, Funding acquisition, Conceptualization. Evelyne Mercure: Writing review & editing, Supervision,
Conceptualization.
Data Availability
Data will be made available upon request for participants whose parents provided consent for their childs information to be shared
with external researchers.
Acknowledgements
We would like to thank all the parents and carers who contributed to this study, as well as Denis Mareschal and the entire team at
the Birkbeck Babylab for graciously allowing us lab space. We are grateful to Jonathan Kominsky for his support with PyHab and to
Janet Werker and Luca Bonatti for discussing study design, directing us to Gisela Pi Casausthesis, and for sharing stimuli. We are also
thankful to the many students who worked on this project, including Anushay Mazhar, Åshild Kummen, Gladys Hui, Sammi-Kenzie
Tam, Sophie Field, and Wing Phoebe Kam.
Copyright statement
For the purpose of open access, the author has applied a CC BY public copyright license to any Author Accepted Manuscript version
arising from this submission.
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