There are two general processes involved in sensation and perception. Bottom-up processing refers to processing sensory information as it is coming in. In other words, if I flash a random picture on the screen, your eyes detect the features, your brain pieces it together, and you perceive a picture of an eagle. What you see is based only on the sensory information coming in. Bottom-up refers to the way it is built up from the smallest pieces of sensory information.
Top-down processing, on the other hand, refers to perception that is driven by cognition. Your brain applies what it knows and what it expects to perceive and fills in the blanks, so to speak. First, let us look at a visual example:
Look at the shape in the box to the right. Seen alone, your brain engages in bottom-up processing. There are two thick vertical lines and three thin horizontal lines. There is no context to give it a specific meaning, so there is no top-down processing involved.
Now, look at the same shape in two different contexts.
Surrounded by sequential letters, your brain expects the shape to be a letter and to complete the sequence. In that context, you perceive the lines to form the shape of the letter “B.” Surrounded by numbers, the same shape now looks like the number “13.” When given a context, your perception is driven by your cognitive expectations. Now you are processing the shape in a top-down fashion.
Next, watch this video for an example of top-down processing with auditory stimuli. Note that at the end, once you have heard the full sentence, you can understand it even when it is broken up again. A “phoneme” is just a basic unit of speech sound.
Watch: Phonemic Restoration Demo / Examples (http://youtu.be/k74KCfSDCn8)
FACE:
To the right is one final example of top-down processing. From a bottom-up perspective, you should see a bunch of meaningless blobs. However, our brain is wired to detect faces, which, from a biosociological perspective, is among the most important stimuli in the world. So the floating blob becomes an eye, and from there we construct a nose and a mouth, and the fact that the picture is labeled as “face” tells your brain that is what it is supposed to see. So here is the twist… instead of a face, now look at the image and see a saxophone player wearing a big hat. Some of you may have noticed that from the beginning, but for most, being told there is another image there will alert your brain to search for the pattern.So again, with these top-down processing example, your brain adds meaning what you perceive based on what it knows or expects.
Visit: Perceptual Comparisons (http://goo.gl/lh4Bs0) so that you can describe the general nature of perceptual contrast. You do not need to focus on the details of perimeters and such, but be able to explain (using the examples provided) how our perceptual experience is influenced by comparisons that we make.
Finally, check out a demonstration of how top-down processing drives your ability to read.
Visit: http://www.ecenglish.com/learnenglish/lessons/can-you-read