The Cinema in Your Head

Experience suggests that the things we see are somehow projected onto asingle mental movie screen. But vision is more like a multiplex theater where each screen shows a slightly different version of the same flick.

That's because vision is a modular process: Different brain areas tackle particular aspects of the job. Some regions, for example, specialize in detecting moving objects, a division of labor vividly demonstrated by certain stroke victims who can see shapes and colors but have trouble seeing objects in motion.

Similarly, the task of tracking spatial locations may also be divvied up in the brain, reports Carol Colby, Ph.D., a cognitive neuroscientist at the National Eye Institute. An object's location in three-dimensional space seems to be represented many times over in the cerebral cortex, with specific regions keeping tabs on where objects are in relationship to specific parts of our anatomy.

For example, circuits in the parietal lobe—a region in the upper rear of our brain that concerns itself with sensory information—monitor the location of objects relative to our head; they signal their neuronal colleagues if we suddenly need to duck. Another brain area helps perceive where things are in relationship to our mouth, allowing us to bite into a hot dog without chomping on air.

"We have the sense that we see the world directly," observes Colby. But vision is a furiously active process in which our brain constructs reality for us. Interpreting the patterns of light that strike our retina requires integrating signals from billions of disparate neurons. Given the complexity of the job, she says, "it is remarkable how easily we do this."

Images seem to be represented in the brains of rhesus monkeys in as many as 20 places--and our own brains may contain even more sites.