The Upside-down Face
Do the eyes and mouth in this picture look normal?
Turn it upside-down to see if you were right?
The brain is an expert at identifying faces. But since we're not used to seeing faces upside-down, the brain sometimes makes mistakes when it processes facial images.
To identify a face, the brain processes facial features and the relationships between them. A normally positioned face with an upside-down mouth and eyes appears distorted because we see a mismatch in the relationship between the features. When we turn the face upside-down, the brain loses its sensitivity to these relationships, and we don't see the distortions.
Our ability to identify faces precisely is important to our survival as human beings, and our brain has developed the expertise to help us. While our standard method for identifying objects is to categorize them ("This is a chair," This is a table," etc.), we identify faces individually ("This is Moshe," not "This is a person"). To make this distinction, the brain must process faces differently than it processes objects.
One of the manipulations used in research on facial processing is to turn a face upside-down. The basic reversal effect shows that people have no trouble identifying pictures of upside-down objects. But they are far worse at identifying upside-down faces. The explanation seem to lie in the fact that we rely on overall shape to identify objects (which helps us categorize them), but we tend to analyze facial features and the distances between them as a means of identifying specific people. We analyze upside-down faces mainly according to their general features; our sensitivity to facial details is weaker. This is the key to the Upside-down Face illusion.