My friend, Heather Sellers, an acclaimed writer and teacher, cannot recognize faces. Her vision is perfect, but her perception is not. In her book, You Don’t Look Like Anyone I Know (2010), she tells of awkward moments resulting from her lifelong prosopagnosia—face blindness.

People sometimes see Sellers as snobby or cold. “Why did you walk past me?” a neighbor might later ask. Hoping to avoid offending others, Sellers sometimes fakes recognition. She smiles at people she passes, in case she knows them. She may pretend to know the person with whom she is talking. But there is an upside to these perception failures. When she runs into someone who previously irritated her, she typically feels no ill will. She doesn’t recognize the person.

Unlike Sellers, most of us have an area on the underside of our brain’s right hemisphere that helps us recognize a familiar human face as soon as we detect it—in only one-seventh of a second (Jacques & Rossion, 2006). This ability is an example of a broader principle. Nature’s sensory gifts enable each animal to obtain essential information. Some examples:

• Frogs, which feed on flying insects, have cells in their eyes that fire only in response to small, dark, moving objects. A frog could starve to death knee-deep in motionless flies. But let one zoom by and the frog’s “bug detector” cells snap awake.
• Male silkworm moths’ odor receptors can detect one-billionth of an ounce of sex attractant per second released by a female one mile away. That is why there continue to be silkworms.
  

  “Time’s fun when you’re having flies.”

  Kermit the Frog

• Human ears are most sensitive to sound frequencies that include human voices, especially a baby’s cry.

In this chapter, we’ll look more closely at what psychologists have learned about how we sense and perceive the world around us. We begin with some basic principles.