Top-down processing is our ability to sort information as we perceive it. Sensation and Perception are more integrated than previously believed. Instead of sensation coming first (collecting data) and perception coming second (extracting meaning), both occur interactively.
There are so many interesting areas of psychology. Here's an intro into physiological psychology. If You Know Nothing About Physiological Psychology
To explain the parts of the eye and how they work, let's take a look at my eyes. You'll learn how your eyes work and how they differ form mine. We'll start at the front and work our way back. Part 1 of An Albino's Guide
If you were designing an eye, here are some of your options. Overall, humans chose well. If I Only Had An Eye
To understand our visual systems, you need to know a bit about light. Here's a quick introduction on the characteristics of light. Show Me The Light