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# Native and non-native vowel discrimination in 6-month-old Norwegian infants

Audun Rosslund, Julien Mayor, Alejandrina Cristia, Natalia Kartushina

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Audun Rosslund, Julien Mayor, Alejandrina Cristia, Natalia Kartushina. Native and non-native vowel discrimination in 6-month-old Norwegian infants. Infant Behavior & Development, 2024, 77, pp.101992. 10.1016/j.infbeh.2024.101992. hal-04874473

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# Native and non-native vowel discrimination in 6-month-old Norwegian infants

Audun Rosslund<sup>a, *, 1</sup>, Julien Mayor<sup>b, 2</sup>, Alejandrina Cristia<sup>c, 3</sup>, Natalia Kartushina<sup>a, 4</sup>

## ARTICLE INFO

### Keywords:

- Speech perception

- Perceptual narrowing

- Habituation

- Eye-tracking

## ABSTRACT

In the current preregistered study, we tested n = 67 6-month-old Norwegian infants' discrimination of a native vowel contrast /y-i/ and a non-native (British) vowel contrast /ʌ-æ/ in an eye-tracking habituation paradigm. Our results showed that, on a group level, infants did not discriminate either contrast. Yet, exploratory analyses revealed a negative association between infants' performance in each experiment, that is, better discrimination of the native contrast was associated with worse discrimination of the non-native contrast. Potentially, infants in this study might have been on the cusp of perceptual reorganisation towards their native language.

## 1. Introduction

Language acquisition is a formidable challenge, yet infants seemingly navigate this task with ease. A developmental process that might facilitate acquisition is that infants' perceptual systems tune in to the specific properties of their ambient environment. This theory, known as perceptual narrowing, posits that infants come into the world sensitive to a wide range of speech contrasts, and gradually, within their first year, become attuned to the speech of their native language through a decline in their ability to discriminate non-native contrasts, while they maintain or improve their discrimination of native contrasts (see review in Werker, 2018).

The shift in broad-to-language-specific speech perception was first demonstrated in infants' discrimination of consonants. In a seminal study, Werker and Tees (1984) reported that English-learning 6–8-month-old infants could distinguish a Hindi and a Nthlakampx consonant contrast, an ability that declined in 8–10-month-olds and was absent in 10–12-month-olds, while Hindi or Nthlakampx-learning 10–11-month-old infants successfully discriminated their respective native contrast. In the wake of Werker and Tees (1984), researchers have now accumulated a rich literature in support of perceptual narrowing not only to consonants, but also to vowels and lexical tones, within infants' first year of life (e.g., Cheour et al., 1998; Gonzalez-Gomez et al., 2021; Götz et al., 2018; Kuhl et al., 2006; Liu & Kager, 2016; Lovčević & Tsuji, 2024; Mattock & Burnham, 2006; Polka & Werker, 1994; Singh et al., 2022a). While

<footnote><sup>a</sup> Center for Multilingualism in Society across the Lifespan, University of Oslo, Norway</footnote>

<footnote><sup>b</sup> Department of Psychology, University of Oslo, Norway</footnote>

<footnote><sup>c</sup> Laboratoire de Sciences Cognitives et de Psycholinguistique, Département d'études Cognitives, ENS, EHESS, CNRS, PSL University, France</footnote>

<footnote>* Correspondence to: Center for Multilingualism in Society across the Lifespan and Department of Linguistics and Scandinavian Studies, University of Oslo, PB 1102, Blindern, 0373 Oslo, Norway.</footnote>

<footnote>E-mail address: audun.rosslund@iln.uio.no (A. Rosslund).</footnote>

<footnote n="1">¹ Orcid ID: 0000-0002-2646-8053</footnote>

<footnote n="2">² Orcid ID: 0000-0001-9827-5421</footnote>

<footnote n="3">³ Orcid ID: 0000-0003-2979-4556</footnote>

<footnote n="4">⁴ Orcid ID: 0000-0003-4650-5832</footnote>
less investigated than consonants, perceptual narrowing to vowels has been found to occur earlier (e.g., Kuhl et al., 1992; Polka & Werker, 1994), perhaps as their acoustic features are more salient, and vowel inventories are typically more sparse in most languages (Granier-Deferre et al., 2011; Gerken, 2002; but see Tsuji et al., 2017).<fnref n="1" /> In a meta-analysis that aggregated 97 effect-sizes from 18 studies, Tsuji and Cristia (2014) reported evidence for an interaction between native and non-native vowel discrimination, that diverges after 6 months of age. That is, around the second half of their first year, infants' ability to discriminate increases for native contrasts, and deteriorates for non-native contrasts.

However, several limitations and inconsistencies in the literature on perceptual narrowing warrant consideration. First, empirical evidence predominantly comes from English-speaking infants, mostly from North America, testing vowels contrasting on height and backness (Singh et al., 2022b), which limits generalisability of the results to other languages, cultures, and phonetic contrasts (see also Kidd & Garcia, 2022). Indeed, it has been argued that phonetic contrasts that are more subtle than typically tested (i.e., less acoustically and articulatory salient), might require more experience with the native language and, hence, postpone the perceptual narrowing in some languages to later in development (see discussions in de Klerk et al., 2019; Narayan et al., 2010; Sundara et al., 2018). Note that specific methodological choices can also affect the results (e.g., discrimination could be modulated by the order of vowels in the task, see Polka & Bohn, 2011; Pons et al., 2012, or potentially by experimental paradigm, see The ManyBabies Consortium, 2020). Second, the meta-analysis by Tsuji and Cristia (2014) also revealed some publication bias in the literature, and the overall age-effect for non-native discrimination was not significant. In fact, recent studies failed to reveal patterns of vowel discrimination fully consistent with perceptual narrowing theory (Kotera et al., 2024; de Klerk et al., 2019; Mazuka et al., 2014; Paillereau et al., 2021). Finally, few studies have examined both native and non-native discrimination within the same infants – a necessity to capture potential fine-grained shifts in perception within individuals as it unfolds (Tsuji & Cristia, 2014). As such, additional research is needed to corroborate the theoretical claim of perceptual narrowing in infants’ vowel discrimination.

To address these limitations and further investigate the perceptual narrowing hypothesis, in this preregistered study, we tested 6-month-old Norwegian infants' ability to discriminate a native and a non-native vowel contrast in a visual habituation paradigm using eye-tracking. Given that perceptual narrowing for the native language has been suggested to occur after 6 months of age (Tsuji & Cristia, 2014), we expected, per preregistration, successful discrimination of both vowel contrasts.

## 2. Method

### 2.1. Participants and general procedure

Parents of 6-month-old infants residing in Oslo, Norway were recruited via birth registries to participate in the current study. The sample comprised n = 67 infants (41 female; 26 male),<fnref n="2" /> with a mean age of 5.99 months/182 days (SD = 8.25, range = 168–204). Infants were monolingual, with no less than 90% reported exposure to Norwegian (reported by parents through an online questionnaire),<fnref n="3" /> born full term (after gestational week 37), and had no reported visual or auditory impairments. Mothers’ highest level of education ranged from secondary school to a doctoral degree, with the median being a master’s degree.

Data collection took place in the infant lab at the Department of Psychology, University of Oslo, and were collected by two different experimenters (native speakers of Norwegian and experienced in infant research). Upon their arrival to the lab, parents and their infants were first acquainted with the lab environment. Then infants proceeded with the native vowel discrimination experiment, and the non-native experiment.<fnref n="4" /> Each experiment lasted for approximately five minutes. There was a 20–30-minute break between the two experiments, where parents’ infant- and adult-directed speech was recorded (as the study was part of a larger project that included assessment of parents’ speech). At the end of each visit, parents were able to choose a small gift for their infant.

The present study was conducted according to the guidelines laid down in the Declaration of Helsinki, with written informed consent obtained from a parent or a guardian for each child before any assessment or data collection. The study was approved by the ethics committee at the Department of Psychology, University of Oslo (ref. 13066519) and by the Norwegian Centre for Research Data (ref. 506879).

### 2.2. Stimuli

The acoustic stimuli for the native experiment consisted of six unique tokens of the syllable /by/, and one token of the vowel contrasting syllable /bi/, produced by a native female speaker of Norwegian from Oslo, Norway (Table 1). Four of the /by/ tokens were used for the habituation phase, while the two remaining tokens, and the contrasting /bi/ token, were used for the test phase. The tested

<footnote n="1">¹ Note that, to the best of our knowledge, no meta-studies have so far compared the onset of perceptual narrowing in consonants versus vowels.</footnote>

<footnote n="2">² As noted in the preregistration, our sample size was set to be minimum n = 23, computed by the power analysis tool at metalab.stanford.edu to obtain 0.80 power for an expected effect size of 0.58. For native and non-native vowel discrimination tasks in 6-month-old infants, the average effect size from studies included at MetaLab is 0.60 and 0.58, respectively. However, we also specified that we aimed for a sample of minimum n = 50, as the sample would be recruited to take part in a longitudinal study where we expected some attrition.</footnote>

<footnote n="3">³ The specific questions were ‘Hva er ditt barns morsmål?’ (‘What is your child’s first language?’), and ‘Hvor stor prosentandel av tiden hører barnet ditt dette språket?’ (‘What percentage of the time does your child hear this language?’).</footnote>

<footnote n="4">⁴ A fixed order was used to reduce model complexity for a separate study that examined the relationship between parents' vowel production and infants' /y-i/ discrimination.</footnote>
subtle contrast /y/-/i/ is phonemic in Norwegian and is commonly considered one of the most challenging vowel contrasts for second-language Norwegian learners (and older Norwegian children, see Rødvik et al., 2019), as the acoustic features are very similar across all formants (cf. Table 1), yet the articulatory gestures differ, with /y/ being rounded and /i/ being unrounded. While /i/ is in the top tertile in terms of relative frequency of Norwegian vowels, /y/ is in the bottom tertile (Grigas & Juskevičienė, 2018).<fnref n="5" /> The non-native experiment similarly consisted of six unique tokens of the syllable /gʌ/ (four for the habituation phase, and two for the test phase), and one token of the vowel contrasting syllable /gæ/ (for the test phase), produced by a native female speaker of Southern British English, from London, UK (Table 1). The /ʌ-æ/ contrast (previously used in L2 research, Baigorri et al., 2019; Georgiou, 2019, and containing vowels used in infant discrimination research; Kotera et al., 2024; Polka & Bohn, 2011) was chosen as it is one of few vowel contrasts not used in the Norwegian language.

All stimuli were recorded in an infant-directed register, with natural variability between habituated tokens, to ensure that discrimination (or lack thereof) would relate to phonological representations and not simply acoustic cues. Formant frequencies, duration, and pitch contours of the individual tokens are depicted in Table 1.

## 2.3. Experimental procedure

Each infant performed two eye-tracking habituation experiments (native and non-native discrimination), inspired by the “hybrid” variant of the visual habituation paradigm Houston and colleagues (2007) claimed had improved detection of individual differences and test-retest reliability (see also Cristia et al., 2016). The experiments took place in a dimly lit room, where infants sat in a car seat, 60 cm away from a monitor (34 × 27 cm in size, 1280 × 1024 resolution) with an eye-tracker at its base. The eye-tracker was an arm-mounted Eye-Link 1000 Plus, set in monocular mode and with a sampling frequency of 500 Hz. The parent was seated outside the infants’ field of vision and wore headphones with a masking noise throughout the experiment.

After a 5-point calibration procedure, the experiment started with the habituation phase. Here, on each trial, infants saw a colourful checkerboard on the monitor (Fig. 1), and heard four unique tokens of a syllable (/by₁₋₄ for the native experiment; /gʌ₁₋₄ for the non-native experiment, cf. Table 1), presented in a pseudo-randomised order (restricted to avoid more than two subsequent presentations of the same token), at 65 dB, with a 1000 ms interstimulus interval. A trial ended after 14 repetitions of a syllable (i.e., across tokens), which corresponded to approximately 20 s, or when the infant had looked away from the monitor consecutively for 2000 ms. A video of colourful circles with the sound of bar-chimes was used as an attention getter to redirect infants to the screen and prompt the onset of a new trial. The habituation phase continued for a maximum of 24 trials, or until infants reached the habituation criterion, which was set to occur when infants had, on average, 50% less looking time across three consecutive trials compared to the average of the three trials with the longest looks. When habituation was reached, the test phase followed, which consisted of four trials. On trial 1 and 3, infants heard the novel, contrasting vowel syllable (/bi/₁ for the native experiment; /gæ/₁ for the non-native experiment) alternating with a token of the habituated syllable (/by/₆ for the native experiment; /gʌ/₆ for the non-native experiment), and on trial 2 and 4, infants heard two alternating tokens of the habituated syllable (/by/₅₋₆ for the native experiment; /gʌ/₅₋₆ for the non-native experiment). Houston and colleagues (2007) proposed that alternating trials (following their “hybrid” paradigm) could increase infants’ looking times and might reduce the chance of habituation to the new category during the test phase. Test trials similarly ended after 14 syllable repetitions, or until the infant had looked away for 2000 ms. A schematic illustration of the experimental paradigm is provided in Fig. 1.

The experimental procedure was fully automatised and gaze contingent, with no manual coding performed by the experimenter. Our measure of interest was infants’ looking time to the alternating vs habituated test trials.

## 3. Results

All statistical analyses were conducted in R (R Core Team, 2023). Preregistration, data, and code are available at the Open Science Framework project's page.<fnref n="6" /> As per preregistration, we first excluded infants who did not reach the habituation criterion within 24 trials, and infants without valid trials in each condition in the test phase, which were set to minimum 1500 ms looking time in a trial (a criterion we revisit later). After applying these criteria, n = 61 and n = 52 infants (of the initial sample of n = 67) were retained for the analyses for the native and non-native experiment, respectively. On average, infants habituated after 8.21 (SD = 3.62, range = 4–20) trials in the native experiment, and 7.14 (SD = 2.92, range = 4–16) trials in the non-native experiment. Descriptive statistics of looking time for each experiment are reported in Table 2.

To test our hypotheses regarding infants’ discrimination of native and non-native vowel contrasts, we fitted, as per preregistration, two separate linear mixed-effect regression models using the *lme4* library (Bates et al., 2015), in which the outcome was infants’ log-transformed looking time in the test phase, and the predictor was trial type (alternating, habituated), with random intercepts of individual infants and random slopes of trial type. Model assumptions, including normality and homogeneity of residuals were evaluated with the *performance* library (Lüdecke et al., 2021) and deemed appropriate, fixed effects were computed via the Anova() function from the *car* library (Fox & Weisberg, 2018), and p-values were obtained from the *lmerTest* library, using Satterthwaite approximation (Kuznetsova et al., 2017).

Contrary to our predictions, our models revealed no significant effect of trial type on infants’ looking time in either experiment,

<footnote n="5">⁵ Note that this study analysed letter frequency, but that /y/ and /i/ are grapheme-phoneme transparent in Norwegian.</footnote>

<footnote n="6">⁶ https://osf.io/edf42</footnote>
<table><caption>**Table 1**
Stimuli characteristics (mean F1–F3 in Hertz, duration in milliseconds, mean F0 and contour patterns as depicted in Praat software).</caption><thead><tr><th>task</th><th>phase</th><th>token</th><th>F1</th><th>F2</th><th>F3</th><th>duration</th><th>F0</th><th>contour</th></tr></thead><tbody><tr><td rowspan="4">native</td><td rowspan="4">habituation</td><td>/by<sub>1</sub></td><td>348</td><td>3160</td><td>3711</td><td>310</td><td>256</td><td>▲</td></tr><tr><td>/by<sub>2</sub></td><td>389</td><td>3022</td><td>3656</td><td>352</td><td>243</td><td>▲</td></tr><tr><td>/by<sub>3</sub></td><td>387</td><td>3008</td><td>3670</td><td>250</td><td>257</td><td>▲</td></tr><tr><td>/by<sub>4</sub></td><td>417</td><td>2980</td><td>3600</td><td>313</td><td>298</td><td>▲</td></tr><tr><td rowspan="3">test</td><td rowspan="3">habituation</td><td>/by<sub>5</sub></td><td>383</td><td>3101</td><td>3651</td><td>355</td><td>281</td><td>▲</td></tr><tr><td>/by<sub>6</sub></td><td>383</td><td>3049</td><td>3686</td><td>338</td><td>271</td><td>▲</td></tr><tr><td>/bi<sub>1</sub></td><td>400</td><td>3066</td><td>3927</td><td>345</td><td>283</td><td>▲</td></tr><tr><td rowspan="4">non-native</td><td rowspan="4">habituation</td><td>/gΛ<sub>1</sub></td><td>830</td><td>1828</td><td>3204</td><td>312</td><td>324</td><td>▲</td></tr><tr><td>/gΛ<sub>2</sub></td><td>864</td><td>1794</td><td>3135</td><td>344</td><td>236</td><td>▲</td></tr><tr><td>/gΛ<sub>3</sub></td><td>864</td><td>1725</td><td>2893</td><td>332</td><td>232</td><td>▲</td></tr><tr><td>/gΛ<sub>4</sub></td><td>847</td><td>1862</td><td>3290</td><td>315</td><td>275</td><td>▲</td></tr><tr><td rowspan="3">test</td><td rowspan="3">habituation</td><td>/gΛ<sub>5</sub></td><td>830</td><td>1759</td><td>2998</td><td>322</td><td>316</td><td>▲</td></tr><tr><td>/gΛ<sub>6</sub></td><td>813</td><td>1776</td><td>3170</td><td>291</td><td>327</td><td>▲</td></tr><tr><td>/gæ<sub>1</sub></td><td>1002</td><td>1966</td><td>3066</td><td>294</td><td>324</td><td>▲</td></tr></tbody></table>

suggesting that, on a group level, infants' looking times were not indicative of discrimination of either the native contrast (habituated trials: *M* = 5070, *SD* = 3700, alternating trials: *M* = 5290, *SD* = 3680, *χ*2 = 0.736, *df* = 1, *p* = .391), or non-native contrast (habituated trials: *M* = 4640, *SD* = 2990, alternating trials: *M* = 4820, *SD* = 3240, *χ*2 = 0.632, *df* = 1, *p* = .427), as illustrated in Fig. 2 (see Appendix 1 for model estimates). Paired-sample Bayesian t-tests, computed via the *BayesFactor* library (Morey et al., 2015) on infants' average looking time between each trial type further suggested that our data provided moderate evidence in favour of the null hypothesis, as indicated with a Bayes Factor of 0.25 (Lee & Wagenmakers, 2014) for both the native and non-native experiment.

To examine the robustness of our null results, we performed a multiverse analysis (Steegen et al., 2016), by fitting a set of plausible (non-preregistered) models, that were run as nested loops by including or not infants' age, gender, and maternal education as fixed effects and in interaction with trial type up to level four (age*gender*education*trial_type). In addition, we fitted these models with different looking time inclusion criteria to increase sample size (850 ms up to 1500 ms, in 50 ms increments). Irrespective of varying model complexity and looking time inclusion criterion, trial type was not predictive of infants' looking time above what would be expected from chance (1.14% of a total of 702 p-values were significant; see Appendix 2). Moreover, varying inclusion criteria for looking time did not change the moderate evidence for the null hypothesis as indicated by Bayes factor (BF10 ranging from 0.20–0.25 for the native experiment, and from 0.21–0.28 for the non-native experiment).

### 3.1. Exploratory analysis of the association between native and non-native discrimination

To examine individual differences in the potential relationship between discrimination of the native and non-native contrasts, we
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Schematic illustration of the visual habituation paradigm consisting of the habituation phase (max 24 trials) and the test phase (4 trials in a fixed order), depicted with vowels used in native vowel discrimination.</figcaption></figure>

<table><caption>**Table 2**

Descriptive statistics of by-trial looking time (in ms).</caption><thead><tr><th rowspan="2"></th><th colspan="3">native experiment (n = 61)</th><th colspan="3">non-native experiment (n = 52)</th></tr><tr><th>M</th><th>SD</th><th>range</th><th>M</th><th>SD</th><th>range</th></tr></thead><tbody><tr><td>habituation phase</td><td>9010</td><td>5590</td><td>116-18900</td><td>6920</td><td>4960</td><td>102-18900</td></tr><tr><td>test phase</td><td>5180</td><td>3680</td><td>1520-16800</td><td>4730</td><td>3110</td><td>1500-17200</td></tr><tr><td>habituated trials</td><td>5070</td><td>3700</td><td>1520-16100</td><td>4640</td><td>2990</td><td>1500-13900</td></tr><tr><td>alternating trials</td><td>5290</td><td>3680</td><td>1530-16800</td><td>4820</td><td>3240</td><td>1530-17200</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Within-participant average looking time to the habituated and alternating test trials in the native (A) and non-native (B) experiment (yellow and blue lines indicate increased and decreased looking time from habituated to alternating test trials, respectively).</figcaption></figure>

computed “discrimination scores” (following Bergmann & Cristia, 2018) for individual infants in each experiment. A discrimination score was the proportion of looking time to the two alternating test trials versus total looking time in the test phase, and as such, a score above 0.50 would indicate discrimination of the vowel contrast. Discrimination scores were computed for infants with valid data for all four test trials in both the native and non-native experiments, lowering the looking time inclusion criterion to 1000 ms to increase
sample size (an evaluation of the impact of using a range of inclusion criteria on discrimination is described below). Infants' average discrimination score was 0.520 (SD = 0.120, range = 0.207-0.754) for the native experiment, and 0.513 (SD = 0.113, range = 0.261-0.711) for the non-native experiment. A correlation analysis indicated that there was a significant, negative relationship between infants' native and non-native discrimination scores: r<sub>s</sub>(31) = -.36, *p* = .037 (see Fig. 3A), suggesting that infants who showed better discrimination of the native contrast showed poorer discrimination of the non-native contrast, and vice-versa. As in our previous analyses, we computed the correlation coefficients with various inclusion criteria for looking time – and in turn different sample sizes – to assess robustness. As illustrated in Fig. 3B, while confidence intervals increase as a function of decreased sample size (ranging from n = 35 infants at 850 ms to n = 19 infants at 1500 ms), the negative correlation coefficient between native and non-native discrimination scores remained relatively stable (r<sub>s</sub> ranging from -.30 to -.41).

4. Discussion

In response to calls for a more diverse literature on perceptual narrowing (Singh et al., 2022b) and a need for within-infant studies on native and non-native speech discrimination (Tsuji & Cristia, 2014), the current study examined 6-month-old Norwegian infants' discrimination of a native and non-native vowel contrast in an eye-tracking visual habituation paradigm. The results of our preregistered analyses suggested that, on a group level, infants did not discriminate either contrast, challenging therefore, the "universal listener" account in infant speech perception, which predicted accurate discrimination of both contrasts at this age (see Narayan et al., 2010 for relevant discussion). Taken at face value, our data points to a need to revise the classic account of perceptual development in infancy, according to which infants start out discriminating all potentially relevant contrasts and start losing non-native vowel discrimination at 6-8 months of age (Polka & Bohn, 1996; a meta-analysis in Tsuji & Cristia, 2014). However, other studies revealed that developmental patterns for non-native contrast discrimination might depend on the specific contrast being tested (Mazuka et al., 2014), with some non-native contrasts counter-theoretically yielding improved discrimination with infant age (de Klerk et al., 2019; Kotera et al., 2024). In fact, Mazuka and colleagues (2014) propose, in contrast to a classic perceptual narrowing account, that non-native speech-sound perception could be influenced by general cognitive abilities and thus improve together with processing speed, attention and memory. As we cannot ascertain that our null result indicates infants "losing" the non-native contrast, this claim is certainly possible, but requires support from further empirical evidence for Norwegian infants. However, the lack of discrimination in either experiment could be attributed to several (non-mutually exclusive linguistic and methodological) factors, to which we turn.

Let us first consider two linguistic factors that may help us interpret the null results obtained for native /i/-/y/ vowel discrimi-
nation, namely, the perceptual similarity of the contrast, and the complexity of Norwegian phonology. The /y-i/ contrast is very subtle
(see e.g., Best et al., 2003). Although previous studies have not tested infants on this contrast, data from Norwegian school-age children
suggest that it may be more difficult to discriminate compared to other vowel contrasts, as indicated by their performance on a vowel
repetition task (Rødvik et al., 2019). Therefore, infants might require additional input exposure to readily perceive it at 6 months of age
(Narayan et al., 2010; although see Sundara et al., 2018), an argument that is further strengthened by the (relatively) reduced fre-
quency of the /y/ vowel in Norwegian (Grigas & Juskevičienė, 2018). Moreover, given that Norwegian /y-i/ is differentiated by lip
roundedness (and infants' vision is still not fully mature at 6 months of age, cf. Johnson, 2011), and perceptual reorganisation might be
multisensory (Werker, 2018), more time might be necessary to develop robust discrimination. Future research, incorporating visual

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** (A) Negative association between native and non-native discrimination within the same infants (values >.50 indicates discrimination), and (B) correlation coefficients and their 95% confidence intervals as a function of minimum looking time inclusion.</figcaption></figure>
stimuli, might find a different result. Finally, as Norwegian has a rich vowel inventory, with a total of 18 vowels (or 19, if schwa is considered; Kristoffersen, 2000), and uses both vowel length and formant frequencies as contrastive cues, infants' perceptual narrowing might be slower-paced and with increased individual variation (Cheour et al., 1998; Paillereau et al., 2021), challenging, again, the classic view of a universal developmental timeline for perceptual tuning.

The lack of discrimination for the non-native /ʌ-æ/ contrast can similarly be attributed to the specificity of the tested contrast. While less so than the native /y-i/ contrast, it is also acoustically subtle (cf. Table 1) and closely positioned in the (Southern British) vowel space (Determining, 1997). We are not familiar with previous studies that have tested this contrast in infants, yet Baigorri and colleagues (2019) demonstrated that adult Spanish-English bilinguals had less accurate discrimination of /ʌ-æ/ compared to other English contrasts, whereas native English monolinguals performed at ceiling levels. This could be taken to suggests that discrimination of the contrast is influenced by linguistic experience (however, see Georgiou, 2019). Still, lack of discrimination of an additional, more acoustically salient contrast by our infants would have strengthened the argument that our null result is indicative of their desensitization to non-native contrasts, as predicted by perceptual narrowing theory, and not confounded by the acoustic subtlety of the contrast itself.

Our null results could also potentially be due to methodological issues. First, while our fully infant-controlled design with an eye-tracking device mitigated potential errors from manual coding, automatic eye-tracking has only recently been used for visual habituation procedures with infants (Bolhuis et al., 2016), and, to the best of our knowledge, has not been applied to vowel discrimination. We can only speculate on the impact that automatic eye-tracking could have had on our results. Potentially, fine-grained sampling of gaze behaviour could be overly sensitive to fluctuations in looking time that is not caused by induced habituation, or novelty preference in the test phase (see Hessels & Hooge, 2019 for a discussion on automatic eye-tracking with infants). Notably, highly sensitive measures such as electroencephalogram, that has been used to demonstrate discrimination of native and non-native speech contrasts in infants (Jansson-Verkasalo et al., 2010), typically use odd-ball paradigms, and not habituation procedures. Habituation might be more robust with “coarse” measures of infant behaviour, such as head turn-preference procedures (e.g., Polka & Bohn, 1996; see also The ManyBabies Consortium, 2020).

Second, to strike a desired balance between ecological validity, task difficulty and fatigue (given that infants would participate in both a native and non-native experiment), our experimental design featured multiple tokens of the habituated category, one token of the novel category, and only four test trials. The lack of variability of the novel category might have led to weak or insufficient activation of the corresponding phonological representations and relied more on acoustic rather than phonological comparison of speech sounds (Gerrits & Schouten, 2004), although our relatively long (1000 ms) interstimulus interval was chosen to buttress this possibility. In addition, the limited number of trials could have made our results more susceptible to random variation (although even fewer test trials have been used in infant habituation tasks; Singh et al., 2022a).

Third, we opted for a fixed order of native and non-native experiments, habituated and novel vowel type, and test trial presentation. This increased the comparability for individual variation between infants (see e.g., Pérez-Edgar et al., 2020; Cristia et al., 2016), and did not compromise the statistical power in our sample (which would have been reduced if we were to resolve order effects). Yet, use of fixed habituation vowel types (including the less frequent /y/ in the native experiment), and the reduced number of trials to habituate and the shorter looking time in the non-native (second) experiment, may have impacted infants' performance. However, we have no clear hypothesis on why a more efficient habituation, which is tailored for each infant, would reduce discrimination abilities.

Our exploratory analyses revealed a negative association between infants' native and non-native discrimination scores, a relationship that could be interpreted through the perceptual narrowing framework. That is, if we assume that perceptual narrowing occurs gradually and asymmetrically and take our group-level results as indicating that the non-native contrast is already “lost” at 6 months of age and that the native contrast is not yet acquired (due to its subtlety), the negative relationship could be capturing narrowing as it unfolds at the individual level. As in previous studies that used such within-infant scores derived from habituation paradigms (Bergmann & Cristia, 2018), we assume that the discrimination score (computed as the proportion of looking to the alternating trials versus total looking – alternating plus habituated trials – in the test phase) is a continuous index of infants' sensitivity to the vowel contrast. Consequently, this negative relationship may suggest that our sample is at the initial stages of perceptual reorganisation, where the perception of (subtle) native vowels is increasing, while that of non-native vowels is decreasing, aligning with the literature's suggested “tipping point” at 6 months of age (Tsuji & Cristia, 2014). Such an inverse relationship between performance in the two experiments (i.e., an imbalance in infants' discrimination scores, whereby lower discrimination scores in either the native or non-native experiment is paired with higher discrimination scores in the other) would likely not be expected if improved native and non-native vowel perception only follow infants' general cognitive abilities (as suggested by Mazuka et al., 2014) and is not influenced by their linguistic environment.

In conclusion, while the above linguistic and methodological aspects must be considered, our study provides novel insights on native and non-native vowel discrimination in a so-far understudied language and vowel contrasts, using automatised eye-tracking methodology, a relatively large sample and preregistered analyses. Our results – from the same group of infants that receive the same language input – are not indicative of native or non-native vowel discrimination at a group level at 6 months, thus aligning with other work that challenges the classic “universal phoneticians” account in early speech perception (Chládková & Paillereau, 2020, de Klerk et al., 2019; Kalashnikova et al., 2024; Kotera et al., 2024; Mazuka et al., 2014; Nam et al., 2024; Paillereau et al., 2021). Yet, our exploratory analyses could be taken to suggest that, at this age, Norwegian infants might be on the verge of tuning to the speech sounds of their native input. Given the so-far sparsity of evidence on Norwegian infants' speech perception, future studies are needed to corroborate this claim. We recommend replications with experimental designs whereby the native and non-native experiments, as well as the habituated and novel vowel within each experiment, are counterbalanced, and infants pre- and post-attention level is assessed. Moreover, the association between native and non-native discrimination should be investigated in different age groups (for example
with older infants that are expected to have fully undergone perceptual narrowing), as well as with more socioeconomic-heterogeneous and gender-balanced samples. Ultimately, our study highlights that, to improve our understanding of perceptual narrowing, future studies are advised to expand its investigation to less-studied linguistic and cultural contexts (Singh et al., 2022b).

### CRediT authorship contribution statement

Audun Rosslund Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Visualization, Writing – original draft, Writing – review & editing. Julien Mayor Conceptualization, Methodology, Supervision, Writing – review & editing. Alejandrina Cristia Conceptualization, Methodology, Writing – review & editing. Natalia Kartushina Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Writing – review & editing.

### Declaration of Competing Interest

The authors declare none.

### Data Availability

Data is shared on OSF (https://osf.io/edf42).

### Acknowledgments

We thank Ada Koleini for her help with data collection and all families for their participation. AR and NK were supported by funding from the Research Council of Norway (project numbers 223265 and 301625). AC was supported by the J. S. McDonnell Foundation Understanding Human Cognition Scholar Award and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (ExELang, Grant agreement No. 101001095).

## Appendix 1

Model estimates, together with standard errors and confidence limits for infants’ log-transformed looking time by trial type for each contrast.

<table><thead><tr><th>contrast</th><th>term</th><th>estimate</th><th>SE</th><th>lower CI</th><th>upper CI</th></tr></thead><tbody><tr><td rowspan="2">native</td><td>intercept</td><td>8.301</td><td>0.070</td><td>8.163</td><td>8.440</td></tr><tr><td>trial type<sup>(1)</sup></td><td>0.059</td><td>0.069</td><td>-0.077</td><td>0.196</td></tr><tr><td rowspan="2">non-native</td><td>intercept</td><td>8.228</td><td>0.076</td><td>8.078</td><td>8.377</td></tr><tr><td>trial type<sup>(1)</sup></td><td>0.062</td><td>0.078</td><td>-0.092</td><td>0.217</td></tr></tbody></table>

Note. (1) dummy coded with “habituated” being the reference category

## Appendix 2

Distribution of n = 702 p-values (ranked from low to high) from a multiverse analysis (Steegen et al., 2016), evaluating the significance (dashed line indicates p = 0.05) of trial type (and any interactions it was involved in) on infants’ looking time across 160 plausible (non-preregistered) model structures, that were run as nested loops by including or not infants’ age, gender, and maternal education as fixed effects and in interaction with trial type, and different looking time inclusion criteria (850 ms up to 1500 ms, in 50 ms increments), across the native and non-native experiments. The distribution shows that very few p-values (1.14 %) were significant – less than what would be expected from chance (5 %, or 1 in 20), indicating that our null-results were robust to variable model complexity or inclusion criteria
<figure><img src="image_8.png" /></figure>

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