Sex Differences in Visual Attention to the Mouth in Infancy: Implications for Language Development and ASD

Background: Development of pre-linguistic communication skills, like gesture, predicts acquisition of language in typically developing infants and toddlers (Iverson & Goldin-Meadow, 2005). Prior to, and concurrent with, such communicative development, infants are attuned to social auditory and visual input, such that their visual behavior serves to index interest in, and predict later proficiency with, social-cognitive abilities (Jones & Klin, 2013). Analysis of social-visual engagement in infants suggests that the degree to which infants visually fixate on eyes of others successfully differentiates between typically-developing (TD) infants and infants subsequently diagnosed with Autism Spectrum Disorder (ASD; Jones & Klin, 2013). Irrespective of diagnosis, a global increase is evident in the second year of life in visual attention to the mouth (mouth-fixation), immediately preceding a period of known vocabulary growth, and a sex difference is evident in the chronology of this increase. The present study suggests mouth-fixation as potentially facilitative of language acquisition, delay of which is a hallmark of ASD.

Objectives: The current study seeks to elucidate the importance of increased mouth-fixation for language development, and to explore apparent sex differences among TD infants for their importance to the male-bias in ASD diagnosis. In keeping with these goals, this study tests the extent to which increased mouth-fixation  correlates with increases in communicative abilities and in specific indices of language acquisition.

Methods: Eye-tracking data were collected longitudinally at 10 time-points from 2 to 24 months of life in TD-infants (26 male, 24 female), and high-risk infants later diagnosed with ASD (13 male). The Mullen Scales of Early Learning (Mullen) and the MacArthur-Bates Communicative Development Inventory (CDI) were administered longitudinally, assessing cognitive and communicative functioning.

Results: Results indicate a phase-shift in mouth-fixation, from increasing with age to decreasing with age, early in the second year of life. The mouth-fixation shift is evident in TD-females at 13.5 months of age, in TD-males at 15 months, and in ASD-males between 15 and 16 months. Concurrent sex differences in trajectories of scores on clinical measures of expressive vocabulary (F(2,47)=7.214, p<0.01), gesture (F(2,47)=5.230, p<0.01),  and language-related cognitive abilities (F(3,46)=9.452, p<0.01), indicate that TD-females are precocious in these areas, and support a relationship between mouth-fixation and language development. Spearman’s rho was used to quantify the relationship between mouth-fixation and measures of communicative development before and after mouth-fixation phase-shift, for each sex and for ASD-males (Table 1).

Conclusions: Measures of communicative development were correlated with age and not with mouth-fixation prior to mouth-fixation phase-shift. After the phase-shift, vocabulary measures and mouth-fixation were negatively correlated in females, during their period of decline in mouth-fixation. As such, vocabulary growth was greatest just after the peak in mouth-fixation. Similar relationships existed for TD-males and ASD-males. Results suggest that sex-dimorphic chronologies of developmental events in typical-development are present in both language and in an apparent behavioral correlate, and can inform our understanding of developmental trajectories that deviate from those of TD-infants in ASD, as TD-females appear chronologically advanced.