I n the past few decades, we have learned a good deal about the brain. We have a fair idea of its structure, and we are discovering what regions become active when specic tasks are performed. On the other hand, this research has produced an aggregate of facts that are interconnected only loosely, if at all. As Francis Crick noted some time ago, what is conspicuously lacking is a broad framework of ideas capable of encompassing all these ndings. Described in this chapter is a rst approximation to such a framework. The framework builds on ideas of earlier researchers and is intended as a general model of how information is processed in the brain. As such, it applies to all sense modalities and cognitive processes. The bulk of the examples, however, have been drawn from the eld of vision.