The shadows are here. Again.

I keep seeing black shapes in the corners of my vision. They are usually circles and polygons but lately i have had a new figure. A bird... I saw it for the first time yesterday, in english it flew over the boundaries of my top vision. I darted my head up to once again, see nothing. It really frustrates me. If what my biology teacher says is correct that our brain makes things up to NOT be there to protect us, then why does mine make things up to BE there? I have not taken any halucenagenics (well as far as i know) so why do i see things? It really is quite disturbing!