The main scientific challenge in the study of sensation and perception is to relate the physical world to the psychological world. The environment contains physical stimuli: light waves, sound waves, heat sources, and so forth. When people encounter them, they have psychological experiences: sights, sounds, feelings of warmth. How are the physical events translated into psychological events?

The translation from physical to psychological begins with transduction, a biological process in which physical stimuli activate cells in the body’s nervous system. Once activated, the cells send nerve impulses toward the brain that function as signals, telling the brain that physical stimulation has occurred. Processing of the signals in the brain itself gives rise to perceptual and sensory experience.

Transduction does not fully determine a person’s sensory and perceptual experiences. Other factors—especially the setting in which the stimulation occurs and the person’s state of mind when it occurs—also are influential. But transduction is the critical first step.

The cells in the nervous system that transduce physical stimuli are called receptor cells. Different types of receptor cells are sensitive to different types of physical stimulation from the environment. When stimulated, they send signals to the brain.

In general, any one type of receptor cell is sensitive to one type of stimulation. For example, receptor cells in the eye, called photoreceptors, are sensitive to stimulation by light. A variety of receptor cells located just under the surface of the skin are sensitive to physical pressure. Receptors in the ear are stimulated by sound waves. By using these impulses, the perceptual system as a whole can gather information about the outside world.

Let’s see exactly how perceptual systems acquire this information by exploring the visual system.