The ability to share attention gradually develops through social interactions with caregivers in the first few years of life. It is known that infants are able to use another's direction of gaze as a cue to find the object. However, limited knowledge about the effect of a tutor's gaze direction and speech on infants' attention during naturalistic triadic interactions exists. Therefore, this study assessed how a tutor's gaze direction and speech affect infants' attention to the face of the tutor and referent using an eye-tracker. As an infant is capable of following an adult's gaze direction, we predicted the following three situations ; (1) the infants would be more attentive to the object when the tutor looked at the object compared with when the tutor looked at the infant ; (2) the frequency of joint attention would increase when the tutor's gaze direction shifted between the object and the infant when compared with a situation wherein the tutor's gaze remained fixed toward either the object or the infant ; and (3) the infants would shift their gaze more frequently from the tutor to the object when the tutor used an infant-directed speech rather than an adult-directed speech. In our experiment, 5- to 7-month-old (n=19) and 10- to 12-month-old (n=15) infants watched three types of movie clips showing a female adult tutor and puppet : the tutor always looks in front (front), the tutor always looks at the object (object), and the tutor often switches her gaze direction between the front and the object (switch). Moreover, the tutor was speaking either in infant- or adult-directed manner. Although our results revealed no difference between the frequencies of joint attention among the different conditions, their looking time toward the puppet was longest in the object condition. Moreover, 10- to 12-month-olds' looking time toward the puppet was significantly longer when the tutor used infant-directed speech rather than adult-directed speech. Furthermore, older infants looked at the puppet longer than at the tutor's face in case of infant-directed speech. The results indicated that infants' looking time toward an object would vary due to a tutor's looking behavior and infants would be able to use not only visual information but also auditory information from a tutor to control their gaze direction at the end of the first year.

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