Vowels, then consonants: Early bias switch in recognizing segmented word forms

Infants' abilities to segment word forms from spectrally degraded speech in the first year of life

Infants begin to segment word forms from fluent speech-a crucial task in lexical processing-between 4 and 7 months of age. Prior work has established that infants rely on a variety of cues available in the speech signal (i.e., prosodic, statistical, acoustic-segmental, and lexical) to accomplish this task. In two experiments with French-learning 6- and 10-month-olds, we use a psychoacoustic approach to examine if and how degradation of the two fundamental acoustic components extracted from speech by the auditory system, namely, temporal (both frequency and amplitude modulation) and spectral information, impact word form segmentation. Infants were familiarized with passages containing target words, in which frequency modulation (FM) information was replaced with pure tones using a vocoder, while amplitude modulation (AM) was preserved in either 8 or 16 spectral bands. Infants were then tested on their recognition of the target versus novel control words. While the 6-month-olds were unable to segment in either condition, the 10-month-olds succeeded, although only in the 16 spectral band condition. These findings suggest that 6-month-olds need FM temporal cues for speech segmentation while 10-month-olds do not, although they need the AM cues to be presented in enough spectral bands (i.e., 16). This developmental change observed in infants' sensitivity to spectrotemporal cues likely results from an increase in the range of available segmentation procedures, and/or shift from a vowel to a consonant bias in lexical processing between the two ages, as vowels are more affected by our acoustic manipulations. RESEARCH HIGHLIGHTS: Although segmenting speech into word forms is crucial for lexical acquisition, the acoustic information that infants' auditory system extracts to process continuous speech remains unknown. We examined infants' sensitivity to spectrotemporal cues in speech segmentation using vocoded speech, and revealed a developmental change between 6 and 10 months of age. We showed that FM information, that is, the fast temporal modulations of speech, is necessary for 6- but not 10-month-old infants to segment word forms. Moreover, reducing the number of spectral bands impacts 10-month-olds' segmentation abilities, who succeed when 16 bands are preserved, but fail with 8 bands.

The influence of language-specific properties on the role of consonants and vowels in a statistical learning task of an artificial language: A cross-linguistic comparison

The contribution of consonants and vowels in spoken word processing has been widely investigated, and studies have found a phenomenon of a Consonantal bias (C-bias), indicating that consonants carry more weight than vowels. However, across languages, various patterns have been documented, including that of no preference or a reverse pattern of Vowel bias. A central question is how the manifestation of the C-bias is modulated by language-specific factors. This question can be addressed by cross-linguistic studies. Comparing native Hebrew and native English speakers, this study examines the relative importance of transitional probabilities between non-adjacent consonants as opposed to vowels during auditory statistical learning (SL) of an artificial language. Hebrew is interesting because its complex Semitic morphological structure has been found to play a central role in lexical access, allowing us to examine whether morphological properties can modulate the C-bias in early phases of speech perception, namely, word segmentation. As predicted, we found a significant interaction between language and consonant/vowel manipulation, with a higher performance in the consonantal condition than in the vowel condition for Hebrew speakers, namely, C-bias, and no consonant/vowel asymmetry among English speakers. We suggest that the observed interaction is morphologically anchored, indicating that phonological and morphological processes interact during early phases of auditory word perception.

An auditory perspective on phonological development in infancy

Introduction

The auditory system encodes the phonetic features of languages by processing spectro-temporal modulations in speech, which can be described at two time scales: relatively slow amplitude variations over time (AM, further distinguished into the slowest <8-16 Hz and faster components 16-500 Hz), and frequency modulations (FM, oscillating at higher rates about 600-10 kHz). While adults require only the slowest AM cues to identify and discriminate speech sounds, infants have been shown to also require faster AM cues (>8-16 Hz) for similar tasks.

Methods

Using an observer-based psychophysical method, this study measured the ability of typical-hearing 6-month-olds, 10-month-olds, and adults to detect a change in the vowel or consonant features of consonant-vowel syllables when temporal modulations are selectively degraded. Two acoustically degraded conditions were designed, replacing FM cues with pure tones in 32 frequency bands, and then extracting AM cues in each frequency band with two different low-pass cut- off frequencies: (1) half the bandwidth (Fast AM condition), (2) <8 Hz (Slow AM condition).

Results

In the Fast AM condition, results show that with reduced FM cues, 85% of 6-month-olds, 72.5% of 10-month-olds, and 100% of adults successfully categorize phonemes. Among participants who passed the Fast AM condition, 67% of 6-month-olds, 75% of 10-month-olds, and 95% of adults passed the Slow AM condition. Furthermore, across the three age groups, the proportion of participants able to detect phonetic category change did not differ between the vowel and consonant conditions. However, age-related differences were observed for vowel categorization: while the 6- and 10-month-old groups did not differ from one another, they both independently differed from adults. Moreover, for consonant categorization, 10-month-olds were more impacted by acoustic temporal degradation compared to 6-month-olds, and showed a greater decline in detection success rates between the Fast AM and Slow AM conditions.

Discussion

The degradation of FM and faster AM cues (>8 Hz) appears to strongly affect consonant processing at 10 months of age. These findings suggest that between 6 and 10 months, infants show different developmental trajectories in the perceptual weight of speech temporal acoustic cues for vowel and consonant processing, possibly linked to phonological attunement.

Consonant bias in adult lexical processing under acoustically degraded listening conditions

Consonants facilitate lexical processing across many languages, including French. This study investigates whether acoustic degradation affects this phonological bias in an auditory lexical decision task. French words were processed using an eight-band vocoder, degrading their frequency modulations (FM) while preserving original amplitude modulations (AM). Adult French natives were presented with these French words, preceded by similarly processed pseudoword primes sharing their vowels, consonants, or neither. Results reveal a consonant bias in the listeners' accuracy and response times, despite the reduced spectral and FM information. These degraded conditions resemble current cochlear-implant processors, and attest to the robustness of this phonological bias.

Maturation of consonant perception, but not vowel perception, predicts lexical skills at 12 months

Consonants and vowels differentially contribute to lexical acquisition. From 8 months on, infants' preferential reliance on consonants has been shown to predict their lexical outcome. Here, the predictive value of German-learning infants' (n = 58, 29 girls, 29 boys) trajectories of consonant and vowel perception, indicated by the electrophysiological mismatch response, across 2, 6, and 10 months for later lexical acquisition was studied. The consonant-perception trajectory from 2 to 6 months (β = -2.95) and 6 to 10 months (β = -.91), but not the vowel-perception trajectory, significantly predicted receptive vocabulary at 12 months. These results reveal an earlier predictive value of consonant perception for word learning than previously found, and a particular role of the longitudinal maturation of this skill in lexical acquisition.

Constraints on infants' ability to extract non-adjacent dependencies from vowels and consonants

Language acquisition requires infants' ability to track dependencies between distant speech elements. Infants as young as 3 months have been shown to successfully identify such non-adjacent dependencies between syllables, and this ability has been related to the maturity of infants' pitch processing. The present study tested whether 8- to 10-month-old infants (N = 68) can also learn dependencies at smaller segmental levels and whether the relation between dependency and pitch processing extends to other auditory features. Infants heard either syllable sequences encoding an item-specific dependency between non-adjacent vowels or between consonants. These frequent standard sequences were interspersed with infrequent intensity deviants and dependency deviants, which violated the non-adjacent relationship. Both vowel and consonant groups showed electrophysiological evidence for detection of the intensity manipulation. However, evidence for dependency learning was only found for infants hearing the dependencies across vowels, not consonants, and only in a subgroup of infants who had an above-average language score in a behavioral test. In a correlation analysis, we found no relation between intensity and dependency processing. We conclude that item-specific, segment-based non-adjacent dependencies are not easily learned by infants and if so, vowels are more accessible to the task, but only to infants who display advanced language skills.

Longitudinal trajectories of electrophysiological mismatch responses in infant speech discrimination differ across speech features

Infants rapidly advance in their speech perception, electrophysiologically reflected in the transition from an immature, positive-going to an adult-like, negative-going mismatch response (MMR) to auditory deviancy. Although the MMR is a common tool to study speech perception development, it is not yet completely understood how different speech contrasts affect the MMR’s characteristics across development. Thus, a systematic longitudinal investigation of the MMR’s maturation depending on speech contrast is necessary. We here longitudinally explored the maturation of the infant MMR to four critical speech contrasts: consonant, vowel, vowel-length, and pitch. MMRs were obtained when infants (n = 58) were 2, 6 and 10 months old. To evaluate the maturational trajectory of MMRs, we applied second-order latent growth curve models. Results showed positive-going MMR amplitudes to all speech contrasts across all assessment points that decreased over time towards an adult-like negativity. Notably, the developmental trajectories of speech contrasts differed, implying that infant speech perception matures with different rates and trajectories throughout the first year, depending on the studied auditory feature. Our results suggest that stimulus-dependent maturational trajectories need to be considered when drawing conclusions about infant speech perception development reflected by the infant MMR.

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Understanding speech perception development requires longitudinal investigations.

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The Mismatch Response (MMR) is valuable to study infants’ speech perception.

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We investigated the infant MMR longitudinally across three assessment points.

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MMRs to different speech features became more mature during infants’ first year.

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Different MMR trajectories imply feature-dependent maturation of speech perception.

Infants exploit vowels to label objects and actions from continuous audiovisual stimuli

Before the 6-months of age, infants succeed to learn words associated with objects and actions when the words are presented isolated or embedded in short utterances. It remains unclear whether such type of learning occurs from fluent audiovisual stimuli, although in natural environments the fluent audiovisual contexts are the default. In 4 experiments, we evaluated if 8-month-old infants could learn word-action and word-object associations from fluent audiovisual streams when the words conveyed either vowel or consonant harmony, two phonological cues that benefit word learning near 6 and 12 months of age, respectively. We found that infants learned both types of words, but only when the words contained vowel harmony. Because object- and action-words have been conceived as rudimentary representations of nouns and verbs, our results suggest that vowels contribute to shape the initial steps of the learning of lexical categories in preverbal infants.

When Too Many Vowels Impede Language Processing: An Eye-Tracking Study of Danish-Learning Children

Research has suggested that Danish-learning children lag behind in early language acquisition. The phenomenon has been attributed to the opaque phonetic structure of Danish, which features an unusually large number of non-consonantal sounds (i.e., vowels and semivowels/glides). The large number of vocalic sounds in speech is thought to provide fewer cues to word segmentation and to make language processing harder, thus hindering the acquisition process. In this study, we explored whether the presence of vocalic sounds at word boundaries impedes real-time speech processing in 24-month-old Danish-learning children, compared to word boundaries that are marked by consonantal sounds. Using eye-tracking, we tested children's real-time comprehension of known consonant-initial and vowel-initial words when presented in either a consonant-final carrier phrase or in a vowel-final carrier phrase, thus resulting in the four boundary types C#C, C#V, V#C, and V#V. Our results showed that the presence of vocalic sounds around a word boundary-especially before-impedes processing of Danish child-directed sentences.

Variation in phonological bias: Bias for vowels, rather than consonants or tones in lexical processing by Cantonese-learning toddlers

Consonants and vowels have been considered to fulfill different functions in language processing, vowels being more important for prosodic and syntactic processes and consonants for lexically related processes (Nespor, Peña, & Mehler, 2003). This C-bias hypothesis in lexical processing is supported by studies with adults and infants in many languages such as English, French, Spanish, although a few studies, on Danish and Mandarin, suggest the existence of cross-linguistic variation. The present study explores whether a C-bias exists in a tone language with a complex tone system, Cantonese, by comparing the relative weight given to consonants, vowels, and also tones during word learning. To do so, looking behaviors of Cantonese-learning 20- and 30-month-olds (24 children per age/condition, 6 groups) were recorded by an eyetracker while they watched animated cartoons in Cantonese to learn pairs of novel words. The words differed minimally by either a consonant (e.g., /tœ6/ vs. /kœ6/), a vowel (e.g., /k^him3/ vs. /k^hɛm3/), or a tone (e.g., T2 vs. T5). Analyses on proportional looking times revealed significant learning in 30-month-olds only, and at that age, only for the vowel contrasts. Growth curve analyses revealed better performance for the vowel condition compared to the other two conditions. The present findings establish a V-bias in Cantonese-learning 30-month-olds, adding new evidence from that tone language that the C-bias in lexical processing is not language-general. Implications for theoretical discussions on the origins of this phonological bias, and the impact of tones in early language acquisition, are discussed.

The role of linguistic experience in the development of the consonant bias

Consonants and vowels play different roles in speech perception: listeners rely more heavily on consonant information rather than vowel information when distinguishing between words. This reliance on consonants for word identification is the consonant bias Nespor et al. (Ling 2:203-230, 2003). Several factors modulate infants' development of the consonant bias, including fine-grained temporal processing ability and native language exposure [for review, see Nazzi et al. (Curr Direct Psychol Sci 25:291-296, 2016)]. A rat model demonstrated that mature fine-grained temporal processing alone cannot account for consonant bias emergence; linguistic exposure is also necessary Bouchon and Toro (An Cog 22:839-850, 2019). This study tested domestic dogs, who have similarly fine-grained temporal processing but more language exposure than rats, to assess whether a minimal lexicon and small degree of regular linguistic exposure can allow for consonant bias development. Dogs demonstrated a vowel bias rather than a consonant bias, preferring their own name over a vowel-mispronounced version of their name, but not in comparison to a consonant-mispronounced version. This is the pattern seen in young infants Bouchon et al. (Dev Sci 18:587-598, 2015) and rats Bouchon et al. (An Cog 22:839-850, 2019). In a follow-up study, dogs treated a consonant-mispronounced version of their name similarly to their actual name, further suggesting that dogs do not treat consonant differences as meaningful for word identity. These results support the findings from Bouchon and Toro (An Cog 2:839-850, 2019), suggesting that there may be a default preference for vowel information over consonant information when identifying word forms, and that the consonant bias may be a human-exclusive tool for language learning.

Emergence of a consonant bias during the first year of life: New evidence from own-name recognition

Recent evidence suggests that during the first year of life, a preference for consonant information during lexical processing (consonant bias) emerges, at least for some languages like French. Our study investigated the factors involved in this emergence as well as the developmental consequences for variation in consonant bias emergence. In a series of experiments, we measured 5-, 8-, and 11-month-old French-learning infants orientation times to a consonant or vowel mispronunciation of their own name, which is one of the few word forms familiar to infants at this young age. Both 5- and 8-month-olds oriented longer to vowel mispronunciations, but 11-month-olds showed a different pattern, initially orienting longer to consonant mispronunciations. We interpret these results as further evidence of an initial vowel bias, with consonant bias emergence by 11 months. Neither acoustic-phonetic nor lexical factors predicted preferences in 8- and 11-month-olds. Finally, counter to our predictions, a vowel bias at the time of test for 11-month-olds was related to later productive vocabulary outcomes.

Is the consonant bias specifically human? Long-Evans rats encode vowels better than consonants in words

In natural languages, vowels tend to convey structures (syntax, prosody) while consonants are more important lexically. The consonant bias, which is the tendency to rely more on consonants than on vowels to process words, is well attested in human adults and infants after the first year of life. Is the consonant bias based on evolutionarily ancient mechanisms, potentially present in other species? The current study investigated this issue in a species phylogenetically distant from humans: Long-Evans rats. During training, the animals were presented with four natural word-forms (e.g., mano, "hand"). We then compared their responses to novel words carrying either a consonant (pano) or a vowel change (meno). Results show that the animals were less disrupted by consonantal alterations than by vocalic alterations of words. That is, word recognition was more affected by the alteration of a vowel than a consonant. Together with previous findings in very young human infants, this reliance on vocalic information we observe in rats suggests that the emergence of the consonant bias may require a combination of vocal, cognitive and auditory skills that rodents do not seem to possess.

Infants' statistical word segmentation in an artificial language is linked to both parental speech input and reported production abilities

Individual variability in infant's language processing is partly explained by environmental factors, like the quantity of parental speech input, as well as by infant-specific factors, like speech production. Here, we explore how these factors affect infant word segmentation. We used an artificial language to ensure that only statistical regularities (like transitional probabilities between syllables) could cue word boundaries, and then asked how the quantity of parental speech input and infants' babbling repertoire predict infants' abilities to use these statistical cues. We replicated prior reports showing that 8-month-old infants use statistical cues to segment words, with a preference for part-words over words (a novelty effect). Crucially, 8-month-olds with larger novelty effects had received more speech input at 4 months and had greater production abilities at 8 months. These findings establish for the first time that the ability to extract statistical information from speech correlates with individual factors in infancy, like early speech experience and language production. Implications of these findings for understanding individual variability in early language acquisition are discussed.

Adult Learning of Novel Words in a Non-native Language: Consonants, Vowels, and Tones

While words are distinguished primarily by consonants and vowels in many languages, tones are also used in the majority of the world's languages to cue lexical contrasts. However, studies on novel word learning have largely concentrated on consonants and vowels. To shed more light on the use of tonal information in novel word learning and its relationship with the development of phonological categories, the present study explored how adults' ability to learn minimal pair pseudowords in a tone language is modulated by their native phonological knowledge. Twenty-four adult speakers of three languages were tested: Cantonese, Mandarin, and French. Eye-tracking was used to record eye movements of these learners, while they were watching animated cartoons in Cantonese. On each trial, adults had to learn two new label-object associations, while the labels differed minimally by a consonant, a vowel, or a tone. Learning would therefore attest to participants' ability to use phonological information to distinguish the paired words. Results first revealed that adult learners in each language group performed better than chance in all conditions. Moreover, compared to native Cantonese adults, both Mandarin- and French-speaking adults performed worse on all three contrasts. In addition, French adults were worse on tones when compared to Mandarin adults. Lastly, no advantage for consonantal information in native lexical processing was found for Cantonese-speaking adults as predicted by the “division of labor” proposal, thus confirming crosslinguistic differences in consonant/vowel weight between speakers of tonal vs. non-tonal languages. These findings establish rapid novel word learning in a non-native language (long-term learning will have to be further assessed), modulated by native phonological knowledge. The implications of the findings of this adult study for further infant word learning studies are discussed.

The consonant bias in word learning is not determined by position within the word: Evidence from vowel-initial words

The current study used an object manipulation task to explore whether infants rely more on consonant information than on vowel information when learning new words even when the words start with a vowel. Canadian French-learning 20-month-olds, who were taught pairs of new vowel-initial words contrasted either on their initial vowel (opsi/eupsi) or following consonant (oupsa/outsa), were found to have learned the words only in the consonant condition and performed significantly better in the consonant condition than in the vowel condition. These results extend to Canadian French-learning infants the consonant bias in word learning previously found in French-learning infants from France and, crucially, shows that vocalic information has less weight than consonant information in new word learning even when it is the initial sound of the target words, confirming the consonant bias at the lexical level postulated by Nespor et al. (2003). The current findings also suggest that French-learning infants are able to segment vowel-initial words as early as 20 months of age.

Consonant and Vowel Processing in Word Form Segmentation: An Infant ERP Study

Segmentation skill and the preferential processing of consonants (C-bias) develop during the second half of the first year of life and it has been proposed that these facilitate language acquisition. We used Event-related brain potentials (ERPs) to investigate the neural bases of early word form segmentation, and of the early processing of onset consonants, medial vowels, and coda consonants, exploring how differences in these early skills might be related to later language outcomes. Our results with French-learning eight-month-old infants primarily support previous studies that found that the word familiarity effect in segmentation is developing from a positive to a negative polarity at this age. Although as a group infants exhibited an anterior-localized negative effect, inspection of individual results revealed that a majority of infants showed a negative-going response (Negative Responders), while a minority showed a positive-going response (Positive Responders). Furthermore, all infants demonstrated sensitivity to onset consonant mispronunciations, while Negative Responders demonstrated a lack of sensitivity to vowel mispronunciations, a developmental pattern similar to previous literature. Responses to coda consonant mispronunciations revealed neither sensitivity nor lack of sensitivity. We found that infants showing a more mature, negative response to newly segmented words compared to control words (evaluating segmentation skill) and mispronunciations (evaluating phonological processing) at test also had greater growth in word production over the second year of life than infants showing a more positive response. These results establish a relationship between early segmentation skills and phonological processing (not modulated by the type of mispronunciation) and later lexical skills.

The Developmental Origins of the Consonant Bias in Lexical Processing

Consonants have been proposed to carry more of the weight of lexical processing than vowels. This consonant bias has consistently been found in adults and has been proposed to facilitate early language acquisition. We explore the origins of this bias over the course of development and in infants learning different languages. Although the consonant bias was originally thought to be present at birth, evidence suggests that it arises from the early stages of phonological and (pre-)lexical acquisition. We discuss two theories that account for the acquisition of the consonant bias: the lexical and acoustic-phonetic hypotheses.