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.
These results are a striking example of the role of action in perception. We are often taught that we sense the world and then react. But our sensing and moving do not happen in sequence. We explore the world by moving our eyes, hands, and other sensory structures. Indeed, we are much better at recognizing an object by touch if we move our fingers over the object rather than have someone else move the object passively over our hands. To see and understand what we are seeing, we must move our eyes in coordinated ways. I learned this concept firsthand through my own vision therapy. Cross-eyed and stereoblind since infancy, I had to learn how to direct my two eyes to the same point in space before I could see in 3D.
Similarly, if SM is to learn to recognize fear in another's face, she has to learn to direct her gaze to another person's eyes. For most of us, we do this without thinking. Our amygdala, a brain structure that SM lacks, directs our eyes to search for environmental cues indicating threat or danger. In so doing, we scan a face by looking at the eyes. In general, our exploratory movements are so automatic that we often don't realize just how much our own actions contribute to the way we sense the world.