Lack of early sensitivity and gradual emergence of native phoneme categories: A pattern from underrepresented language learners.

Perceptual narrowing of speech perception supposes that young infants can discriminate most speech sounds early in life. During the second half of the first year, infants' phonetic sensitivity is attuned to their native phonology. However, supporting evidence for this pattern comes primarily from learners from a limited number of regions and languages. Very little evidence has accumulated on infants learning languages spoken in Asia, which accounts for most of the world's population. The present study examined the developmental trajectory of Korean-learning infants' sensitivity to a native stop contrast during the first year of life. The Korean language utilizes unusual voiceless three-way stop categories, requiring target categories to be derived from tight phonetic space. Further, two of these categories-lenis and aspirated-have undergone a diachronic change in recent decades as the primary acoustic cue for distinction has shifted among modern speakers. Consequently, the input distributions of these categories are mixed across speakers and speech styles, requiring learners to build flexible representations of target categories along these variations. The results showed that among the three age groups-4-6 months, 7-9 months, and 10-12 months-we tested, only 10-12-month-olds showed weak sensitivity to the two categories, suggesting that robust discrimination is not in place by the end of the first year. The study adds scarcely represented data, lending additional support for the lack of early sensitivity and prolonged emergence of native phonology that are inconsistent with learners of predominant studies and calls for more diverse samples to verify the generality of the typical perceptual narrowing pattern. RESEARCH HIGHLIGHTS: We investigated Korean-learning infants' developmental trajectory of native phoneme categories and whether they show the typical perceptual narrowing pattern. Robust discrimination did not appear until 12 months, suggesting that Korean infants' native phonology is not stabilized by the end of the first year. The prolonged emergence of sensitivity could be due to restricted phonetic space and input variations but suggests the possibility of a different developmental trajectory. The current study contributes scarcely represented Korean-learning infants' phonetic discrimination data to the speech development field.

Sensitivity to visual cues within motion events in monolingual and bilingual infants.

It is well known that infants undergo developmental change in how they respond to language-relevant visual contrasts. For example, when viewing motion events, infants' sensitivities to background information ("ground-path cues," e.g., whether a background is flat and continuous or bounded) change with age. Prior studies with English and Japanese monolingual infants have demonstrated that 14-month-old infants discriminate between motion events that take place against different ground-paths (e.g., an unbounded field vs a bounded street). By 19 months of age, this sensitivity becomes more selective in monolingual infants; only learners of languages that lexically contrast these categories, such as Japanese, discriminate between such events. In this study, we investigated this progression in bilingual infants. We first replicated past reports of an age-related decline in ground-path sensitivity from 14 to 19 months in English monolingual infants living in a multilingual society. English-Mandarin bilingual infants living in that same society were then tested on discrimination of ground-path cues at 14, 19, and 24 months. Although neither the English nor Mandarin language differentiates motion events based on ground-path cues, bilingual infants demonstrated protracted sensitivity to these cues. Infants exhibited a lack of discrimination at 14 months, followed by discrimination at 19 months and a subsequent decline in discrimination at 24 months. In addition, bilingual infants demonstrated more fine-grained sensitivities to subtle ground cues not observed in monolingual infants.

Reduced perceptual narrowing in synesthesia.

Synesthesia is a neurologic trait in which specific inducers, such as sounds, automatically elicit additional idiosyncratic percepts, such as color (thus "colored hearing"). One explanation for this trait-and the one tested here-is that synesthesia results from unusually weak pruning of cortical synaptic hyperconnectivity during early perceptual development. We tested the prediction from this hypothesis that synesthetes would be superior at making discriminations from nonnative categories that are normally weakened by experience-dependent pruning during a critical period early in development-namely, discrimination among nonnative phonemes (Hindi retroflex /d̪a/ and dental /ɖa/), among chimpanzee faces, and among inverted human faces. Like the superiority of 6-mo-old infants over older infants, the synesthetic groups were significantly better than control groups at making all the nonnative discriminations across five samples and three testing sites. The consistent superiority of the synesthetic groups in making discriminations that are normally eliminated during infancy suggests that residual cortical connectivity in synesthesia supports changes in perception that extend beyond the specific synesthetic percepts, consistent with the incomplete pruning hypothesis.

Effects of interracial experience on the race preferences of infants.

Over their first year of life, infants express visual preferences for own- versus other-race faces. This developmental transition has primarily been investigated in monoracial societies where infants have limited personal or societal contact with other races. We investigated whether previously reported visual preferences for race generalize to a multiracial society (i.e., Singapore). In addition, we investigated effects of caregiver race on visual preferences for race. In Experiment 1, race preferences were measured at 3, 6, and 9 months of age for own-race (Chinese) versus other-race (Indian) faces in infants with no regular interaction with Indian-race individuals. Singaporean infants displayed a significant visual preference for Indian-race faces at each age group. Furthermore, infants raised with other-race caregivers demonstrated an age-related increase in other-race visual preferences. The visual preferences of infants for other-race faces were predicted by the extent of other-race contact. In Experiment 2, we confirmed that an other-race visual preference was not exclusive to Indian faces in a sample of 6-month-old Singaporean Chinese infants who demonstrated a similar other-race visual preference for Caucasian faces over Chinese faces. Findings are discussed in terms of the influence of other-race contact on visual preferences for race in infants.

Infants' visual exploration strategies for adult and child faces.

By the end of the first year of life, infants' discrimination abilities tune to frequently experienced face groups. Little is known about the exploration strategies adopted to efficiently discriminate frequent, familiar face types. The present eye-tracking study examined the distribution of visual fixations produced by 10-month-old and 4-month-old singletons while learning adult (i.e., familiar) and child (i.e., unfamiliar) White faces. Infants were tested in an infant-controlled visual habituation task, in which post-habituation preference measured successful discrimination. Results confirmed earlier evidence that, without sibling experience, 10-month-olds discriminate only among adult faces. Analyses of gaze movements during habituation showed that infants' fixations were centered in the upper part of the stimuli. The mouth was sampled longer in adult faces than in child faces, while the child eyes were sampled longer and more frequently than the adult eyes. At 10 months, but not at 4 months, global measures of scanning behavior on the whole face also varied according to face age, as the spatiotemporal distribution of scan paths showed larger within- and between-participants similarity for adult faces than for child faces. Results are discussed with reference to the perceptual narrowing literature, and the influence of age-appropriate developmental tasks on infants' face processing abilities.

Selectivity in bilingual nonspeech label learning.

This research investigates selectivity in word learning for bilingual infants. Previous work demonstrated that bilingual infants show greater openness to non-native language sounds in object labels than monolinguals (Hay et al., 2015; Singh, 2018). It remains unclear whether bilingual openness extends to nonspeech sounds. We presented 14- and 19-month-old bilinguals with object labels consisting of nonspeech tones. Monolinguals recently displayed learning of the same labels at 14 months, but not 19 months (Graf Estes et al., 2018). In contrast, bilinguals failed to learn the labels. We propose that hearing phonological variation across two languages helps bilinguals reject nonspeech word forms.

Diverse sensory cues for individual recognition.

To be social, the ability to recognize and discriminate conspecific individuals is indispensable in social animals, including primates, rodents, birds, fish, and social insects which live in societies or groups. Recent studies using molecular biology, genetics, in vivo and in vitro physiology, and behavioral neuroscientific approaches have provided detailed insights into how animals process and recognize the information of individuals. Here, we review the most distinct sensory modalities for individual recognition in animals, namely, olfaction and vision. In the case of rodents, two polymorphic gene complexes have been identified in their urine as the key and essential pheromonal components for individual recognition: the major histocompatibility complex (MHC) and the major urinary protein (MUP). Animals flexibly utilize MHC and/or MUP, which are detected by the main olfactory epithelium (MOE) and/or the vomeronasal organ (VNO) for various types of social recognition, such as strain recognition, kin recognition, and individual recognition. In contrast, primates, including humans, primarily use facial appearance to identify others. Face recognition in humans and other animals is naturally unique from genetic, cognitive, developmental, and functional points of view. Importantly note that nurture effects during growth phase such as social experience and environment can also shape and tune this special cognitive ability, in order to distinguish subtle differences between individuals. In this review, we address such unique nature and nurture mechanisms for individual recognition.

Narrowing in face and speech perception in infancy: Developmental change in the relations between domains.

Although prior research has established that perceptual narrowing reflects the influence of experience on the development of face and speech processing, it is unclear whether narrowing in the two domains is related. A within-participant design (N = 72) was used to investigate discrimination of own- and other-race faces and native and non-native speech sounds in 3-, 6-, 9-, and 12-month-old infants. For face and speech discrimination, whereas 3-month-olds discriminated own-race faces and native speech sounds as well as other-race faces and non-native speech sounds, older infants discriminated only own-race faces and native speech sounds. Narrowing in face and narrowing in speech were not correlated at 6 months, negatively correlated at 9 months, and positively correlated at 12 months. The findings reveal dynamic developmental changes in the relation between modalities during the first year of life.

Neural sensitivity to natural texture statistics in infancy.

During infancy, vision becomes tuned to environmental statistics. For example, infant face recognition "narrows" in response to the frequency of face categories in the visual world, inducing out-group effects that disadvantage other-race, other-species, and other-age face recognition. There are many other low-level statistical regularities in visual experience that infants may also become tuned to during this period. In particular, natural scenes have lawful properties that adults and children are sensitive to. To what extent do infants become tuned to these regularities during the first year of life? In particular, do infants exhibit evidence of perceptual narrowing that excludes atypical images from fluent processing? We examined this question by measuring 6- and 9-month-old infants' event-related potentials (ERPs) to natural and artificial textures created by: (a) Disrupting local statistics via contrast negation, (b) Disrupting global statistics via parametric texture synthesis, or (c) both of these. We predicted that younger infants' would be sensitive to both manipulations of natural appearance, but that older infants might not distinguish between different kinds of atypical images. Instead, we found that sensitivity to synthetic appearance is only evident late in infancy. We discuss what these results imply for our understanding of visual statistical learning in infancy.

Sibling experience modulates perceptual narrowing toward adult faces in the first year of life.

During the first year of life face discrimination abilities narrow toward adult human faces of the most frequently encountered ethnic group/s. Earlier studies showed that perceptual learning under laboratory-training protocols can modulate this narrowing process. Here we investigated whether natural experience acquired in everyday settings with an older sibling's face can shape the trajectory of perceptual narrowing towards adult faces. Using an infant-controlled habituation procedure we measured discrimination of adult (Experiment 1) and child faces (Experiment 2) in 3- and 9- month-old infants with and without a child sibling. Discrimination of adult faces was observed for infants at both ages, although accompanied by posthabituation preferences in opposite directions, whereas at both ages the discrimination of child faces critically depended on sibling experience. These results provide the first evidence that natural experience acquired with siblings affects the tuning properties of infant face representation.

Oscillatory Dynamics Underlying Perceptual Narrowing of Native Phoneme Mapping from 6 to 12 Months of Age.

During the first months of life, human infants process phonemic elements from all languages similarly. However, by 12 months of age, as language-specific phonemic maps are established, infants respond preferentially to their native language. This process, known as perceptual narrowing, supports neural representation and thus efficient processing of the distinctive phonemes within the sound environment. Although oscillatory mechanisms underlying processing of native and non-native phonemic contrasts were recently delineated in 6-month-old infants, the maturational trajectory of these mechanisms remained unclear. A group of typically developing infants born into monolingual English families, were followed from 6 to 12 months and presented with English and Spanish syllable contrasts varying in voice-onset time. Brain responses were recorded with high-density electroencephalogram, and sources of event-related potential generators identified at right and left auditory cortices at 6 and 12 months and also at frontal cortex at 6 months. Time-frequency analyses conducted at source level found variations in both θ and γ ranges across age. Compared with 6-month-olds, 12-month-olds' responses to native phonemes showed smaller and faster phase synchronization and less spectral power in the θ range, and increases in left phase synchrony as well as induced high-γ activity in both frontal and left auditory sources. These results demonstrate that infants become more automatized and efficient in processing their native language as they approach 12 months of age via the interplay between θ and γ oscillations. We suggest that, while θ oscillations support syllable processing, γ oscillations underlie phonemic perceptual narrowing, progressively favoring mapping of native over non-native language across the first year of life. SIGNIFICANCE STATEMENT: During early language acquisition, typically developing infants gradually construct phonemic maps of their native language in auditory cortex. It is well known that, by 12 months of age, human infants move from universal discrimination of most linguistic phonemic contrasts to phonemic expertise in their native language. This perceptual narrowing occurs at the expense of the ability to process non-native phonemes. However, the neural mechanisms underlying this process are still poorly understood. Here we demonstrate that perceptual narrowing is, at least in part, accomplished by decreasing power and phase coherence in the θ range while increasing activity in high-γ in left auditory cortex. Understanding the normative neural mechanisms that support early language acquisition is crucial to understanding and perhaps ameliorating developmental language disorders.

Visual selective attention biases contribute to the other-race effect among 9-month-old infants.

During the first year of life, infants maintain their ability to discriminate faces from their own race but become less able to differentiate other-race faces. Though this is likely due to daily experience with own-race faces, the mechanisms linking repeated exposure to optimal face processing remain unclear. One possibility is that frequent experience with own-race faces generates a selective attention bias to these faces. Selective attention elicits enhancement of attended information and suppression of distraction to improve visual processing of attended objects. Thus attention biases to own-race faces may boost processing and discrimination of these faces relative to other-race faces. We used a spatial cueing task to bias attention to own- or other-race faces among Caucasian 9-month-old infants. Infants discriminated faces in the focus of the attention bias, regardless of race, indicating that infants remained sensitive to differences among other-race faces. Instead, efficacy of face discrimination reflected the extent of attention engagement.

Can human eyes prevent perceptual narrowing for monkey faces in human infants?

Perceptual narrowing has been observed in human infants for monkey faces: 6-month-olds can discriminate between them, whereas older infants from 9 months of age display difficulty discriminating between them. The difficulty infants from 9 months have processing monkey faces has not been clearly identified. It could be due to the structural characteristics of monkey faces, particularly the key facial features that differ from human faces. The current study aimed to investigate whether the information conveyed by the eyes is of importance. We examined whether the presence of Caucasian human eyes in monkey faces allows recognition to be maintained in 6-month-olds and facilitates recognition in 9- and 12-month-olds. Our results revealed that the presence of human eyes in monkey faces maintains recognition for those faces at 6 months of age and partially facilitates recognition of those faces at 9 months of age, but not at 12 months of age. The findings are interpreted in the context of perceptual narrowing and suggest that the attenuation of processing of other-species faces is not reversed by the presence of human eyes.

Perceptual narrowing: retrospect and prospect.

Research is reviewed demonstrating perceptual narrowing across a variety of domains. Research is also reviewed showing that the temporal window of perceptual narrowing can be extended and, in some cases, perceptual narrowing can be reversed. Research is also reviewed highlighting the neurophysiological correlates of perceptual narrowing as well as some of the individual neurophysiological differences associated with perceptual narrowing. Various methodological issues associated with perceptual narrowing are also discussed. The broader purpose of this paper, however, is to argue that the term perceptual narrowing fails to capture the dynamic nature of this perceptual process. Finally, it is argued that just as other concepts associated with experience and development are refined and modified as new evidence emerges, likewise we need to evaluate and refine how we conceptualize perceptual narrowing.

Age-related face processing bias in infancy: evidence of perceptual narrowing for adult faces.

Recent data demonstrate a perceptual processing advantage for adult faces in both adults and young children, suggesting that face representation is shaped by visual experience accumulated with different face-age groups. As for species and race, this age bias may emerge during the first year of life as part of the general process of perceptual narrowing, given the extensive amount of social and perceptual experience accumulated with caregivers and/or other adult individuals. Using infant-controlled habituation and visual-paired comparison at test, two experiments were carried out to examine 3- and 9-month-olds' ability to discriminate within adult and infant faces. Results showed that, when they are provided with adequate time to visually compare the stimuli during test trials (Experiment 2), 3-month-olds exhibit above-chance discrimination of adult and infant faces. Instead, 9-month-olds discriminate adult faces but not infant faces (Experiments 1 and 2). Results provide the first evidence of age-related face processing biases in infancy, and show that by 9 months face representations tune to adult human faces.

Own- and other-race face scanning in infants: implications for perceptual narrowing.

The present study investigated how 6- and 9-month-old Caucasian infants scan Caucasian and Chinese dynamic faces using eye-tracking methodology. Analyses of looking times revealed that with increased age, infants decreased their looking time to other-race noses, while maintaining their looking time for own-race noses. From 6 to 9 months, infants increased their looking time for the eyes of both races of faces. Analyses of scan paths showed that infants were no more likely to shift their fixation between the eyes of own-race faces than other-race faces. Similarity between participants' scan paths suggested that facial information was collected more efficiently for own- versus other-race faces at 9 months of age. Combined with previous eye-tracking studies of infants' face scanning (Liu et al. [2011] Journal of Experimental Child Psychology, 108, 180-189; Wheeler et al. [2011] PLoS ONE, 6, e18621. doi: 10.1371/journal.pone.0018621; Xiao et al. [2013] International Journal of Behavioral Development, 37, 100-105), the findings are interpreted in the context of perceptual narrowing and suggest differential contributions of visual experience, facial physiognomy, and culture in accounting for similarity and difference in infants scanning of own- and other-race faces.

Development of non-native vowel discrimination: Improvement without exposure.

The present study tested Japanese 4.5- and 10-month old infants' ability to discriminate three German vowel pairs, none of which are contrastive in Japanese, using a visual habituation-dishabituation paradigm. Japanese adults' discrimination of the same pairs was also tested. The results revealed that Japanese 4.5-month old infants discriminated the German /bu:k/-/by:k/ contrast, but they showed no evidence of discriminating the /bi:k/-/be:k/ or /bu:k/-/bo:k/ contrasts. Japanese 10-month old infants, on the other hand, discriminated the German /bi:k/-/be:k/ contrast, while they showed no evidence of discriminating the /bu:k/-/by:k/ or /bu:k/-/bo:k/ contrasts. Japanese adults, in contrast, were highly accurate in their discrimination of all of the pairs. The results indicate that discrimination of non-native contrasts is not always easy even for young infants, and that their ability to discriminate non-native contrasts can improve with age even when they receive no exposure to a language in which the given contrast is phonemic.

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

The perceptual assimilation model (PAM; Best, C. T. [1995]. A direct realist view of cross-language speech perception. In W. Strange (Ed.), Speech perception and linguistic experience: Issues in cross-language research (pp. 171-204). Baltimore, MD: York Press.) accounts for developmental patterns of speech contrast discrimination by proposing that infants shift from untuned phonetic perception at 6 months to natively tuned perceptual assimilation at 11-12 months, but the model does not predict initial discrimination differences among contrasts. To address that issue, we evaluated the Articulatory Organ Hypothesis, which posits that consonants produced using different articulatory organs are initially easier to discriminate than those produced with the same articulatory organ. We tested English-learning 6- and 11-month-olds' discrimination of voiceless fricative place contrasts from Nuu-Chah-Nulth (non-native) and English (native), with one within-organ and one between-organ contrast from each language. Both native and non-native contrasts were discriminated across age, suggesting that articulatory-organ differences do not influence perception of speech contrasts by young infants. The results highlight the fact that a decline in discrimination for non-native contrasts does not always occur over age.

Learning to differentiate individuals by their voices: Infants' individuation of native- and foreign-species voices.

The ability to discriminate and identify people by their voice is important for social interaction in humans. In early development, learning to discriminate important differences in a number of socially relevant stimuli, such as phonemes and faces, has been shown to follow a common pattern of experience-driven perceptual narrowing, where the discrimination of native stimuli improves, while the discrimination of foreign stimuli worsens. The aim of the present study was to investigate whether similar perceptual narrowing occurs for discriminating individuals by voice. We tested the ability of English-speaking adults and English-learning 6- and 12-month-olds to discriminate either native-species (human) or foreign-species (rhesus monkey [Macaca mulatta]) individuals by their vocalizations. Between 6 and 12 months of age, the ability to discriminate monkey voices decreased significantly and there was a non-significant trend for improved ability to discriminate human voices. The results support the hypothesis of experience-driven perceptual narrowing for voice individuation during the first year after birth.

Infant perceptual development for faces and spoken words: an integrated approach.

There are obvious differences between recognizing faces and recognizing spoken words or phonemes that might suggest development of each capability requires different skills. Recognizing faces and perceiving spoken language, however, are in key senses extremely similar endeavors. Both perceptual processes are based on richly variable, yet highly structured input from which the perceiver needs to extract categorically meaningful information. This similarity could be reflected in the perceptual narrowing that occurs within the first year of life in both domains. We take the position that the perceptual and neurocognitive processes by which face and speech recognition develop are based on a set of common principles. One common principle is the importance of systematic variability in the input as a source of information rather than noise. Experience of this variability leads to perceptual tuning to the critical properties that define individual faces or spoken words versus their membership in larger groupings of people and their language communities. We argue that parallels can be drawn directly between the principles responsible for the development of face and spoken language perception.