Vision provides us with an enormous amount of information. It tells us about shapes and colors, about spatial arrangements, and about objects both near and far away. We also tend to put great trust in our vision—it’s why we say things like “seeing is believ-ing.” And it’s easy to document this trust in vision. We can, for example, arrange things so that you see a person speaking off to the left but hear their voice from your right. In this setting, you’re likely to believe what you see and thus (mis)perceive the voice to be coming from the left. Common experience confirms this point: In large lec-ture halls, the speaker’s voice sounds like it’s coming from the front of the room— where the plainly visible lecturer is standing. But in many cases, the sound waves are actually reaching you from loudspeakers positioned around the room; you can check this by closing your eyes and paying careful attention to where the sounds are coming from. The moment you open your eyes, though, the sounds again seem to be coming from the front of the lecture hall—the visual information is overruling the evidence you receive from your ears.
How does vision function? In tackling this broad question, we’ll focus on three issues. First, what are the structures for gathering the stimulus, and how do they work? Second, what is the nature of the transduction process that converts the physical energy of the stimulus into a neural signal? Third, what are the coding processes that allow us to discriminate—and then recognize—the millions of shapes, colors, and pat-terns of movement that make up our visual world?