The brain is a complex perceptual-motor system. To assemble an understanding of this system we need some ploy to get a grip on this complexity. Beginning at the level of individual muscles, a functional model for organizing the brain is developed. This forms the basis for an approach to the design of intelligent robotic systems based on the idea of composing complex systems from primitive functional modules. This paper addresses the sensor fusion aspects of this composition. We identify perceptual components of primitive functional modules, and address the problems of sensor fusion when composing complex perceptual systems employing an array of environmental sensory modalities. The discussion is centered on the human perceptual-motor system.