Some studies analyse how aroused a baby is and whether she/he smiles or frowns while seeing the emotions of other people.
In order to study the role of facial expressions in our everyday life we need precise ways of measuring them. Electromyagrphy (EMG) uses little sensors that are placed on the face to assess the activity of facial muscles. These sensors are very light in weight in order not to interfere with natural movements. In the picture you can the sensors places on the cheek to record the activity of the muscle that makes us smile. Facial EMG allows us to measure facial activity in more detail than it might be possible by simply observing a child, and can even detect changes in our faces that are too subtle to be visible to the naked eye.
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