I can't tell you how nice it was to find this site. It's the Scientific American and it's a fantastic Magazine that I used to love reading through in my earlier grade school years. They had it in our library then and that was my first all time favorite hang-out place. I both cut classes to go there and later skipped school altogether opting to stay in there rather than do classes at all.
This post is a representation of their article called "Reading between the lines: How we see hidden objects". That title is a link to the original post, as is this [link here]. You will want to read theirs because it is three pages long and mine is just a primer for it.
Here also is their site [The Scientific American Journal], enjoy it.
Imagine that you are looking at a dog that is standing behind a picket fence. You do not see several slices of dog; you see a single dog that is partially hidden by a series of opaque vertical slats. The brain’s ability to join these pieces into a perceptual whole demonstrates a fascinating process known as amodal completion.
It is clear why such a tendency would have evolved. Animals must be able to spot a mate, predator or prey through dense foliage. The retinal image may contain only fragments, but the brain’s visual system links them, reconstructing the object so the animal can recognize what it sees. The process seems effortless to us, but it has turned out to be one of those things that is horrendously difficult to program computers to do. Nor is it clear how neurons in the brain’s visual pathways manage the trick.
In the early 20th century Gestalt psychologists were very interested in this problem. They devised a number of cunningly contrived illusions to investigate how the visual system establishes the continuity of an object and its contours when the object is partially obscured. A striking example of amodal completion is an illusion devised by Italian psychologist Gaetano Kanizsa. In one view, you see a set of “chicken feet” arranged geometrically. But if you merely add a set of opaque diagonal bars, a three-dimensional cube springs into focus seemingly by magic, the chicken feet becoming cube corners.
The astonishing thing is you do not even need to overlay real bars—even illusory ones will do. Here the otherwise inexplicable absence of boundaries terminating the chicken feet leads the brain to automatically infer the presence of opaque bars. So you see an illusory cube occluded by illusory bars!
The term “amodal completion” was coined to distinguish it from modal completion. Modal completion is the brain’s tendency to see the full outline of a nonexistent object, as occurs in Kanizsa’s classic triangle illusion. The brain regards it as highly improbable that some sneaky visual scientist has placed three black disks with pie-shaped wedges cut out of them precisely in this manner, preferring instead to see an opaque white triangle that is partially covering three black disks.