The Binding Problem
Suppose you watch lightning strike near you. You experience a single unified event, although what reaches your senses is both sound and light. How do sound and light come together in experience? Light reaches your eyes before sound reaches your ears. Then, within the brain, auditory processing occurs at a slightly different rate than visual processing. Your brain is faced with a double discrepancy between the speeds of sound and light and between auditory and visual processing. As Libet’s experiments indicate, the brain also backdates your experience to make subjective time seem like real time. At a sufficient distance, your brain loses synchronization and you see lightning before you hear it.
What goes for hearing and vision goes for other senses—the brain is challenged to make sensory integration result in a coherent whole. That, roughly speaking, is the binding problem. When you see an object, different parts of the brain work on generating the visual experience. One part of the brain is responsible for color and another for shape; the brain must bind these qualities into a coherent whole. With some neurological disorders, the binding fails. People may see free-floating patches of color, disconnected from objects.
Some people, known as synesthetes, make perceptual bindings by adding features to objects of thought and perception. A synesthete may, for instance, add colors to digits. What are the boundaries for synesthetes—what could or could not be added to objects of perception? Some synesthetes smell words, others see sound as colored, and so on. It is estimated that one in 200 people is a synesthete, and it is thought to be more common among artists.