Percepts are epistemologically first because they are what our conscious mind actually deals with. Sensations are integrated into percepts unconsciously, automatically, before they reach our conscious mind. In terms of physics, sensations come first, interacting with our senses, and then are integrated into percepts automatically by our brains. When we look out, we don’t see or hear individual sensations, we see them already integrated into percepts. This is important to understand because the fact that percept formation is automatic means that it cannot err. It occurs according to deterministic processes. Our conscious mind can misinterpret percepts, but there cannot be a contradiction within the physical process itself that produces percepts.
David Kelley’s book “The Evidence of the Senses” goes into great detail concerning perception and sensations. Grames, one of our frequent forum users, has posted his notes on the relevant section here: http://forum.objectivismonline.com/index.php?showtopic=17927&view=findpost&p=237133. It’s quite involved stuff.