Mouth Motion and Growing Interest in Speech Drives the Developmental Shift in Infant Attention to the Mouth of a Talking Face

Lewkowicz & Hansen-Tift found that when 4-month-old infants see and hear a person
talking, they look more at her eyes but that 8- and 10-mo infants look more at her mouth. The
developmental attentional shift to the mouth reflects infants’ growing interest in speech.
Attention to the mouth enables infants to gain access to redundant and maximally salient
audiovisual cues which then facilitate speech and language acquisition.
We investigated the separate role of mouth movement and vocalization cues in the attentional
shift from a talker’s eyes to the talker’s mouth. In 3 experiments, we used an eye-tracker to
measure the proportion of attention infants, 4-, 8-, and 10-mo, allocate to the eyes and mouth of a
static/silent face, a static/talking face, and a silently talking face. We found that when infants see
a static person, they attend to the eyes. Lewkowicz & Hansen-Tift found that when infants see
and hear a person talking, 4-mos look at the eyes whereas 8- and 10-mos look at the mouth.
When infants see a silently talking person, only 10-mos look at the mouth. These findings
demonstrate that the shift from the eyes to the mouth is mediated by three factors: dynamic
visual speech cues, an emerging interest in speech, and the redundancy of audiovisual speech.
Thus, younger infants are not interested in speech so they focus on the eyes, whereas older
infants become interested in speech, shifting their focus to the mouth, but initially at 8 m, this
shift requires that speech be multisensory.