Parallel processing is the brain's ability to process multiple incoming stimuli at the same time. For example, when the brain processes elements of vision, such as color, motion, shape, and depth, and compares these elements to memories, it creates an image of what you are seeing and it allows you to comprehend what you are seeing because it puts all of the elements and comparisons together into a "whole".
There are two main ways that the brain processes, bottom-up and top-down. Bottom-up processing is where your brain puts together a "whole" from all of the parts. Top-down processing is where your beliefs and expectations influence the "whole". Thinking about real life people, top-down can be compared to the person who sees the larger picture picture when they look at things and bottom-up is the person who sees the little pieces when they look at things. Often, these two types of processing work together, but sometimes one of these processes is relied on more than the other.
Looking at ambiguous figures is one way to identify which one of the processes is being relied on most heavily. For example, this figure could be interpreted as either a duck or a rabbit.
(here is the link to a photo of the rabbit/duck if it doesn't appear below)
Depending on our expectations (like if we read a caption that said "duck" or "rabbit"), we may perceive the figure as one of the two because our perception would be biased. Our top-down would influence our bottom-up to put together a "whole" that goes accordingly with the thought that the caption ("duck" or "rabbit") creates.
I found this topic interesting and significant in life because we are constantly perceiving things whether we are aware of it or not, and without parallel processing, the brain would have to analyze each piece of information one at a time, which would take such a long time! Say the rabbit/duck had big fangs and was going to attack. It wouldn't really matter if the person perceived it as a rabbit with fangs or a duck with fangs first as long as the person was able to perceive it quickly. If the brain could only process one piece of information at a time, the rabbit/duck with fangs would have attacked by the time the brain was able to put together a "whole".