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I remember learning in previous biopsych classes that our eyes do not fully perceive the visual world and that our brains "fill in the gaps" to give us the picture that we "see" when we look at the world. This concept of "filling in the gaps" has always been mind-boggling to me as it is so fascinating how all humans seem to fill in the gaps in the exact same way. We learned in class that our brains picks up information about edges and is able to fill in what goes between those edges. I believe it is the ganglion cells that are responsible for this ability as they are able to compare a particular visual point to its surroundings. However, in one of the blind spot tests that we did where our brains "filled in" a yellow dot that was in our blind spot with a red dot (the yellow dot was surrounded by other red dots), it seems as if the brain not only fills in edges but is also able to create edges. This concept is interesting because it shows that our brains don't just simple "fill in" gaps but can create new boundaries within those gaps. So how does our brain do this? How does it know where to create new edges and not just fill in the existing ones?

Moreover, I understand that each of our eyes has a blind spot where the optic nerve endings are. But, in all of the blind spot tests that we did, we had to close one eye to get the "blind spot effect". So doesn't one eye see what is in the blind spot of the other eye (as each eye's blind spot is in a different location, creating a complete visual picture that doesn't need any mental "filling in"? I believe this question was raised during class, but I forgot how it was addressed. Since most people can see with both eyes, when is this mental filling in of the visual picture necessary?