Development of visual and somatosensory attention of the reach-to-eat movement in human infants aged 6 to 12 months.

The reach-to-eat movement is a natural act in which an object or food item is grasped and brought to the mouth. It is one of the earliest forelimb behaviours displayed by human infants, who bring almost all grasped objects to the mouth, and is used daily by adults. In adults, there is a tight coupling between visual attention and the advance phase of the reach-to-eat movement. The target is visually engaged just as hand advance is initiated and visually disengaged just as the target is grasped. This coupling of vision and hand advance suggests that advance is mediated by visual attention and withdrawal by somatosensation. The present study examined when the tight coupling between visual attention and the advance phase of the movement develops in infancy. In a longitudinal study, eight infants, aged 6-12 months, and 20 adults reached for familiar inanimate objects and food items. Visual gaze, hand movement and hand accuracy were measured using frame-by-frame video scoring and 2D kinematic analysis. The study found that the youngest infants (6-8 months) visually engaged the target well before initiating a reaching movement and continued to fixate on the target after it was grasped and as it was brought to the mouth. Between 10 and 12 months of age, infants began to visually engage the target just as the reaching movement was initiated and visually disengaged the target as it was grasped, as did the adults. Over the same developmental time period, the infants developed rotatory hand shaping movements, precision grasping, and improved targeting accuracy both for grasping the object and placing it into the mouth. The results suggest that visual guidance of advance and somatosensory guidance of withdrawal develop together and in concert with hand movement ability and skill.