When we need to record with precision where a baby is looking, we use an instrument called eye-tracker. This instrument relies on a systems of cameras to record images of the eyes. Based on the eyes’ orientation, the eye-tracker calculates with millimeter precision where the eye-gaze lands. The video below illustrates an eye-tracker in action. The red dot shows in real-time where the baby is looking and for how long. The bigger the dot, the longer the baby is looking. You will notice that although this baby is very young, only 7-months-old, the eye movements are quite precise.
In this video you can see which parts of happy face a 7-months-old baby is exploring.