Emerging sensitivity to talking mouth in infants with low and elevated likelihood of autism spectrum disorder: A longitudinal study

The talker’s mouth provides significant multimodal information that supports language development. Studies have revealed that infants over 6 months of age show increased attention to the mouth of a talking face, which results in vocabulary growth. However, few studies have reported the relationship between early gaze behavior and later language development in infants with an elevated likelihood of autism spectrum disorder (ASD) (EL infants). Since ASD cannot be diagnosed in infancy, these infants provide a valuable opportunity to investigate early developmental differences in visual attention that may be associated with ASD. Therefore, we conducted a longitudinal eye-tracking experiment at ages 6, 9, 12, 18, and 24 months to investigate differences in gaze behavior between infants with a low likelihood of ASD (LL infants) and EL infants. We found that, exclusively in LL infants, the proportion of mouth-looking time significantly increased from 6 months to 9 months of age and remained relatively constant thereafter. In contrast, in EL infants, although the proportion of mouth-looking time gradually increased with age up to 24 months, their attentional change to talking mouth started later than that in LL infants. Furthermore, our results showed a conditionally positive relationship between mouth-looking and expressive vocabulary size in LL infants but not in EL infants. These findings indicate that EL infants may not utilize audiovisual information as valuable cues for language acquisition during infancy and toddlerhood, which may be a factor in their slow language development.