that applies to restricted field of vision only. however, if one has wide FOV, and lets say its 2pi wide then there is no direction unseen frome where one is looking from. still one could talk about front/back or/left/right or top/down, only that all are present. but all are seen at once. like in the following images of an interior project for my friend.
or this sketch of my girlfriend's livingroom:
http://i24.photobucket.com/albums/c18/t ... ngroom.jpg
Edit by Rob: removed enormous image
so the thing of absence of part of visual field is a matter of optical interface of the perceiving agent. if we have eyes facing forward with our skin and head occluding backView, ok. but if the perceiver is a point of consciousness, or a spherical lens, then there is no occluded region in the visual field just different gestalts of the whole field of vision within its margin which is a point.