Verified by 
When I was in high school, my friend switched sodas on me one day. I thought I was taking a swig of Coca Cola, but he had replaced it with cream soda. I immediately spit it out, thinking it tasted horrible. But I generally like cream soda. Why is this? The short answer is that my “top-down” processing conflicted with the “bottom-up” sensory input. To understand what I mean, let’s think about perception. After clarifying, I will help you “see” what I mean with some common perceptual illusions.
Look around you. What could be easier and more automatic than seeing the computer or the desk or the trees blowing in the wind outside? And yet, despite occurring so effortlessly and automatically, our ability to perceive the world is a truly remarkable thing. As computer scientists have discovered, it is enormously complicated to figure out how to build an object detector. And not only are we able to detect and identify objects and events, but we have an actual first-person experience of them.
The basic outline of how perception works: Through experience, the mind-brain builds perceptual categories of objects. These categories emerge from basic interaction with the object and, in humans, via conceptual knowledge and naming. These perceptual (and, to a lesser extent, conceptual) categories serve as schema or templates, and perception occurs via the process of matching sensory input patterns to perceptual templates.
The matching process is what gives us the experience of figure-ground relationships. Cognitive and neuroscientists attempt to explore the rules by which bottom-up sensory inputs are matched to top-down perceptual templates to give rise to the experience of the object.
Here is the basic schematic. Perceptual illusions provide a great way to experience the template matching process firsthand.
Let’s start with a simple example. When you look at a picture, your eyes will scan around. Subconsciously, your mind is bringing templates to match patterns. When the match happens, the object “pops” out at you. Interestingly enough, once you see it you can’t “unsee” it. It just is there.
Another great way to see perception as the intersection of bottom-up and top-down processing is to take a look at pictures that have dual objects or dual perspectives in them. Because you perceive via template matching, you will experience these illusions as flipping back and forth between different objects or the same object from different angles. You can’t see them both simultaneously because experience emerges from the matching process. So, the figure and ground keep flipping back and forth.
In the above map outlining the processes, you will see that there are “rules” next to the “hypothesis generator” box. These refer to the ways in which basic sensory inputs are categorized and organized. For example, when looking at black and white lines, some of our vision receptors fire at vertical lines and others at horizontal lines. The rules form the patterns that organize the basic sensory input. For example, some neurons fire in response to contrast, which in turn can get processed as motion if patterned in particular ways.
Conceptual knowledge is key. If you play a lot of chess, a certain depiction will immediately jump out at you as a checkmate, whereas if you do not play the game, it will be a confusing arrangement of vaguely familiar pieces. (If you are somewhat familiar with the game, you might question if you are in checkmate). Likewise, you are able to see words without effort, but if it had been written in Chinese, the experience likely would be of odd shapes and symbols with no meaning.
The bottom line is that as you go about your life, you are constantly (and unconsciously) organizing sensory inputs via rules and then matching them to your perceptual knowledge, which in turn is then translated, analyzed, and manipulated via conceptual knowledge, at least when you are surprised or in need of deeper understanding about what you are seeing. It is one of those everyday things that we should occasionally remind ourselves about how awesome it really is.
