It is a limited but interesting research about neurocognitive science. According to Churches et al. In Social Neuroscience, human brain reacts to emoticons as same as to a real face.
Social Neuroscience: Emoticons in mind: An event-related potential study
The researchers showed the subjects some images of face, emoticon, and random characters sequentially. Then the brain activity of the subjects was evaluated with Event Related Potential (ERP) some kind of specific electroencephalogram. They also use the inverse images of each picture to assess the difference in the reaction of the subjects.
As a result, the amplitude of the N170 (a particular ERP) was altered when the image was inversed in the case of both face and emoticon, but no change observed in random characters. This data suggests that emoticon stimulates human brain activity in a similar process as the real face.
Actually, they used the most popular emoticon :-), which most Americans have already seen many times. Nonetheless, emoticon is quite different from the real face. To begin with, it is located by 90 degrees! Even if the ability to recognize a face is congenital, cognition of emoticon is never. Thus, the same reaction of both face and emoticon means that the human brain learns that emoticon is the same as a face.
I think it is an interesting result, but the evidence is still weak. It would be better to evaluate brain blood flow during the reaction to the images. It is also curious whether people other than in European countries will react the emoticons which they have not seen ever before. I hope sequel studies.