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Infants increasingly gaze at the mouth of talking faces during the latter half of the first postnatal year. This study investigated mouth-looking behavior of 120 full-term infants and toddlers (6 months-3 years) and 12 young adults (21-24 years) from Japanese monolingual families. The purpose of the study included: (1) Is such an attentional shift to the mouth in infancy similarly observed in Japanese environment where contribution of visual speech is known to be relatively weak? (2) Can noisy conditions increase mouth-looking behavior of Japanese young children? (3) Is the mouth-looking behavior related to language acquisition? To this end, movies of a talker speaking short phrases were presented while manipulating signal-to-noise ratio (SNR: Clear, SN+4, and SN-4). Expressive vocabulary of toddlers was obtained through their parents. The results indicated that Japanese infants initially have a strong preference for the eyes to mouth which is weakened toward 10 months, but the shift was later and in a milder fashion compared to known results for English-learning infants. Even after 10 months, no clear-cut preference for the mouth was observed even in linguistically challenging situations with strong noise until 3 years of age. In the Clear condition, there was a return of the gaze to the eyes as early as 3 years of age, where they showed increasing attention to the mouth with increasing noise level. In addition, multiple regression analyses revealed a tendency that 2- and 3-year-olds with larger vocabulary increasingly look at the eyes. Overall, the gaze of Japanese-learning infants and toddlers was more biased to the eyes in various aspects compared to known results of English-learning infants. The present findings shed new light on our understanding of the development of selective attention to the mouth in non-western populations.

Japanese 8-month-olds were tested to investigate the matching of particular lip movements to corresponding non-canonical sounds, namely a bilabial trill (BT) and a whistle (WL). The results showed that the infants succeeded in lip-voice matching for the bilabial trill, whereas they failed to do so for the whistle.

Japanese infants at the ages of 6, 12, and 18 months were tested on their ability to discriminate three nonsense words with different phonotactic status: canonical keetsu, noncanonical but possible keets, and noncanonical and impossible keet. The results showed that 12 and 18 months olds discriminate the keets/keetsu pair, but infants in all age groups fail to discriminate the keets/keet pair. Taken together with the findings in our previous study [Kajikawa et al., J. Acoust. Soc. Am. 120(4), 2278-2284 (2006)], these results suggest that Japanese infants develop the perceptual sensitivity for native phonotactics after 6 months of age, and that this sensitivity is limited to canonical patterns at this early developmental stage.

This study explored sensitivity to word-level phonotactic patterns in English and Japanese monolingual infants. Infants at the ages of 6, 12, and 18 months were tested on their ability to discriminate between test words using a habituation-switch experimental paradigm. All of the test words, neek, neeks, and neekusu, are phonotactically legitimate for English, whereas the first two words are critically noncanonical in Japanese. The language-specific phonotactical congruence influenced infants' performance in discrimination. English-learning infants could discriminate between neek and neeks at the age of 18 months, but Japanese infants could not. There was a similar developmental pattern for infants of both language groups for discrimination of neek and neeks, but Japanese infants showed a different trajectory from English infants for neekusu/neeks. These differences reflect the different status of these word patterns with respect to the phonotactics of both languages, and reveal early sensitivity to subtle phonotactic and language input patterns in each language.

This study investigated vowel length discrimination in infants from 2 language backgrounds, Japanese and English, in which vowel length is either phonemic or nonphonemic. Experiment 1 revealed that English 18-month-olds discriminate short and long vowels although vowel length is not phonemically contrastive in English. Experiments 2 and 3 revealed that Japanese 18-month-olds also discriminate the pairs but in an asymmetric manner: They detected only the change from long to short vowel, but not the change in the opposite direction, although English infants in Experiment 1 detected the change in both directions. Experiment 4 tested Japanese 10-month-olds and revealed a symmetric pattern of discrimination similar to that of English 18-month-olds. Experiment 5 revealed that native adult Japanese speakers, unlike Japanese 18-month-old infants who are presumably still developing phonological perception, ultimately acquire a symmetrical discrimination pattern for the vowel contrasts. Taken together, our findings suggest that English 18-month-olds and Japanese 10-month-olds perceive vowel length using simple acoustic?phonetic cues, whereas Japanese 18-month-olds perceive it under the influence of the emerging native phonology, which leads to a transient asymmetric pattern in perception.

Faces convey primal information for our social life. This information is so primal that we sometimes find faces in non-face objects. Such illusory perception is called pareidolia. In this study, using infants’ orientation behavior toward a sound source, we demonstrated that infants also perceive pareidolic faces. An image formed by four blobs and an outline was shown to infants with or without pure tones, and the time they spent looking at each blob was compared. Since the mouth is the unique sound source in a face and the literature has shown that infants older than 6 months already have sound-mouth association, increased looking time towards the bottom blob (pareidolic mouth area) during sound presentation indicated that they illusorily perceive a face in the image. Infants aged 10 and 12 months looked longer at the bottom blob under the upright-image condition, whereas no differences in looking time were observed for any blob under the inverted-image condition. However, 8-month-olds did not show any difference in looking time under both the upright and inverted conditions, suggesting that the perception of pareidolic faces, through sound association, comes to develop at around 8 to 10 months after birth.

Recent studies using a violation-of-expectation task suggest that preverbal infants are capable of recognizing basic arithmetical operations involving visual objects. There is still debate, however, over whether their performance is based on any expectation of the arithmetical operations, or on a general perceptual tendency to prefer visually familiar and complex displays. Here we provide new evidence that 5-month-old infants recognize basic arithmetic operations across sensory modalities. Using a violation-of-expectation task that eliminated the possibility of the familiarity and complexity preference, 5-month-old infants were presented alternatively with two types of arithmetical events: the expected, correct outcomes of operations (1 object+1 tone=2 objects and 1 object+2 tones=3 objects) and the unexpected, incorrect ones (1 object+2 tones=2 objects and 1 object+1 tone=3 objects). Results showed that subjects looked significantly longer at the unexpected events than at the expected events, suggesting that infants are able to recognize basic arithmetic operations across sensory modalities.