Firstly your eyes are not connected directly to your "brain". You don't see what is in front of you.
Your eyes write a rough scan of elements of what is in front of you to a "frame buffer". Your brain looks at this "frame buffer" and interprets what it thinks you are seeing. At this stage it can fill in blanks and do all sorts of manipulations to make it look smooth and complete.
What can also happen is your brain can write to this "frame buffer", this is indistinguishable from the information the eyes have put there. This is pretty much how psychosis works - those monsters/spiders/people/objects seem as real as if they are there.