SM is a woman with a very rare disease that destroyed her amygdala, a structure in the forebrain that is involved with our experience of fear. Essentially all other parts of SM's brain are intact. She has normal intelligence and language function and no movement or sensory deficits. However, SM does not show normal fear responses. She is indiscriminately trusting and friendly. While she is able to sketch a picture of a face expressing happiness, sadness, surprise, anger, and disgust, she cannot draw a face showing fear. Nor could she recognize fear in photographs of human faces.
Most of us judge facial expressions by looking primarily at the eyes. Even seven week old infants explore the face by fixating on the eyes. When scientists monitored SM's eye movements as she looked at photos of human faces, they discovered that she did not direct her gaze at the eyes. It is possible to judge other emotions, such as happiness, by looking at the mouth, but to recognize fear, you must look at the eyes. Indeed, if the eyes are erased from photos of a human face, normal subjects lose their ability to recognize fearful expressions. Moreover, when SM was instructed specifically to look at the eyes in the photos, her ability to recognize fear increased to normal levels.